Igneous Rock Associations 7. Arc magmatism I: relationship between subduction and magma genesis.SUMMARY
Dehydration of subducted oceanic lithosphere lithosphere (lĭth`əsfēr '), brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle. releases fluid into the overlying overlying
suffocation of piglets by the sow. The piglets may be weak from illness or malnutrition, the sow may be clumsy or ill, the pen may be inadequate in size or poorly designed so that piglets cannot escape. mantle wedge and initiates a chain of events culminating in the generation of magma that rises to form volcanic arcs. Arc magmas are distinct from magmas in other settings because of the different geothermal regimes at destructive plate margins, the presence of fluids/volatiles derived from dehydration of the subducted slab and the generally compressive com·pres·sive
Serving to or able to compress.
com·pressive·ly adv. tectonic regime that inhibits the ascent of magma, thereby promoting extensive interaction with the adjacent wall rocks. As a result, most arc magmas solidify as intrusive bodies, ranging from sills and dykes to large plutonic plu·ton·ic
Of deep igneous or magmatic origin: plutonic rocks.
[From Latin Pl complexes. Several mechanisms facilitate the rise of arc magmas. Diapirs rising from a larger pool of buoyant magma are important in the ductile lower crust. The rapid rise and the expansion of magma results in the propagation of fractures, that facilitate stoping and assimilation in the brittle upper crust. Fracture zones are repeatedly exploited, and the net result may be formation of a composite batholith batholith, enormous mass of intrusive igneous rock, that is, rock made of once-molten material that has solidified below the earth's surface (see rock). Batholiths usually are granitic (see granite) in composition, have steeply inclined walls, have no visible floors, .
Water plays an important role in all stages of arc-magma evolution. Water lowers the temperatures that are required for the partial melting in the mantle wedge, which produces mafic magma, and in the crust, produces felsic fel·sic
Containing a group of light-colored silicate minerals that occur in igneous rocks.
[fel(dspar) + s(ilica) + -ic. magma. Arc magmatism is intimately related to metamorphism metamorphism, in geology, process of change in the structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock being changed remains , although this relationship is complicated largely because maximum pressures and temperatures are attained at different times. Arcs under compression undergo rapid thickening, followed by erosional or tectonic exhumation. Crustal crust·al
Of or relating to a crust, especially that of the earth or the moon.
Adj. 1. crustal - of or relating to or characteristic of the crust of the earth or moon melting is triggered by a variety of processes, including relaxation following crustal thickening. Melting initiates at the base of the crust, but eventually occurs at shallower crustal levels. Arcs under extension have a steep geothermal gradient and underplating of the crust by mafic magma may transfer sufficient heat to induce anatexis. During a prolonged history of subduction sub·duc·tion
A geologic process in which one edge of one crustal plate is forced below the edge of another.
[French, from Latin subductus, past participle of , the dip and location of the subduction zone subduction zone, large-scaled narrow region in the earth's crust where, according to plate tectonics, masses of the spreading oceanic lithosphere bend downward into the earth along the leading edges of converging lithospheric plates where it slowly melts at about 400 may vary causing the locus of arc magmatism to migrate and causing intermittent switching from compressional to extensional environments.
Le phenomene de deshydratation qui accompagnent la subduction de la lithosphere oceanique relache des fluides dans le prisme mantelique susjacent et initie une suite d'evenements qui conduit i la generation de magmas qui forment des structures d'iles en arc. Les magmas d'iles en arc different des magmas d'autres contextes parce que les regimes geothermaux aux lieux de destruction de marges tectoniques sont differents, etant donne la presence de fluides et/ou de volatiles issus de la deshydratation de la plaque en subduction, et du regime tectonique generalemerit compressif qui inhibe l'ascension du magma, d'ou l'importance de Finteraction avec la roche La Roche may refer to:
L'eau joue tin role important a toutes les etapes de l'evolution des magmas d'iles en arc. La presence d'eau abaisse les temperamres de fusion partielle dans le prisme mantelique, ce qui conduit a la formation de magmas mafiques, et de magmas felsiques dans la croute. Le magmatisme d'ile en arc est intimement lie au metamorphisme, bien que cette relation soit compliquee, surtout Sur`tout´
n. 1. A man's coat to be worn over his other garments; an overcoat, especially when long, and fitting closely like a body coat.
Noun 1. parce que les pressions et les temperatures maximales sont atteintes a des moments differents. Les magmas d'iles en arc en situation de compression s'epaississent rapidement, puis sont erodes ou exhumes tectoniquement. La fusion de la croute est declenchee par une variete de processus, dont la detente dé·tente
1. A relaxing or easing, as of tension between rivals.
2. A policy toward a rival nation or bloc characterized by increased diplomatic, commercial, and cultural contact and a desire to reduce tensions, as through accompagnant l'epaississement crustal. La fusion se produit d'abord a la base de la croute, mais elle atteint eventuellement des niveaux moins profonds de la croute. Les magmas d'ile en arc en situation d'extension ont un gradient geothermal tres pentu, et le placage de la croute par un magma mafique peut y apporter assez de chaleur pour provoquer l'anatexie. Si l'histoire de la subduction se prolonge, le contexte d'un systeme d'iles en arc peut alterner episodiquement d'un regime en compression a un regime en extension.
INTRODUCTION: A REVIEW OF THE TECTONICS OF DESTRUCTIVE PLATE MARGINS
This is the first of two articles on arc magmatism intended for geoscientists who do not specialize in petrology petrology, branch of geology specifically concerned with the origin, composition, structure, and properties of rocks, primarily igneous and metamorphic, and secondarily sedimentary. or geochemistry, but who are interested in the relationship between the tectonic evolution of arcs and magma petrogenesis pet·ro·gen·e·sis
The branch of petrology that deals with the origin of rocks, especially igneous rocks.
pet . This article is intended to explore that relationship and to set the stage for a companion article that focuses on the geochemical, isotopic and petrological evolution of arc magmas and that be published later by this journal. A review of the role that plate tectonics plate tectonics, theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history. plays, including the effects of subduction on the upper plate (the trench, the fore-arc, arc and backarc), the contrasting heat-flow regimes, and the effect of changes in the profile of the subduction zone with time is first presented here. The products of arc magmatism, from the pyroclastic py·ro·clas·tic
Composed chiefly of rock fragments of volcanic origin.
Composed chiefly of rock fragments of explosive origin, especially those associated with explosive volcanic and lava deposits that erupt at the surface to the intrusive rocks (Geol.) rocks which have been forced, while in a plastic or melted state, into the cavities or between the cracks or layers of other rocks. The term is sometimes used as equivalent to plutonic rocks ltname>. It is then contrasted with effusive or volcanic rocks.
See also: Intrusive in the plumbing system at depth, are described, as are the varying processes governing the ascent of arc magma and the controversies associated with the mechanisms of intrusion. The critical role of water, derived from dehydration of the subducted slab, is considered in all stages of arc-magma evolution, from the generation of the magma in the mantle and crust, during its rise toward the surface, to its final cooling and crystallization Crystallization
The formation of a solid from a solution, melt, vapor, or a different solid phase. Crystallization from solution is an important industrial operation because of the large number of materials marketed as crystalline particles. . This article concludes with an examination of the varying relationships between arc magmatism, metamorphism and deformation during subduction-related orogenesis o·rog·e·ny also or·o·gen·e·sis
The process of mountain formation, especially by a folding and faulting of the earth's crust.
PLATE TECTONIC SETTING
Plate tectonics plays a crucial role in the generation of arc magmas. On a globe of constant volume, the creation of lithosphere at constructive margins (where two plates move apart) is balanced by the destruction of lithosphere at destructive margins (where two plates collide). This balance is apparent on paleocontinental reconstructions for the past 200 million years, which show that the Atlantic Ocean Atlantic Ocean [Lat.,=of Atlas], second largest ocean (c.31,800,000 sq mi/82,362,000 sq km; c.36,000,000 sq mi/93,240,000 sq km with marginal seas). Physical Geography
Extent and Seas
expanded by seafloor spreading seafloor spreading, theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H. as the Pacific and Tethys oceans contracted by subduction (Fig. 1). Subduction eventually resulted in closure of the Tethys Ocean about 35 million years ago as the collision of Africa and India into Eurasia formed the Alpine-Himalayan mountain chain. However, subduction has persisted along most of the circum-Pacific region, where the abundance of arc-related volcanic activity has resulted in this region being called the "Ring of Fire" (Fig. 2). Petrologic pe·trol·o·gy
The branch of geology that deals with the origin, composition, structure, and alteration of rocks.
pet studies from this region provide the foundation for our understanding of arc processes.
[FIGURES 1-2 OMITTED]
Although the detailed mechanisms are still a matter of debate, in general terms, subduction of oceanic lithosphere is caused by the sinking of a dense oceanic plate beneath an adjacent, less dense plate (Figs. 3a, b). A long, narrow, curvilinear curvilinear
a line appearing as a curve; nonlinear.
see curvilinear regression. trench in the ocean floor develops where the subducting plate starts its angled descent into the mantle. The deepest trench occurs in the western Pacific, where the Marianas Trough is 11.5 km deep.
