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Emplacement of the Fogo Island Batholith, Newfoundland.

ABSTRACT

The Siluro-Devonian Fogo Island Batholith is a high-level, bimodal, sill-like unit about 7 km thick, intruded by a slightly younger, heterogeneous mafic unit. Stratigraphic evidence suggests emplacement of the batholith by raising of the roof without strong deformation of the host rocks. Large composite gabbro-microgranite dykes below the sill, intruded along axial planar cleavage, probably served as feeders. The upper 3-4 km of the sill comprise homogeneous to slightly zoned, coarse, hastingsite-biotite granite. Ignimbrite sheets of similar composition above the sill suggest that the granite was emplaced beneath cover rocks no more than 1 km thick. The lower 3 km of the sill consist of heterogeneous, locally layered, mafic rocks. Metre-scale, non-intrusive sheets of contrasting composition mark the contact between the granitic upper and mafic lower part of the sill. A late influx of mafic magma disrupted older mafic rocks and hybridised with granite, producing complex, heterogeneous "diorite". The local presence of A-type compositions in the granitic portion of the batholith may reflect diffusion during this process. A model for emplacement of the batholith assumes crustal anatexis above mantle-derived, underplated mafic magma, followed by passive emplacement of composite magma along tensional fractures related to movements on the Dog Bay Line, a dextral terrane boundary. Later movements on this feature controlled emplacement of further batches of magma, and finally tipped the batholith about 25[degrees] to the north, producing its present configuration.

RESUME

Le Batholithe siluro-devonien de l'ile Fogo forme une unite a l'aspect d'un filon-couche bimodale de niveau eleve et d'environ sept kilometres d'epaisseur, dans laquelle fait intrusion une unite mafique heterogene legerement plus recente. Les observations stratigraphiques laissent supposer une mise en place du batholithe par soulevement du toit sans deformation marquee des roches hotes. Des dykes composites de gabbro-rhyolite de fortes dimensions au-dessous du filon-couche, ayant subi une intrusion le long de la schistosite de plan axial, ont probablement servi de voles de passage. Les trois a quatre kilometres superieurs du filon-couche sont constitues de granite a hastingsite-biotite grossier allant d'homogene a legerement zone. Des couches de tuf consolide d'une composition semblable au-dessus du filon-couche permettent de supposer que la mise en place du granite s'est faite sous des roches de couverture d'au plus un kilometre d'epaisseur. Les trois kilometres inferieurs du filon-couche sont constitues de roches mafiques heterogenes, localement stratifiees. Des nappes non intrusives de quelques metres d'une composition differente marquent la zone de contact entre le sommet granitique et la base mafique du filon-couche. Un afflux tardif de magma mafique a perturbe les roches mafiques agees et a entraine une hybridation avec le granite, produisant des << diorites >> heterogenes complexes. La presence locale de compositions de type A dans la partie granitique du batholithe pourraient temoigner d'une diffusion au tours de ce processus. Un modele de mise en place du batholite suppose une anatexie crustale au-dessus d'un magma mafique reposant sur des plaques et d'origine mantellique, suivie de la raise en place passive de magma composite le long de fractures de tension apparentees a des mouvements survenus le long de la ligne Dog Bay, une ligne de demarcation de terrane dextre. Des mouvements ulterieurs le long de cette ligne ont determine la mise en place d'autres melanges de magma et ont finalement incline le batholithe d'environ 25 degres vers le nord, pour produire sa configuration existante.

[Traduit par la redaction]

INTRODUCTION

Batholith emplacement, by definition, takes place deep within the crust under conditions which are difficult to determine precisely, and even more difficult to scale accurately in experiment or simulation, rendering dubious the applicability of conclusions reached from experimental and modelling studies. Compare, for example, thermal modelling studies indicating that diapiric ascent of magma in the crust is impossible (Weinberg 1996; Petford 1996) with structural studies of felsic plutons indicating diapir-like emplacement (Sylvester 1964; Paterson and Vernon 1995). However, conditions of emplacement of a batholith can, in favourable cases, be constrained by stratigraphic, structural, and geochemical data. For the Fogo Island Batholith of northeast Newfoundland, these data provide valuable information on the evolution and emplacement of the batholith, including its geometric form and a mechanism of creating sufficient space for emplacement.

GEOLOGICAL SETTING

The Fogo Island Batholith lies within a curved, northeast-trending fault slice dominated by mid-Ordovician to Silurian strata (Fig.1) which underwent polyphase deformation in Silurian time (Karlstrom et al. 1982; Currie 1997c). Southwest of Fogo Island, isoclinal, steeply dipping, northeast-trending, northwest-verging structures dominate the outcrop pattern within this slice, but on Fogo Island these folds are open, with limb dips rarely exceeding 40[degrees]. The folded strata comprise back-are volcanogenic rocks of the mid-Ordovician Exploits Group (O'Brien et al. 1997), late Ordovician early Silurian, westerly-derived, greywacke and conglomerate turbidites of the Late Ordovician-early Silurian Badger Group (Williams et al. 1995), and clastic and volcanic rocks of the Silurian Botwood Group (Williams 1972). The Dog Bay Line (Williams et al. 1993), a dextral terrane boundary marked by tectonic melange, juxtaposes the Badger and Botwood groups against Silurian limestone and limy coralline siltstone of the Indian Islands Group (Currie 1995). Along the Dog Bay Line, the bimodal Fogo Island Batholith abuts coeval peraluminous anatectic plutons (Currie and Pajari 1981) and the metaluminous early Devonian Deadman's Bay megacrystic pluton (D'Lemos and Holdsworth 1995). Metaluminous Siluro-Devonian bimodal batholiths similar to the Fogo Island Batholith occur between the Dog Bay and Red Indian lines (Mount Peyton, Loon Bay, Long Island), but northwest of the Red Indian Line only sparse Siluro-Devonian salic dykes occur (Elliot et al. 1991)

[FIGURE 1 OMITTED]

DESCRIPTION OF UNITS

Botwood Group

Fogo Island (Fig. 2) comprises salic and mafic igneous rocks of the Fogo Island Batholith and its host, the Botwood Group, which on Fogo Island comprises siltstone, sandstone, and matrix-supported conglomerate of the Fogo Harbour Formation overlain by rhyolite ignimbrite, rhyolitic tuff, and minor tuffaceous sandstone of the Brimstone Head Formation. The lowest unit of the Botwood Group, the Llandovery Lawrenceton Formation (Williams 1972), does not outcrop on Fogo Island, but appears on nearby islands. This formation, consisting of about equal amounts of basalt (flows and breccias) and rhyolite (flows, tuffs, and breccias), reaches thicknesses in excess of 2000 m on Change Islands, 10 km west of Fogo Island (Currie 1997a), but elsewhere does not exceed 300 m in thickness, suggesting presence of a vent area just west of Fogo Island. The presence of several rhyolite ignimbrite sheets within the Lawrenceton Formation indicates subaerial emplacement. The top 100 m of the Lawrenceton Formation contains metre-scale lenses of pale green, thinly laminated siltstone identical to the conformably overlying Fogo Harbour Formation.

