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Mineralogical and geochemical evidence of magma mingling/mixing in the Sierra de las Cruces volcanic range, Mexican Volcanic Belt/Evidencias mineralogicas y geoquimicas de mezcla incompleta de magmas en la Sierra de las Cruces, Cinturon Volcanico Mexicano.

1. Introduction

The Mexican Volcanic Belt (MVB; Fig. 1) is a geologic province of ~8000 volcanic centers (stratovolcanoes, cinder cones, maars, etc.) of Miocene to present-day, predominantly having an andesitic to dacitic composition. The MVB is ~1000 km long and 50-300 km wide, extending approximately east-west from Veracruz to Puerto Vallarta (Gomez-Tuena et al., 2007). The MVB origin has generally been viewed as related to the subduction of the Cocos and Rivera plates subduction beneath the North American plate (Pardo and Suarez, 1995; Ferrari et al., 1999). However, some geological, geochemical and geophysical observations do not completely agree with a unique subduction-related origin for the MVB, as summarized by Marquez et al. (1999a,b) and Sheth et al. (2000). Alternative models have been proposed to explain the MVB origin and these include: (a) hybrid models involving both mantle and slab sources (Luhr, 1997; Wallace and Carmichael, 1999; Ferrari, 2004); (b) the activity of a west-east propagating mantle plume coexisting with the subduction of the Cocos and Rivera plates (Marquez et al., 1999a); and (c) the ongoing extensional processes along the MVB (Sheth et al., 2000; Verma, 2002, 2009).

On the other hand, several geochemical and isotopic studies reveal the crust participation in the genesis of the MVB intermediate and silica-rich magmas. Besch et al. (1995) determined that an assimilation of lower crust can explain the chemical and isotopic differences between primitive mafic and evolved rocks in the eastern MVB. Verma (1999) reported that andesites and dacites from the Sierra de Chichinautzin volcanic field (central MVB) were derived from partial melting of a heterogeneous mafic granulite from the lower crust. Later, Marquez and De Ignacio (2002) suggested that the magmatic diversity of Sierra de Chichinautzin can be explained by magma mixing between mafic magmas derived from a heterogeneous upper mantle and two different felsic crustal magmas. Chelsey et al. (2002), based on geochemical and isotopic data (particularly Os isotopic ratios), reported that the Michoacan-Guanajuato volcanism (central MVB) shows assimilation of the lower crust. Magma mixing processes, between magmas of different composition, have also been inferred from the phenocryst assemblages and geochemistry of the Iztaccihuatl (Nixon, 1988a,b),Tequila (Wallace and Carmichael, 1994), Tancitaro (Ownby et al., 2011), and Popocatepetl volcanoes (Straub and Martin-Del Pozzo, 2001; Schaaf et al., 2005; Witter et al., 2005; Sosa-Ceballos et al., 2012), as well as in Amealco caldera (Aguirre-Diaz, 2001).

Therefore, it is clear that detailed studies involving mineralogical, geochemical, and isotopic data are required to explain the magmatic diversity observed in the MVB and to try to solve the controversies related to this significant geologic province. This paper describes an initial petrogenetic evaluation of magmatism in the Sierra de las Cruces (SC) volcanic range, based on mineralogical, geochemical, and Sr-Nd isotopic ratios of intermediate and felsic volcanic rocks. The lithological features of SC volcanic range represent an opportunity to test the role of the continental crust in the origin and evolution of magmas in the central MVB.


2. Geological setting

2.1. Regional stratigraphy

Vazquez-Sanchez and Jaimes-Palomera (1989) and Garcia-Palomo et al. (2002) have reported detailed stratigraphic information for the SC surrounding regions (Mexico and Toluca basins), covering the Cretaceous to present-day period. Stratigraphic record begins with a calcareous sedimentary sequence (marine to shelf facies) that was deposited in central Mexico during the Cretaceous (Fries, 1960). Rocks from this sequence include massive limestone with black chert lenses, beds of gypsum, massive to thickly bedded limestones, greywacke interbedded with limolite and shale beds. This Cretaceous sedimentary sequence (thickness ~ 3000 m) was folded and uplifted during the Laramide orogenic event (Fries, 1960) and was subsequently intruded by granitic or granodioritic dykes dated at 50 [+ or -] 10 Ma (De Cserna et al. 1974). The Eocene--Oligocene stratigraphy that overlies the Cretaceous sequence consists of calcareous conglomerates, lava flows, sandstones, volcanic siltstones, and lacustrine deposits, with a maximum thickness of 500 m.

This sedimentary sequence is unconformably overlain by about 38 to 7.5 Ma rhyolite, rhyodacite, dacitic lava flows and pyroclastic flow deposits (Moran-Zenteno et al. 1998; Garcia-Palomo et al. 2002), and by a Pliocene to Holocene volcanism of the MVB, that include the Las Cruces eruptive period (Delgado-Granados and Martin del Pozzo 1993).

2.2. Central MVB Pliocene to Holocene volcanism

The Pliocene to Holocene volcanism in the central MVB was generated during three eruptive periods (Delgado-Granados and Martin del Pozzo, 1993): Las Cruces, Ajusco, and Chichinautzin.