[FIGURE 3 OMITTED]
As oceanic crust oceanic crust
See under crust. is denser than continental crust continental crust
See under crust. , subduction preferentially removes the oceanic lithosphere along convergent plate boundaries where continental and oceanic crust is juxtaposed jux·ta·pose
tr.v. jux·ta·posed, jux·ta·pos·ing, jux·ta·pos·es
To place side by side, especially for comparison or contrast. . Where two oceanic plates meet at a convergent boundary In plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move toward one another. , the older (and therefore denser) oceanic lithosphere is preferentially subducted. The net effect is that subduction efficiently removes old oceanic lithosphere. So, despite the fact that the stratigraphic stra·tig·ra·phy
The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks.
strat record of marine deposits indicates the existence of oceans as far back as the Early Archean, the oldest intact oceanic lithosphere preserved in modern ocean basins is less than 200 million years old. Continental crust has a much greater preservation potential than oceanic crust. Continental crust older than 200 million years is common, and Archean crust between 3.5 and 4.0 billion years old is preserved in the ancient shield areas of several continents (e.g. Bowring and Williams 1999).
The location of subductlon zones beneath the surface is defined by an inclined zone of seismic activity, which extends to about 700 km depth. Around the Pacific "Ring of Fire", the angle of subduction varies from near vertical to subhorizontal, and is generally steeper in the western Pacific than in the eastern Pacific (Pilger 1981). This angle depends on a number of factors, including the age and thickness of the subducting lithosphere, and the motion of the upper plate (e.g. Bird 1988; Gutscher et al. 2000; Gutscher 2002). The shallower angles of subduction on the eastern Pacific margin are generally attributed to the subduction of younger (and therefore more buoyant) oceanic lithosphere, subducting oceanic plateaus (e.g. Dixon and Farrar 1980; Pilger 1981; Murphy et al. 1998; Gutscher et al. 2000; Yanez et al. 2001; Collins 2002) and/or the westerly migration of the North American North American
named after North America.
North American blastomycosis
see North American blastomycosis.
North American cattle tick
see boophilusannulatus. and South American plates (Bird 1988; Ramos et al. 2002). Irrespective of irrespective of
Without consideration of; regardless of.
preposition despite the subductlon angle, the descending slab overwhelmingly consists of oceanic lithosphere. Sediments deposited in the trench, however, can also be dragged down the subduction zone to considerable depths. In addition, in some regions, such as in the Andes of western South America South America, fourth largest continent (1991 est. pop. 299,150,000), c.6,880,000 sq mi (17,819,000 sq km), the southern of the two continents of the Western Hemisphere. , parts of the upper plate can be sliced off and dragged down the subduction zone, a process known as subduction erosion (Stern 1991). Upper plate crust can be entrained in the subduction zone and contribute to the chemistry of arc-related magmatism.
The effect of subduction zones on crustal evolution is preserved in the tectonic evolution of the plate above the subduction zone (upperplate) where there is a relatively predictable zonation zo·na·tion
1. Arrangement or formation in zones; zonate structure.
2. Ecology The distribution of organisms in biogeographic zones. (Fig. 3). At some distance from the trench, an arcuate arcuate /ar·cu·ate/ (ahr´ku-at) arc-shaped; arranged in arches.
Formed in the shape of an arc. chain of volcanoes, or volcanic arc, is developed above the subduction zone, fuelled by magma rising from above the subducting slab.
Modern arcs tend to expose the surface or near-surface expression of arc magmas and so are dominated by lava and pyroclastic deposits and some feeder dykes and sills. Beneath a volcanic arc, however, are magma chambers that are a complex labyrinth of intrusions, which represent the interior of the arc plumbing system (Marsh 1988). In many ancient arc systems, uplift and erosion have removed much of the volcanic arc, so that the crystallized crys·tal·lize also crys·tal·ize
v. crys·tal·lized also crys·tal·ized, crys·tal·liz·ing also crys·tal·iz·ing, crys·tal·liz·es also crys·tal·iz·es
1. products of the magma chambers are exposed as large plutonic bodies, typically of granitic to dioritic di·o·rite
Any of various dark, granite-textured, crystalline rocks rich in plagioclase and having little quartz.
[French, from Greek diorizein, to distinguish : dia-, composition. Therefore, to obtain a comprehensive view of arc magmatism, it is necessary to combine data from modern and ancient arcs.
There are two types of volcanic arcs; i) island arcs (Fig. 3a), which are an arcuate chain of volcanic islands outboard of a continental landmass land·mass
A large unbroken area of land.
a large continuous area of land
landmass , like those in the modern western Pacific Ocean, which are situated in the overriding upper plate above the subduction zone; and ii) continental arcs (Fig. 3b) like the modern Andes, which are a chain of active volcanoes on the edge of a continental landmass.
Island arcs may be subdivided depending on whether they are capped by a mafic crust, like the Philippines, or by continental crust, like Japan or northern New Zealand New Zealand (zē`lənd), island country (2005 est. pop. 4,035,000), 104,454 sq mi (270,534 sq km), in the S Pacific Ocean, over 1,000 mi (1,600 km) SE of Australia. The capital is Wellington; the largest city and leading port is Auckland. (Fig. 3a). This distinction has a profound affect on the petrological evolution of the arc magmas (Gill 2001). Island arcs capped by mafic crust tend to be dominated by mafic to intermediate magmas; those capped by continental crust have a higher proportion of intermediate to felsic lavas, and are compositionally similar to continental arcs (e.g. Miyashiro 1974). Although island arcs are typically 200-300 km wide and up to 2000 km long, volcanism volcanism
Any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam, including volcanoes, geysers, and fumaroles. at any one time is typically concentrated in a region about 10 km wide, and about 110 km above the subduction zone. However, as the dip of the subduction zone varies over its lifespan, so too does the location of the volcanism. Over a long period of time, therefore, a broad band of arc-volcanic rocks can be produced, even though only a relatively narrow band is active at any one time.
Also, island arcs are segmented into belts up to 300 km long, which are terminated by deep-penetrating fractures that accommodate along-strike variations in the dip of the subduction zone. Magmatism in the vicinity of these fractures has chemical and isotopic features that are different from typical arc magmas, and may reflect local upwelling up·well·ing
1. The act or an instance of rising up from or as if from a lower source: an upwelling of emotion.
2. of underlying asthenosphere asthenosphere (ăsthēn`əsfēr), region in the upper mantle of the earth's interior, characterized by low-density, semiplastic (or partially molten) rock material chemically similar to the overlying lithosphere. (e.g. Marsh 1979, 1982; Kay et al. 1982). Other patterns of arc volcanism are rarer. For example, a secondary narrow chain of volcanoes may occur about 175 km above the subduction zone (Marsh 1979, Tatsumi and Eggins 1995).
Products of Arc Magmatism
Because of their lower viscosity, eruptions of mafic lavas are generally not explosive and produce broad volcanic structures that have gentle slopes. Intermediate and felsic magmas in volcanic arcs, however, are viscous, and move so slowly that most of the magma cools underground forming large plutons of granite and diorite diorite
Medium- to coarse-grained igneous rock that commonly is composed of about two-thirds plagioclase feldspar and one-third dark-coloured minerals, such as hornblende or biotite. (e.g. Clarke 1992, Pitcher 1993). As viscous felsic magma rises toward the surface, the reduced pressure In thermodynamics, the reduced pressure of a fluid is defined as its actual pressure divided by its critical pressure.
1. to force out, or to occupy a position distal to that normally occupied.
2. in dentistry, to occupy a position occlusal to that normally occupied. , its high gas content usually causes violent explosions, which produce steep-sided volcanoes that eject far more airborne fragments than lava, and result in pyroclastic deposits such as volcanic breccia breccia: see conglomerate.
Coarse sedimentary rock consisting of angular or nearly angular fragments larger than 0.08 in. (2 mm). Breccia commonly results from processes such as landslides or geologic faulting, in which rocks are fractured. , tuff and ash. Such explosive activity propels mixtures of gases, or aerosols, together with fragments of hot magma and wall-rock into the air. When this mixture becomes too dense to be vented upward, it collapses under its own weight, producing a rapidly moving, ground-hugging avalanche known as a pyrodastic flow that can spread as far as 100 kilometres from the site of the eruption (Sparks et al. 1973). Because of their explosive character and high velocity (typically between 50 and 200 km/hr), pyroclastic flows have claimed many lives in recorded history Recorded history can be defined as history that has been written down or recorded by the use of language, whereas history is a more general term referring simply to information about the past. It starts in the 4th millennium BC, with the invention of writing. .
