Journal of South American Earth Sciences 13 (2000) 355±375 www.elsevier.nl/locate/jsames

Geologic and geochronologic data from the Guerrero terrane in the Tejupilco area, southern Mexico: new constraints on its tectonic interpretation

M. ElõÂas-Herrera*, J.L. SaÂnchez-Zavala, C. Macias-Romo

Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Ciudad Universitaria, DelegacioÂn CoyoacaÂn, Mexico DF 04510, Mexico

Abstract The eastern part of the Guerrero terrane contains two tectonically juxtaposed metavolcanic-sedimentary sequences with island arc af®nities: the lower, Tejupilco metamorphic suite, is intensely deformed with greenschist facies metamorphism; the upper, Arcelia-Palmar Chico group, is mildly to moderately deformed with prehnite-pumpellyite facies metamorphism. A U±Pb zircon age of 186 Ma for the Tizapa metagranite, and Pb/Pb isotopic model ages of 227 and 188 Ma for the conformable syngenetic Tizapa massive sul®de deposit, suggest a Late Triassic±Early Jurassic age for the Tejupilco metamorphic suite. 40Ar/39Ar and K±Ar age determinations of metamorphic minerals from different units of the Tejupilco metamorphic suite in the Tejupilco area date a local early Eocene thermal event related to the emplacement of the undeformed Temascaltepec granite. The regional metamorphism remains to be dated. 40Ar/39Ar ages of 103 and 93 Ma for submarine volcanics support an Albian±Cenomanian age for the Arcelia-Palmar Chico group, although it may extend to the Berriasian. U±Pb isotopic analyses of zircon from the Tizapa metagranite, together with Nd isotopic data, reveal inherited Precambrian zircon components within units of the Tejupilco metamorphic suite, precluding the generation of Tejupilco metamorphic suite magmas from mantle- or oceanic lithosphere-derived melts, as was previously considered to be the case. Instead, these data, together with high-grade gneiss xenoliths with Grenvillian Nd isotopic af®nity in Oligocene subvolcanics, indicate the presence of pre-Mesozoic continental crust beneath at least the eastern part of the Guerrero terrane. As a Late Triassic±Early Jurassic basement unit in the eastern part of the Guerrero terrane, the Tejupilco metamorphic suite may therefore represent an evolved volcanic arc developed on old crust with assimilated craton-derived sediment. This would imply a tectonic cycle of deformation, metamorphism and erosion during the Middle±early Late Jurassic that was probably related to the accretion and consolidation of part of the Guerrero terrane into the AcatlaÂn Complex, the pre-Mississippian poly- deformed and metamorphosed basement of the Mixteco terrane. q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Guerrero terrane; Tectonic interpretation; Arcelia-Palmer Chico group

1. Introduction and geologic setting and southern Mexico. The same units were subsequently assigned to the Upper Jurassic±Lower Cretaceous Guerrero Greenschist and subgreenschist facies volcanic-sedimen- terrane by Campa and Coney (1983), a composite terrane tary metamorphic rocks are exposed in the Balsas River made up of the Teloloapan-Ixtapan, Zihuatanejo and basin region of southern Mexico (Fig. 1). Because of uncer- Huetamo subterranes. Relations among these subterranes tainties in their geochronologic, stratigraphic and structural have not yet been established; however, according to relations, these rocks were informally grouped into the Campa and Coney (1983), the eastern margin of the Guerrero Tierra Caliente complex (Ortega-GutieÂrrez, 1981). The terrane is thrust against the Cretaceous calcareous Morelos- complex was interpreted to record the tectonic juxtaposition Guerrero platform of the Mixteco terrane (Fig. 1). of two separate terranes, one belonging to an oceanic-trench Additional subdivisions based on geochemical, Sm±Nd setting and the other to a marginal sea-island arc environ- isotopic and limited paleontologic data have also been ment (Ortega-GutieÂrrez, 1981) in a similar fashion to that proposed for the Guerrero terrane (Talavera-Mendoza et proposed previously by de Cserna (1971) for the ªUpper al., 1993; Centeno-GarcõÂa et al., 1993a). The Mesozoic Triassic eugeosynclinal assemblageº in northern, central submarine volcanic and sedimentary sequences of island arc af®nity in southwestern Mexico were regrouped into the Hauterivian (?)±Aptian Teloloapan subterrane, the * Corresponding author. Tel.: 152-5-622-4288/4290; fax: 152-5-550- 6644. Albian±Cenomanian Arcelia-Palmar Chico subterrane, E-mail address: [email protected] (M. ElõÂas-Herrera). and the Ladinian to Albian Zihuatanejo-Huetamo subterrane