Las Cruces eruptive period

It is considered as the oldest one, from Late Pliocene to Early Pleistocene (Fries, 1960; Schaelpfer, 1968; Sanchez-Rubio, 1984; Delgado-Granados and Martin del Pozzo, 1993). Mora-Alvarez et al. (1991) reported K-Ar dates from 2.87 to 1.92 Ma for this volcanism, whereas Osete et al. (2000), combining magnetostratigraphic and K-Ar radiometric data, established that this eruptive period can be best constrained between 3.6 and 1.8 Ma. The main mass of SC range (Fig. 2) was formed by consecutive episodes of faulting accompanied by the development of volcanic edifices, constructed by lavas and associated pyroclastic products (Mooser et al., 1974; Fries, 1960; Schlaepfer, 1968; Delgado-Granados and Martin del Pozzo, 1993; Garcia-Palomo et al., 2002, 2008). The volcanic range is elongated, extending in a NNW-SSE direction for ~65 km, with a width varying from 47 km to the north and 27 km to the south. The SC constitutes the western margin of the Mexico basin (2220 m.a.s.l.) and the eastern border of the Toluca Valley (2400 m.a.s.l.), both being graben/horst depressions. According to Garcia Palomo et al. (2008), the SC volcanic field has been divided in three sectors: northern, central, and southern, all of them bounded by E-W faults. Each sector is characterized by specific altitudes, slopes, trends of morpholineaments, and drainage patterns. Morphostructure of the northern and central sectors is controlled by N-S and NE-SW faults, whereas E-W faults have ruled the southern sector. An N-S spatial-temporal evolution of SC magmatic activity was confirmed by paleomagnetic and K-Ar data (Mora Alvarez et al., 1991; Osete et al., 2000; Fig. 2). The SC consists of eight overlapped stratovolcanoes, which are named, from south to north: Zempoala, La Corona, San Miguel, Salazar, Chimalpa, Iturbide, La Bufa, and La Catedral (Garcia-Palomo et al., 2008). These volcanic edifices underwent alternated episodes of effusive and explosive activity, during which they were affected by faulting. Effusive products comprise gray andesitic to dacitic porphyritic lava flows (< 4 m thick) with planar fracturing sub parallel to the surface, labeled as Lava Dacitica Apilulco (total thickness ~400 m) by Delgado-Granados and Martin del Pozzo (1993). Occasionally, these lava flows contain magmatic enclaves (see Table 1 in Appendix A), which are dark gray on fresh surfaces and may appear lighter than the host on weathered surfaces. The enclaves comprise <2 to 10 vol% of the rock at a given outcrop. Their shape is commonly spherical to ellipsoidal and only rarely angular on 2D outcrop, with a finely crystalline and vesicular texture in hand specimen. They are randomly distributed over the volcanic range, although their number and size apparently increase towards the north. This fact could be related to an increase in fault and fracture density in this direction (Garcia-Palomo et al., 2008), a favorable condition for magma mingling/mixing processes. The most common enclaves are small, ranging from a few millimeters to 4 centimeters in diameter, although in the northern sector they can reach a maximum dimension of ~20 centimeters. Enclave contacts with their host dacite are generally sharp and crenulated, showing chilled margins. Their morphology and vesicularity of the ME indicate that they were partially molten before they were entrained into and quenched against relatively cooler host dacitic melt. On the other hand, lava flows are sometimes intercalated by pyroclastic deposits (Brecha Piroclastica Cantimplora after Delgado-Granados and Martin del Pozzo, 1983; total thickness = 1- 4 m) conformed by gray dacitic blocks (20-30 cm), pumice clasts (<15 cm), and ashes.

Ajusco eruptive period

During this event, in the Middle Pleistocene, the Ajusco volcano was formed by the extrusion of several andesitic domes. Mora Alvarez et al. (1991) reported a K-Ar age of 0.39 [+ or -] 0.15 Ma for a sample from the Ajusco southern flank.

Chichinautzin eruptive period

This last eruptive stage was characterized by monogenetic activity generating scoria cones with associated lava flows and shield volcanoes. 14C dating yields ages <40,000 years for the Chichinautzin eruption products (Marquez et al., 1999b; Wallace and Carmichael, 1999; Velasco-Tapia and Verma, 2001a,b, 2013).

2. Sampling and analytical techniques

On the basis of the morphostructural classification by Garcia-Palomo et al. (2008) and geochronological K-Ar data reported by Mora Alvarez et al. (1991) and Osete et al. (2000), we have divided the SC volcanic range in four sectors (Fig. 2): (a) Northern sector (SCN; 2.9-3.7 Ma); (b) Central sector (SCC; 1.9-2.9 Ma); (c) Southern sector (SCS; 0.7-1.9 Ma), and (d) Las Cruces -Chichinautzin transition sector, which includes the Ajusco volcano (SCT; < 0.6 Ma). Fifty-two samples were collected across the range, being complemented by eight previously collected ones by Osete et al. (2000; Fig. 2 and Table 1 in Appendix A Supplementary Material).Most of the samples collected correspond to massive lava flows several meters thick, although some dykes and dome structures were also considered. Faulting in the area (e.g., Garcia-Palomo et al., 2008) together with the dense vegetal cover, make difficult to clearly identify which of the proposed stratovolcanoes is the source of the sampled structures. In seven of the sampled outcrops ellipsoidal magmatic enclaves, up to several centimetres diameter, have been collected in the field.