[FIGURE 2 OMITTED]

The bulk of the Fogo Harbour Formation (Baird 1958) consists of grey-green to brown siltstone and sandstone, laminated on centimetre-scale, and exhibiting grading, ripple marks, and cross-bedding. Most exposures contain no obvious volcanic material, but a few beds contain 2 to 10% by volume of pink feldspar lapilli and crystal fragments up to a centimetre in diameter. Near Rogers Cove one bed contains volcanic bombs. Conglomerate lenses with rounded siltstone cobbles in a homogenised siltstone matrix occupy erosive channels up to 2 m thick. Composite stratigraphic sections for the Fogo Harbour Formation suggest a relatively constant thickness of 1000 to 1300 m. Observations indicate a shallow marine origin for the formation, with the conglomerate intervals attributed to slumping during deposition. No fossils have been found in the Fogo Harbour Formation, but interbedding of its lower part with the Llandovery Lawrenceton Formation fixes a maximum age, and a precise U-Pb zircon age of 422 [+ or -] 2 Ma (Ludlow) from a composite dyke cutting the formation on Dog Bay (Elliot et al. 1991) provides a minimum age.

A sharp, conformable contact of the Fogo Harbour Formation with the overlying Brimstone Head Formation (Baird 1958) can be traced across northern Fogo Island at the base of a cliff of rhyolite ignimbrite (Currie 1997b). A single metre-scale lens of tuffaceous sandstone resembling the underlying sedimentary section occurs about 60 m above the base of the Brimstone Head Formation at Brimstone Head. On Fogo Island, the Brimstone Head Formation comprises several sheets of densely welded, brown, rhyolite ignimbrite with a foliation marked by elongate pale streaks of less welded material (fiamme). Offshore islands consist of moderately welded crystal ruff and tuff breccia with a few lenses of red, subaerial sandstone up to 10 m thick. The thickness of the formation is unknown, but locally exceeds 1000 m. The composition of the ignimbrite sheets resembles that of salic parts of the Fogo Island Batholith, and the Brimstone Head Formation may represent an erupted portion of the batholith, as suggested by Sandeman and Malpas (1995).

All of the supracrustal rocks on Fogo Island are openly folded about axial planes trending northeast and dipping about 65[degrees] to the south. Limb dips rarely exceed 40[degrees], and fold amplitudes vary from a few metres to a few hundred metres. All of the folds exhibit strong axial planar cleavage, a cleavage which is developed on a regional scale, extending tens of kilometres southwest from Fogo Island. An envelope to the folds dips north-northwest at about 25[degrees], so that stratigraphically higher levels are exposed to the north.

Fogo Island Batholith

The Fogo Island Batholith, which underlies 80% of Fogo Island (about 250 [km.sup.2]) and also appears on small islands up to 8 km offshore to the east and southeast, intrudes the Fogo Harbour Formation. The batholith comprises five distinct lithologies. In order of decreasing age they are (1) agmatite containing blocks of host rock in a coarse-grained igneous matrix, (2) felsite, rhyolite porphyry, and microgranite, (3) layered and massive gabbro, (4) medium- to coarse-grained amphibolebiotite granite, (5) heterogeneous mafic rocks with a generally dioritic to tonalitic matrix containing schliers and blocks ranging from layered gabbro, to monzonitic, syenitic and granitic varieties. In the Tilting Harbour area, Aydin (1995) reported an emplacement age of 422 [+ or -] 2 Ma for the amphibole-biotite granite, and 408 [+ or -] 2 Ma for agmatitic diorite, consistent with field observations.

Layering in the batholith, commonly present in intermediate to mafic units and locally in granitic units, parallels bedding in nearby sedimentary rocks, and exhibits open folds congruent to those in the host rocks. However, folds in the igneous rocks do not exhibit axial planar cleavage, suggesting that folding in the host rocks pre-dated emplacement of the batholith, at least in part. Both top and bottom contacts of the batholith with the Fogo Harbour Formation can be observed in the western part of the island, dipping moderately north. These observations indicate a sill-like form for the exposed part of the batholith. Geological mapping shows the batholith to be about 6.5 km thick at its western end increasing to 8 km or more toward the east. The batholith comprises a homogeneous granitic upper part, 3 4 km thick, and a texturally complex intermediate to mafic lower part about 3 km thick.

Minor intrusive bodies related to the batholith occur both above and below the main mass. Dykes related to the batholith tend to strike parallel to bedding in the host rocks, but dip at high angles to bedding, lying in the axial planar cleavage of open folds. Below the body, on the southwest corner of the island, composite dykes up to 60 m thick (Fig.3) have a core of gabbro up to 20 m thick, commonly plagioclase-porphyritic, with thick rims of porphyry or microgranite chemically indistinguishable from granite of the main body. The contacts between mafic and salic phases are sharp, but lobate, with occasional rounded blobs or streaks of one lithology in the other, indicating some degree of mingling. These distinctive dykes occur up to 20 km from Fogo Island. One gave a precise U-Pb zircon date of 422 [+ or -] 2 Ma (Elliot et al. 1991), identical to that of the main batholith. In some cases these dykes connect to small microgranite sills or laccoliths. Above the main body near Fogo Harbour, porphyry and microgranite dykes abound, but they lack mafic cores and rarely exceed 15 m in width. Many of these dykes connect to sills up to 10 m thick, with the dykesill transition forming prominent rock ridges, possibly due to thickening of the erosionally resistant igneous bodies at this point. Sandeman and Malpas (1995) considered these sills to be ignimbrite sheets, but their intrusive nature is demonstrated by (1) equal and intense hornfelsing of the host on both sides of the sills, and (2) presence of laminated siltstone on both sides with no increase in tuffaceous material.

[FIGURE 3 OMITTED]

Agmatite

Although some small-scale contacts of the batholith with its host rocks appear knife-sharp and generally conformable, agmatite (Fig. 4) commonly appears along internal and external contacts of the Fogo Island Batholith and may be up to 100 m thick. Agmatite in contact with the Fogo Harbour Formation consists of 50 to 80% of centimetre to metre-scale hornfelsed blocks of Fogo Harbour Formation in a medium- to coarse-grained matrix which is granitoid along the northern and southwestern margins of the complex, but becomes more tonalitic to the south. Similar agmatite forms a semi-continuous fringe, 10 to 100 m thick, separating granite bodies between Joe Batts Arm and Barr'd Islands. Within the agmatite, the blocks become larger and more coherent with distance from more massive igneous rocks, passing outward into irregularly folded sedimentary rocks with a few granite dykes. Throughout the agmatite, the blocks exhibit a strong preferred orientation, in many examples preserving only slightly disrupted stratigraphy. Within 50 m of the main body, some centimetre-scale beds appear to have melted to thin layers of brownish aphanitic material which locally forms a selvedge around blocks. Despite intense hornfelsing and possible local partial melting, new mineral growth was confined to sparse growth of fine biotite, suggesting that cooling took place rapidly enough that thermal equilibrium was never attained.

[FIGURE 4 OMITTED]

Felsite, rhyolite porphyry, and microgranite

The upper margin of the granite consists of an almost continuous layer of fine-grained, pink, felsic rock varying from featureless felsite to microgranite with abundant granophyric intergrowth and drusy miarolitic cavities, to rhyolite porphyry that is commonly spherulitic. Similar to identical lithologies occur as sills along the base of the granite where it contacts the mafic phase, and as sills and dykes above and below the batholith. In general felsite occurs close to the coarse-grained granite, whereas microgranite tends to form sills below the batholith, and rhyolite porphyry forms dykes above the batholith. However, there is no sharp division and all these rocks have similar or identical compositions (Appendix 1). A typical feature of all these rocks is presence of thin seams or clots of amphibole and magnetite. These segregations occur erratically in the felsic rocks (which otherwise contain virtually no mafic minerals) and also in hornfelsed country rocks adjacent to them.