In contrast to the gentle slopes associated with mafic volcanoes, eruption of intermediate to felsic lavas produces dramatic volcanic edifices, including composite volcanoes, which consist of alternating layers of pyroclastic deposits and lava flows that may be erupted from either the central vent or along the flanks of the volcano. This alternation alternation /al·ter·na·tion/ (awl?ter-na´shun) the regular succession of two opposing or different events in turn.
alternation of generations metagenesis. reflects the varying style of eruption over time and is a characteristic of the Cascade Range Cascade Range, mountain chain, c.700 mi (1,130 km) long, extending S from British Columbia to N Calif., where it becomes the Sierra Nevada; it parallels the Coast Ranges, 100–150 mi (161–241 km) inland from the Pacific Ocean. , in the Pacific Northwest. Calderas up to 100 km across are found at the summits of these volcanoes. The volume of pyroclastic material associated with a single eruption can be substantial and its effects can be global. In 1991, the eruption of Mount Pinatubo Noun 1. Mount Pinatubo - a volcano on Luzon to the northwest of Manila; erupted in 1991 after 600 years of dormancy
Pinatubo in the Philippines jettisoned an estimated 5 cubic kilometres of debris into the atmosphere, 5 times more than the volume of debris associated with the 1980 eruption of Mount St. Helens. An eruption in the vicinity of Lake Taupo Lake Taupo is a lake situated in the North Island of New Zealand. It has a perimeter of approximately 193 kilometres, a deepest point of 186 metres and a surface area of 616 square kilometres. , New Zealand in 1881, is estimated to have ejected between 30-50 cubic kilometres of debris in just a few minutes. Some pre-historic eruptions are estimated to have ejected as much as 2500 cubic kilometres of debris and may have had significant impact on climate.
The region between the volcanic arc and the trench is known as the fore-arc (or "arc-trench gap", Fig. 3a), the width of which depends on the dip of the subduction zone. In steeply dipping subduction zones, the fore-arc is narrow, whereas in more shallowly dipping subduction zones, the fore-arc is wide. The fore-arc, therefore, is generally much narrower in Western Pacific, compared to Eastern Pacific, arc systems. This relationship is generally interpreted to indicate that the subduction zones must attain a certain depth (about 110 km) before processes in them trigger the generation of arc magmas. The frontal part of the fore-arc may contain pyroclastic deposits, lavas or immature volcaniclasric sediments, all derived from the active part of the arc, which rest upon a basement of igneous ig·ne·ous
1. Of, relating to, or characteristic of fire.
a. Formed by solidification from a molten state. Used of rocks.
b. Of or relating to rock so formed; pyrogenic. and metamorphic rocks metamorphic rocks: see rock. . Near the trench, the fore-arc exhibits complex deformation as sediments deposited in the trench are scraped off the downgoing plate and are plastered onto the inner trench wall to form an accretionary prism. As the accretionary prism grows, the trench migrates, which in turn can result in migration of the locus of arc magmatism. Alternatively, fore-arc material can be transferred from the overriding plate to the subducting plate by subduction erosion and may then be transported down the subduction zone (cf. Stern 1991, 2002). Subduction erosion occurs on more than half of the modern convergent plate boundaries, especially where the overriding plate is moving rapidly towards the subduction zone and the width of fore-arc in these regions is less than that predicted by the dip of the subduction zone. High pressure, coesite- (e.g. Schreyer 1988) and diamond-beating (Xu et al. 1992) metamorphic rocks in subduction zone complexes indicate that crustal material can be subducted to depths of at least 100 km. The preservation of high pressure minerals in low-grade metamorphic rocks requires more rapid exhumation of arcs than the relatively slow erosion-controlled exhumation, and is generally interpreted to reflect tectonic thinning processes (e.g. Brown 1993). Additional evidence that subducted material is recycled by the subduction zone processes is provided by the chemical and isotopic composition of arc magmas (Murphy 2007).
Although the precise mechanisms are controversial, the region behind the arc may be either in extension or compression. There may be several mechanisms responsible for backarc extension, but the currently popular model is the roll-back model (Dewey 1980). This model proposes that as the disconnected end of the dense slab collapses into the asthenosphere, it may "roll-back" oceanward (Fig. 4). The retreat of the slab is associated with seaward migration of the trench and causes extension in the upper plate. Therefore, subduction zones may move appreciably over timescales of 100 million years relative to some other reference frame. Subducting slabs sink more steeply than the inclinations of Benioff seismic zones, which mark transient positions, not trajectories, of slabs. The convergent margin can move faster than the roll-back if the upper plate is also moving over the descending one, as in the case of the westward drift of North America North America, third largest continent (1990 est. pop. 365,000,000), c.9,400,000 sq mi (24,346,000 sq km), the northern of the two continents of the Western Hemisphere. since the break-up of Pangaea. Episodic subduction occurred along the western margin of North America over the past 100 million years, demonstrating significant motion of subduction zones.
[FIGURE 4 OMITTED]
Roll-back is most effective where old oceanic lithosphere is subducted, and extension in the upper plate results in backarc spreading, in which a backarc basin (also known as a marginal basin) opens up behind the arc. This style of subduction predominates in the western Pacific Ocean, where backarc basins separate island arcs from the Asian continent. For example, the Sea of Japan and the West Philippine Basin are backarc basins that separate the island arcs of Japan and the Philippines from the Asian mainland. As these basins are floored by oceanic crust, it is not surprising that the pillow lavas produced are similar mineralogically min·er·al·o·gy
n. pl. min·er·al·o·gies
1. The study of minerals, including their distribution, identification, and properties.
2. A book or treatise on mineralogy. and texturally to mid-ocean ridge mid-ocean ridge: see plate tectonics. basalts (MORB MORB Mid-Ocean Ridge Basalt
MORB Medical Officer Retention Bonus
MORB Male O-ring Boss (fitting)
MORB Multicast Object Request Broker ). Backarc basins, however, lack the classical, well-defined magnetic anomaly Magnetic Anomaly may refer to:
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. Collins (2002), extension and decompression associated with roll-back may result in asthenospheric upwelling and voluminous magmatism in the overlying mantle and crust. In most cases, geochemical and isotopic investigations reveal the influence of a nearby arc on the composition of the mantle source.
In the geological record, vestiges of ancient backarc basins also may be distinguished from mid-ocean ridge settings by the character of the sedimentary strata associated with the basalts. As backarc basins fringe volcanic arcs on one side and a continental land mass on the other, they may contain abundant volcaniclastic sediments that are cannibalized from the adjacent arc or clastic clastic /clas·tic/ (klas´tik)
1. undergoing or causing division.
2. separable into parts.
1. sediments derived from the continental hinterland. In contrast, most mature mid-ocean ridge settings are sediment-starved; consequently, chemical (e.g. chert chert: see flint. ) and pelagic sediments Pelagic sediments, also known as marine sediments, are those that accumulate in the abyssal plain of the deep ocean, far away from terrestrial sources that provide terrigenous sediments; the latter are primarily limited to the continental shelf, and deposited by rivers. predominate.
For continental arcs with backarcs in extension, regions of thinned continental crust are produced in which the rise of hot mantle towards the surface generates crustal melts as well as melts from the decompressed mantle. For example, in the Taupo Volcanic Zone The Taupo Volcanic Zone is an active volcanic area in the North Island of New Zealand. It is named after Lake Taupo, which is the flooded caldera of the largest volcano in the zone. of northern New Zealand, the crust is as little as 5 km thick with partially molten material, thought to be the source of the magma, immediately below (Cole et al. 1998, 2005). Magmas in continental backarcs are commonly bimodal bi·mod·al
1. Having or exhibiting two contrasting modes or forms: "American supermarket shopping shows bimodal behavior in composition (i.e. they are dominated by basalt basalt (bəsôlt`, băs`ôlt), fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. and rhyolite rhyolite, fine-grained light-colored acidic volcanic rock. Rhyolite is chemically the equivalent of granite, and is thus composed primarily of quartz and orthoclase feldspar with subordinate amounts of plagioclase feldspar, biotite mica, amphiboles, and pyroxenes. , with few "intermediate" rocks) and are associated with high temperature-low pressure metamorphic met·a·mor·phic
1. also met·a·mor·phous Of, relating to, or characterized by metamorphosis.
2. Geology Changed in structure or composition as a result of metamorphism. Used of rock. belts and possibly metamorphic core complexes (e.g. Hyndman et al. 2005).
Profile of the Subduction Zone
Subduction zones are not static; they may migrate (shallow or steepen steep·en
tr. & intr.v. steep·ened, steep·en·ing, steep·ens
To make or become steep or steeper.
to become or cause (something) to become steep or steeper
) as the age or thickness of the subducted lithosphere, or as the relative motion of the converging plates, changes (e.g. Molnar and Atwater 1978; Hynes and Mott 1985; Ramos and Aleman 2000; Gutschner et al. 2002). As the dip and location of a subduction zone change, so too do the width of the arc-trench gap and the locations of the volcanic arc and the backarc basin. If plate convergence is high and young, and/or buoyant oceanic crust is being subducted, the subduction zone becomes subhorizontal, and is too shallow to generate arc magmas (Dickinson and Snyder 1979; Pilger 1981; Bird 1988; Ramos et al. 2002; English et al. 2003). Such an environment occurs in several segments of the modern Andes (Fig. 5). Although much of the Andes is a classic continental arc, there are narrow segments where no modern magmatism occurs. Seismic images reveal that beneath these segments the subduction zones are sub-horizontal. Pilger (1981) drew attention to the correspondence between these zones and the subduction of thick and buoyant oceanic plateau (Fig. 5).