Modal compositions were determined by point counting on thin sections using a Prior Scientific petrographic microscope. Approximately 500 points per sample were counted in order to obtain a representative mode (Table 1 in Appendix A Supplementary Material). Mineral chemistry was analysed on thin sections of selected samples, using the WDS JXA-8900 JEOL microprobe system of Centro de Microscopia Electronica, Universidad Complutense (Madrid, Spain). Measurement conditions were 15 kV and 20 nA, with a beam diameter of ~1[micro]m. The apparent concentrations were corrected for atomic number (Z), absorption (A), and fluorescence (F) effects, automatically applying the internal ZAFJEOL software. Microprobe system calibration was carried out using reference minerals from the Smithsonian Institution (Jarosewich et al., 1980). Analytical precision and detection limit data have been reported by Verma et al. (2009). Zoning in different mineral phases was studied by means of profile analysis and scanning probe microanalysis (SEM) images. Amphibole and pyroxene compositions were recalculated for [Fe.sup.+3] using the charge balance method described by the equation of Droop (1987) equation. Representative analyses of the different mineral types are shown in Tables 2-7 (Appendix A Supplementary Material).


Major and trace element composition of representative SC volcanic rocks (Tables 8 and 9 in Appendix A Supplementary Material) were determined in ActLabs laboratories (Ancaster, Canada), using the "4LithoRes" methodology (for details consult webpage: www.actlabsint. com). Major elements were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES) with an analytical precision <2% and accuracy typically better than 5% at 95% confidence level, based on analysis of diverse geochemical reference materials (GRM). Trace element concentrations were determined by inductively coupled plasma-mass spectrometry (ICP-MS) with an analytical precision <10% and accuracy typically better than 7% for most elements at the 95% confidence level, based on analysis of diverse GRM.CIPW norms were calculated on a100% anhydrous adjusted basis of major elements, with [Fe.sub.2][O.sub.3]/FeO ratios adjusted depending on the rock type (Middlemost, 1989). Rock classification was based on the total alkali-silica (TAS) scheme (LeBas et al., 1986; LeBas, 1989). All computations (anhydrous and iron-oxidation ratio adjustments, norm compositions, and rock classifications) were automatically done using the SINCLAS software (Verma et al., 2002, 2003).

Sr and Nd isotope analysis (Table 10 in Appendix A Supplementary Material) for eleven SC rocks were performed at Pacific Centre for Isotopic and Geochemical Research, University of British Columbia (Vancouver, Canada). Analyses were carried out on a Thermo Finnigan Triton thermal ionization mass spectrometer. Detailed analytical procedures were reported by Weiss et al. (2006). Repeated analysis of SRM987 Sr standard (n = 5) and La Jolla standard (n = 7) gave average values of 0.710248 [+ or -] 15 and 0.511854 [+ or -] 17. The analytical errors for [sup.87]Sr/[sup.86]Sr and [sup.143]Nd/[sup.144]Nd measured ratios are directly quoted for each sample.

4. Petrography and mineral chemistry

Petrographic and electron microprobe analysis reveal the existence of three mineralogical groups: (a) felsic rocks of dacitic composition (F); (b) intermediate and felsic rocks (andesite to dacite) with disequilibrium evidence (FDE); and (c) chilled magmatic enclaves with an andesitic composition (ME) hosted in some lavas with disequilibrium evidence, whose distinctive features are reported in the next paragraphs.

4.1. Felsic rocks (F)

This group (Table 1 in in Appendix A) includes samples showing porphyritic or vesicular-porphyritic textures. Four different phenocryst assemblages are observed: (a) Pl + Qz +Amp, (b) Pl + Amp, (c) Opx + Pl + Amp, and (d) Opx + Cpx + Pl. Euhedral to subhedral phenocrysts are embedded in an essentially vitreous groundmass with plagioclase and pyroxene microlites. Plagioclase (0.8-4.0 mm), displaying andesine core compositions ([An.sub.31-40]; Table 2 in Appendix A, and Fig. 3A), is the most abundant phenocryst type (60-75 modal %). It generally shows normal zoning and polysynthetic and Carlsbad twins, and contains apatite and Fe-Ti oxides inclusions. Amphibole occurs as strongly pleochroic, yellow to brown, euhedral or subhedral crystals (0.7-2.0 mm in length), occasionally forming clots. It has been classified as calcic amphibole (ferri-magnesiohastingsite; Table 4 in Appendix A, and Fig. 4A,B) using the NEWAMPHCAL software (Yavuz, 1999), based on the Leake et al. (1997) scheme, with a relatively high concentration of Si (6.7-6.9, apfu) and low Al content (1.2-1.4, apfu). Prismatic orthopyroxene (0.5-1.0 mm) display hypersthene core composition ([En.sub.62-68][Fs.sub.28-37]; Table 5 in Appendix A, and Fig. 5A). SC10 and SC20 rocks, from SCS sector, also include euhedral to subhedral clinopyroxene (0.5-0.8 mm) and a few hypersthene phenocrysts showing a narrow continuously zoned clinopyroxene rim.