Sandeman and Malpas (1995) reported that microgranite sheets cut the batholith, and that coarse-grained granite grades to microgranite. However, contacts between coarse and fine-grained to aphanitic felsic rocks are always sharp where exposed, and where intrusive relations can be determined, the coarse-grained phase invariably cuts the fine-grained one. Chemically no significant differences are present in major element composition between coarse-grained and fine-grained felsic rocks (Appendix 1). Sandeman and Malpas (1995) drew attention to enrichment of the fine-grained rocks in high-field-strength elements and Ga/Al ratio, suggesting an A-type affinity. These tendencies are evident in all the felsic compositions (Fig. 5), and there appear to be no systematic differences in composition between fine-grained rocks and coarse-grained rocks, with the possible exception of slightly lower Fe[O.sup.*]MgO in the latter.

[FIGURE 5 OMITTED]

Gabbro and basalt

Baird (1958) noted the presence of layered mafic rocks around Tilting Harbour and Seldom Come By. The Tilting Layered Suite, which formed the subject of studies by Cawthorn (1978) and Aydin et al. (1994), underlies an oval area about 2 by 1.5 km in size, surrounded by hybrid dioritic rocks into which it grades, and by which it is intruded. Centimetre to metre-scale layering faces and dips 35-85[degrees] to the north, and consistently pinches out to the southwest. Layers exhibit modal and textural variation, including repeated cycles from websterite bases (cumulate ortho- and clinopyroxene [+ or -] olivine) to gabbro or leucogabbro tops (cumulate clinopyroxene and plagioclase). Large poikilitic hornblende and in some cases plagioclase overgrow the cumulates. Local convolute layering and prominent size and density sorting in some layers suggest deposition from magmatic currents, although much of the fine-scale layering may be due to in situ recrystallisation (Aydin et al. 1994). The top of the section, northwest of Tilting, consists of coarse-grained, unfoliated plagioclase-hornblende rocks which grade to quartz dioritic agmatite containing metre-scale fragments of layered mafic rocks.

Gabbroic rocks around Seldom Come By form two ovoid masses. At Burnt Point, massive, pegmatitic gabbro contains cumulus ortho- and clinopyroxene, and large poikilitic post-cumulus amphibole and plagioclase. The body contains about 10% of sinuous, boudinaged basalt dykes up to 50 cm wide. At the head of Seldom Harbour irregularly layered gabbro and diorite grade to heterogeneous mafic rocks which contain enclaves of layered gabbro, some sharply bounded in metre-scale blocks, others apparently gradational to larger areas of massive gabbro.

The layered mafic rocks of the Fogo Island Batholith resemble large layered mafic intrusions, and imply that such a body was formed or disrupted during emplacement of the Fogo Island Batholith. Within the layered mafic rocks, abrupt reversals to a more primitive crystallisation order (clinopyroxene+plagioclase to orthopyroxene+olivine) imply repeated influx of primitive magma into a relatively stable magma chamber (Brown 1956; Wager and Brown 1968).

Gabbroic compositions commonly occur in dykes and sheets below the main batholith in the southwest corner of the island. Distinctive plagioclase-porphyritic gabbro, resembling the layered gabbro around Seldom Come By, occurs as discrete dykes, in the cores of composite dykes, as synplutonic dykes within the granite, and as synplutonic dykes within gabbro at Burnt Point.

Chemical analyses of non-cumulate gabbroic rocks from Tilting and Seldom show a considerable range of composition, ranging from subalkaline basalt across the andesite/basalt and andesite fields (Fig. 6d), a range approaching that of the apparently much more heterogeneous diorite complex (Appendix 1). On chemical discrimination diagrams (Fig. 6), compositions fall into both arc (Fig. 6a) and non-arc fields (Fig. 6b), with a large amount of scatter in some plots (Fig. 6c).

[FIGURE 6 OMITTED]

Granite

Coarse-grained granite of the Fogo Island Batholith occurs as an east-west belt across the northern part of the island, partially separated into three lobes by narrow belts of hornfelsed host rocks, agmatite, and sheets of felsite. All three lobes exhibit hastingsitic amphibole with minor biotite, and a colour index consistently <15. The central feldspar-porphyritic lobe contains 1 to 2% of digested mafic inclusions up to 5 cm across. The more equigranular eastern and western lobes lack mafic inclusions. All three lobes tend to be locally homogeneous, but become slightly more potassium feldspar-rich and leucocratic toward the north. All three lobes contain sinuous synplutonic basalt dyke segments exhibiting chilled margins, commonly 1 to 2 m wide and 5 to 10 m long (Fig. 7). Dyke lithologies include plagioclase-porphyritic varieties similar to the cores of composite gabbro-rhyolite dykes. Felsic dykes and pegmatite are absent and quartz veins extremely rare (one observed).

[FIGURE 7 OMITTED]

South of Fogo Harbour, granite cuts across a carapace of felsite and rhyolite sills, and is in essentially conformable contact with strongly hornfelsed sedimentary rocks. This region exhibits crude sheeting parallel to the contact, and prominent joints perpendicular to the contact.

Chemically, all analyses of the granite fall in a restricted field (Fig. 5). On the diagram of Batchelor and Bowden (1985) this region straddles the boundary between late orogenic and post-orogenic granites (Fig. 5a), whereas in the diagrams of Maniar and Piccoli (1989), it clearly falls in the post-orogenic granite field (Fig. 5b, c). The trend to more differentiated compositions from south to north in all lobes can be seen in Fig. 5c (increasing Si[O.sub.2] and Fe[O.sup.*]/(Fe[O.sup.*]+MgO))The tendency toward A-type compositions noted by Sandeman and Malpas (1995), seen in Fig. 5d, results from relatively high contents of(Zr+Nb+Ce+Y) combined with extreme depletion in MgO (producing extreme Fe[O.sup.*]/MgO ratios). Alkali contents are not unusually high.

Diorite complex

The diorite complex forms the largest and most complex unit of the Fogo Island Batholith. Some dioritic rocks exhibit coarse, massive textures comparable to those of the gabbro and granite. However, large exposures invariably include patchy, inequigranular textures, streaks of one lithology in another, and schlieren and blocks of more mafic lithologies in a dioritic to granitic matrix. Characteristic gradational, centimetre-scale pegmatitic patches with large amphibole and feldspar crystals have widely varying alkali feldspar/plagioclase ratios, and may contain a significant amount of quartz. Igneous breccias of diverse mafic blocks in a more salic matrix form a major part of the diorite complex (Fig. 8). The mafic component varies from rare examples of layered gabbro, to hornblenderich amphibolite to hornblende-plagioclasc rocks with minor quartz. In coastal exposures at Wild Cove, Cape Fogo, and Kippen Cove some mafic blocks exhibit intimate, amoeboid interfingering with the matrix, and form pillow-like masses, phenomena suggestive of coexisting magmas. The composition of the more homogeneous parts of the diorite complex ranges from gabbroic through hornblende diorite and quartz diorite to tonalite, monzodiorite, and rare syenite. Mafic varieties contain mafic inclusions, whereas more silicic varieties contain more silicic inclusions. Much of the interior part of the diorite complex comprises locally homogeneous, fine to medium-grained, granoblastic hornblende-plagioclase rocks with variable but minor amounts of clinopyroxene, quartz, and potash feldspar. Many of these rocks exhibit nebulous pegmatitic patches of hornblende+plagioclase, commonly associated with healed, epidote-filled fractures.

