Origin of rhyolitic lavas in the Mesa Central, Mexico, by crustal melting related to extension

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<ul><li><p>Origin of rhyolitic lavas in the Mesa Central, Mexico, bycrustal melting related to extension</p><p>M.T. Orozco-Esquivel , A.F. Nieto-Samaniego, S.A. Alaniz-AlvarezUnidad de Investigacion en Ciencias de la Tierra, Instituto de Geolog|a, Universidad Nacional Autonoma de Mexico,</p><p>Campus Juriquilla, P.O. Box 1-742, 76001 Queretaro, Qro., Mexico</p><p>Received 15 December 2000; accepted 8 January 2002</p><p>Abstract</p><p>The emplacement of a voluminous sequence of rhyolitic lava flows and domes characterizes Oligocene volcanismin the Mesa Central (MC) of Mexico. Its dominant effusive style of emplacement contrasts deeply with thepredominantly explosive volcanism of the Sierra Madre Occidental Volcanic Province toward the west. Whole rockgeochemical (major- and trace elements) and Sr^Nd isotopic data of the MC Oligocene rhyolitic lavas document amarked change in magma composition at around 30 Ma, allowing us to distinguish between a lower and an uppersequence. Lavas from the lower sequence are geochemically similar to the high-K rhyolitic rocks of the eastern SierraMadre Occidental. Major- and trace-element variations are characteristic of mantle-derived magmas evolving throughfractional crystallization. The initial 87Sr/86Sr and ONd values are nearly constant (0.70644^0.70770 and 31.2 to 32.1respectively) and indicate some contribution from crustal material. Lavas from the upper sequence are high-silica,peraluminous rhyolites, with strong enrichment in fluorine and in some incompatible lithophile elements (Rb, La, Sm,Yb, Y, Th, U, Nb, Ta), and strong depletion in the feldspar-compatible elements Sr, Ba, Eu. Initial 87Sr/86Sr ratios ofthe upper sequence lavas are high and variable (0.70812^0.72190), and decrease as silica content increases, whereas theONd values are relatively constant (31.4 to 32.8). The trace element behavior indicates an origin by variable degrees ofnon-modal partial melting of granulitic low-crustal rocks and chemical disequilibrium during melting processes. Thehigh and variable Sr isotopic ratios could also be related to isotope disequilibrium melting processes if the isotopicheterogeneities between individual mineral phases were preserved during heating of the source rocks. The changes ingeochemical compositions are related to the onset of crustal extension at high strain rates documented for the MC.Crustal extension promoted crustal melting at high melting rates, high melt segregation rates, rapid ascent of low-viscosity fluorine-rich magmas, and inhibited melt stagnation in magma chambers. Such conditions favored theeffusive volcanic style and support the possibility of melting under disequilibrium conditions.B 2002 Elsevier Science B.V. All rights reserved.</p><p>Keywords: disequilibrium melting; crustal partial melting; geochemistry; Nd^Sr isotopes; extension tectonics; Mexico</p><p>1. Introduction</p><p>The subduction-related mid-Tertiary volcanismof the Sierra Madre Occidental Volcanic Province(SMOVP) represents one of the most voluminous</p><p>0377-0273 / 02 / $ ^ see front matter B 2002 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 2 4 9 - 4</p><p>* Corresponding author. Fax: +52-442-2381100.E-mail address: torozco@unicit.unam.mx</p><p>(M.T. Orozco-Esquivel).