[FIGURE 5 OMITTED]
Backarc regions in compression occur predominantly in continental settings where compression of a thermally softened crust can lead to polyphase Pol´y`phase
a. 1. (Elec.) Having or producing two or more phases; multiphase; as, a polyphase machine, a machine producing two or more pressure waves of electro-motive force, differing in phase; a deformation and both thick-skinned and thin-skinned fold-and-thrust belts (Ramos and Aleman 2000). Much of the backarc region of the Andes is currently in compression and is undergoing active deformation. Continental backarcs are characterized by thin hot lithosphere and have Moho temperatures of ca. 850[degrees]C and lithospheric thickness of ca. 55 km, compared to ca. 450[degrees]C and 150-300 km, respectively, for adjacent cratons (Fig. 6, Hyndman et al. 2005). Continental backarcs are hotter, and therefore, an order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc. weaker than adjacent lithosphere. In addition, these basins may contain thick deposits of sediments that are fertile material for the formation of crustal melts. Taken together, backarc basins may well be the sites where magmatism and high-grade metamorphism develop more readily than in adjacent, drier rocks, and so may exert a fundamental control on the petrology of magmas produced in arc systems (Collins 2002; Hyndman et al. 2005). In support of this model, Collins (2002) points out that the mineralogy mineralogy
Scientific study of minerals, including their physical properties, chemical composition, internal crystal structure, occurrence and distribution in nature, and origins or conditions of formation. of many granulite facies complexes requires geothermal gradients between 20[degrees] and 50[degrees]C per km, which is much higher than gradients in collisional environments, but is consistent with those expected in backarc settings.
[FIGURE 6 OMITTED]
Backarc regions in continental environments may alternate between periods of extension and compression (known as "tectonic switching", Collins 2002). Backarc basins that result from extension induced by periods of rollback and slab retreat can intermittently become inverted inverted
reverse in position, direction or order.
inverted L block
a pattern of local filtration anesthesia commonly used in laparotomy in the ox. and polydeformed by compression, induced by flat-slab subduction related to arrival at the trench of an oceanic plateau (Fig. 4). During the extensional phase, the high geothermal gradient and ascending mafic magma induce crustal melting and high-grade (granulite-facies) metamorphism.
In a general sense, subduction zones, and therefore arc magmatism, can last for hundreds of millions of years. The processes that result from prolonged subduction profoundly affect the composition, thickness and strength of the lithosphere. Uplift associated with this tectonothermal activity is not continuous, but instead appears to be episodic. For example, although subduction and magmatism have persisted along the western margin of South America for most of the last 200 million years, it was only about 15 million years ago that the rate of uplift was sufficient to reverse the flow of the Amazon river from its ancestral westward path into the Pacific Ocean to its current eastward path into the Atlantic Ocean (e.g. Hoorn et al. 1995). Many geoscientists attribute this rapid uplift to the effect of higher rates of compression between the converging South American and Pacific plates on a crust that was thermally weakened by prolonged arc-related magmatism (Ramos and Aleman 2000). Arc magmatism can be pre-kinematic, syn-kinematic and post-kinematic with respect to discrete deformational events.
The subducting slab, the region of arc magma generation, and the varied components of backarc regions can all be detected by surface heat flow measurements across oceanic and continental arc systems (Fig. 6). Heat flow is low, both in the vicinity of the trench and in the arc-trench gap (fore-arc), reflecting the descent of cold oceanic lithosphere into the mantle and the absence of convecting mantle between the fore-arc and the subducting plate. The effect is similar to that of immersing an ice cube into hot tea. For the time it takes for the heat to be transferred from the tea into the ice, the ice remains anomalously cold. Similarly, the temperature in, and adjacent to, the subduction zone is lower than the surrounding mantle at the same depth. As a result, the geothermal gradient (the increase in temperature with depth), and therefore, the heat flow to the surface, are low in the vicinity of subduction zones. The geothermal gradient can be as low as 10[degrees]C/km, and the heat flow as low as 0.05 W/[m.sup.2].
In contrast, the region beneath volcanic arcs is dominated by high heat flow. Volcanic activity accounts for about 10% of this anomaly and the remainder is attributed to the ascent of hot magma towards the surface and/or mantle convection (Gill 1981). As the region below the arc is hotter than the surrounding rocks, heat flow measurements are high (up to 0.1 W/[m.sup.2]) and the geothermal gradient is steep (typically 30[degrees]C/km).
Until recently, it was thought that heat flow in the backarc depended on whether it is under extension (high heat flow) or compression (low heat flow). Extensional backarcs are hot, irrespective of whether they are floored by oceanic (e.g. Western Pacific) or continental (e.g. Basin and Range) crust (Wiens and Smith 2003). However, Hyndman et al. (2005) point out that many modern non-extensional backarcs have heat flow values that are comparable with the region beneath volcanic arcs, and are twice that of the adjacent cratons (0.04 W/[m.sup.2], Chapman and Furlong 1992). The reason for the high heat flow in such backarcs is unclear. However, as many geodynamic models treat shortening as being geologically instantaneous (e.g. England and Thompson 1986), it is possible that some of these backarcs may have inherited the high heat flow from a recent history of extension (from which the heat has yet to dissipate) and switched to compression by the arrival of a microcontinent or oceanic plateau at the subduction zone.
ASCENT OF ARC MAGMA
Three fundamental properties influence the rate at which arc magma ascends: i) magma is less dense than the wall rocks and is, therefore, buoyant, ii) a magma with lower viscosity rises with greater ease than a more viscous magma (therefore, mafic magma generally rises more readily than felsic magma, which is likely to be trapped in magma chambers where it cools to form a pluton plu·ton
A body of igneous rock formed beneath the surface of the earth by consolidation of magma.
[German, back-formation from plutonisch, plutonic, from Latin ), and iii) magma rises more rapidly in an extensional environment than in a compressional setting because widening fractures create space for rising magma to exploit.
Mechanisms of Intrusion
In arc environments, vast volumes of magma rise toward the surface, and this magma must occupy space that was previously occupied by solid rock. Unlike continental rifts, in which regional crustal extension provides space for ascending magma, in arc environments the "room problem" (see Paterson et al. 1991) is still controversial after more than a century of debate. Although, to some extent, this space problem applies to all tectonic settings, it is particularly acute in arc settings where compressional tectonics predominates, and crustal thickening impedes the ascent of magmas. The less efficient ascent of magmas in arc settings has important petrological consequences because such environments promote extensive magma-wall-rock interaction and cooling, factors that explain the abundance of plutonic rocks in arc environments.
Where sufficient magma is produced (> 15%), it congregates and attains buoyancy because it is less dense than the surrounding rock, particularly in the lower crust which is more ductile than the upper crust. Experiments show that magma may attain enough buoyancy to rise from large magma chambers as tear-drop shaped bodies (diapirs) with regular spacing between them (Fig. 7; Marsh 1982; Miller and Paterson 1999; Johnson et al. 2001) or as nested diapirs (Paterson and Vernon 1995). For example, volcanic centres in the Central American arc are typically spaced about 25 km apart, and are thought to be the result of diapiric uprise from a larger pool of magma that parallels the length of the arc (Marsh 1979; Gill 1981). Although field observations indicate that diapirs can arch the overlying layers, most petrologists contend that this process acting alone is unlikely to enable magma to rise into the shallow brittle crust (Hutton 1988; Pitcher 1993). The rise of a diapir di·a·pir
An anticlinal fold in which a mobile core, such as salt or gypsum, has pierced through the more brittle overlying rock.
[French, from Greek diapeirein, to push through : is self-limiting because its buoyancy decreases as it rises (the density of rocks surrounding the magma decreases as the surface is approached and magma becomes denser as it cools).
[FIGURE 7 OMITTED]
An alternative mechanism for the upper crust proposes that magma rises by assimilating the overlying rocks. The occurrence of partially digested xenoliths in many arc-related intrusive rocks lends support to this hypothesis. However, most petrologists are still swayed by Bowen's (1928) elegant arguments that magmas do not have sufficient heat content to digest large quantities of host rock, and the loss of energy in the digestion process causes the magma to cool and crystallize crys·tal·lize also crys·tal·ize
v. crys·tal·lized also crys·tal·ized, crys·tal·liz·ing also crys·tal·iz·ing, crys·tal·liz·es also crys·tal·iz·es
1. , rather than rise. Geochemical and isotopic data from many plutons also argue against significant assimilation of local wall rocks (e.g. Glazner and Bartley 2006).
Magma may be able to create and exploit cracks and fractures in the overlying crust. Experiments show that melting results in a 15 percent increase in volume, and this expansion may produce enough pressure to fracture the overlying rock and allow the magma to rise. The presence of water in the arc magmas enhances its ability to create fractures (see Downey and Lentz 2006). Magma will also increase in volume if it rises sufficiently fast that it does not lose much heat (i.e. adiabatically). The resulting expansion of the magma may also result in fracture propagation that weakens the resistance of overlying rock.