[FIGURE 8 OMITTED]

Near the contact between the diorite complex and granite, diorite and granite are typically observed in alternate outcrops, or in small adjacent areas with few exposed contact relations. In a large highway quarry, felsite and diorite form conformable, non-intrusive, metre-scale sheets. Coastal exposures at Cape Fogo and Cape Cove exhibit a variety of complex relations ranging from complexes of sills with contrasting compositions, through diorite cross-cut by granite, or pillows of mafic in salic phases, to completely gradational contacts over a few tens of centimetres. In the absence of definitive contact relations, Baird (1958) and Sandeman and Malpas (1995) interpreted the salic phases of the batholith to intrude the mafic phases. However, field observations during the present study generally suggested originally non-intrusive contacts within a zone of mingling.

Although the textural range of the diorite complex is bewilderingly large, chemical analyses (Appendix 1) show a limited range of composition, with Si[O.sub.2] content ranging fairly continuously from 52 to 61%. Two samples yielded Si[O.sub.2] contents of 64 and 75%. The former is the only one of 65 analyses of the batholith with an Si[O.sub.2] content between 61 and 70%. In general, analyses of the diorite complex plot in the same fields as the layered mafic rocks (Fig. 8) but contain much higher concentrations of K, Zr, and U, and lower contents of Mg. An obvious possible cause for these anomalies could be mixing with the granitic component of the batholith, as suggested by field observations. The strongly bimodal nature of the analyses shows that if this model is correct, mixing involved incorporation of the salic component into the mafic phase but little incorporation of the mafic component into the salic phase.

A MODEL FOR EMPLACEMENT OF THE FOGO ISLAND BATHOLITH

The Fogo Island Batholith consists of a homogeneous salic portion and a texturally heterogeneous mafic portion which includes layered rocks. A model for emplacement must explain the presence and mutual relations of these components. Field observations relevant to emplacement of the batholith may be summarised as follows. (1) Most mafic dyke emplacement and all salic dyke emplacement preceded emplacement of the main batholith and followed a near-vertical cleavage ,axial planar to pre-batholith open folds. (2) Composite dykes with mafic cores occur below the batholith but not above it. (3) The older part of the batholith, a thick sill, is gravitationally stratified, with mafic rocks at the base and salic rocks at the top. (4) Layered mafic rocks are disrupted by and included within heterogeneous mafic to intermediate rocks which show evidence of magma mixing and assimilation. (5) The bimodal Fogo Island Batholith developed close to a vent area of slightly older bimodal volcanic rocks of the Lawrenceton Formation.

These observations suggest a three-stage model for emplacement, namely (i) generation of a composite magma, here assumed to be due to mafic underplating of the crust with subsequent crustal anatexis, (ii) buoyant rise of the composite magma followed by gravitational differentiation to produce the sill-like mass, and (iii) a late influx of mafic magma. These stages are cartooned in Fig. 9, which uses the time scale of Tucker et al. (1990) for comparison of stratigraphic and absolute ages.

[FIGURE 9 OMITTED]

Eruption of the bimodal, subduction-related (Currie 1995) Lawrenceton Formation (Fig. 9a) had almost ceased by Late Llandovery time (430 Ma) as motion along the Dog Bay Line passed from subduction to dextral transpression (Karlstrom et al. 1982). Reduction of magmatic activity led to subsidence due to cooling of the crust, permitting a marine incursion and deposition of the Fogo Harbour Formation. Magmatism had not completely ceased, as shown by presence of tuffaceous material in the Fogo Harbour Formation. The model assumes that mafic magma, no longer able to erupt, underplated the crust where dextral transpression was converted into extension at the bend in the Dog Bay Line (Fig. 9b). The need for such a deep-seated precursor to the high-level Fogo Island Batholith follows from the regional distribution of the composite dykes which appear to have fed the batholith.

Large scale emplacement of mafic magma eventually caused anatexis of surrounding crustal host rocks, initially producing a gravitationally stable system with light salic magma above dense mafic magma. Such a system is unstable relative to its host because both components have positive buoyancy with respect to continental crust (Glazner 1994). If the volume of magma becomes sufficiently large, it will rise buoyantly along fractures. Analogs of basalt-rhyolite systems experimentally studied by Huppert and Sparks (1988) showed that fingers of hot mafic rock rise through the overlying salic melt carrying salic material along with them. The composite dykes below the Fogo Island Batholith suggest such a process, with magma following cleavage axial planar to slightly earlier folds (Fig. 9c). The large size of the dykes, and lack of mixing of salic and mafic components, suggest that these fractures may have opened during emplacement, allowing passive emplacement with the mafic magma rising to its neutral buoyancy level, and spreading laterally.

Glazner (1994) estimated the density of mafic magma near surface to be about 2.75 g/[cm.sup.3], in reasonable accord with the measurement of 2.73 g/[cm.sup.3] for erupting Kilauea basalt (Macdonald 1963). Both values are higher than the density of Fogo Harbour Formation (2.68 g/[cm.sup.3]) but lower than the density of basalt of the Lawrenceton Formation (2.89 g/[cm.sup.3]). Currie (1995) found the latter to be underlain by a thick section of Ordovician mafic volcanic rocks and a thin section of Ordovician-Silurian greywacke (Badger Group). It is therefore plausible that mafic magma would rise buoyantly to the top of the mafic volcanic rocks, but stop near the base of the low-density Fogo Harbour Formation. The abrupt transition from submarine deposition of the Fogo Harbour Formation to the subaerial ignimbrite sheets of the Brimstone Head Formation required rapid uplift which could be explained by floating of the Fogo Harbour Formation on underlying magma, combined with thermal expansion.

The thick composite dykes intruding the Fogo Harbour Formation strongly suggest that the magma was composite when emplaced. Gravitational differentiation of such composite magma would lead to formation of salic upper and mafic lower parts. Because the density of the salic portion would be less than that of the host Fogo Harbour Formation, it could continue to rise, hypothetically producing the three observed domes and their dividing screens of agmatite. Given sufficient heat content and appropriate channels, it could eventually reach the surface and erupt to form ignimbrite sheets (Brimstone Head Formation).

The proposed model requires passive conditions of emplacement. Layered mafic rocks, the presence of composite dykes, and apparent large-scale gravitational differentiation of composite magma all indicate stable conditions. Repetition of cumulus assemblages (olivine +orthopyroxene) in the layered gabbro suggests that minor influxes of fresh mafic magma did not disturb these conditions. However, influx of a large volume of mafic magma (Fig. 9d) would upset the delicate thermal steady state producing vigorous convection and hybridisation with slightly older, still hot, phases of the batholith. Along contacts with salic rocks, remelting could form some salic liquid, leading to the formation of complex salic-mafic relations, including pillow-like masses of mafic rocks in salic matrices, and some cross-cutting of mafic rocks by salic dykes. Conditions along such interfaces would favour extensive double diffusive transfer of material across the boundary, leading to A-type tendencies in the overlying granite according to the model of Wiebe (1994).

The present superb exposure of internal structure of the Fogo Island Batholith results from tipping of the batholith at a moderate angle to the northwest subsequent to emplacement by minor late sinistral movement on the Dog Bay Line (Williams et al. 1993; Piasecki 1992) which produced compression in the Fogo Island region in early to mid-Devonian time.
Appendix 1. Geochemical data for the Fogo Island Batholith.