4.2. Felsic rocks with disequilibrium evidence (FDE)

Most of the collected SC rocks (~70% distributed along all sectors; Table 1 in Appendix A) have been integrated in this group. FDE lavas are generally porphyritic, showing flow texture and sometimes vesicles. Phenocrysts are embedded in a microlitic groundmass, mainly constituted by plagioclase and amphibole. However, some rocks display either vitreous or trachytic matrix. Considering phenocryst assemblages, these rocks can be divided into five major groups: (a) Ol + Pl [+ or -] Amp [+ or -] Opx [+ or -] Qz; (b) Pl + Opx [+ or -] Cpx [+ or -] Qz; (c) Pl + Amp + Qz; (d) Pl + Amp + Qz [+ or -] Opx [+ or -] Cpx; (e) Pl + Amp + Opx [+ or -] Cpx.


Additionally, FDE rocks exhibit the following distinctive characteristics: (a) diverse disequilibrium textures including sieved plagioclase, normal and sieved plagioclases in the same sample, rounded and embayed crystals, and reaction rims (Fig. 6A-D); (b) occurrence of vesicular, rounded or ellipsoidal andesitic magmatic enclaves (Fig. 6E-F); and (c) unusual mineral compositions, such as heterogeneous plagioclase core and rim compositions, and complex mineral zoning (reverse and oscillatory zoning in crystals or normally and reversely zoned crystals in the same sample).


Plagioclase occurs as euhedral or subhedral phenocrysts (0.5-8 mm), showing Carlsbad or polysynthetic twinning. Crystals with clear and sieved texture can be observed in the same lava (Fig 6C,D). Core compositions in clear texture plagioclase vary from [An.sub.30] to [An.sub.51] (Table 2 in Appendix A, and Fig. 3B). They show normal zoning with rims of [An.sub.32-43]. Sieved plagioclase shows disequilibrium or resorption rims, although occasionally sieved-cored crystals are also observed. In comparison with clear texture crystals, sieved plagioclase exhibits a restricted range composition of labradorite ([An.sub.51-61]; Table 2 in Appendix A). Additionally, an oscillatory zoning has been detected in several sieved plagioclase crystals (Table 3 in Appendix A), where the more calcic composition is observed in the dusty zones (Fig. 7). Some plagioclase phenocrysts show acicular apatite inclusions. On the other hand, FDE lavas include acicular Na-rich plagioclase microphenocrysts, one of them displaying an Or-rich composition ([Ab.sub.59-75] [Or.sub.3-19]; Table 2, Fig. 3B).

Amphibole phenocrysts (0.8-4.0 mm), with intense green to reddish brown pleochroism, cover a variety of compositions, probably as a product of a polybaric crystallization (Hammarstrom and Zen, 1986). In accordance with Leake et al. (1997) classification scheme, they are calcic, showing Fe-tschermakite, Fe-Mg-hastingsite or K-edenite compositions (Table 4 in Appendix A Supplementary Material, and Fig. 4A,B). Thin opaque rims, presumably formed by oxidation, are found on many crystals and locally Fe-Ti oxides appear to have replaced entire phenocrysts.

Euhedral or subhedral orthopyroxene (0.5-2.0 mm) occurred as isolated crystals or glomeroporphyritic aggregates, displaying bronzite ([En.sub.82-86]) or hypersthene ([En.sub.64-79]) core compositions (Table 5 in Appendix A Supplementary Material, and Fig. 5B). SC FDE lavas include both normally and reversely zoned orthopyroxene (rim: [En.sub.64-80]). Some orthopyroxene crystals may also support narrow augitic rims ([En.sub.50-57] [Fs.sub.10-12] [Wo.sub.50-57]; Table 6 in Appendix A Supplementary Material). Isolated calcic pyroxenes (0.5-2.0 mm) are included as euhedral phenocrysts with augitic composition ([En.sub.45-48] [Fs.sub.9-12] [Wo.sub.42-44]; Table 6, Fig. 5B).


Anhedral quartz (< 2.0 mm) occurs in some SC FDE lavas, showing an undulated extinction and sometimes reaction rims of acicular augite ([En.sub.44-47][Fs.sub.9-13][Wo.sub.43-45]; Table 6, Fig. 5B). These coronae generally occur a short distance away from the edge of the quartz crystal suggesting nucleation within the boundary layer melt (Nixon, 1988a).


Mineralogical assemblage in this group is complemented with anhedral Fe-Ti oxides microphenocrysts (< 250 [micro]m). According to the recalculation method suggested by Stormer (1983), titanomagnetite and ilmenite compositions have the range %[mol.sub.usp] = 13-19 and %[mol.sub.lim] = 67-89, respectively (Table 7 in Appendix A Supplementary Material). They occur separately as well as in intimate contact. However, thermodynamic data have not been calculated as the Mg/Mn partition test of Bacon and Hirschmann (1988) has not been validated by the oxide pairs.


SC FDE lavas collected in Cantimplora (SC4 and SC5) and Rancho Agustin localities (SC14 and SC15) from SCT sector have a high content (70-80% in phenocryst mode) of euhedral to subhedral (skeletal) olivine phenocrysts (0.7-2.0 mm). Additionally, olivine (<1.5 mm) has also been observed in Pena de Lobos (SC49 from SCN sector) and Ajusco34 (SC1 from SC-SCh transition sector) outcrops, although its modal abundance is ~5%. Some SC olivine phenocrysts exhibit partial iddingsitization indicated by the development of red-clay and Feoxyhydroxide. Olivine compositions extend from [Fo.sub.87-90] (core) to [Fo.sub.75-88] (rim), generally showing normal zoning (Table 7 in Appendix A).