</p><p>VOLGEO 2485 14-10-02</p><p>Journal of Volcanology and Geothermal Research 118 (2002) 37^56</p><p>www.elsevier.com/locate/jvolgeores</p></li><li><p>rhyolitic volcanic events on Earth (McDowell andKeizer, 1977). Most ignimbrites were eruptedfrom large caldera complexes during two periodsof intense volcanic activity, between 34^27 Maand 23^21 Ma (McDowell and Clabaugh, 1979;Nieto-Samaniego et al., 1999).In the southeastern part of the SMOVP, the</p><p>mid-Tertiary volcanism of the Mesa Central(MC) physiographic province (Fig. 1) presentsdistinctive characteristics in style and composi-tion, suggesting dierent mechanisms of magmageneration, ascent and emplacement. In contrastto the explosive volcanic nature of the SMOVPlocated toward the west, the mid-Tertiary volcan-ism in the southern MC shows an intense eusivephase, comprising a large volume of lavas anddomes emplaced over an area of ca. 10 000 km2.Pyroclastic deposits are also present, but theyhave much lower thickness than in the SMOVP.Eusive volcanism in the MC took place between32 and 27 Ma (Nieto-Samaniego et al., 1996,1999).Most lavas are high-K rhyolites ; intercalated</p><p>volcanics of intermediate composition are minor.Based on geochemical variations reported in thiswork, the rhyolites have been divided into a lowerand an upper sequence. The younger lavas are</p><p>high-K and high-silica rhyolites and locally con-tain topaz. Independently of the topaz content,the high-silica rhyolites have the distinctive chem-ical characteristics of the so-called topaz or tinrhyolites. These rhyolites are rich in silica anduorine, strongly enriched in some incompatiblelithophile elements (Rb, U, Th, Li, Be and Sn)and depleted in compatible elements such as Srand Ba (Christiansen et al., 1983, 1986). Topazrhyolites are related to economic deposits ofsuch lithophile elements as Be, U, F, Li, and Sn(e.g. Christiansen et al., 1986).The origin of topaz rhyolites has been related</p><p>to magmas generated by partial melting of lowercrustal granulites, followed by evolution throughextensive fractional crystallization enroute to thesurface or in small magma chambers (Christian-sen et al., 1986). Other authors (e.g. Reece et al.,1990) consider that the topaz rhyolite magmascontain a major component of lower crustal ma-terial partially mixed with mantle-derived macmagma, and that the lavas represent the highlydierentiated uppermost part of a large magmachamber. However, such processes cannot explainthe high and variable 87Sr/86Sr ratios often foundin topaz rhyolites, and late-stage upper crustalcontamination has been invoked as responsible</p><p>Fig. 1. (A) Present-day geodynamic setting and location of the study area (in black) in the southeastern portion of the SMOVP(stippled area). Physiographic provinces: MC: Mesa Central ; SMOc: Sierra Madre Occidental; MVB: Mexican volcanic belt;SMOr: Sierra Madre Oriental. (B) Distribution of Oligocene magmatic rocks based on Ferrari et al. (1999) and our own isotopicages compilation. Black dots indicate the location of dated uorine-rich high-silica rhyolites. Tertiary extension north of theMVB is based on Henry and Aranda-Gomez (2000).</p><p>VOLGEO 2485 14-10-02</p><p>M.T. Orozco-Esquivel et al. / Journal of Volcanology and Geothermal Research 118 (2002) 37^5638</p></li><li><p>for those variations (Christiansen et al., 1986;Reece et al., 1990).Occurrences of uorine-rich rhyolites have been</p><p>described for the western USA and for the easternedge of the SMOVP. In the western USA, theserhyolites are spatially and temporally related tothe extensional regimes of the Rio Grande Riftand the Basin and Range Province (Christiansenet al., 1986). Topaz rhyolites in Mexico are ofOligocene age (32^27 Ma) and coincide with theclimax of mid-Tertiary volcanism in the SMOVP.In the MC, the emplacement of widely distributedhigh-silica rhyolites was simultaneous with docu-mented three-dimensional strain, that generatedan orthogonal fault pattern (Nieto-Samaniegoet al., 1999). The domes are aligned along frac-tures and their emplacement shows a close spa-tial relationship with major graben faults ; nocaldera structures have been identied in thearea (Tristan-Gonzalez, 1986; Aguillon-Robleset al., 1994). No clear relationship between rhyo-lite emplacement and extensional features hasbeen documented in the northern portion of