0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0895-9811(00)00029-8 356 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375

Fig. 1. Simpli®ed geologic map of southwestern Mexico showing the location of the Tejupilco area and the Guerrero (GT), Mixteco (MT), and Xolapa (XT) terrane boundaries: (1) terrane boundary between Guerrero and Mixteco terranes after Campa and Coney (1983); (2) terrane boundary between Guerrero and Mixteco terranes after Sedlock et al. (1993). Division of the Guerrero terrane into subterranes (after Coney and Campa-Uranga, 1987; Talavera-Mendoza et al., 1995): A ˆ Arcelia, Ar ˆ Arteaga, H ˆ Huetamo, I-Z ˆ Ixtapa-Zihuatanejo, LO ˆ Las Ollas locality, P ˆ Palmar Chico, PO ˆ Placeres del Oro, Pp ˆ Pepechuca locality, SP ˆ San Pedro LimoÂn, Tl ˆ Teloloapan, Tj ˆ Tejupilco, Tx ˆ Taxco, VB ˆ Valle de Bravo, Zt ˆ ZitaÂcuaro. The Index Map shows the Oligocene volcanics of the La Sierra Madre Occidental (SMOCC), the Miocene-Quaternary volcanics of the Transmexican volcanic belt (TMBV), and the terrane bondaries of the southern Mexico: (1) Guerrero terrane (NaÂhuatl, Sedlock et al., 1993), (2) Xolapa terrane (Chatino, Sedlock et al., 1993), (3) Mixteco terrane, (4) Zapoteco terrane.

(Fig. 1), with inferred or observed tectonic relationships basin located in the back-arc position (Huetamo sequence) between them (Talavera-Mendoza et al., 1993; Centeno- which also formed on previously deformed ocean-¯oor GarcõÂa et al., 1993a). These subterranes are suggested to (Placeres complex), and a subduction complex (Las Ollas form a Mesozoic multi-arc system accreted to the North complex) (Talavera-Mendoza et al., 1993; Centeno-GarcõÂa American plate. Thus, the Teloloapan subterrane, the east- et al., 1993a). The paleogeographic relations and tectonic ernmost part of the Guerrero terrane (Fig. 1), is thought to evolution of these subterranes, and the relationships with the correspond to an evolved intra-oceanic island-arc system Mixteco terrane are obscure. (Talavera-Mendoza, 1993; Talavera-Mendoza et al., 1995) The Mixteco terrane comprises the AcatlaÂn Complex and and the Arcelia-Palmar Chico subterrane is considered to its cover. The AcatlaÂn Complex is a pre-Mississippian represent a primitive intra-oceanic island-arc (Ortiz- poly-deformed and metamorphosed basement unit HernaÂndez et al., 1991; Talavera-Mendoza, 1993) and an (Ortega-GutieÂrrez, 1978, 1981; Sedlock et al., 1993; oceanic back-arc basin (Talavera-Mendoza et al., 1993; Ortega-GutieÂrrez et al., 1994). The metamorphic basement Centeno-GarcõÂa et al., 1993a). The Zihuatanejo-Huetamo rocks are interpreted to record early Paleozoic subduction subterrane has been interpreted as a complex assemblage and obduction of an ophiolite onto a subduction complex, made up of an island-arc (Zihuatenejo sequence) developed early to middle Paleozoic collision of the oceanic rocks with on previously deformed oceanic crust (Arteaga complex), a continental crust, and middle to late Paleozoic deformation M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 357

