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

M.T. Orozco-Esquivel �, A.F. Nieto-Samaniego, S.A. Alaniz-AlvarezUnidad de Investigacio¤n en Ciencias de la Tierra, Instituto de Geolog|¤a, Universidad Nacional Auto¤noma de Me¤xico,

Campus Juriquilla, P.O. Box 1-742, 76001 Quere¤taro, Qro., Mexico

Received 15 December 2000; accepted 8 January 2002

Abstract

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.

Keywords: disequilibrium melting; crustal partial melting; geochemistry; Nd^Sr isotopes; extension tectonics; Mexico

1. Introduction

The subduction-related mid-Tertiary volcanismof the Sierra Madre Occidental Volcanic Province(SMOVP) represents one of the most voluminous

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

* Corresponding author. Fax: +52-442-2381100.E-mail address: [email protected]

(M.T. Orozco-Esquivel).

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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

mid-Tertiary volcanism of the Mesa Central(MC) physiographic province (Fig. 1) presentsdistinctive characteristics in style and composi-tion, suggesting di¡erent 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 e¡usivephase, 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.E¡usive volcanism in the MC took place between32 and 27 Ma (Nieto-Samaniego et al., 1996,1999).Most lavas are high-K rhyolites ; intercalated

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

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 and£uorine, 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

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 ma¢cmagma, and that the lavas represent the highlydi¡erentiated 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

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-Go¤mez (2000).

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for those variations (Christiansen et al., 1986;Reece et al., 1990).Occurrences of £uorine-rich rhyolites have been

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 identi¢ed in thearea (Trista¤n-Gonza¤lez, 1986; Aguillo¤n-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

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 (Aguillo¤n-Robles et al., 1994; Webster etal., 1996). Labarthe-Herna¤ndez 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-

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 in£uence of deformation on the geochemicaland isotopic composition of the lavas is as-sessed.

2. Stratigraphy

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 Antun‹ano 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.PaleoceneÊocene magmatism in the area is

scarce. The oldest post-orogenic rocks in thearea are granitic intrusives of Paleocene age(Mugica-Mondrago¤n and Jacobo-Albarra¤n, 1983),which form isolated bodies in the Sierra deGuanajuato (southwestern edge of Fig. 2). Con-tinental psamitic and conglomeratic sedimentswith intercalated ma¢c volcanic rocks of Eoceneage outcrop near the cities Guanajuato and SanLuis Potos|¤ (Labarthe-Herna¤ndez et al., 1982;Aranda-Go¤mez 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-na¤ndez et al., 1982), and a rhyolitic ignimbritereported in the Guanajuato area (Mart|¤nez-Reyes,1992).In the early Oligocene, a voluminous volcanic

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

two sequences according to their stratigraphicposition, their spatial distribution relative to theVilla de Reyes graben, their ¢eld and petrographicdescriptions, and principally, their di¡erences inchemical composition reported in this work (Ta-ble 1).The lower sequence includes rocks emplaced

before the formation of the Villa de Reyes graben(Trista¤n-Gonza¤lez, 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

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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 KÂr dates(Labarthe-Herna¤ndez et al., 1982, Cerca-Mart|¤nezet al., 2000), including a new date (Table 1).The upper sequence rocks are located adjacent

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

mainly in the Bledos graben. The major volumeof rocks was formed during a well-de¢ned 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

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

Fig. 2. Simpli¢ed geologic map of the southern MC showing the main stratigraphic units. The map is based on geologic chartspublished by Mart|¤nez-Reyes (1992), Alvarado-Me¤ndez 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-Herna¤ndez et al., 1982 for references).

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along faults or fractures parallel to the directionof major fault systems (Trista¤n-Gonza¤lez, 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(Trista¤n-Gonza¤lez, 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.

The age of the rhyolitic domes and lavas iswell-constrained by KÂr 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 (Aguillo¤n-Robles et al.,1994) (Table 1). These dates overlap with agesreported for the lower sequence, but the strati-graphic relationships clearly de¢ne a youngerage for the upper sequence units.The rhyolitic ignimbrites and pyroclastic rocks

of the upper sequence cover the western half ofthe study area. No calderas have been identi¢ed inthe MC as sources for those deposits. During¢eldwork, we found several examples of pyroclas-tic dikes within the underlying rhyolites. Thesedikes have orientations similar to those of the

Table 1Oligocene stratigraphic units of the southern MC

Units Age Lithostratigraphic unit Chemicalclassi¢cationa

Upper sequenceRhyolitic ignimbrites 27.6R 0.6 Ma (San)3 Riolita Panalillo with intercalated La Placa

basaltRhyolite1;6

Rhyolite Zapote 27.0R 0.7 Ma (San)2 Riolita Zapote Rhyolite1

Ignimbrites and otherpyroclastic rocks

28.2R 0.7 Ma (San)2,29.9R 0.6 Ma (San)3

Ignimbrita Cantera, Ignimbrita Cuatralba Rhyolite1

Rhyolitic lavas anddomes

29.2R 0.8 Ma (Bt)4,30.1R 0.8^30.8R 0.8Ma (San)2

Riolita San Miguelito, Riolita Chich|¤ndaro Rhyolite1;6

Lower sequenceRiolita Quelital Rhyolite1

30.6R 1.5 Ma (WR)1 Latita Portezuelo Dacite toRhyolite1;6

Andesita Estanco Andesite1

Traquita Ojo Caliente Rhyolite1;6

Serie Potrerillo, And. Golondrinas Andesite todacite1

29.5R 1.5 Ma (WR)1 Ignimbrita Santa Mar|¤a, IgnimbritaEl Organo

Rhyolite1

32.8R 0.9 Ma (San)6 Intrusivo Palo Verde, Riodacita delCarmen

Trachyte torhyolite1;6

30.7R 0.4 Ma (WR)5 And. Salitrera, And. Agua Fr|¤a, SerieAtotonilco, And. Cedro

Basalticandesite todacite1;5

Age determinations by KÂr method. Dated material in parentheses: WR: whole rock; San: sanidine; Bt: biotite.a Data from: (1) Labarthe-Herna¤ndez et al. (1982); (2) Nieto-Samaniego et al. (1996); (3) Labarthe-Herna¤ndez and Jime¤nez-

Lo¤pez (1992); (4) Aguillo¤n-Robles et al. (1994); (5) Cerca-Mart|¤nez et al. (2000); (6) this work.