4.3. Chilled magmatic enclaves (ME)

In the present study, seven ME (SCN sector: SC43a, SC49a, and SC49b; SCC sector: SC35a and SC37a; SCS sector: SC29a and SC57a; SC-SCh transition sector: SC24a; Table 1) were separated from their host dacites. They are darker than their host rock and display a porphyritic texture, with plagioclase, orthopyroxene, amphibole, and quartz phenocrysts in a microlitic groundmass mainly composed of acicular plagioclase and accessory opaque minerals. Some ME (SC49a,b) additionally contain scarce olivine phenocrysts ([Fo.sub.88-90]; Table 7 in Appendix A).

Euhedral plagioclase (< 3.0 mm) displays core compositions ranging from andesine to labradorite ([An.sub.33-48]; Table 2, Fig. 3C).Some plagioclase phenocrysts include acicular apatite inclusions. Some plagioclase phenocrysts show oscillatory zoning and are sieve-ringed, with a clear core mantled by a resorption zone ([An.sub.40-59]). As observed in andesitic-dacitic lavas showing disequilibrium features, acicular K-rich plagioclase microphenocrysts ([Or.sub.14]) occur in the enclaves.


Orthopyroxene (>1.5 mm) occurs as sparse phenocrysts, displaying a hypherstene and bronzite composition ([En.sub.73-87][Fs.sub.11-24][Wo.sub.2-6]; Fig. 5C). An amphibole phenocryst (1.1 mm) is calcic, following the Leake et al. (1997) classification scheme, with a tschermakite composition (Table 4 in Appendix A, Fig. 4A,B). This crystal display Si (5.9, apfu) and Al (2.2, apfu) values similar to tschermakite from dacites without disequilibrium features. Subhedral to anhedral quartz xenocrysts (<1.0 mm) with irregular cracks and undulated extinction complete the enclave mineral assemblage. Some of them display augitic thin rims ([En.sub.44-46][Fs.sub.12-13] [Wo.sub.42-44]; Table 6 in Appendix A).


5. Whole-rock geochemistry and isotopic ratios

5.1. Major elements

The F lavas have a dacitic composition (Fig. 8), covering a range of Si[O.sub.2] = 63.7-69.4 %and ([Na.sub.2]O + [K.sub.2]O) = 6.4-6.9 wt%, with MgO = 0.5-2.4 wt%. In contrast, the ME could be considered as andesites (Fig. 8) displaying, in comparison to F lavas, lower Si[O.sub.2] (= 58.3-61.3 %) and ([Na.sub.2]O + [K.sub.2O]) (= 5.1-5.9 %) values, as well as higher MgO contents (= 3.9-6.6 %). FDE lavas display a range of major element compositions varying from andesite (restricted to SCC and SCS sectors) to dacite. These rocks have Si[O.sub.2] (= 60.6-68.2 %), [Na.sub.2]O + [K.sub.2]O (= 5.9-6.8 %), and MgO (= 1.3-5.1 %) concentrations that are intermediate between the F lavas and ME.

On Harker diagrams, all major elements contents decrease with increasing Si[O.sub.2], except alkalis and [Al.sub.2][O.sub.3] (Fig. 9). [Na.sub.2]O and [Al.sub.2][O.sub.3] do not change whereas [K.sub.2]O increases with Si[O.sub.2]. It is important to note that the most silicic FDE lavas overlap the F dacites without disequilibrium texture.


5.2. Trace elements

F dacites exhibit higher abundances of trace elements compared to most ME, with the exception of compatible elements, Sr, and Y However, SC37a enclave (SCC sector) represents a special case, due to a marked enrichment in REE, Ba, Sr, Zr and Y compared to the other enclaves. Compatible and LILE variation diagrams versus Si[O.sub.2] show that the SC rocks plot along a mixing line, although a considerable scatter exists (Fig. 10). REE (e.g., La, Yb) and HFSE (e.g., Nb, Zr) variation diagrams versus Si[O.sub.2] display relatively flat trends showing a significant data dispersion.

Chondrite-normalized REE diagrams of all SC rocks (Fig. 11) are characterized by enrichment in light REE and a flat pattern for heavy REE. F dacites display significantly higher [(La/Yb).sub.N] ratios (= 5.7-10.3) than the ME (= 4.7--7.8), whereas FD-E lavas have ratios (= 4.7-12.0) that overlap or exceed those of the other groups. All rocks lack a significant negative Eu anomaly, as would be expected by a prolonged fractionation of plagioclase, the most abundant phenocryst of the SC rocks. However, a slightly negative Ce anomaly is observed in some cases, which can be evaluated by the [Ce.sub.N]/[Ce.sup.*.sub.N] parameter (= [Ce.sub.N]/[[[La.sub.N]*[Pr.sub.N]].sup.05]; Seto and Akagi, 2008). Dacites lacking disequilibrium texture display a [Ce.sub.N]/[Ce.sup.*.sub.N] = 0.74-0.89, a range comparable to the observed in FDE (= 0.60-0.93) and ME (= 0.81-0.93) rock types.