 Hare Bay lobe
Unit
Sample # K05 K06 K07 K10 K19

Si[O.sub.2] 77.35 77.25 77.15 76.70 76.05
Ti[O.sub.2] 0.17 0.11 0.12 0.11 0.18
[Al.sub.2][O.sub.3] 11.14 11.68 11.65 11.53 12.49
[Fe.sub.2][O.sub.3] 0.43 0.98 1.22 0.69 0.46
FeO 1.96 1.09 0.70 1.25 1.52
MnO 0.05 0.03 0.03 0.03 0.03
Mg0 0.09 0.03 0.03 0.03 0.24
CaO 0.70 0.40 0.28 0.49 0.59
[Na.sub.2]O 3.85 3.84 3.52 3.88 3.43
[K.sub.2]O 3.11 4.34 4.21 4.29 4.26
[P.sub.2][O.sub.5] 0.02 0.01 0.02 0.01 0.03
LOI 0.65 0.58 0.50 0.63 0.65
Total 99.52 100.34 99.43 99.64 99.93

Trace element contents in ppm
Li 9.2 10.3 3.2 13 12.3
Be 3.4 3.8 4.1 4.3 2.9
F 131 541 99 669 451
V 1 1 1 1 8
Cr 2 2 1 1 3
Co 1 1 1 2 2
Ni 3 1 2 3 1
Cu 9 7 7 12 5
Zn 56 54 42 67 26
Ga 20 22 20 21 22
Rb 115 150 150 140 140
Sr 109 43 43 42 69
Y 71 93 93 98 63
Zr 330 270 269 271 174
Nb 16 22 19 17 12
Mo 5 4 4 4 3
Ba 659 612 543 555 565
La 35 61 67 62 74
Ce 76 134 143 134 162
Pb 1 5 5 13 1
Th 9 17 18 16 20
U 3 4.6 5 4.5 3.6

 Shoal
 Bay
 Hare Bay lobe lobe
Unit
Sample # K21 K31 K34 K42 K03

Si[O.sub.2] 75.95 75.40 74.25 71.80 77.54
Ti[O.sub.2] 0.13 0.17 0.35 0.36 0.09
[Al.sub.2][O.sub.3] 11.80 12.48 12.32 13.62 12.21
[Fe.sub.2][O.sub.3] 0.50 1.15 0.29 0.44 0.42
FeO 1.24 0.82 2.69 2.77 0.85
MnO 0.03 0.03 0.06 0.07 0.02
Mg0 0.03 0.06 0.57 0.57 0.14
CaO 0.59 0.70 1.48 2.15 0.38
[Na.sub.2]O 3.83 4.09 3.87 4.25 3.44
[K.sub.2]O 3.97 3.94 3.15 2.66 4.39
[P.sub.2][O.sub.5] 0.03 0.02 0.05 0.07 0.01
LOI 0.47 0.59 0.76 0.46 0.35
Total 98.57 99.45 99.84 99.22 99.84

Trace element contents in ppm
Li 14.8 4 28.7 26.4 11.9
Be 4.3 3.7 2.5 3.3 2.7
F 399 101 404 380 64
V 4 5 22 28 2
Cr 2 3 7 9 2
Co 2 3 5 6 1
Ni 1 1 5 4 3
Cu 8 9 4 10 8
Zn 63 41 60 72 25
Ga 22 22 19 22 20
Rb 141 82 95 80 175
Sr 45 92 134 146 24
Y 77 62 65 50 40
Zr 266 301 327 420 132
Nb 19 20 15 16 9
Mo 3 3 7 4 4
Ba 561 620 793 657 102
La 55 27 52 36 24
Ce 117 87 111 78 34
Pb 12 9 3 9 11
Th 17 13 13 11 30
U 5.4 3.3 2.7 2.9 9

 Shoal Bay lobe
Unit
Sample # K09 K16 K18 K26 K27

Si[O.sub.2] 76.80 76.40 76.15 75.90 75.75
Ti[O.sub.2] 0.12 0.20 0.19 0.20 0.21
[Al.sub.2][O.sub.3] 11.78 11.91 12.14 12.34 12.48
[Fe.sub.2][O.sub.3] 0.98 0.48 2.03 0.39 0.06
FeO 1.14 1.58 <.10 1.68 2.08
MnO 0.03 0.03 0.03 0.03 0.04
Mg0 0.04 0.27 0.25 0.30 0.31
CaO 0.55 0.90 0.94 0.93 0.93
[Na.sub.2]O 3.75 3.40 3.78 3.54 3.54
[K.sub.2]O 3.98 3.65 3.63 4.17 4.10
[P.sub.2][O.sub.5] 0.01 0.03 0.03 0.02 0.02
LOI 0.83 0.39 0.67 0.52 0.54
Total 100.01 99.24 99.84 100.02 100.06

Trace element contents in ppm
Li 5.8 43.3 22.9 36.2 37.8
Be 3.1 3.3 3.6 3.6 3.6
F 656 386 339 630 523
V 1 9 10 9 9
Cr 1 5 3 5 5
Co 3 2 2 2 2
Ni 2 1 3 6 2
Cu 6 7 5 11 7
Zn 43 42 42 42 43
Ga 20 22 20 19 23
Rb 115 145 125 160 155
Sr 70 55 67 58 56
Y 97 66 80 83 84
Zr 258 247 199 219 219
Nb 18 14 15 14 13
Mo 4 4 4 4 5
Ba 700 475 477 463 456
La 66 47 50 58 61
Ce 152 106 107 112 122
Pb 1 1 3 8 5
Th 17 17 16 18 18
U 4.5 4.1 5.1 5.3 6.1

 Shoal Bay lobe Joe Batts lobe
Unit
Sample # K40 K44 K46 K11 K12

Si[O.sub.2] 72.95 71.50 70.60 76.65 76.65
Ti[O.sub.2] 0.32 0.40 0.38 0.16 0.15
[Al.sub.2][O.sub.3] 13.91 13.68 14.28 12.62 12.67
[Fe.sub.2][O.sub.3] 0.48 0.59 0.68 0.65 0.61
FeO 2.07 2.30 2.25 0.84 0.87
MnO 0.05 0.06 0.05 0.03 0.04
Mg0 0.66 0.93 0.82 0.15 0.12
CaO 1.99 2.12 2.39 0.62 0.61
[Na.sub.2]O 3.96 3.94 3.97 3.70 3.93
[K.sub.2]O 3.41 2.94 3.15 4.05 3.86
[P.sub.2][O.sub.5] 0.06 0.06 0.07 0.02 0.01
LOI 0.56 0.75 0.74 0.69 0.59
Total 100.42 99.27 99.35 100.18 100.11

Trace element contents in ppm
Li 36.4 41.3 33.7 14.6 11.3
Be 3.8 2.3 2.5 1.9 1.9
F 555 551 133 127 499
V 26 34 34 4 2
Cr 8 11 10 2 1
Co 5 5 6 1 1
Ni 5 6 4 1 1
Cu 6 4 4 8 9
Zn 41 53 46 36 51
Ga 23 19 23 16 16
Rb 140 105 110 135 135
Sr 99 160 128 66 58
Y 57 58 56 26 36
Zr 186 261 228 149 160
Nb 14 11 11 6 9
Mo 4 4 4 5 4
Ba 347 678 397 419 394
La 34 38 35 23 27
Ce 83 80 82 50 59
Pb 3 1 3 10 9
Th 18 4 13 12 13
U 4 1.8 3.4 4.1 3.2