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regional faults, suggesting a possible eruption ofthe ignimbrites through ¢ssures.There are two major pyroclastic events, the old-

er and more widespread was dated between29.9 R 0.6 Ma (Labarthe-Herna¤ndez and Jime¤-nez-Lo¤pez, 1992) and 28.2 R 0.7 Ma (Nieto-Sama-niego et al., 1996); and the rocks of the youngerpyroclastic event have been dated at 27.6 R 0.6 Ma(Labarthe-Herna¤ndez and Jime¤nez-Lo¤pez, 1992)(Table 1). Between these ignimbrites there is alarge rhyolite dome (Riolita Zapote) dated in27R 0.7 Ma (Nieto-Samaniego et al., 1996).The distribution of the lower and upper sequen-

ces indicates that prior to 30 Ma the volcanismwas located to the east of Villa de Reyes graben.After 30 Ma the volcanism expanded widely inthe southern MC (Fig. 2), synchronously withthe peak of extension and the growth of the Villade Reyes graben (Trista¤n-Gonza¤lez, 1986; Nieto-Samaniego et al., 1999).Over the upper sequence there is an unconform-

ity. The volcanic activity started again in Miocenewith the emplacement of ma¢c to intermediaterocks of the Mexican volcanic belt in the southernarea of Fig. 2. Scattered cinder cones, maars andlava £ows of early to middle Pleistocene age out-crop northeast of San Luis Potos|¤ (Aranda-Go¤-mez and Luhr, 1996). Their composition includesnephelinite, basanite, alkaline basalt and basalt.Alluvial and lacustrine deposits with intercalatedbasalts and ash-fall deposits ¢ll the basins formedby Cenozoic extension. Their ages range from Oli-gocene to Pliocene^Pleistocene (Carranza-Casta-n‹eda et al., 1994; Nieto-Samaniego et al., 1996).

3. Sampling and analytical procedures

Fifteen samples from rhyolitic domes and £owswere collected for geochemical analysis. The sam-ples have well-constrained ages within a narrowrange around 30 Ma and were collected in abroad zone with the aim of ¢nding rhyoliteswith similar geochemical characteristics. A limit-ing factor for sampling in the area is the commonalteration of the rhyolites that made regular spac-ing of sampling sites di⁄cult. In the ¢eld weselected dense felsites, with no lithophysae or

obvious silici¢cation, kaolinitization and/or pheno-cryst alteration. The ¢nal selection of samples forgeochemical analysis was based on the petro-graphic study of thin sections.Whole rock samples were analyzed for major

elements at the Laboratorio Universitario de Geo-qu|¤mica Isoto¤pica (LUGIS), of the UniversidadNacional Auto¤noma de Me¤xico, using a SiemensSRS 3000 X-ray sequential spectrometer. Analyt-ical precision was estimated at R 2% for SiO2,TiO2, Al2O3, Fe2O3 tot., MgO, and CaO; andR 6% for the other major elements.Analysis of the rare earth elements (REE) and

other trace elements was performed in 14 samplesby inductively coupled plasma-mass spectrometry,and those of £uorine with ion-selective electrodein Activation Laboratories Ltd., ON, Canada.Reported analytical precision varies between 4and 11%.Sr, Sm and Nd isotopic compositions and ele-

ment concentrations were obtained at the LUGISusing a Finnigan MAT-262 multicollector massspectrometer operating in static mode. Rb isotoperatios were measured in the same laboratory withan NBS-type single collector mass spectrometer.A detailed description of the analytical procedureis given in Schaaf et al. (2000). La Jolla Nd stan-dard 143Nd/144Nd= 0.511882; SRM987 Sr stan-dard 87Sr/86Sr = 0.710233. Relative reproducibil-ities (1c) of the isotope ratios of these standardsare 0.049x and 0.024x respectively. Experi-ence-based values for relative uncertainties (1c)are R 2% for 87Rb/86Sr and R 0.5% for 147Sm/144Nd. Relative reproducibilities (1c) for Rb, Sr,Sm and Nd element concentration are R 1.5%,R 0.7%, R 2.6% and R 1.2% respectively.

4. Geochemistry

4.1. Major and trace elements

The compositions of the analyzed samples arelisted in Table 2. Following the total alkalies vs.silica classi¢cation of LeBas et al. (1986), the sam-ples are mainly classed as rhyolites, with one sam-ple of trachydacitic composition; all samples be-long to the high-K series as de¢ned by Peccerillo

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Table 2Major- and trace-element analysis of lavas from the MC

Lower sequence Upper sequence

RIO-27

RIO-45 RIO-29 RIO-22 RIO-18 RIO-47 RIO-24 RIO-16 RIO-12 RIO-9 RIO-44 RIO-41 RIO-46 RIO-43 RIO-7

SiO2 66.4 71.3 71.9 74.3 75.1 75.5 76.2 76.0 76.1 76.7 76.7 77.7 78.0 78.2 77.7TiO2 0.83 0.45 0.31 0.19 0.20 0.21 0.18 0.07 0.06 0.06 0.08 0.10 0.08 0.10 0.06Al2O3 14.4 13.5 13.3 12.3 13.0 12.6 11.8 13.1 13.2 12.8 12.6 12.2 11.7 11.7 12.4Fe2O3* 5.13 4.62 3.57 2.23 2.50 1.95 2.23 1.45 1.38 1.53 1.33 1.25 1.07 1.22 1.46MnO 0.07 0.06 0.05 0.03 0.03 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02MgO 1.07 0.31 0.43 0.56 0.31 0.15 0.23 0.20 0.21 0.19 0.13 0.20 0.13 0.14 0.21CaO 2.53 1.69 2.19 0.87 0.74 0.70 0.86 0.51 0.44 0.31 0.29 0.46 0.51 0.51 0.19Na2O 3.8 2.8 2.9 1.9 2.6 2.6 2.6 3.3 2.8 2.7 3.3 2.5 2.6 2.5 2.0K2O 4.6 5.0 5.0 5.8 5.5 4.8 5.1 5.4 5.5 4.9 4.8 5.1 5.0 5.1 4.8P2O5 0.23 0.15 0.07 0.03 0.03 0.03 0.10 0.01 0.02 0.01 0.01 0.02 0.03 0.02 0.01L.O.I. 1.08 0.83 0.43 2.13 0.94 0.83 0.68 0.66 0.86 1.14 0.57 0.69 0.61 0.75 1.95