The foundering of overlying blocks into the magma, (Fig. 7), called stoping, is enhanced by the tendency for magma to invade fractures in the host rocks and pry them loose so that they drop into the magma (e.g. Marsh 1982). Evidence supporting this process includes the occurrence of large xenoliths in plutonic rocks. However, it is unclear how important this process is in facilitating the ascent of magma over long distances (see Clarke et al. 1998). In addition to the lack of geochemical and isotopic evidence for significant assimilation, the fact that xenoliths are volumetrically vol·u·met·ric
Of or relating to measurement by volume.
[volu(me) + -metric.]
vol small, even on pluton floors and that large plutons grow incrementally from small batches of magma (blocks can only fall through liquid magma) together suggest that stoping is not a significant method of pluton emplacement (Glazner and Bartley 2006).
Field observations demonstrate a close relationship between igneous intrusions and adjacent faults (Fig. 7) suggesting that magma exploits large-scale crustal weaknesses (e.g. Sawyer 1994). Fractures associated with faulting may, therefore, be favourable locations for magma emplacement (Fitch 1972) and significant volumes of magma can migrate through them (e.g. Clemens 1998; Brown and Solar 1998). A magma intruded into an extensional tectonic environment, for example, can exploit opening fractures that create space for it. Extensional environments can occur locally in arc settings, especially where a fault within the arc bends or jogs (e.g. Guineberteau et al. 1987; Hutton 1988; Murphy and Hynes 1990; Koukouvelas et al. 2002; Fig. 7) and magma ascent is especially rapid in such environments.
A number of recent studies on modern arcs have indicated the importance of extensional regimes in the generation of arc magmas (e.g. Hyndman et al. 2005) and that the composition and ascent of these magmas are profoundly influenced by faults and fractures whose orientation reflects plate boundary conditions (Tibaldi 1992; Spinks et al. 2005). For example, Tibaldi (1992) shows that these environments generate wide belts of magmatism, and Spinks et al. (2005) show that the regions with the greatest extension in the Taupo Volcanic Zone of New Zealand produce active calderas up to 50 km wide that are dominated by felsic magmatism, whereas areas with the greatest transtension produce less active andesitic stratovolcanoes A list of stratovolcanoes follows below. The Smithsonian's Global Volcanism Program database of geologically recent volcanoes lists over 700 stratovolcanoes.
Several studies emphasize the importance of intra-arc strike-slip tectonics in controlling the rise of magmas in modern arcs (e.g. central Andes: De Silva 1989; NE Japan: Sato 1994; Ecuador: Tibaldi and Ferrari 1990; Mexico: Tibaldi 1992; Kamchatka: Baluyev and Perepelov 1988). This is similar in principle to example D in Figure 7, where the strike-slip fault occurs within an arc setting. Magmatism in the Kamchatka Volcanic Belt, for example, is associated with regional transcurrent trans·cur·rent
Extending or running transversely. faults that reach a depth of 40 km and it is suggested that the depth of magma generation is controlled by these faults (Baluyev and Perepelov 1988). Tibaldi (1992) attributes the obliquity obliquity /obliq·ui·ty/ (ob-lik´wit-e) the state of being inclined or slanting.oblique´
Litzmann's obliquity between the Mexican Volcanic Belt and the Middle American Trench to transcurrent motion along adjacent faults. According to Ferrari et al. (1989), deep penetrating faults in intra-arc environments create favourable conditions for rapid ascent of mafic magmas with little crustal contamination.
Field studies of large batholiths in arc regimes show that they represent the vestiges of voluminous magma chambers in the middle and upper crust (e.g. Clarke 1992; Pitcher 1993; Coleman et al. 2004; Miller and Paterson 2001; Vigneresse and Bouchez 1997) that were constructed incrementally by repeated injection from elongated e·lon·gate
tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates
To make or grow longer.
adj. or elongated
1. Made longer; extended.
2. Having more length than width; slender. dykes (Hutton 1992; Miller and Paterson 2001) or were emplaced as nested diapirs (Allen 1992; Paterson and Vernon 1995). These batholiths, therefore, are composite and have numerous internal contacts that vary from sharp and planar to gradational gra·da·tion
a. A series of gradual, successive stages; a systematic progression.
b. A degree or stage in such a progression.
2. and irregular. A recent study of the Tuolumne Batholith of the Sierra Nevada (Zak and Paterson 2005) proposes that its evolution involved fractional crystallization, magma mixing on a kilometrescale, stoping along sharp contacts, and downward return flow (and/or margin collapse) of older magma units, with dyke injections becoming important locally during the later stages.
It appears that several mechanisms may act in concert to facilitate the rise of arc magmas. Many diapirs are located near fault zones and appear to have exploited zones of crustal weakness. Magma exploiting such zones may rapidly rise and expand, resulting in fracture propagation, facilitating stoping and assimilation. As fracture zones are repeatedly exploited, the net result may be a composite batholith that typifies arc systems.
EFFECT OF WATER
Perhaps the most unifying characteristic of arc magmas is the important role played by water in all stages of magma evolution, including magma genesis, ascent toward the surface, cooling and crystallization, and during volcanic eruptions volcanic eruptions
discharging of fumes, dust and lava from volcanoes. They have damaging potential in addition to those of being physically overpowering by the lava flow or the ash or dust fallout. . The explosive nature of arc volcanic eruptions, the abundance of vesiculated products such as pumice pumice (pŭm`ĭs), volcanic glass formed by the solidification of lava that is permeated with gas bubbles. Usually found at the surface of a lava flow, it is colorless or light gray and has the general appearance of a rock froth. and phenocrysts of hydrous hydrous
containing water. mineral phases are all indicative of magmas with high volatile content.
Water In the Mantle Wedge
The subducted slab of oceanic lithosphere contains abundant water, and as it descends into the mantle it is warmed and undergoes prograde prograde
Having a rotational or orbital movement that is the same as most bodies within a celestial system. In our solar system, prograde movement for both rotating and orbiting bodies is in a counterclockwise direction when viewed from a vantage point dehydration reactions. According to Tatsumi and Eggins (1995), the release of water from prograde dehydration reactions occurring in the slab (amphibolechlorite assemblage at 110 km depth, phlogopite phlog·o·pite
A yellow to dark brown mica, K(Mg,Fe)3AlSi3O10(OH)2, used in insulation.
[Greek phlog at 175 km depth) may be responsible for two parallel volcanic arcs. The released water is a supercritical fluid under mantle conditions, and has the capacity to extract soluble elements from the slab (e.g. K, Rb, Ba, Sr, U, Pb, Th and Cs, known as the large ion lithophile elements or L/L L/L Lids & Lashes
L/L Land Line ), which are transported to, and become enriched in, the overlying mantle wedge (e.g. Spandler et al. 2003; Leibscher 2004).
The fluid that invades the mantle wedge plays a pivotal role in generating arc magma (Fig. 3, Peacock et al. 1994). It is a general principle of igneous petrology that water lowers solidus temperatures of the mantle. So the relentless invasion of water into the mantle wedge promotes melting and the generation of arc magmas. The presence of water affects the stability of silicate silicate, chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids. and accessory minerals at the site of melting and during subsequent cooling. Water content influences magma composition, chemical evolution and the kinetics of mineral growth. The combination of these effects can be recognized in the mineralogy, textures, geochemistry and isotopic signature of arc magmas.
Even a small amount of water lowers the melting temperature (or the solidus) of rocks (Fig. 8). Common mantle minerals such as garnet, olivine olivine (ŏlĭv`ēn), an iron-magnesium silicate mineral, (Mg,Fe)2SiO4, crystallizing in the orthorhombic system. , orthopyroxene orthopyroxene
Any variety of the mineral pyroxene that crystallizes in the orthorhombic system and contains no calcium and little or no aluminum. Enstatite is an orthopyroxene. and clinopyroxene clinopyroxene
Any variety of the mineral pyroxene that crystallizes in the monoclinic system. Diopside and augite are clinopyroxenes. do not readily admit water into their crystal structure (this tendency is reflected by the lack of water in their mineral formulae). Because the water molecule is polar, the negative end of the molecule tugs on the cations of minerals, weakening their bonds. The weakened crystal structure requires less energy to break the bonds, and so magma can form at a lower temperature than under dry conditions. This effect is especially profound in the mantle, where dry rocks are resistant to melting because the elevated pressure reduces bond lengths and favours more dense mineral structures. However, at these pressures, water's ability to weaken crystal structures becomes even greater, so that wet rocks become less resistant to melting with increasing depth.
[FIGURE 8 OMITTED]
In addition to weakening the crystal structures, water ascending from depth is a heat flux because it rises rapidly and does not lose much heat to its surroundings. The combination of weakened mineral structures and additional heat promotes melting and the formation of magma in the overlying mantle wedge and in the crust.
Potential Significance of Mineral-Fluid Interaction In the Mantle Wedge
In the mantle wedge, fluid rising from the subduction zone interacts with minerals and fluids already present. Thus, the fluid composition at the site of melting probably has components of both the invading fluid from the subduction zone, the indigenous fluid previously added to the mantle wedge, and fluid released by mantle minerals. In addition, some elements are released from the mineral into the fluid but other elements present in the fluid may be incorporated in a solid phase.