Primitive mantle-normalized multi-element diagrams for SC groups (Fig. 12) are characterized by a zig-zag pattern, with negative anomalies in HFSE (Th, Nb, Ta, and Ti) and P, and a general trend where the abundance diminishes as compatibility increases.

5.3. Nd and Sr isotopic ratios

Available Sr and Nd isotopic information for SC rock types are summarized in Table 10 (in Appendix A Supplementary Material). [sup.87]Sr/[sup.86]Sr values of these rocks range from 0.703917 to 0.704268, whereas [sup.143]Nd/[sup.144]Nd ratios cover the narrow interval from 0.512812 to 0.512903. All samples fall within the "mantle array", shown schematically by small dashed lines in the [sup.87]Sr/[sup.86]Sr--[sup.143]Nd/[sup.144]Nd diagram (Fig. 13), overlapping with the Mexican lower crust (Patchett and Ruiz, 1987; Ruiz et al., 1988a,b; Roberts and Ruiz, 1989; Schaaf et al. 1994) and crustal xenoliths of Popocatepetl stratovolcano (Schaaf et al., 2005).

6. Discussion

Several mineralogical and geochemical features observed in SC rocks strongly indicate the involvement of magma mingling processes during the 3.6 to 0.4 Ma period of development of the SC stratovolcanoes.

6.1. Magma mingling mineralogical evidence

Sieve texture, as observed in FDE (for example, Fig. 6D) and ME (for example, Fig. 6F), has been described (Tsuchiyama, 1985; Shelley, 1993) as the small, interconnected inclusions of glass or other matrix material giving the crystal a porous appearance. Heating above the plagioclase liquidus temperature causes fusion of the phenocryst and a rounding off (resorption) of the crystal shape.

Normal and oscillatory zoning in plagioclase crystals in the same sample (for example, dacite SC29; Tables 2-3; Fig. 7) has been interpreted as consequence of a magma mingling process (Nakada, 1991). Nixon and Pearce (1987) reported repeated major sharp reversals in plagioclase crystals from Iztaccihuatl dacites due to magma mixing caused by injection of fresh basic magma. This phenomenon has also been documented in other MVB localities (for example, Luhr and Carmichael, 1980; Wallace and Carmichael, 1994). Additionally, inverse zoning of orthopyroxene (for example, core/rim analysis for dacites SC31 and SC40; Table 5 in Appendix A) has been especially quoted as evidence of magma mingling behavior (Sakuyama, 1981; Bloomfield and Arculus, 1989).


Acicular plagioclase microphenocrysts displaying a K-rich composition (Fig. 3C) could be interpreted as micro-crystals produced by quenching, where Na atoms have been partially substituted by ion exchange K atoms at high temperature (e.g., Viswanathan, 1971; Kroll and Bambauer, 1981). K migration (diffusion coefficient ~ [2.10.sup.-7] [cm.sup.2]/sec) from felsic to mafic magmas during mingling, before the ME undercooling, has been reported by Kumar and Pieru (2010).

An effective blending is inhibited when temperature and viscosity contrasts between two magmas are large and when the proportion of the mafic/intermediate end-member magma is small (typically < 50%), because the mafic/intermediate magma is undercooled to form isolated magmatic inclusions or enclaves in the felsic magma (Eichelberger, 1980; Bacon, 1986; Sparks and Marshall, 1986; Vernon et al., 1988; Stimac et al., 1990; Clyne, 1999; Gencalioglu Kuccu and Floyd, 2001; Coombs et al., 2002; Alpaslan et al., 2005). Some of the FDE rocks (see Table 1 and Fig. 6E,F) include rounded to ellipsoidal magmatic enclaves, which generally maintain coherent and sharp contacts with their host lavas. ME can be interpreted as quenched blobs of an andesitic magma that was injected into relatively cooler dacitic magma, displaying a non-Newtonian behavior within their host magma. A disparity in physical properties between andesitic enclaves and their host dacitic magmas probably leads to fast crystallization of the former magmatic enclaves, as indicated by the acicular character of some crystals (Sparks and Marshall, 1986; Blake and Fink, 2000).



6.2. Magma mingling geochemical evidence

Eichelberger et al. (2006) suggested that, rather than a liquid line of descent, the linear trends as those observed in the Harker major-element diagrams for SC lavas (Fig. 9) reflect a spectrum of discrete magma batches product of complex mingling processes between intermediate and silicic end-members. The lack of coherent trends on Harker trace-element diagrams (Fig. 10) also supports the argument that analyzed compositions did not follow simple liquid lines of descent, but rather reflect complex open-system processes including partial mixing or mingling.

Trace element (including rare earths) Harker patterns, however, show partial to complete equilibration, most likely governed by different degrees of elemental diffusion. Additionally, at least one end-member must be compositionally heterogeneous, as reflected by the differences in trace element geochemistry of SC37a and the rest of andesitic enclaves.