F 1082 1336 322 250 207 489 7603 1463 1289 2840 1296 1149 1039 356Rb 189 159 193 170 181 155 581 407 307 601 359 330 322 186Sr 141 129 62 91 78 90 6 9 9 7 8 9 13 20Zr 501 457 278 277 304 250 125 113 86 143 144 112 145 113Nb 18 16 13 13 14 15 42 19 24 30 23 18 17 16Ba 1340 1630 1300 1480 1410 1360 50 28 49 89 20 49 70 137La 53.6 59.0 61.2 61.6 77.8 53.5 75.8 17.9 10.0 24.3 21.7 35.3 28.7 19.6Pr 14.1 14.9 15.4 15.0 19.2 12.7 25.2 5.71 2.58 6.19 4.25 11.3 7.39 7.16Nd 56.6 59.2 59.2 58.1 73.9 50.0 105 22.3 10.8 23.8 14.8 47.0 26.5 32.1Sm 10.8 11.0 10.9 10.5 13.2 9.61 30.7 6.31 3.47 6.07 4.08 12.0 6.22 8.47Eu 1.69 1.60 1.10 1.30 1.35 1.18 0.15 0.07 0.08 0.08 0.14 0.12 0.19 0.33Gd 10.1 10.2 10.0 9.79 12.7 9.31 29.1 7.10 4.01 6.74 4.46 12.2 6.87 8.31Tb 1.60 1.61 1.59 1.52 2.02 1.40 5.16 1.41 1.02 1.27 1.25 2.30 1.57 1.37Dy 8.65 9.12 8.93 8.52 11.5 7.72 31.3 9.01 7.57 7.76 8.25 13.7 10.2 7.89Ho 1.73 1.81 1.79 1.70 2.30 1.56 5.99 1.84 1.66 1.62 1.74 2.80 2.13 1.55Er 4.71 5.01 4.94 4.73 6.27 4.45 18.2 5.19 5.40 4.71 5.03 7.72 6.00 4.49Tm 0.72 0.76 0.76 0.71 0.94 0.64 2.94 0.88 0.92 0.80 0.83 1.23 1.01 0.66Yb 4.49 4.80 4.57 4.42 5.67 3.93 19.2 5.47 6.23 5.00 5.17 7.21 6.01 3.93Lu 0.65 0.67 0.64 0.61 0.80 0.56 2.88 0.73 1.00 0.70 0.69 0.98 0.81 0.57Hf 12.0 11.6 7.8 7.6 8.3 7.1 7.5 5.6 5 7.2 5.8 5.1 5.9 5.2Ta 1.8 1.9 1.8 1.7 1.5 1.8 6.2 3.9 3.2 5.5 3.3 2.9 2.7 1.7Th 17 16 18 16 18 19 85 36 30 57 33 28 32 16U 4.4 3.9 3.7 3.6 4.3 3.3 8.4 8.6 4.0 7.0 5.7 7.7 7.8 3.9

A/CKN 0.91 1.03 0.94 1.12 1.13 1.16 1.05 1.08 1.17 1.24 1.14 1.17 1.11 1.10 1.41Eu/Eu* 0.48 0.45 0.31 0.38 0.31 0.38 0.01 0.03 0.05 0.04 0.08 0.03 0.08 0.11K/Rb 183 246 248 253 229 256 63 103 120 66 111 119 120 197

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and Taylor (1976). Based on the observed changein the geochemical characteristics, the lavas weredivided in two sequences. Samples from the lowersequence are trachydacitic to rhyolitic in compo-sition, have silica contents between 66.4 and 76.2wt% (normalized to 100% volatile-free), and rangefrom metaluminous to weakly peraluminous (A/CKN: 0.91^1.16; A/CKN=mol. Al2O3/CaO+K2O+Na2O). Lavas from the upper sequenceare rhyolites signi¢cantly richer in silica (SiO2 :76.0^78.2 wt%) and peraluminous (A/CKN:1.08^1.41).In both sequences, the content of major ele-

ments, except potassium, decreases with increas-ing SiO2. The main di¡erence between the two

sequences is a stronger depletion and relativelyconstant content of TiO2, Fe2O3(tot), CaO,MgO, and P2O5 and slightly higher contents ofK2O, Al2O3 and Na2O at similar silica contentin the upper sequence.The trace element abundances are shown in a

multi-element diagram (Fig. 3), normalized againstthe reference sample RGM-1, a sub-alkaline high-silica rhyolite (Govindaraju, 1984). The samplesfrom the lower sequence are chemically similarto the reference rhyolite, whereas the lavas fromthe upper sequence are enriched in £uorine andsome incompatible lithophile elements (Rb, La,Sm, Yb, Y, Th, Nb, Ta), and depleted in Sr,Ba, Eu, and Zr with respect to RGM-1. In this

Fig. 3. Multi-element diagrams for lavas of the lower and upper sequences normalized to RGM-1 rhyolite from Govindaraju(1984). Shaded area represents the composition of topaz rhyolites from western USA (Christiansen et al., 1986). Numbers in pa-rentheses are silica contents in wt% for each sample, recalculated to 100% on a volatile-free basis.