Several experiments have demonstrated that the composition of igneous melts is profoundly influenced by fluid composition at the site of melting (e.g. Mysen and Boettcher 1975; Boettcher 1977) with higher [H.sub.2]O content resulting in silica-saturated basalts, and higher C[O.sub.2] content leading to the generation of silica-undersaturated basalts. In addition, fluids derived from the dewatering Dewatering (dē′wöd·ər·iŋ) is the removal of water from solid material or soil by wet classification, centrifugation, filtration, or similar solid-liquid separation processes. of the slab invade and react with minerals in the mantle wedge, promoting the formation of minerals that permit water into their crystal structure, such as hornblende hornblende: see amphibole.
Any of a subgroup of amphibole minerals that are calcium-iron-magnesium-rich and monoclinic in crystal structure. and phlogopite. For the most part, these fluids migrate along grain boundaries. The resulting solution and precipitation of mantle minerals affects the bulk partitioning of trace elements Trace elements
A group of elements that are present in the human body in very small amounts but are nonetheless important to good health. They include chromium, copper, cobalt, iodine, iron, selenium, and zinc. Trace elements are also called micronutrients. between mantle minerals, fluid and the melt.
These findings suggest that an understanding of the factors that affect this fluid composition is essential. Although these factors are poorly understood, generalized phase relationships shown schematically on isothermal-isobaric diagrams illustrate the importance of understanding fluidmineral interaction (Fig. 9; modified after Eggler 1978; Thompson 1983; Murphy 1989). These diagrams focus on the relationship between nonvolatile phases (A), hydrous phases (H) and carbonate phases (C). Hydration reactions are represented on the AH join, carbonatization reactions on the AC join, and fluid composition on the CH join (Fig. 9a). Mantle rocks may contain several hydrous and carbonate phases (e.g. phlogopite, amphibole amphibole (ăm`fəbōl'), any of a group of widely distributed rock-forming minerals, magnesium-iron silicates, often with traces of calcium, aluminum, sodium, titanium, and other elements. , calcite calcite (kăl`sīt), very widely distributed mineral, commonly white or colorless, but appearing in a great variety of colors owing to impurities. , and dolomite dolomite (dō`ləmīt', dŏl`ə–).
1 Mineral, calcium magnesium carbonate, CaMg (CO3)2. ) with varying degrees of hydration hydration /hy·dra·tion/ (hi-dra´shun) the absorption of or combination with water.
1. The addition of water to a chemical molecule without hydrolysis.
2. and carbonatization. For simplicity, individual hydrate hydrate (hī`drāt), chemical compound that contains water. A common hydrate is the familiar blue vitriol, a crystalline form of cupric sulfate. Chemically, it is cupric sulfate pentahydrate, CuSO4·5H2O. and carbonate phases are not displayed as they do not affect the following arguments. The stippled stippled /stip·pled/ (stip´'ld) marked by small spots or flecks.
covered with many small dots.
see basophilic stippling. triange in Figure 9a represents an assemblage with no free fluid phase (i.e. volatile-absent, see Thompson 1983). For bulk compositions within that triangle, fluids injected become absorbed in hydrate (H) or carbonate (C) phases. With increasing temperature, the reaction
C+H [right arrow] A + fluid
occurs and anhydrous an·hy·drous
Without water, especially water of crystallization.
adj without water.
containing no water. phases exist in equilibrium with a fluid (as shown in Fig. 9b).
[FIGURE 9 OMITTED]
Incremental input of a C[O.sub.2]-rich fluid (G, Fig. 9a) into a rock of bulk composition J would result in progressive dissolution of anhydrous phases (path 1, Fig. 9a). If the ratio of hydrate to carbonate phases in the host rock is less than the C[O.sub.2]/[H.sub.2]O ratio of the fluid, then dissolution of the hydrate-bearing phases (H) will occur via the continuous reaction:
A+H+G (fluid) [??] C
A free fluid phase would only occur after expiration of A (X, Fig. 9a). Further input of fluid G would result in continued precipitation of C and dissolution of H (path X to Y). The fluid produced by this reaction is buffered (at constant pressure and temperature) to constant composition P, which is different from the composition of the invading fluid. This continuous reaction is:
H+G (fluid) [??] C + P(fluid) i.e. fluid G is spontaneously converted to fluid P.
Note that in this case the reaction may be irreversible if fluid P is lost (e.g. through partitioning into magma). If fluid loss occurs when the bulk composition is at R, then the bulk composition is deflected from path 1 along path 2 which projects through R to P (i.e. a path of constant C/H C/H Card Holder
C/H Characters per Hour ). If the bulk composition continues along path 1, then buffering of the fluid phase to composition P is lost when the hydrate phase expires (Y, path 1). At this stage, magma of different composition may be produced.
Input of C[O.sub.2]-rich fluid (G, Fig. 9b) to the higher temperature assemblage (e.g. S) initially causes dissolution of H (path 1) and precipitation of A as the fluid is buffered to constant composition M via the continuous reaction:
H + G (fluid) [??] A + M(fluid) Buffering of the fluid phase would continue until H is consumed (at h, Path 1). Continued addition of fluid G would eventually lead to renewal of buffering at the first appearance of a carbonate phase C at d (path 1). Fluid loss, at an early stage along path 1, results in precipitation of A and H (between S and h), precipitation of A (between h and d) or precipitation of A and C (between d and e). Where partial melting occurs, the fluid extracted by the magma would be significantly more enriched in [H.sub.2]O. Extraction of a fluid F from bulk composition S (path 2, Fig. 9b), would result in continued precipitation of A and dissolution of H. Situations in which buffering is either achieved or lost involve sudden changes in the composition of the fluid, and therefore, may promote enhanced dissolution or precipitation of mantle minerals (depending on how the composition of the fluid changes).
Partial Melting In the Mantle Wedge
Although the introduction of water lowers solidus temperatures, there is never enough energy to completely melt the mantle wedge. Instead the mantle undergoes partial melting. Although partial melting of the mantle typically produces basaltic ba·salt
1. A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, and often having a glassy appearance.
2. A kind of hard unglazed pottery. magma, in detail, the overall chemical composition of the basalt (especially the trace element and isotopic content) is profoundly influenced by the relative stability of minerals. The crystal structures of some minerals are more weakened than others and so some minerals are more prone to melting than others. Partial melting, therefore, proceeds in a regular fashion, and melt compositions can be predicted by phase equilibria. For example, at high mantle pressures, garnet is more stable than olivine, and so garnet stays behind in the residue, and the basalts produced are depleted de·plete
tr.v. de·plet·ed, de·plet·ing, de·pletes
To decrease the fullness of; use up or empty out.
[Latin d in the components of garnet relative to olivine. A hydrated hy·drat·ed
Chemically combined with water, especially existing in the form of a hydrate.
Adj. 1. hydrated - containing combined water (especially water of crystallization as in a hydrate)
hydrous mantle wedge enhances the stability of several accessory minerals (e.g. zircon zircon
Silicate mineral, zirconium silicate, ZrSiO4, the principal source of zirconium. Zircon is widespread as an accessory mineral in acid igneous rocks; it also occurs in metamorphic rocks and, fairly often, in detrital deposits. , titanite ti·tan·ite
See sphene. , and ilmenite ilmenite (ĭl`mĕnīt), black mineral, iron titanium oxide, FeTiO3, crystallizing in the hexagonal system. It is sometimes found as tabular hexagonal crystals but occurs more commonly as small grains in igneous and metamorphic ). Because several important trace elements and REE elements are concentrated in these phases, their stability in a hydrated mantle means that arc magmas are depleted in these elements relative to, for example, rift related magmas. This is an important chemical fingerprint for distinguishing among magmas of different tectonic environments and will be discussed in a later publication (Murphy 2007).
Partial Melting of the Crust In Continental Arcs
Volatile components such as water and carbon dioxide carbon dioxide, chemical compound, CO2, a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. have a strong tendency to enter magmas generated in both the mantle and the crust. In many tectonic settings, the partitioning of water into magma reduces [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] in the source rock, thereby raising the solidus temperature and limiting the amount of melt produced. This effect is schematically shown in a simplified phase diagram in the system [K.sub.2]O-[Na.sub.2]O-[Al.sub.2][O.sub.3]Si[O.sub.2]-[H.sub.2]O (KNASH, after Thompson and Algor 1977, Fig. 10a). The reactions depicted (Fig. 10) involve the breakdown of muscovite muscovite: see mica.
or common mica or potash mica or isinglass
Abundant silicate mineral that contains potassium and aluminum and has a layered atomic structure. It is the most common member of the mica group. during the melting of pelites. Reactions 2 and 5 define the curve for minimum melting of petite and the formation of felsic magma under water-saturated conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]). In some tectonic environments (e.g. continental rifts), the condition ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) may develop in the petites such that the activity of water is less than unity ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]). In this scenario, the univariant reactions involving water become divariant and progressively migrate as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is reduced, so that the stability field of the assemblage containing water is expanded. Reactions 2, 3 and 5 move to the right, reactions 1 and 4 to the left, but reaction 6, which is the fluidabsent reaction is unchanged. Since the invariant (programming) invariant - A rule, such as the ordering of an ordered list or heap, that applies throughout the life of a data structure or procedure. Each change to the data structure must maintain the correctness of the invariant. point for a given [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is the intersection of the univariant curves, the invariant point slides along the vapour-absent curve (V) towards higher P and T as a [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] decreases (see Thompson and Algor 1977). The net effect is that the temperature required for melting petites increases as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] decreases causing the amount of melt generated to be small (< 10%), i.e. below the critical volume required to attain the buoyancy to rise to higher levels in the crust. However, reactions involving the breakdown of biotite biotite (bī`ətīt'), iron-rich variety of phlogopite, most abdunant of the mica minerals.
or black mica
Silicate mineral in the common mica group. (Fig. 10b; generally between 750 and 780[degrees]C) may generate more magma (up to 60%, Clarke 1992), leaving a residue of granulite gran·u·lite
A fine-grained metamorphic rock often banded in appearance and composed chiefly of feldspar, quartz, and garnet.
gran that is typical of arc environments (Collins 2002).