 Joe Batts lobe
Unit
Sample # K20 K29 K35 K36 K37

Si[O.sub.2] 76.05 75.50 74.05 74.00 73.90
Ti[O.sub.2] 0.13 0.15 0.28 0.28 0.27
[Al.sub.2][O.sub.3] 12.48 12.41 13.09 12.96 12.99
[Fe.sub.2][O.sub.3] 0.63 0.48 0.52 0.49 0.52
FeO 0.68 0.98 1.85 1.59 1.74
MnO 0.03 0.03 0.06 0.05 0.05
Mg0 0.06 0.12 0.52 0.47 0.50
CaO 0.66 0.81 1.52 1.25 1.38
[Na.sub.2]O 3.96 3.88 3.71 3.71 3.59
[K.sub.2]O 3.82 3.66 3.31 3.41 3.39
[P.sub.2][O.sub.5] 0.05 0.01 0.05 0.05 0.04
LOI 0.66 0.47 0.78 0.83 0.81
Total 99.21 98.50 99.74 99.09 99.18

Trace element contents in ppm
Li 7.5 20.2 26.8 25.2 18.8
Be 2.7 1.7 2.4 2.3 2.6
F 267 323 301 473 376
V 4 2 19 17 18
Cr 1 3 5 5 5
Co 2 3 4 4 4
Ni 1 1 5 3 8
Cu 10 4 27 10 6
Zn 41 38 75 51 37
Ga 17 14 19 21 21
Rb 128 115 120 130 125
Sr 64 58 136 107 115
Y 36 32 42 40 46
Zr 164 142 156 168 174
Nb 10 6 10 9 10
Mo 3 4 5 4 4
Ba 403 381 405 438 499
La 30 30 36 31 33
Ce 64 65 74 65 71
Pb 17 1 18 11 9
Th 15 8 14 13 16
U 5.7 3.5 3.1 3.2 4.3

 Joe Batts lobe Minor felsic bodies
Unit
Sample # K41 K47 K04 K14 K15

Si[O.sub.2] 72.55 70.05 77.45 76.45 76.40
Ti[O.sub.2] 0.30 0.30 0.16 0.17 0.16
[Al.sub.2][O.sub.3] 14.15 14.07 11.49 11.76 11.78
[Fe.sub.2][O.sub.3] 0.59 0.80 0.59 0.78 1.43
FeO 0.94 2.49 1.35 1.56 1.57
MnO 0.08 0.09 0.04 0.10 0.04
Mg0 0.62 0.37 0.02 0.05 0.03
CaO 2.43 1.82 1.29 1.55 0.71
[Na.sub.2]O 2.94 4.01 4.00 3.80 3.92
[K.sub.2]O 3.29 3.42 2.36 3.15 3.80
[P.sub.2][O.sub.5] 0.09 0.07 0.02 0.01 0.01
LOI 1.42 0.69 0.20 0.98 0.31
Total 99.40 98.18 98.97 100.36 100.16

Trace element contents in ppm
Li 15.1 46.5 2.4 6.4 0.6
Be 2.1 2.1 3.4 2.8 3.3
F 298 318 78 101 65
V 16 13 1 1 1
Cr 6 3 1 3 2
Co 3 3 1 2 1
Ni 5 1 1 2 6
Cu 8 5 11 14 12
Zn 87 63 26 204 45
Ga 15 22 21 21 21
Rb 115 110 25 70 115
Sr 138 132 216 141 67
Y 42 34 91 79 86
Zr 192 222 353 356 322
Nb 9 11 20 19 19
Mo 3 5 3 4 5
Ba 535 431 776 914 629
La 32 36 55 48 52
Ce 65 81 133 118 125
Pb 40 2 1 26 1
Th 14 9 14 13 14
U 3.5 3.1 4.4 4.1 4.6

 Minor felsic bodies
Unit
Sample # K22 K24 K25 K28 K38

Si[O.sub.2] 75.95 75.95 75.95 75.75 73.25
Ti[O.sub.2] 0.17 0.18 0.16 0.19 0.34
[Al.sub.2][O.sub.3] 11.55 11.62 11.48 11.31 13.53
[Fe.sub.2][O.sub.3] 0.99 2.68 0.45 1.22 0.25
FeO 1.52 <.10 2.14 1.77 2.44
MnO 0.04 0.05 0.03 0.04 0.05
Mg0 0.03 0.04 0.06 0.06 0.74
CaO 0.72 1.12 0.77 0.68 2.10
[Na.sub.2]O 3.98 3.85 3.84 3.86 3.89
[K.sub.2]O 3.65 3.31 3.68 2.94 3.25
[P.sub.2][O.sub.5] 0.01 0.01 0.01 0.01 0.06
LOI 0.56 1.05 0.69 0.39 0.49
Total 99.17 99.86 99.26 98.22 100.39

Trace element contents in ppm
Li 1.8 4.4 2.2 2.8 33.5
Be 3.3 3.2 3.7 2.7 2.8
F 132 108 81 95 511
V 1 1 1 1 30
Cr 1 7 4 1 9
Co 1 1 1 1 5
Ni 1 2 1 1 8
Cu 10 17 20 8 8
Zn 47 44 106 28 54
Ga 21 21 21 21 20
Rb 100 70 110 65 110
Sr 82 106 65 107 109
Y 88 78 80 66 71
Zr 302 355 328 322 208
Nb 18 17 20 16 12
Mo 3 4 4 4 4
Ba 686 744 618 672 483
La 53 45 48 43 32
Ce 129 117 121 97 83
Pb 1 1 1 1 4
Th 12 13 14 10 10
U 4 4 4.3 2.8 2.6

 Minor
 felsic
 bodies Rhyolite, felsite, porphyry
Unit
Sample # K43 K01 K02 K08 K13

Si[O.sub.2] 71.50 77.95 77.85 77.00 76.45
Ti[O.sub.2] 0.30 0.15 0.11 0.16 0.15
[Al.sub.2][O.sub.3] 14.35 11.07 12.22 11.90 11.83
[Fe.sub.2][O.sub.3] 0.88 0.52 0.16 0.52 1.52
FeO 2.22 1.59 0.78 1.32 0.95
MnO 0.08 0.03 0.03 0.04 0.06
Mg0 0.54 0.12 0.07 0.09 0.01
CaO 1.95 0.60 0.62 0.66 0.81
[Na.sub.2]O 4.42 3.23 3.03 3.68 4.63
[K.sub.2]O 2.49 3.33 5.18 3.85 2.56
[P.sub.2][O.sub.5] 0.08 0.01 0.01 0.01 0.03
LOI 0.80 0.33 0.34 0.38 0.39
Total 99.61 98.93 100.40 99.61 99.39

Trace element contents in ppm
Li 13.6 8.9 3.8 6.7 1.8
Be 1.9 2.2 1.9 3.3 4.1
F 260 122 37 489 85
V 12 5 5 2 1
Cr 6 2 2 2 1
Co 5 1 1 1 1
Ni 2 2 1 1 1
Cu 15 12 4 4 29
Zn 67 55 21 57 67
Ga 21 20 21 21 22
Rb 75 85 150 115 52
Sr 150 49 73 49 88
Y 37 69 35 78 78
Zr 204 202 201 234 325
Nb 9 12 6 18 20
Mo 3 3 4 4 3
Ba 285 499 948 572 753
La 26 48 45 10 50
Ce 56 110 93 116 117
Pb 1 6 8 11 8
Th 9 14 9 16 14
U 2.3 2.8 1.8 4 3.8