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regard, upper sequence rhyolites resemble the to-paz rhyolites from the western USA and sharemany characteristics with anorogenic (A-type)granites (Christiansen et al., 1986).Samples from the two sequences show distinct

trace element trends against silica (Fig. 4). For thelower sequence, incompatible elements such asRb, Nb, Ta, and Th remain approximately con-stant at variable silica content, while compatibleelements like Sr and Zr decrease as silica contentincreases. This is the pattern expected for a seriesevolving through fractional crystallization ofphases observed in the samples (e.g. feldspars,zircon).Trace elements in the upper sequence are the

opposite of those expected from fractional crys-tallization processes. The enriched elements Rb,Nb, Ta, and Th decrease and Sr and Ba slightlyincrease with silica content, whereas Zr remainslow and relatively constant. Fluorine in the uppersequence shows a large variation (0.1^0.8 wt%),with the highest contents at lower silica, and apositive correlation with the enriched elements

Rb, Nb, Ta, and Th. As £uorine is partly lostduring devitri¢cation of rhyolitic glasses (Christi-ansen et al., 1986; Webster et al., 1996), the re-ported £uorine contents probably do not re£ectthe original magma composition, but give someinformation about relative abundances.In chondrite-normalized REE diagrams (Fig. 5),

samples from the lower sequence are light REE(LREE) enriched with (La/Yb)n= 8.0^9.3, with arelatively £at pattern for the heavy REE (HREE)and a small negative Eu anomaly (Eu/Eu*= 0.31^0.48; Eu*=Sm/((Sm/Tb)1=3). REE patterns forsamples from the upper sequence are £at with(La/Yb)n= 1.08^3.33 and strongly depleted inEu (Eu/Eu*= 0.01^0.11). Such characteristics aretypical for topaz rhyolites and other high-silicarhyolites. The Eu depletion in upper sequencesamples (expressed as Eu/Eu*) is more pro-nounced as silica decreases (Fig. 5).

4.2. Sr and Nd isotopes

The change in lava chemistry between the lower

Fig. 4. Trace element variation diagrams. Silica contents are recalculated to 100% on a volatile-free basis.

VOLGEO 2485 14-10-02


and upper sequence is also re£ected in pro-nounced changes in isotopic ratios (Tables 3 and4). 87Rb/86Sr and 147Sm/144Nd values are plottedin Fig. 6 against silica content. For the lower se-quence, 87Rb/86Sr ratios increase with silica con-tent (3.5^7.8), whereas 147Sm/144Nd is rather con-stant (0.116^0.124). In samples from the uppersequence, both ratios are higher and displaystrong variation (87Rb/86Sr: 25.3^299.7, 147Sm/144Nd: 0.155^0.193), with a trend to lower valuesat higher silica contents.Such di¡erences are also observed in the initial

87Sr/86Sr values (Fig. 7). Lavas from the lowersequence have 87Sr/86Sr initial ratios that are ap-

proximately constant (0.70644^0.70770), while forthe upper sequence, the initial 87Sr/86Sr ratiosare signi¢cantly higher and variable (0.70812^0.72190). The initial 143Nd/144Nd ratios are similarfor both sequences, with slightly greater variationin samples from the upper sequence (Fig. 7). Ini-tial ONd values for the lower sequence vary from31.2 to 32.1, and for the upper sequence from31.4 to 32.8.Because of their high 87Rb/86Sr ratios, initial

87Sr/86Sr values in samples from the upper se-quence are strongly controlled by the age correc-tion. Ages reported for the MC were determinedby the K/Ar method in sanidine or biotite sepa-

Fig. 5. Chondrite-normalized REE abundances for lavas of the lower (A) and upper (B) sequences. The insets in each diagramshow the variation of the Eu anomalies, expressed as Eu/Eu* (Eu*=Sm/((Sm/Tb)1=3)), with silica content. Silica contents are re-calculated to 100% on a volatile-free basis.

VOLGEO 2485 14-10-02


rates, with a relatively high error (VR1 Ma). Theuncertainties in 87Sr/86Sr initial values were calcu-lated by propagation of individual uncertainties inage, 87Rb/86Sr and 2c of 87Sr/86Sr measurementsby means of a squared partial di¡erential expres-

sion of the decay equation. Values of the calcu-lated uncertainties are listed in Table 3. For thesample with the highest 87Rb/86Sr ratio, a varia-tion of 1 Ma in age and 2% in 87Rb/86Sr changesthe calculated initial 87Sr/86Sr ratios by 0.00262.

Table 3Whole rock Rb^Sr isotopic compositions of lavas from the MC

Rb Sr 87Rb/86Sr 87Sr/86SrR 1c n 87Sr/86Sr(i) R u(ppm) (ppm)

Lower sequenceRIO-45 228 144 4.58 0.708323R 36 58 0.70644R 4RIO-29 168 139 3.49 0.708841R 45 56 0.70740R 4RIO-22 196 73 7.79 0.710905R 45 57 0.70770R 9RIO-18 180 101 5.16 0.709414R 39 52 0.70729R 7RIO-47 178 71 7.23 0.709867R 48 57 0.70689R 9RIO-24 166 99 4.85 0.709224R 44 59 0.70723R 7

Upper sequenceRIO-16 705 6.9 299.7 0.841091R 40 54 0.72190R 262RIO-12 443 11.7 110.3 0.762045R 41 56 0.71661R 96RIO-9 344 10.2 98.0 0.760363R 42 58 0.71998R 86RIO-44 609 8.0 222.9 0.799892R 36 55 0.71125R 195RIO-41 379 8.2 134.9 0.764112R 47 58 0.70856R 117RIO-46 352 9.5 107.5 0.754400R 48 59 0.71013R 94RIO-43 357 13.4 77.6 0.740087R 37 47 0.70812R 68RIO-7 207 23.7 25.3 0.719511R 41 56 0.70909R 24

Rb and Sr concentrations were determined by isotope dilution techniques. 1c : one standard deviation of repeated measurements,represents internal relative reproducibility in the last two ¢gures; n : number of measurements; u : uncertainty in calculated initial87Sr/86Sr in the last ¢gures.