It is important to note that, the Harker diagram linear arrays are product of several magma mingling events that occurred during the SC geological history, (e.g., Si[O.sub.2] vs. MgO Harker plots, (Fig. 14). Taking into account the morphostructural SC sectors, the best defined linear arrangement is observed in the SCN sector, where: (a) the compositional contrast between the magmatic end-members is more pronounced, and (b) the magmatic enclave density per area is maximum, and its size reaching 20 cm. Harker diagrams of SCC and SCS sectors are characterized by a greater dispersion, as: (a) the difference in composition of ME in relation to F rocks is lower, and (b) the number and size of enclaves decreases southward.

Low HFSE concentrations showed by SC rocks could be related to amphibole and ilmenite fractional crystallization, whereas P negative anomaly could be related to apatite fractionation (Rollinson, 1993). On the other hand, Th/Cs (= 2.5-3.5) and Rb/Cs (= 23-45) ratios in ME are similar to those of the continental crust (Th/Cs = 2.1-4.0, Rb/Cs = 17.1-36.7; Rudnick and Gao, 2003). Crustal ratios are also observed in F lavas (Th/Cs = 1.52.8; Rb/Cs = 15-40). This similarity in incompatible trace element ratios strongly suggests the crust as the potential source region for both rock types, although magma generation probably occurred at different depths. Nevertheless, crystal fractionation processes are not sufficient in order to explain the correlation between Th/Cs and Rb/ Cs ratios. Therefore, it is suggested that a partial mixing relationship superimposed on fractional crystallization is required to explain the SC volcanic rocks geochemistry.

Negative Ce anomalies, observed in chondrite-normalized REE diagrams, could result from incipient weathering (e.g., Patino et al., 2003), although no petrographic evidence has been detected in the SC volcanic rocks. Borg and Clyne (1998) suggested that upper-crustal assimilation might explain the negative Ce anomalies observed in felsic calc-alkaline magmas from the southernmost Cascades. However, REE patterns of crustal xenoliths from central MVB (Urrutia-Fucugauchi and Uribe-Cifuentes, 1999; Aguirre-Diaz et al., 2002) do not display negative Ce anomalies. Gomez-Tuena et al. (2006) have reported intermediate to felsic lavas from the Nevado de Toluca volcano characterized by negative Ce anomalies, being interpreted as evidence for slab-derived sedimentary contributions in the petrogenesis of the central MVB. However, REE normalized patterns for pelagic sediments from Site 487 located in the Cocos plate also show a significant negative Eu anomaly (Verma, 2000), which is not observed in the SC rock types. In such a way, negative Ce anomaly remains unexplained, as additional Pb isotopic information for SC lavas is necessary to understand this phenomenon.

Unfortunately, approximations of physical properties of magmas (e.g., pressure and temperature) cannot at present be evaluated because thermodynamic data are not still available for SC host dacitic and intruded andesitic magmas.

The formation of enclaves probably took place later in the mingling process, because the co-mingled lavas exhibit disequilibrium mineral textures and a geochemistry between the andesitic enclaves and the dacites. The partial mixing origin of FDE rocks can also be modeled based on the geochemical compositions of two end-members, particularly using trace elements (Rollinson, 1993). For example, in the Rb/Sr and K/Ti ratios plotted versus Rb diagrams (Fig. 15A,B), lavas with disequilibrium textures (FDE) fall on the representative mixing curves drawn by using average F dacites as a silicic end-member and considering two basic ME end-members (E1: average of SC27a, SC35a, and SC49a,b; E2: SC37a andesitic enclave). Shifting of some points from representative mixing curves can be interpreted as previous or coeval plagioclase, amphibole, and pyroxene fractional crystallization, although may also been affected by mineral sorting and/or diffusion processes.

6.3. Proposed magma mingling/mixing scenario

Magmatic processes such as differentiation, eruption style, vesiculation, and fluid flow are mainly ruled by chemical composition and inter-related melt physical properties including temperature, density, viscosity, and crystallinity. When two contrasting magmas interact several scenarios may unfold (Donoghue et al., 1995): (a) the melts may mix physically, thereby forming a hybrid magma; (b) one magma may freeze against the host magma, giving a net-vein complex, and abruptly arresting the mixing process; (c) the melts may mix partially but incompletely, particularly when the two end-member components are porphyritic, leaving phenocrysts from both magmas with strong disequilibrium textures; and (d) the melts may intermingle but not mix, forming a banded rock. Where mixing has been efficient, the only evidence for this process may be disequilibrium textures and/or linear arrays in Harker diagrams. However, where mixing is limited or incomplete, the development of magmatic enclaves or banded rocks may occur, which could retain the identity of the end-members. These phenomena have operated commonly in volcanic and plutonic environments (e.g., Nakamura, 1995; Eichelberger et al., 2000; Coombs et al., 2002; Brown et al., 2006; Kumar and Rino, 2006; Dokukina et al., 2010).