 Rhyolite, felsite, porphyry
Unit
Sample # K17 K32 K33 K39 K45

Si[O.sub.2] 76.25 75.25 75.10 73.20 71.20
Ti[O.sub.2] 0.16 0.26 0.28 0.34 0.34
[Al.sub.2][O.sub.3] 12.22 12.31 12.27 13.31 13.46
[Fe.sub.2][O.sub.3] 0.71 0.54 0.57 0.08 0.29
FeO 1.12 1.09 2.23 2.56 2.40
MnO 0.03 0.01 0.05 0.06 0.05
Mg0 0.08 0.24 0.36 0.72 0.78
CaO 0.62 0.61 1.22 1.88 1.74
[Na.sub.2]O 4.01 4.06 3.66 3.66 3.79
[K.sub.2]O 3.93 4.00 3.50 3.39 3.38
[P.sub.2][O.sub.5] 0.02 0.04 0.05 0.06 0.06
LOI 0.33 0.60 0.51 0.86 0.86
Total 99.48 99.01 99.80 100.12 98.35

Trace element contents in ppm
Li 6.7 4.1 13.3 26.5 31.3
Be 3.5 3.3 2.2 2.9 2.8
F 161 293 213 494 469
V 3 9 15 30 32
Cr 2 2 6 9 9
Co 1 1 3 6 5
Ni 1 11 2 7 7
Cu 13 13 16 15 8
Zn 58 16 64 53 49
Ga 20 18 22 20 19
Rb 125 105 75 110 120
Sr 48 186 92 142 143
Y 52 65 75 59 65
Zr 232 366 331 214 180
Nb 20 18 16 11 11
Mo 4 5 4 4 3
Ba 588 828 785 534 472
La 33 46 50 38 40
Ce 92 116 114 79 88
Pb 6 1 7 5 8
Th 16 12 8 13 17
U 2.6 3.7 1.7 3.5 3.7

 Gabbro
Unit
Sample # K57 K58 K59 K60 K62

Si[O.sub.2] 53.55 53.05 52.65 51.00 50.50
Ti[O.sub.2] 0.49 1.56 0.61 1.80 1.79
[Al.sub.2][O.sub.3] 4.40 16.33 18.04 18.85 17.09
[Fe.sub.2][O.sub.3] 1.61 0.05 1.54 3.67 2.96
FeO 5.14 8.79 4.07 4.87 6.35
MnO 0.14 0.21 0.12 0.13 0.15
Mg0 18.88 6.23 7.33 4.35 6.49
CaO 12.80 9.40 11.89 7.89 10.60
[Na.sub.2]O 0.92 3.14 2.71 4.17 3.23
[K.sub.2]O 0.44 0.88 0.30 1.38 0.39
[P.sub.2][O.sub.5] 0.05 0.20 0.14 0.42 0.14
LOI 1.50 0.98 0.67 2.02 0.91
Total 99.92 100.82 100.07 100.55 100.60

Trace element contents in ppm
Li 11.4 16.8 6 13.5 7.2
Be 0.4 1.2 1.7 1.2 0.7
F 241 279 132 417 120
V 101 260 106 207 272
Cr 1270 167 167 82 196
Co 69 26 30 29 40
Ni 522 1 35 50 41
Cu 94 14 28 34 37
Zn 58 91 51 76 70
Ga 12 24 22 24 22
Rb 10 20 10 35 5
Sr 114 239 689 897 654
Y 12 33 12 21 16
Zr 39 129 29 141 53
Nb 1 4 1 6 1
Mo 4 6 4 5 4
Ba 84 127 126 508 159
La 8 15 11 37 10
Ce 24 35 25 75 23
Pb 5 1 2 3 1
Th 1 1 1 5 1
U 0.5 1.5 0.3 1.1 0.4

 Gabbro Tonalite, grandt.
Unit
Sample # K63 K64 K66 K30 K48

Si[O.sub.2] 49.45 49.25 45.15 75.40 64.25
Ti[O.sub.2] 5.03 3.36 0.24 0.26 0.78
[Al.sub.2][O.sub.3] 18.17 7.96 13.37 12.87 15.54
[Fe.sub.2][O.sub.3] 2.75 3.47 2.94 0.37 3.40
FeO 6.29 12.21 8.24 1.89 3.06
MnO 0.13 0.26 0.18 0.07 0.06
Mg0 4.32 12.70 19.40 0.54 0.61
CaO 9.05 6.72 6.42 2.13 3.91
[Na.sub.2]O 3.79 1.48 1.55 4.26 3.93
[K.sub.2]O 0.50 0.51 0.20 1.44 3.29
[P.sub.2][O.sub.5] 0.11 0.16 0.06 0.05 0.19
LOI 0.63 1.92 2.90 1.11 0.82
Total 100.22 100.00 100.65 100.39 99.84

Trace element contents in ppm
Li 9.8 8.2 5.3 21.9 33.2
Be 1.5 1.0 0.3 1.7 2.1
F 56 167 97 294 334
V 293 329 38 14 72
Cr 97 487 406 26 11
Co 35 77 97 6 8
Ni 76 206 298 15 6
Cu 61 109 12 60 115
Zn 54 115 89 146 52
Ga 25 23 15 19 23
Rb 10 10 10 55 65
Sr 594 212 365 132 247
Y 12 20 4 40 43
Zr 116 79 21 150 300
Nb 10 5 1 9 10
Mo 5 4 4 4 5
Ba 132 74 82 264 535
La 9 10 6 34 35
Ce 16 27 12 71 80
Pb 1 8 1 50 11
Th 1 1 1 16 10
U 0.6 0.7 0.3 4 4.5

 Diorite complex
Unit
Sample # K49 K50 K51 K52 K53

Si[O.sub.2] 59.45 59.35 58.60 57.75 57.65
Ti[O.sub.2] 0.68 0.71 0.15 1.33 2.14
[Al.sub.2][O.sub.3] 17.17 15.99 18.58 17.66 13.17
[Fe.sub.2][O.sub.3] 0.87 1.29 1.75 0.27 1.83
FeO 4.79 4.11 0.37 6.51 9.43
MnO 0.10 0.10 0.21 0.15 0.22
Mg0 3.23 5.57 0.24 2.80 2.44
CaO 6.54 6.42 15.17 7.02 5.66
[Na.sub.2]O 3.31 3.39 1.71 3.72 3.35
[K.sub.2]O 1.29 1.61 2.26 1.52 1.40
[P.sub.2][O.sub.5] 0.13 0.17 0.02 0.20 0.81
LOI 1.93 1.56 1.40 1.37 2.26
Total 99.49 100.27 100.46 100.30 100.36

Trace element contents in ppm
Li 56.1 18.3 2.2 36.5 23.7
Be 0.9 1.2 9.7 1.8 2.1
F 190 291 391 350 683
V 112 107 124 171 240
Cr 43 217 4 76 3
Co 21 26 1 13 23
Ni 20 128 4 10 8
Cu 3 28 11 21 27
Zn 60 66 234 88 116
Ga 22 21 43 26 27
Rb 75 20 55 45 45
Sr 258 484 573 275 296
Y 16 11 25 36 52
Zr 99 112 86 191 548
Nb 1 2 5 7 9
Mo 4 4 5 5 5
Ba 121 286 753 245 514
La 10 18 20 25 27
Ce 23 38 36 51 77
Pb 1 1 274 1 5
Th 1 1 5 1 5
U 1.2 1.2 2.3 3 2.1