Table 4Whole rock Sm^Nd isotopic compositions and of lavas from the MC

Sm Nd 147Sm/144Nd 143Nd/144NdR1c n 143Nd/144Nd(i) ONdðiÞ(ppm) (ppm)

Lower sequenceRIO-45 9 46 0.124 0.512517R 18 57 0.51249 32.09RIO-29 11 58 0.119 0.512535R 33 57 0.51251 31.72RIO-22 13 64 0.119 0.512520R 29 59 0.51250 32.01RIO-18 12 61 0.116 0.512525R 20 58 0.51250 31.91RIO-47 12 62 0.115 0.512560R 18 57 0.51254 31.22RIO-24 11 56 0.118 0.512528R 19 58 0.51251 31.85

Upper sequenceRIO-16 40 132 0.185 0.512510R 21 59 0.51247 32.45RIO-12 8 26 0.177 0.512490R 18 57 0.51246 32.82RIO-9 5 14 0.193 0.512567R 23 57 0.51253 31.37RIO-44 5 20 0.162 0.512488R 17 59 0.51246 32.80RIO-41 3 12 0.182 0.512497R 26 59 0.51246 32.70RIO-46 11 39 0.166 0.512547R 17 59 0.51252 31.66RIO-43 6 23 0.155 0.512506R 19 57 0.51248 32.42RIO-7 12 44 0.161 0.512540R 14 57 0.51251 31.78

Sm and Nd concentrations were determined by isotope dilution techniques. 1c : one standard deviation of repeated measure-ments, represents internal relative reproducibility in the last two ¢gures; n : number of measurements.

VOLGEO 2485 14-10-02


Although high, this uncertainty is much less thanthe total range of initial 87Sr/86Sr ratios observedin the samples (Fig. 7), and should not consider-ably a¡ect the interpretation of the data.

4.3. Alteration

Element mobilization due to devitri¢cation andpost-magmatic alteration is often observed inrhyolitic rocks. Fluorine and the large-ion litho-phile elements (LILE) Rb, Sr and Ba are moresusceptible to be mobilized, while the high ¢eldstrength elements Ti, P, Th, Zr, Nb, Hf, Y andthe REE are commonly immobile (Webster andDu⁄eld, 1991; Riley et al., 2001). Within theLILE, alteration causes in most cases an enrich-ment in Rb and depletion in Sr (Matheney et al.,1990; Riley et al., 2001).The strong Rb enrichment coupled to Sr deple-

tion (Fig. 8), and the high and variable 87Sr/86Srratios in the upper sequence could be interpretedas a result of post-emplacement processes. Never-theless, the immobile elements Nb, Th, and Tahave a similar behavior as the mobile elementRb, and also correlate negatively with Sr as exem-pli¢ed by Ta in Fig. 8. This similarity in the be-havior of mobile and immobile elements arguesagainst post-emplacement mobilization of Rb inthese lavas. Besides, alteration commonly causes87Sr to be preferentially lost relative to Rb, whichwould give low age-corrected Sr isotopic ratios(Matheney et al., 1990), which is opposite to theobserved high 87Sr/86Sr initial ratios in upper se-quence rhyolites.

5. Origin of rhyolites

The geochemical and isotopic data indicate animportant change in the composition of magmasgenerated in the MC during the Oligocene. Thischange is accompanied by changes in the spatialdistribution and style of magma emplacement.The di¡erences between lower and upper sequencesuggest changes in the processes of genesis, ascentand emplacement of the magmas. In the next sec-tions we discuss possible petrogenetic processes

Fig. 6. Variation in 87Rb/86Sr and 147Sm/144Nd ratios withsilica content for lavas of the lower (A) and upper (B) se-quences. The inset in diagram (A) shows the 87Rb/86Sr ratiosof samples from the lower sequence. Silica contents are recal-culated to 100% on a volatile-free basis. Symbols as in Fig.4.

Fig. 7. (A) Initial ONd vs. initial 87Sr/86Sr for samples of theMC. Uncertainties in initial 87Sr/86Sr calculation are shownas bars, when they exceed symbol size. The composition oflower crustal xenoliths is shown in the closed ¢eld (crosses:ma¢c granulites; squares: intermediate granulites; circles:metasedimentary rocks). Xenolith data from Schaaf et al.(1994) and Ruiz et al. (1988a,b). Symbols of MC lavas as inFig. 4.

VOLGEO 2485 14-10-02


and their relation to the structural history of theMC.

5.1. Lower sequence

Lavas from the lower sequence are geochemi-cally similar to high-K rhyolitic rocks of similarage in the eastern Sierra Madre Occidental (e.g.Cameron et al., 1980). The petrogenesis and inparticular the nature of melt sources of theserocks are controversial. On the basis of geochem-ical and isotopic studies, some authors propose amantle origin for the melts, followed by evolutionthrough fractional crystallization and assimilationof crustal material (e.g. Cameron et al., 1986;Wark, 1991; Smith et al., 1996). By contrast,Ruiz et al. (1988a) proposed an origin throughpartial melting of lower crustal material withoutany contribution of the mantle.The negative ONd values and high Sr isotopic

ratios indicate a contribution of crustal materialfor both MC sequences. Sr^Nd isotopic ratios forthe lower sequence lie within the ¢eld of MC low-er crustal compositions (Fig. 7) as de¢ned bygranulitic xenoliths (Ruiz et al., 1988a,b; Schaafet al., 1994). The coincidence in the isotopic com-positions of rhyolites and lower crustal xenoliths

was the main argument of Ruiz et al. (1988a,b) topropose a crustal origin of the melts. However,isotopic similarities alone do not preclude the pos-sibility of a large mantle component in these meltsbecause the isotopic compositions could be pro-duced through assimilation of a crustal compo-nent into a mantle-derived magma. Due to thewide variation in the composition of the crustalxenoliths, it is not possible to estimate the pro-portion of crustal material involved in any crust^mantle mix, and simple mixing calculations indi-cate crustal contributions between 10 and 70%,depending upon the crustal composition consid-ered.Although geochemical data for intermediate

lavas from the MC are not yet available, the pres-ence of compositions such as andesites to rhyo-dacites in the lower sequence would support theidea of an evolution of mantle-derived magmasthrough fractional crystallization. At the felsicend of the compositional range (which we havestudied here), fractional crystallization is sup-ported by the progressive depletion and large var-iation in compatible elements (e.g. Sr, Zr, Eu) andminor variation in incompatible elements (Rb, Ta,Nb, Th) with increasing silica content (Fig. 4).Fractionation of feldspar and other phases, likebiotite and zircon, could account for the observedelement variations. The constant isotopic ratios inthe rhyolitic lavas from the lower sequence arecharacteristic of such a process.