Therefore, the following model is proposed for magma mingling/mixing in Sierra de las Cruces volcanic range, that include the recurrent operation of the (b) and (c) scenarios by Donoghue et al. (1995) from 3.5 to 0.4 Ma. Fix (1975) reported a decrease in the seismic velocities at the base of the crust in the central MVB. Campos-Enriquez and Sanchez-Zamora (2000) have indeed interpreted gravity data to infer the presence of partial melts in the lower crust beneath this region. These magmas might be stored at the base of the crust, transferring heat to shallower crustal levels and provoking their partial melting. SC dacitic rocks were probably generated by partial melting of the upper continental crust (depth at the base ~10 km; Ortega-Gutierrez et al., 2008), and were subsequently stored in a shallow level magma chamber. The combined Sr-Nd isotope data from the Mexican lower crust and crustal xenoliths from Popocatepetl stratovolcano (Fig. 13) are in general consistent with a significant crustal involvement in the genesis of SC rocks. Sodic plagioclase, Mg-hastingsite amphibole, hypherstene and minor quartz were crystallized prior to the injection of the andesitic magma. Subsequently, a small volume of andesitic magma, probably generated at a deeper crustal level (lower crust (?): 25-45 km; Ortega-Gutierrez et al., 2008), intruded in the shallow magma chamber, losing heat to the surroundings and starting to vesiculate. Eichelberger et al. (2000) suggested that, if ponding mafic magma operates like a rising piston, co-mingled lavas are slowly expulsed reaching the surface by a volatile-poor and non-explosive effusive eruption. Felsic minerals in the host dacite became round and embayed, plagioclase crystals were sieved, and mafic minerals developed reaction rims as a response to an increase in temperature. On the other hand, quenching of the andesitic magma resulted in discrete enclave development, reflecting an incomplete physical mixing process. Mingling process occurred due to differences in physical properties between the two magmas and the relatively small volume proportion of the intermediate magma. Additionally, limited exchange of xenocrysts (labradorite, edenitic amphibole, and bronzite in dacites, oligoclase and andesine in andesitic enclaves) might have occurred during this entrainment stage.

7. Conclusions

This study presents detailed petrography, mineral chemistry, and whole-rock geochemical data for Sierra de las Cruces. Magma mingling/mixing processes have played a significant role in the evolution of this volcanic range. Most dacites display several disequilibrium features that imply the involvement of open system magmatic evolution processes over the 3.6 to 0.4 Ma period. Several mineralogical lines of evidence confirm the interaction between andesitic magmas, probably originated in the interface between upper and lower crust, and felsic magmas possibly derived from upper crust partial melting. These lines of evidence include: (a) chilled magmatic enclaves which probably represent the material that mingled with felsic magma, causing thermal and compositional disequilibrium; (b) presence of more and less silicic amphiboles in the same sample, some of them showing reaction textures and inverse zoning; (c) bronzite and hypersthene orthopyroxenes displaying augitic reaction rims, and (d) sieved plagioclase crystals and their occurrence together with clear normal plagioclase, as well as oscillatory zoning of sieved plagioclase. Magma mixing processes in Sierra de las Cruces are also supported by linear arrays in bivariate plots of major elements. Diagrams based on trace element ratios also point out to an origin by mixing between dacitic and andesitic melts for the Sierra de las Cruces volcanic rocks. Results from this study confirm the significance of magma mixing processes in the intermediate-felsic volcanic rocks petrogenesis in the central MVB.

Appendix A--Supplementary material

Tables 1 to 10 cited in this paper can be downloaded from


This study was funded by PAICYT-UANL program (Project CT786-02). Thanks are due to CONACyT and UANL for studentships (PRS, SGMG, and OMRA). We would like to thank Francisco Anguita, Jose Gonzalez del Tanago and Alfredo Fernandez Larios (Universidad Complutense) for laboratory assistance during microprobe analyses. We are also grateful to Maria Luisa Osete (Universidad Complutense) for having provided some samples utilized in this paper. The first author (FVT) wishes to express his gratefulness to Academic exchange office--UANL for institutional and financial support. Comments by A. Aparicio, S. Kumar, and an anonymous reviewer significantly improved the manuscript.


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F. Velasco-Tapia (1) *, P. Rodriguez-Saavedra (1), A. Marquez (2), I. Navarro de Leon (1), C. De Ignacio (3), S. G. Marroquin Guerra (4), J. Quintanilla-Garza (5), O.M. Rangel-Alvarez (1)

(1) Universidad Autonoma de Nuevo Leon, UANL, Facultad de Ciencias de la Tierra, Av Universidad S/N, Ciudad Universitaria, 66451 San Nicolas de los Garza, Nuevo Leon, Mexico.;;

(2) Grupo de Geologia, Escuela Superior de C.C. Experimentales y Tecnologia, Universidad Rey Juan Carlos, c/Tulipan s/n, Mostoles, 28933 Madrid, Spain.

(3) Departamento de Petrologia y Geoquimica, Facultad de Ciencias Geologicas, Universidad Complutense, Cd. Universitaria, c/Jose Antonio Novais No. 2, 28040 Madrid, Spain.

(4) Universidad Autonoma de Nuevo Leon, UANL, Escuela Preparatoria No. 4, Av. Universidad S/N, Ciudad Universitaria, 66451 San Nicolas de los Garza, Nuevo Leon, Mexico.

(5) First Majestic--Silver Corp. Unidad San Martin de Bolanos. c/Fanny Anitua No. 2700, Col. Los Angeles, 34076 Durango, Durango, Mexico.

* corresponding author:

Received: 19/05/2011 / Accepted: 12/04/2013
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Title Annotation:articulo en ingles
Author:Velasco-Tapia, F.; Rodriguez-Saavedra, P.; Marquez, A.; de Leon, I. Navarro; De Ignacio, C.; Guerra,
Publication:Journal of Iberian Geology
Date:Jan 1, 2013
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