 Diorite complex Mafic dykes
Unit
Sample # K54 K55 K56 K61 K65

Si[O.sub.2] 55.45 54.60 54.20 50.60 48.45
Ti[O.sub.2] 2.25 1.28 1.58 3.08 0.42
[Al.sub.2][O.sub.3] 13.07 16.18 16.48 14.66 4.30
[Fe.sub.2][O.sub.3] 1.51 1.47 0.93 2.55 6.96
FeO 10.55 9.44 7.82 10.19 12.88
MnO 0.24 0.24 0.23 0.22 0.08
Mg0 2.48 3.91 5.09 4.10 12.78
CaO 6.45 7.10 7.82 8.37 11.22
[Na.sub.2]O 3.22 3.24 3.00 3.20 0.77
[K.sub.2]O 1.46 1.01 1.44 0.70 0.44
[P.sub.2][O.sub.5] 1.00 0.29 0.19 0.66 0.05
LOI 1.42 1.22 1.72 0.20 2.01
Total 99.10 99.98 100.50 98.53 100.36

Trace element contents in ppm
Li 29.7 13 41.6 14.2 14
Be 1.7 1.3 1.5 2 1.7
F 750 271 412 429 3369
V 208 110 235 462 72
Cr 2 87 201 26 11
Co 19 25 19 3 55
Ni 1 22 2 21 33
Cu 27 13 15 24 9
Zn 106 99 102 101 148
Ga 28 29 25 36 25
Rb 50 25 45 18 5
Sr 272 366 218 317 22
Y 44 24 36 27 56
Zr 597 280 135 251 55
Nb 7 3 6 1 7
Mo 5 5 5 4 5
Ba 249 330 414 123 27
La 23 13 19 14 55
Ce 54 30 44 39 131
Pb 9 1 1 1 11
Th 3 1 4 1 1
U 1.4 1 2.4 0.8 0.6

Appendix 2. Sample locations.

 # Easting Northing Description

Hare Bay lobe
K05 693360 5504100 granite, shore south of Hare Bay Head
K06 695200 5503000 granite, Leveret Islands
K07 693700 5505800 granite, Hare Bay Head
K10 694850 5506400 granite, Leveret Islands
K19 695930 5504600 granite, road cut at Deep Bay
K21 696000 5504200 granite, Island Harbour-Deep Bay junction
K31 694750 5503000 granite, road to island Harbour
K34 697280 5504200 granite, head of Hare Bay
K42 699050 5503950 granite, road to Island Harbour 2 km west
 of highway

Shoal Bay lobe
K03 700520 5509460 granite, west side of Shoal Bay
K09 695100 5508100 granite, head northeast of Hare Bay
K16 700520 5509460 granite, west side of Shoal Bay
K18 696650 5505470 granite, east side of Hare Bay
K26 697910 5507660 granite, 4 km southeast of Fogo
K27 699640 5505300 granite, Higway 333 1 km north of
 Highway 334
K40 702350 5509580 grained granite, Highway 334 2 km north of
 Shoal Bay
K44 701280 5505840 grained granite, Highway 334 2 km southwest
 of Shoal Bay
K46 702400 5506500 grained granite, Higway 334 1. km south of
 Shoal Bay

Joe Batts Arm lobe
K11 706580 5510800 granite, Highway 333 4 km east of Joe
 Batts Arm
K12 705450 5511530 granite, Highway 334, Joe Batts Arm
K20 707250 5511100 granite, Highway 333 5 km east of Joe
 Batts Arm
K29 709130 5510530 granite, Highway 333 7 km east of Joe
 Batts Arm
K35 708160 5514290 granite, shore west of Round Head
K36 704420 5513850 granite, shore north of Joe Batts Arm
K37 705320 5514420 granite, head 3 km north of Joe Batts Ann
K41 708840 5514100 granite, Round Head
K47 708040 5510600 granite, Highway 333 6- km east of Joe
 Batts Arm

Minor felsic intrusions
K04 697460 5494290 granite, west side of Stag Harbour
K14 697180 5494300 granite, west side of Stag Harbour
K15 700150 5497450 granite, highway northwest corner of Little
 Seldom Cove
K22 697910 5495500 granite, Cobb Cove
K24 695110 5494350 granite sill, ferry landing at Man
 O'War Cove
K25 697530 5496000 granite sill, highway north of Cobb Cove
K28 699890 5497660 granite, dump west of Little Seldom
K38 715560 5505400 granite, 2 km northwest of Cape Fogo
K43 712900 5508730 granite, Olivers Cove

Felsite, porphyry and microgranite
K01 693570 5597700 Felsite, coast south of island Harbour
K02 713890 5503840 felsite, Cape Cove
K08 701310 5501780 porphyry, Highway 333 at Mile Pond
K13 702050 5500880 microgranite, highway 2 km northwest of
 Seldom
K17 700200 5497750 rhyolite dyke, highway at Little Seldom
 Cove
K32 716000 5503600 felsite, coast 1 km west of Cape Fogo
K33 712140 5501750 porphyritic rhyolite, West Head
K39 714090 5504650 microgranitc, central part of Cape Fogo
 peninsula
K45 713160 5503800 felsite, Cape Cove

Diorite complex, tonalite
K30 708540 5514100 tonalite, Round Head
K48 712140 5501750 heterogeneous granodiorite, Western Head
K49 701280 5506840 diorite dyke in granite, south tip of
 Shoal Bay
K50 712220 5510000 diorite, east side of Tilting Harbour
K51 708540 5514100 diorite agmatite, Round Head
K52 700350 5503100 diorite, highway quarry near Island Harbour
 turnoff
K53 697910 5495500 diorite, dyke or minor intrusion, Cobb Cove
K54 701650 5497760 diorite, highway at Little Seldom

Massive and layered gabbro
K55 704360 5498700 gabbro, east side of Seldom Harbour
K56 713160 5503800 gabbro inclusion in diorite, Cape Cove
K57 713000 5509600 gabbro, head shore of Tilting Harbour
K58 714060 5503330 heterogeneous gabbro, Cape Cove
K59 710450 5510000 layered gabbro, Sandy Cove
K60 710820 5510310 gabbro, Sandy Cove
K62 710700 5510060 gabbro, Sandy Cove
K63 711800 5510150 gabbro, northwest corner of Tilting Harbour
K64 711800 5510150 gabbro, northwest corner of Tilting Harbour
K66 711000 5510360 gabbro, shore between Tilting and Sandy
 Cove

Mafic dykes
K61 700300 5498580 basalt dyke cutting microgranite, north of
 Little Seldom Cove
K65 712140 5501750 basalt dyke cutting diorite complex,
 Western Head

UTM grid, zone 21, NAD 27 projection, NTS 1:50,000 sheet 2E/9 (Fogo),
edition 3


ACKNOWLEDGEMENTS

This is Geological Survey of Canada Contribution Number 1997066. The author wishes to acknowledge constructive reviews of a previous version of the paper by Louise Corriveau and two anonymous referees, and a concise and helpful review by an anonymous journal reviewer.

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Editorial responsibility: Sandra M. Barr

K.L. CURRIE *

Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 * Correspondence address: 9 Tiverton Drive, Nepean, ON K2E 6L4

Date received: July 28, 2003 & Date accepted: October 11, 2003
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Author:Currie, K.L.
Publication:Atlantic Geology
Geographic Code:1CNEW
Date:Jul 1, 2003
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