5.2. Upper sequence

The geochemical characteristics of the topazrhyolites and A-type granites have been inter-preted as the result of low degree partial meltingof residual lower crustal granulites (Christiansenet al., 1983, 1986). Although not all upper se-quence rhyolites contain topaz, their chemicalsimilarity to topaz rhyolites and the presence ofPrecambrian granulitic lower crust in the MC sug-gest a similar origin is at least possible. The stron-ger argument for this hypothesis is the behaviorof trace elements, which corresponds to that ex-pected in magmas generated by partial meltingprocesses, as described below.The low and relatively constant Sr and Ba con-

Fig. 8. Variations of Rb (A) and Sr (B) with Ta content inlavas from the MC. Symbols as in Fig. 4.

VOLGEO 2485 14-10-02


tents (Fig. 4), the pronounced negative Eu anom-aly, which correlates positively with silica content(Fig. 5), and the positive correlation of K/Rb withsilica (Fig. 9), are indicative of feldspar being re-sidual to melting. Such feldspar-bu¡ered melts areconsidered characteristic of partial melting pro-cesses in the lower crust. The low and relativelyconstant Zr contents (Fig. 4) indicate bu¡ering ofthis element by residual zircon during partialmelting. As Zr is an essential structural constitu-ent (ESC) in zircon, its abundance in anatecticmelts will be mainly controlled by the solubilityof zircon, and remain constant at saturation levelas long as some zircon is present in the residue.The behavior of the incompatible elements (Rb,

Nb, Ta, Th, F), which negatively correlate withsilica, and show strong abundance variation(Fig. 4), is also typical for partial melting process-es. The fact that the enriched elements negativelycorrelate with Sr (e.g. Ta vs. Sr; Fig. 8) and withEu/Eu* points to variable degrees of non-modal

partial melting processes with variable contribu-tions of the involved phases as melting advances.Given the strong enrichment observed in incom-patible elements, the melting process is mostprobably fractional.In previous studies, water-undersaturated dehy-

dration melting of crustal rocks containing £uo-rine-rich biotite has been proposed as a mecha-nism to produce F-rich granitic liquids withcompositions within the range of topaz rhyolitesand A-type granites (Christiansen et al., 1986;Skjerlie and Johnston, 1993). Hydrous mineralslike biotite and amphibole, present in high-grademetamorphic rocks, are £uorine- and Rb-rich,and their preferential breakdown at low degreesof partial melting would produce melts enrichedin those elements. The Rb and F enrichment ofupper sequence rhyolites, coupled to pronouncednegative Eu anomaly and low Sr (Figs. 4, 5 and 8)at lower silica content, would indicate a preferen-tial contribution from biotite to the melt and feld-spars acting mainly as residual phases in the earlystages of melting. At higher degrees of partialmelting, feldspars in the granulitic source increas-ingly contribute to the melt leading to lower Fand Rb contents, less pronounced Eu anomaliesand higher silica and Sr contents.A further argument that supports an origin by

crustal partial melting is the scarcity of ma¢c tointermediate rocks in the upper sequence. Almostthe whole sequence is constituted by high-silicarhyolites and ignimbrites, and only a thin lava£ow of basaltic composition has been observedintercalated in the uppermost ignimbrite deposit,which represents the last volcanic pulse in theOligocene sequence.Although the behavior of trace elements in the

upper sequence supports an origin by crustal par-tial melting processes, the isotopic ratios are some-how di⁄cult to explain. The upper sequence sam-ples plot outside the lower crustal trend (Fig. 7),due to the higher initial 87Sr/86Sr ratios relative tothe ONd values. Therefore, the 87Sr/86Sr ratios ofthe rhyolites could not originate by equilibriumpartial melting processes of a source similar tothe lower crustal xenoliths or by simple mixingof lower crust and mantle components. This fea-ture of high and variable initial 87Sr/86Sr ratios

Fig. 9. Variation in initial 87Sr/86Sr (A), and K/Rb (B) withsilica content. K/Rb calculated on atomic basis; SiO2 recal-culated to 100% on a volatile-free basis. Symbols as in Fig. 4.

VOLGEO 2485 14-10-02


and relatively constant initial ONd values has beenreported for other localities of silica- and £uorine-rich rhyolites and granites (e.g. Reece et al., 1990;Bikerman et al., 1992; Liu et al., 1999), and ap-parently represents a characteristic of such rocks.The high and variable 87Sr/86Sr ratios in rocks

with chemical characteristics resembling those ofthe upper sequence were explained by minor de-gree of late-stage upper crustal assimilation of ra-diogenic Sr-rich wall and roof rocks that had thegreatest e¡ect on the Sr-poor high-silica rhyolites.In that interpretation, the constant Nd isotopicratios result from small or non-existent contrastsbetween magma and wall rocks (Christiansen etal., 1986; Reece et al., 1990).Crustal contamination should produce a posi-

tive correlation of 87Sr/86Sr values and a negativecorrelation of K/Rb with SiO2. By contrast, frac-tional crystallization processes do not signi¢cantlyalter K/Rb and 87Sr/86Sr ratios. In the upper se-quence lavas, K/Rb ratios show a broad range(63^197), with a trend to increasing values asSiO2 increases, whereas 87Sr/86Sr correlates nega-tively with SiO2 (Fig. 9). These observations donot support assimilation of radiogenic, silica-richcrust or fractional crystallization processes.Thus, the model of upper crustal assimilation

could explain the association of high 87Sr/86Sr atlower Sr contents in the MC upper sequence onlyif assimilation of upper crust material a¡ectedalone the Sr isotopic ratios and did not signi¢-cantly alter other elements (K/Rb, SiO2). If thisis the case, a problem arises to explain the corre-lations between 87Sr/86Sr and SiO2 (Fig. 9).As an alternative explanation for the Sr iso-

topic composition of upper sequence lavas, thepossibility of crustal partial melting occurringunder disequilibrium conditions can be consid-ered. Recent experimental and ¢eld studies indi-cate that the isotopic compositions of crustalmelts do not necessary re£ect the bulk composi-tion of source rocks, and instead may trace therelative contribution of individual mineral phasesto the melt during disequilibrium non-modal par-tial melting (Hammouda et al., 1996; Knesel andDavidson, 1999). This statement is especially im-portant when the source rocks are old and theywere not re-equilibrated by a thermal event, so