[FIGURE 10 OMITTED]
If no water is added, (e.g. Spear et al. 1999), then reactions that generate magma require higher temperatures to proceed as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] decreases, but in arc settings, the supply of water may be continuously replenished (either directly or indirectly) by ongoing dehydration of the subducting slab. As a consequence, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] may remain close to unity, and anatexis of crustal rocks in arc environments generally proceeds at lower temperatures, and is more voluminous than in continental rifts, and spans longer periods of geologic time, routinely tens of millions of years.
Water and the Ascent of Magma
Although the details are controversial, water probably plays an important role in the ascent of magmas, because it reduces the viscosity of the melt (e.g. Mysen 1988). Furthermore, water pressure also promotes fracturing in overlying rocks, particularly in the crust. In the deviatoric stress field that is typical of arc regimes, water pressure reduces the effective stresses, which induces fracturing, as demonstrated by classic Mohr circle analysis. As water pressure builds up, the Mohr circle migrates towards the left as effective pressures are reduced, until the failure envelope is intersected. At this point fractures develop in the host rock, which facilitate the rise of magma and fluid (Fig. 11). This process is cyclic, as water pressure builds up and is then released when fracturing occurs (e.g. Sibson 1987).
[FIGURE 11 OMITTED]
Cooling History and Stability of Minerals
The presence of water also affects the sequence of crystallization in mafic magmas. Water content also affects stability of hydrated phases such as hornblende and biotite. Hornblende phenocrysts are common in arc lavas, and both hornblende and biotite are common in arc plutons. Experiments on the cooling history of basaltic magma formed in arc environments (Foden and Green 1992) indicate that the stability field of amphibole relative to olivine, pyroxene pyroxene (pī`rŏksēn), name given to members of a group of widely distributed rock minerals called metasilicates in which magnesium, iron, and calcium, often with aluminum, sodium, lithium, manganese, or zinc occur as X in the chemical and plagioclase plagioclase
Any member of the series of abundant feldspar minerals that usually occur as light- to medium-grey-coloured, transparent to translucent grains or crystals. Plagioclase ranges in composition from albite to anorthite. , is expanded with increasing partial pressure of water (see also Yoder and Tilley 1962). The implication is that amphibole may, in some situations, crystallize before olivine or pyroxene.
As water is an oxidizing agent, it also affects the stability of minerals such as magnetite magnetite (măg`nətīt), lustrous black, magnetic mineral, Fe3O4. It occurs in crystals of the cubic system, in masses, and as a loose sand. that contain [Fe.sup.3+]. In oxidized oxidized
having been modified by the process of oxidation.
see absorbable cellulose. magmas, magnetite is an early crystallizing phase, reflecting the higher [Fe.sup.3+]/[Fe.sup.2+] content of the melt (e.g. Miyashiro 1974). The enhanced stability of these minerals greatly affects the chemical composition of arc magmas. In contrast to continental tiffing environments, K-feldspar phenocrysts are rare, even in felsic rocks, because most arc magmas are not enriched in alkalies.
Water content in magmas also plays a key role in phenocryst phe·no·cryst
A conspicuous, usually large, crystal embedded in porphyritic igneous rock.
[pheno- + cryst(al). development because it reduces the viscosity of the melt by breaking Si-O bonds (a process known as depolymerization depolymerization /de·po·lym·er·iza·tion/ (de?po-lim?er-i-za´shun) the conversion of a polymer into its component monomers.
depolymerization ). As a result, ions can diffuse more easily and minerals can grow more rapidly (Mysen 1988). Arc lavas of all compositions are characterized by relatively high proportion of phenocrysts, commonly up to 20%. Plagioclase is by far the most common phenocryst in arc magmas of all compositions and typically exhibits normal, oscillatory oscillatory
characterized by oscillation.
see pendular nystagmus. or reverse zoning (e.g. Stewart and Fowler 2001). At constant pressure, the composition of plagioclase is controlled by the temperature, composition and water content of the melt. If, as a plagioclase crystal grows, these conditions change, so too may the composition of the next growth increment, leading to zoning. Zoning requires that chemical equilibrium between the melt and the plagioclase is not maintained. This may be because the strength of Si-O and Al-O bonds inhibits the dissolution of plagioclase, which is required to maintain equilibrium with the cooling melt, or because of the slow diffusion of A1. Normal zoning is thought to reflect cooling of the magma, oscillatory zoning reflects abrupt changes in melt composition, possibly brought on by a new influx of magma into the chamber, and reverse zoning is thought to represent a small component of oscillatory zoning (e.g. Shelley 1993).
In summary, water derived from the subducted slab plays a crucial role in all stages of arc development, starting with magma generation, then during its ascent, and finally during its cooling history. It also influences the stability of minerals in the source rock, thereby affecting bulk partition coefficients and magma chemistry. Water imparts chemical, mineralogical min·er·al·o·gy
n. pl. min·er·al·o·gies
1. The study of minerals, including their distribution, identification, and properties.
2. A book or treatise on mineralogy. and textural characteristics at all stages of magma evolution.
RELATIONSHIP BETWEEN MAGMATISM, METAMORPHISM AND DEFORMATION IN ARCS
Over the last 30 years, it has become clear that maximum pressures and temperatures are acquired at different times during the progressive evolution of orogenic belts (e.g. Houseman et al. 1981; Brown 1993). This conclusion has profound implications for the genesis of arc magmas and their relationship to metamorphism and deformation of crustal rocks. Through a combination of thermodynamic ther·mo·dy·nam·ic
1. Characteristic of or resulting from the conversion of heat into other forms of energy.
2. Of or relating to thermodynamics. modelling, Laboratory experiments and isotopic and fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium bearing minerals and glasses. Uranium-238 undergoes spontaneous fission decay at a known rate. , changes in pressure and temperature during the temporal evolution of orogenic belts :an be documented (Pressure-Temperature-time paths, or P-T-t). Some arcs have clockwise, others have counterclockwise P-T-t paths.
Clockwise (CW) P-T-t paths are generated by crustal thickening followed by erosional or tectonic exhumation. Because thrusting is viewed to be geologically instantaneous, maximum P is reached in the crust before maximum T and so magma generation typically post-dates thrust-related deformation, thereby initiating a CW path. In compressional zones in arc systems, the thermal regime that gives rise to arc magmas is greatly affected by thrust and reverse faults, particularly in zones with theologies that have been weakened by rising isotherms. The thermal effects of crustal thickening due to thrusting have been modelled (Figs. 12, 13) by England and Thompson (1986) and Thompson and Connelly (1995). For a thrust sheet 35 km thick, the net effect is to put the lower half of the thickened thick·en
tr. & intr.v. thick·ened, thick·en·ing, thick·ens
1. To make or become thick or thicker: Thicken the sauce with cornstarch. The crowd thickened near the doorway.
2. crust under conditions of melting. Initial magma generation takes place at the base of the crust and migrates upwards with time (Fig. 13) as successively higher structural levels cross the minimum melting curve. Note that the paths for initial post-thrusting depths of about 50 km cross the minimum melting curve about 15 million years after thrusting. Should extensional exhumation accompany thrusting, the time interval between thickening and magmatism would be reduced in duration. The above model provides insights into the spatial and temporal association of magmatism, metamorphism and deformation exhibited in many arc regimes.
[FIGURES 12-13 OMITTED]
Counter-clockwise (CCW (Continuous Composite Write) A magneto-optic disk technology that emulates a WORM (Write Once Read Many) disk. It uses firmware in the drive to ensure that data cannot be erased and rewritten. ) PT-t paths are generated when heating (and magmatism) precedes crustal thickening. Such paths can be initiated by underplating of the crust by mafic magma, possibly during a local or regional extensional event (Fig. 14). Underplating probably occurs because of viscosity contrasts at the crust-mantle boundary. As its ascent is arrested, the mafic magma spreads out laterally and heat is transferred to the base of the crust as the magma cools. If weaknesses in the crust are exploited, mafic magmas may rise to relatively shallow levels. In both situations, significant heat is transferred from the cooling mafic magma into the crust and may be enough to cause crustal melting. These tendencies may be amplified, if at the same time, the crust is undergoing extension, such as in back arc regions. The resulting crustal thinning and pressure reduction leads to a steepening of isotherms and induces partial melting in the mantle and crust. This thermal phase is followed by crustal thickening which may be coeval co·e·val
Originating or existing during the same period; lasting through the same era.