theSMOVP.Previous geochemical studies of topaz rhyolites</p><p>in Mexico have been of local scale and mainlyrelated to tin or uorite deposits (Ruiz et al.,1980, 1985; Huspeni et al., 1984; Tuta et al.,1988). Other studies in the MC are restricted toisolated rhyolitic domes carrying topaz in litho-physae (Aguillon-Robles et al., 1994; Webster etal., 1996). Labarthe-Hernandez et al. (1982) re-ported analysis of major and trace element (Sr,Rb, Zr) compositions for some Tertiary volcanicunits in the MC.In this study we present geochemical and iso-</p><p>topic data for Oligocene rhyolites from the MC.Our data show that in the MC a large volume oflavas geochemically similar to topaz rhyolites wasemplaced in close relation to the main event ofOligocene normal faulting. With regard to theorigin of the rhyolites, previous petrogenetic mod-els are discussed and the geochemical data areinterpreted in the light of recent experimentalwork on crustal melting processes. In addition,the inuence of deformation on the geochemicaland isotopic composition of the lavas is as-sessed.</p><p>2. Stratigraphy</p><p>The Mesozoic rocks in the southern MC arerepresented by two main lithologies (Fig. 2) : ma-rine calcareous rocks of the Sierra Madre Orientalsequence (e.g. Eguiluz de Antunano et al., 2000),and volcanic and ysch sequences of the Sierra deGuanajuato volcano^sedimentary Complex (Mar-t|nez-Reyes, 1992; Centeno-Garc|a et al., 1993).Shortening during Laramide orogeny deformedthose rocks.Paleocene^Eocene magmatism in the area is</p><p>scarce. The oldest post-orogenic rocks in thearea are granitic intrusives of Paleocene age(Mugica-Mondragon and Jacobo-Albarran, 1983),which form isolated bodies in the Sierra deGuanajuato (southwestern edge of Fig. 2). Con-tinental psamitic and conglomeratic sedimentswith intercalated mac volcanic rocks of Eoceneage outcrop near the cities Guanajuato and SanLuis Potos| (Labarthe-Hernandez et al., 1982;Aranda-Gomez and McDowell, 1998). The vol-ume of volcanic rocks of Eocene age is small ;they consist of isolated bodies of a unit containingandesite lavas and breccias with subordinateamounts of pyroclastic deposits (Labarthe-Her-nandez et al., 1982), and a rhyolitic ignimbritereported in the Guanajuato area (Mart|nez-Reyes,1992).In the early Oligocene, a voluminous volcanic</p><p>event took place forming a thick volcanic cover ofandesitic to rhyolitic composition. Commonly,these rocks rest directly over the Mesozoic units.We grouped the Oligocene volcanic cover into</p><p>two sequences according to their stratigraphicposition, their spatial distribution relative to theVilla de Reyes graben, their eld and petrographicdescriptions, and principally, their dierences inchemical composition reported in this work (Ta-ble 1).The lower sequence includes rocks emplaced</p><p>before the formation of the Villa de Reyes graben(Tristan-Gonzalez, 1986). The composition variesfrom andesite to rhyolite and each outcrop areapresents one or more andesite-to-rhyolite cycles.At least 50% of the lower sequence rocks are an-desitic lavas, which commonly show intense alter-ation, and therefore are unsuitable for chemical</p><p>VOLGEO 2485 14-10-02</p><p>M.T. Orozco-Esquivel et al. / Journal of Volcanology and Geothermal Research 118 (2002) 37^56 39</p></li><li><p>analysis. Apart from some trachytic to rhyoliticlavas, the remainder of the sequence is dominatedby pyroclastic deposits of rhyolitic composition.Only few relatively small outcrops of the lowersequence are present to the west of the Villa deReyes graben between San Luis Potos| and SanFelipe. Ages of 32.8 R 0.9 to 29.5 R 1.5 Ma for thelower sequence are provided by four K^Ar dates(Labarthe-Hernandez et al., 1982, Cerca-Mart|nezet al., 2000), including a new date (Table 1).The upper sequence rocks are located adjacent</p><p>to the major structures and cover more than ahalf of the study