that the Sr isotopic ratios in the individual min-erals evolve to very di¡erent values depending onthe Rb content.The trace element behavior in upper sequence

rhyolites indicates the involvement of biotite andK-feldspar. Hydrous minerals like biotite, presentin high-grade metamorphic rocks, show high87Rb/86Sr and Sr isotopic ratios. Experimentalstudies (Knesel and Davidson, 1999; Davies andTomasini, 2000) demonstrate that preferentialbreakdown of such mineral phases at low degreesof partial melting produces melts with high 87Rb/86Sr and 87Sr/86Sr ratios at low silica content. Athigher degrees of partial melting, feldspars in thegranulitic source increasingly contribute to themelt, lowering its 87Rb/86Sr and 87Sr/86Sr ratiosand increasing SiO2.This inferred melt evolution coincides with the

geochemical features observed in the upper se-quence rhyolites. If isotopic disequilibrium oc-curred during partial melting, melts generated atlow degrees of partial melting, with high biotitecontribution, should have high 87Sr/86Sr ratios,and are those most enriched in incompatible ele-ments and strongly depleted in Sr, Ba and Eu. Asmelting advances, the higher feldspar contributionto the melt leads to lower 87Sr/86Sr ratios andhigher Sr, Ba and Eu contents. This evolutionalso accounts for the negative correlation between87Sr/86Sr ratios and silica. In this model, the rela-tively constant ONd values could result from littledi¡erences in Sm/Nd between individual mineralphases in the protolith, and from the long half-lifeof 147Sm (106 Byr).Partial melting in the lower crust under isotope

disequilibrium conditions is still a matter of dis-cussion. One of the main arguments against thismodel is that source rocks should re-equilibrateisotopically as they approach melting tempera-tures. Isotope equilibrium between individualmineral during prograde metamorphism will de-pend on the rate of elemental di¡usion betweenmineral phases, and on the time elapsed betweenthe opening of system to di¡usional exchange andmelting onset (Tomasini and Davies, 1997; Ham-mouda et al., 1996). Di¡usion calculations at tem-peratures typical of anatexis indicate that feldspargrains will achieve Sr isotopic equilibrium in times

VOLGEO 2485 14-10-02


between a few million and in excess of 100 Madepending on grain size (0.1 to s 1 cm) (Toma-sini and Davies, 1997). Besides, the absence of anintergranular £uid and a rapid heating could havethe e¡ect of increase the temperature required toopen the system for Sr di¡usion above 650‡C(Hammouda et al., 1996). Thus, isotopic hetero-geneities in source rocks could be preserved ifheating under £uid-absent conditions increasedrapidly the temperature of rocks, from values be-low isotopic closing temperatures to the temper-atures of melting, not allowing re-equilibrationbetween mineral phases.For the MC, there is a poor knowledge of the

physical conditions and mineral composition oflower crustal rocks, but it seems probably thatboth conditions are ful¢lled: a rapid heating aswill be discussed later, and the presence of drygranulitic rocks, suggesting the possibility of iso-tope disequilibrium being preserved in sourcerocks.With regard to trace elements, Davies and

Tomasini (2000) conclude that, although isotopedisequilibrium can be preserved, near equilibriumelemental partition coe⁄cients operate for anydisequilibrium melting model. Other studies sug-gest that due to slow intracrystalline di¡usion andhigh melting rates, e¡ective mineral-melt partitioncoe⁄cients at the temperatures of crustal meltingare close to unity, implying that crustal melts areusually generated at chemical disequilibrium (Bea,1996).In the upper sequence rhyolites, possible evi-

dences of elemental disequilibrium during meltingare provided by ESCs of accessory mineral phases(Zr in zircon, P in apatite, and LREE in mona-cite), whose abundance in anatectic melts ismainly controlled by the solubility and availabil-ity of the mineral. Saturation concentrations ofthese elements in peraluminous silica-rich meltsat 800‡C, calculated from empirical equations(Watson and Harrison, 1983; Harrison and Wat-son, 1984; Watson, 1987), are Zr: 156 ppm,4LREE (La^Gd): 201 ppm, P: 176 ppm. Thecontents of those elements in upper sequencerhyolites are mostly below these saturation values(Fig. 10), with the exception of 4LREE in onesample. Subsaturation in these elements can be

interpreted as a result of fast melt segregationrates that prevented equilibration (Barbero etal., 1995). It is also noteworthy that those ele-ments show a more or less de¢ned trend to lowercontents as initial Sr isotopic ratios increase,which would be consistent with the isotope dis-equilibrium model discussed earlier.The upper sequence rhyolites show variable

REE enrichment and £at patterns (low LREE/HREE) in chondrite-normalized diagrams (Fig. 5),and high Sm/Nd ratios, which are features fre-quently observed in felsic rocks (Miller and Mit-tlefehldt, 1982, and references therein). In crus-tally derived melts, such characteristics havebeen ascribed to variable dissolution of LREE-rich accessory phases present in the protolith(Bea, 1996; Ayers and Harris, 1997), which couldaccount for the stronger variation for most uppersequence samples in LREE abundances compared

Fig. 10. Contents of elements mainly controlled by accessoryphases against initial 87Sr/86Sr. 4LREE is the sum of LREEcontents from La to Gd. The stippled line represents satura-tion values of the respective elements in the melt.

VOLGEO 2485 14-10-02


with HREE abundances (Fig. 5). The low LREE/HREE in upper sequence rhyolites can then berelated to the low solubility of LREE-rich acces-sory phases in dry peraluminous melts, and todisequilibrium promoted by water-undersaturatedmelting and fast extraction of melt from residue(Watt and Harley, 1993).Furthermore, calculations of Nd crustal resi-

dence ages (TDM) for the upper sequence yieldestimates between 1.7 and 4.3 Ga. The lower crustin the MC is of Precambrian age and was a¡ectedby the Grenville Orogeny. A Sm^Nd isochron ageof 1248R 69 Ma, determined for lower crustal xe-noliths from the MC, is interpreted as intrusionage of the magmatic precursor (Schaaf et al.,1994). Variable and unrealistic TDM values as ob-tained for the upper sequence rhyolites are ex-pected from anatectic melts generated under dis-equilibrium conditions (Ayers and Harris, 1997).In this discussion, we present the available evi-

dence that supports an origin of upper sequencerhyolites through non-modal melting of granuliticlower crustal rocks by fuid-absent dehydrationmelting at chemical and probably isotope dis-equilibrium conditions. Numerical modelling ofdisequilibrium processes in the MC is limited bythe poor knowledge of lower crustal composi-tions, as individual mineral phase compositionsare required to obtain model melt compositions.