One of the same era or period; a contemporary. with mantle thinning (e.g. Sandiford and Powell 1991; Brown 1993).
[FIGURE 14 OMITTED]
According to Collins (2002), however, as most granulite terranes record higher geothermal gradients (25-50[degrees]C [km.sup.-1]) than those reported in simple crustal thickening models (Fig. 13), voluminous magmatism and high-grade metamorphism may preferentially occur during episodes of extension in the arc during slab roll-back. However, the inevitable arrival of microcontinents and/or oceanic plateaux at the trench may induce temporary compression, in which the rheologically softened crust undergoes thickening driving the P-T-t path to higher pressures (into a CCW path). In this scenario, crustal melts become deformed as they cool below near-solidus temperatures in a deviatoric stress field (e.g. Hibbard and Watters 1985).
There are two types of volcanic arcs--intraoceanic island arcs that may be underlain un·der·lain
Past participle of underlie. by either oceanic or continental crust, and continental arcs that form along the edge of a continent and are underlain by continental crust. Island arcs underlain by oceanic crust are dominated by mafic magmas, whereas continental arcs and island arcs underlain by continental crust are dominated by intermediate to felsic magmas. Arc magmatism is closely related to metamorphism, which may follow either clockwise or counterclockwise P-T-t paths. Clockwise P-T-t paths are initiated by crustal thickening, associated with thrusting, followed by a temperature increase due to the thermal effects of thickening. Magma forms first at the base of the crust but over tens of millions of years forms at successively higher crustal levels. Counter-clockwise P-T-t paths are initiated when heating (e.g. underplating of the crust by mafic magma) precedes crustal thickening. Clockwise P-T-t paths occur when arcs are under compression and counter-clockwise paths occur when an arc is under extension. Both conditions can occur during the lifespan of a given arc.
Arc volcanism occurs in a belt about 10 km wide, which is approximately 110 km above the subduction zone. However, subduction zones can last for hundred of millions of years and changes in the location and dip of the subduction zone during that time result in arcs that are typically 200-300 km wide. The width of the fore-arc region i.e. between the arc and the trench, also changes with the clip of the subduction zone. Fore-arc crust and overlying sediments may become imbricated imbricated /im·bri·cat·ed/ (im´bri-kat?id) overlapping like shingles.
overlapping like shingles or roof slates or tiles. and transferred to the subducting slab, a process known as subduction erosion.
The backarc region, another important site of magmatism, is characterized by high heat flow, and undergoes repeated episodes of extension and compression. During the extensional phase, basins develop that are filled with sediments and a steepened geothermal gradient produces extensive magmatism in the rheotogically weakened and thinned crust. During a prolonged history of subduction, the basin fill provides a fertile chemistry for the generation of magmas during subsequent phases of compression or extension.
Water plays a fundamental role at all stages of magma evolution. Water lowers the solidus temperatures in the mantle and crust because it weakens mineral structures. In addition, rising water acts as a heat flux. The composition of the fluid at the site of melting has a major influence on the composition of the magmas produced. This fluid probably has components of the invading fluid from the subduction zone, the indigenous fluid previously added to the mantle wedge, and the fluid released by mantle minerals. Thus, reaction between the invading fluid, mantle minerals and previously added fluid may influence the mineral composition of the mantle, as well as change the composition of the fluid. The composition of the fluid at the time of melting influences the chemistry of the magma produced, and therefore, the composition of the resulting arc rocks.
Water also plays an important role in the ascent and cooling of magma. High pore-water pressure can induce fracturing in the crust subjected to deviatoric stress, which allows magma and fluids to rise more rapidly. Water depolymerises melt structure, and the resulting decrease in viscosity allows minerals to grow more rapidly. Arc lavas, especially those of intermediate composition, are characterized by plagioclase phenocrysts with oscillatory zoning that records variations in temperature, composition and water content of the melt during magma crystallization. As volatiles partition into magma, source rocks may become dehydrated de·hy·drate
v. de·hy·drat·ed, de·hy·drat·ing, de·hy·drates
1. To remove water from; make anhydrous.
2. To preserve by removing water from (vegetables, for example). so that [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], which causes melt-generating reactions to migrate to higher temperatures, and thus, inhibits the formation of significant quantifies of magma. If, however, water is continually replenished, directly or indirectly, by ongoing dehydration of the subducting slab, then magma generation can proceed under water-saturated conditions at lower temperatures.
Several mechanisms may combine to facilitate the rise of arc magmas towards the surface. Large plutons appear to represent the end result of multiple intrusions. Buoyant magma has the tendency to form teardropshaped diapirs, particularly in the ductile lower crust. Many diapirs appear to have exploited zones of structural weakness in the crust, where rapid ascent resulted in expansion and further fracture propagation in the roof above the magma chamber, facilitating further rise by stoping and assimilation. The presence of water in magmas also aids in fracture propagation. Volatile overpressure overpressure,
n excessive pressure applied at the end of a physiologic joint range to confirm the severity of pain, thus helping determine the manual treatments. and extensional tectonic regimes in the arc (e.g. local pull-apart structures) permit rapid rise of magma. Significant volumes of magma migrate through a variety of structural weaknesses and the net result may be a composite batholith that is typical of arc systems.
Much of what I have learned about arc magmatism has been through collaboration and conversations with colleagues including Alan Anderson, Randy Cormier, Richard D'Lemos, Jarda Dostal, Andrew Hynes, Duncan Keppie, Damian Nance, Georgia PePiper, Cecilio Quesada, Rob Strachan and participants in various UNESCO-IGCP projects. I am grateful to NSERC NSERC Natural Sciences and Engineering Research Council (Canada)
NSERC Naval Systems Engineering Resource Center Canada for research funding, for which arc systems are a major part, to Randy Cormier for reading an early draft, to Randy, Derek Thorkelson and Steve Johnston for thorough, constructive reviews, Georgia Pe-Piper, Steve McCutcheon and Sonya Dehler for editorial advice, and to Matt Middleton (funded by NSERC Research Capacities Development grant) for cheerful technical assistance.
Accepted as revised, 5 January 2007.
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Consisting of or relating to stone or rock.
Adj. 1. lithic - of or containing lithium
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Large, bowl-shaped volcanic depression that forms when the top of a volcanic cone collapses into the space left after magma is ejected during a violent volcanic eruption. The term is Spanish for “caldron. complex, Taupo volcanic centre, New Zealand: Journal of Volcanology volcanology
Scientific discipline concerned with all aspects of volcanic phenomena. Volcanology deals with the formation, distribution, and classification of volcanoes, as well as their structure and the kinds of materials ejected during an and Geothermal Research, v. 80, p. 217-237.
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tr.v. e·duced, e·duc·ing, e·duc·es
1. To draw or bring out; elicit. See Synonyms at evoke.
2. To assume or work out from given facts; deduce. , and the Neogene tectonics of southwestern North America: Tectonophysics Tectonophysics, a branch of geophysics, is the study of rock deformation and of dynamic geophysical processes. This includes small individual crystals as well as the larger scales which include tectonic plates and forces in the mantle. , v. 67, p. 81-99.
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Any one of the sciences, such as geology or geochemistry, that deals with the earth.
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Coarse-grained, heavy, igneous rock that contains at least 10% olivine, other iron- and magnesium-rich minerals (generally pyroxenes), and not more than 10% feldspar. [H.sub.2]O-C[O.sub.2] system: American Journal of Science The American Journal of Science (AJS) is America's longest-running journal, having been published continuously since its conception in 1818, by Professor Benjamin Silliman, who edited and financed it himself. , v. 278, p. 305-343.
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EGS El Goonish Shive (webcomic)
EGS Environmental Goods and Services
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Any member of a large family of rocks that occur in most of the world's volcanic areas, mainly as surface deposits and to a lesser extent as dikes and small plugs. : some experimental and natural evidence: Contributions to Mineralogy and Petrology, v. 109, p. 479-493.
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The frequency or magnitude of earthquake activity in a given area.
The frequency or magnitude of earthquake activity in a given area. and tomographic constraints from the Andean margin: Tectonics, v. 19, p. 814-833.
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Relating to, resembling, containing, or derived from silica or silicon. magma chambers, implications for lithospheric magmatism: Journal of Geophysical Research, v. 86, p. 10153-10192.
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The compounded annualized rate of growth of a company's revenues, earnings, dividends, or other figures.
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1. Of, facing, or located on the right side; right.
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The occurrence of returns on asset classes diverging from their normal pattern of correlation.
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A bluish schist that gets its color from the presence of a sodic amphibolite, glaucophane, or a variety of glaucophane called crossite. to eclogite eclogite
Any member of metamorphic rocks whose original composition is similar to that of basalt. Eclogites consist primarily of green pyroxene (omphacite) and red garnet (pyrope), with small amounts of various other minerals such as kyanite and rutile. facies facies /fa·ci·es/ (fa´she-ez) pl. fa´cies [L.]
1. the face.
2. surface; the outer aspect of a body part or organ.
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