area (Fig. 2). The upper se-quence is overwhelming rhyolitic, with the excep-tion of small outcrops of basaltic lavas located</p><p>mainly in the Bledos graben. The major volumeof rocks was formed during a well-dened vol-canic phase, which started with the emplacementof a large volume of rhyolitic lavas, followed byan ignimbritic event of rhyolitic composition. Alater minor second volcanic phase producedignimbrites and a subordinated amount of rhyo-litic lavas located close to the Bledos graben.The petrographic characteristics of the rocks</p><p>produced in both phases are similar. In the eld,rhyolitic lavas and domes of the upper sequenceare white to pink-colored, typically containingabundant phenocrysts of quartz and sanidine,and locally biotite. The groundmass shows well-developed ow foliation. The domes are aligned</p><p>Fig. 2. Simplied geologic map of the southern MC showing the main stratigraphic units. The map is based on geologic chartspublished by Mart|nez-Reyes (1992), Alvarado-Mendez et al. (1997), and the Geological Institute of the San Luis Potos| Univer-sity between 1977 and 1995 (see Nieto-Samaniego et al., 1996 and Labarthe-Hernandez et al., 1982 for references).</p><p>VOLGEO 2485 14-10-02</p><p>M.T. Orozco-Esquivel et al. / Journal of Volcanology and Geothermal Research 118 (2002) 37^5640</p></li><li><p>along faults or fractures parallel to the directionof major fault systems (Tristan-Gonzalez, 1986).Major outcrops are located along the Villa deReyes graben between San Luis Potos| and Gua-najuato. We have not much information aboutthe extent of the domes within the grabens, butthey probably underlie the sedimentary cover(Tristan-Gonzalez, 1986). To the east of the studyarea, dikes and spines related to the upper se-quence intrude the lower sequence (Fig. 2). Theseoutcrops suggest that upper sequence rocks mayhave covered a large area toward the east of theVilla de Reyes graben. In many localities therhyolite domes are associated with tin ore depos-its, and some of them contain topaz.</p><p>The age of the rhyolitic domes and lavas iswell-constrained by K^Ar dates ranging between30.1 R 0.8 and 30.8 R 0.8 Ma (Nieto-Samaniego etal., 1996) to 29.2 R 0.8 Ma (Aguillon-Robles et al.,1994) (Table 1). These dates overlap with agesreported for the lower sequence, but the strati-graphic relationships clearly dene a youngerage for the upper sequence units.The rhyolitic ignimbrites and pyroclastic rocks</p><p>of the upper sequence cover the western half ofthe study area. No calderas have been identied inthe MC as sources for those deposits. Duringeldwork, we found several examples of pyroclas-tic dikes within the underlying rhyolites. Thesedikes have orientations similar to those of the</p><p>Table 1Oligocene stratigraphic units of the southern MC</p><p>Units Age Lithostratigraphic unit Chemicalclassicationa</p><p>Upper sequenceRhyolitic ignimbrites 27.6R 0.6 Ma (San)3 Riolita Panalillo with intercalated La Placa</p><p>basaltRhyolite1;6</p><p>Rhyolite Zapote 27.0R 0.7 Ma (San)2 Riolita Zapote Rhyolite1</p><p>Ignimbrites and otherpyroclastic rocks</p><p>28.2R 0.7 Ma (San)2,29.9R 0.6 Ma (San)3</p><p>Ignimbrita Cantera, Ignimbrita Cuatralba Rhyolite1</p><p>Rhyolitic lavas anddomes</p><p>29.2R 0.8 Ma (Bt)4,30.1R 0.8^30.8R 0.8Ma (San)2</p><p>Riolita San Miguelito, Riolita Chich|ndaro Rhyolite1;6</p><p>Lower sequenceRiolita Quelital Rhyolite1</p><p>30.6R 1.5 Ma (WR)1 Latita Portezuelo Dacite toRhyolite1;6</p><p>Andesita Estanco Andesite1</p><p>Traquita Ojo Caliente Rhyolite1;6</p><p>Serie Potrerillo, And. Golondrinas Andesite todacite1</p><p>29.5R 1.5 Ma (WR)1 Ignimbrita Santa Mar|a, IgnimbritaEl Organo</p><p>Rhyolite1</p><p>32.8R 0.9 Ma (San)6 Intrusivo Palo Verde, Riodacita delCarmen</p><p>Trachyte torhyolite1;6</p><p>30.7R 0.4 Ma (WR)5 And. Salitrera, And. Agua Fr|a, SerieAtotonilco, And. Cedro</p><p>Basaltic...</p></li></ul>