6. Melting, segregation and ascent

Available ages indicate that the change in mag-ma composition from the lower to the upper se-quence and the emplacement of the voluminousupper sequence took place over a short interval(Table 1). Although high precision age determina-tions are needed to better constrain the event du-ration, the change in magma generation processesoccurred in 6 1 Ma, suggesting an event of rapidheating of source rocks which led to high meltingrates. Such fast crustal melting processes couldoccur when mantle-derived basalts intrude be-neath or into continental crust; time scales of102^103 yr for generation of crustal melts by ba-salt underplating were suggested by Huppert andSparks (1988).

We suggest that the well-documented Oligocenecrustal extension in the MC (Nieto-Samaniego etal., 1999) allowed basaltic melts to invade thecrust, which acted as heat source for crustal melt-ing. In the MC, granulite facies metamorphism ofOligocene age has been documented for the base ofthe crust (Hayob et al., 1989), and this would beassociated with the heating event and melting pro-cess that generated the upper sequence rhyolites. Inother localities of the eastern SMOVP to the north,where rhyolites of similar geochemistry and ageoccur, U^Pb zircon ages between 25 and 37 Mareveal Oligocene granulite facies metamorphismin the lower crust (Rudnick and Cameron, 1991).Melting occurring at high rates causes a rapid

increase in pore £uid pressure that reduces rockstrength and promotes embrittlement. Such con-ditions enhance rock permeability and rapid meltsegregation at low degrees of melting before equi-librium is attained (Petford, 1995; Knesel andDavidson, 1999). The same e¡ect of enhancedpermeability would be produced by dehydrationmelting of the granulites under water-undersatu-rated conditions (Sawyer, 1994; Rushmer, 1996).Rapid heating could also account for the isotopicheterogeneities in the source rocks. This couldoccur if the time elapsed between the opening ofthe system to isotopic exchange and the beginningof melting is not enough for the system to re-equilibrate.Another factor promoting rapid melt segrega-

tion and ascent is low melt viscosity (McKenzie,1984). The typically high viscosity of graniticmelts can be signi¢cantly reduced by the presenceof £uorine, which depolymerizes the melt andlowers viscosity (Dingwell et al., 1985). Fluorinecontents in our rhyolite samples vary between 0.1and 0.8 wt%, and the presence of topaz in somedomes and £ows indicates that high £uorine con-tents are characteristic of the upper sequence.Fluorine has also the e¡ect of increasing water

solubility and reducing the solidus temperature inmelts (Koster van Groos and Wyllie, 1968; Man-ning, 1981). The predominance of e¡usive magmaemplacement could be related to such e¡ects, byallowing magmas with relatively low viscosity toreach the surface before they crystallize and/orwater exsolves.

VOLGEO 2485 14-10-02


Upper sequence rhyolite emplacement in theMC was synchronous with crustal deformationat high strain rates (Nieto-Samaniego et al.,1999), which would favor high melt segregationrates. High extension rates would also promoterapid magma ascent and probably inhibit meltstagnation in crustal magma chambers. Extensionin the MC began before the emplacement of theupper sequence (Trista¤n-Gonza¤lez, 1986), but arelatively short phase of accelerated deformation,indicated by the stratigraphy, is supported by thegeochemical changes in the magmas and theemplacement of large lava volumes, mainly asdomes, in close relation to major graben faults.The conditions of a short-lived event of melt

generation in an extensional stress ¢eld associatedto rapid heating of source rocks, high meltingrates, and fast melt segregation, support thepossibility of an origin through disequilibriummelting of the upper sequence magmas. The pres-ervation of geochemical disequilibrium features re-quires a relatively rapid segregation of melt fromresidue with respect to di¡usion rates, in order toprevent homogenization of the successive meltbatches and equilibration between solid and melt.

7. Conclusions

Crustal extension processes at high strain rateduring Oligocene caused a change in the mecha-nisms of magma generation, segregation and as-cent in the MC. This change is re£ected in thegeochemical and isotopic composition, in the styleof emplacement, and in the spatial distribution ofthe emplaced magmas.Studied lower sequence lavas are geochemically

similar to high-K rhyolitic rocks of similar age inthe eastern SMOVP. The geochemical and iso-topic characteristics indicate an evolution of man-tle-derived magmas through fractional crystal-lization and variable contributions of crustalmaterial. Trace element behavior in upper se-quence rhyolites supports an origin throughnon-modal crustal melting of granulitic lowercrustal rocks by fuid-absent dehydration melting.Some evidences indicate processes of chemicaland probably isotope disequilibrium during parti-

al melting, which could have been originated byfast segregation rates of melt from residuum pro-moted by rapid extension, high melting rates, de-hydration melting reactions and the lower viscos-ity of the £uorine-rich rhyolitic magmas.The predominance of e¡usive magma emplace-

ment in upper sequence rhyolites was related tothe increased water solubility and reduction ofsolidus temperatures in £uorine-rich melts. Fur-thermore, the reduced viscosity of the magmasand the extensional stress ¢eld allowed the mag-mas to ascent rapidly to the surface and probablyinhibited its stagnation in magma chambers. Theemplacement of magmas was controlled by exten-sional features, determining the spatial distribu-tion of the rhyolitic lava £ows and domes alongfaults.

Acknowledgements

This work was supported by CONACyT 33087-T research grant. We thank Gabriela Sol|¤s, Ma.del Sol Herna¤ndez, Juan Julio Morales and Teo-doro Herna¤ndez for isotopic measurements, aswell as Ru¢no Lozano and Patricia Giro¤n forX-ray £uorescence analysis. Roland Maas andIan Nicholls provided helpful comments to im-prove this manuscript.

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