Fig. 2. Simpli®ed geologic map of the Tejupilco area showing the localities that were sampled for geochronology. In phyllite and muscovite schist the NW±N- trending regional foliation is generally subhorizontal (5±308) with W±SW- or E±NE-dipping directions. and metamorphism (Sedlock et al., 1993; Ortega-GutieÂrrez 1994) and, according to its geologic relationships, is pre- et al., 1994). The AcatlaÂn Complex is unconformably Late Jurassic in age (Pantoja-Alor, 1990). The Arteaga and covered by unmetamorphosed marine siliciclastic and Placeres complexes are pre-Upper Jurassic basement units carbonate strata of Mississippian±Permian ages, ignimbrite of the Guerrero terrane (Centeno-GarcõÂa, 1994). In Fig. 1, and andesite of apparent Triassic, epicontinental and marine the Placeres complex is not differentiated from the Artega strata of Middle Jurassic and Cretaceous ages, and Cenozoic complex. continental clastic and volcanic rocks (Corona-Esquivel, The existence of deformed Triassic basement introduces 1981/3; VillasenÄor-MartõÂnez, 1987; GonzaÂlez-Arreola et al., a new dimension to those tectonic interpretations (Campa, 1994). The westernmost exposure of the AcatlaÂn Complex is 1978; Campa and RamõÂrez, 1979; Ortiz-HernaÂndez et al., thrust westward over the Cretaceous calcareous Morelos- 1991; Tardy et al., 1991, 1992, 1994; Lapierre et al., 1992) Guerrero platform along the Papalutla fault (de Cserna et for the Guerrero terrane, in which the Upper Jurassic± al., 1980), which has been suggested as the boundary Lower Cretaceous island-arc assemblages were assumed between the Mixteco and NaÂhuatl terranes (Sedlock et al., to be constructed over oceanic lithosphere. However, it is 1993; Ortega-GutieÂrrez et al., 1994) (Fig. 1). The NaÂhuatl consistent with those interpretations that propose a pre- terrane is thus equivalent to the southern part of the Upper Jurassic metamorphic basement in the Taxco- Guerrero terrane of Campa and Coney (1983) plus the Tejupilco region (Fries, 1960; de Cserna and Fries, Morelos-Guerrero platform (Fig. 1). 1981; de Cserna, 1982; ElõÂas-Herrera and SaÂnchez-Zavala, For our purposes, it is convenient to emphasize the base- 1990; ElõÂas-Herrera, 1993). In this paper, new geologic and ment units: the Arteaga and Placeres complexes of the geochronologic data support the presence of an intensely Zihuatanejo-Huetamo subterrane. The Arteaga complex deformed pre-Upper Jurassic basement in the easternmost consists of deep-marine sediments and basaltic pillow part of the Guerrero terrane. lavas with a poorly constrained Triassic age based on Ladinian±Carnian radiolaria in cherts (Campa et al., 1982). The Placeres complex (RõÂo Placeres Formation of 2. Geology of the Tejupilco area Pantoja-Alor, 1990) is lithologically similar to the Arteaga complex (Centeno-GarcõÂa et al., 1993a; Centeno-GarcõÂa, The Tejupilco area is located near the apparent eastern 358 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375

Table 1 Table 1 (continued) Sample descriptions and locations Mineral abbreviations: Ab ˆ albite, Apy ˆ arsenopyrite, Aug ˆ augite, Sample Description Location Bt ˆ biotite, Cal ˆ calcite, Ccp ˆ chalcopyrite, Cchl ˆ clinochlore, Chl ˆ chlorite, Clpx ˆ pyroxene, Crd ˆ cordierite, Ep ˆ epidote, Fre ˆ 04-1 Calc-muscovite schist. Cal, Qtz, Cerro de la Pila, freibergite, Gn ˆ galena, Hbl ˆ hornblende, Hem ˆ haematite, Kfs ˆ K- Ms and Hem after Py. Lower north slope, 5.5 km feldspar, Mag ˆ magnetite, Mc ˆ microcline, Ms ˆ muscovite, Na-Pl ˆ plate of overthrust; SSE from sodic plagioclase, Or ˆ orthoclase, Phl ˆ phlogopite, Pl ˆ plagioclase, phyllonitic(?)schist; small Zacazonapan. Lat. Pmp ˆ pumpellyite, Prh ˆ prehnite, Py ˆ pyrite, Qtz ˆ quartz, Rt ˆ rutile, lenticular body overlying 1980105400N, Long. Sa ˆ sanidine, Sp ˆ sphalerite, Spn ˆ sphene, Tor ˆ tourmaline rhyolitic metatuff 10081405600W Sample Description Location ME26-1 Basaltic andesite pillow lava. Los Epazotes, along Holocrystalline to the Tejupilco- G102 Tizapa metagranite. Granitic Arroyo El Ahogado, Hypocrystalline rock; Luvianos old road. augen gneiss. Porphyroclasts of near Tizapa mine, hypidiomorphic Pl in Lat. 188500000N, Kfs and Qtz surrounded by Zacazonapan. Lat. groundmass of microcrystals of Long. 10081304500W foliated aggregates of Kfs, 1980200000N, Long. Pl and Clpx, palagonite, and Bt and Qtz 10081303800W amygdules ®lled by Cal, Chl, AGN1 Tizapa metagranite. Granitic Arroyo FrõÂo, near La and/or Ab, Prh and Pmp augen gneiss. The augen Finca and San Pedro ME26-2 Basaltic andesite pillow lava. El LimoÂn, along the structure is slightly developed Tenayac. Lat. Holocrystalline to San Juan AcatitlaÂn- and gradually changes into 1980103600N, Long. Hypocrystalline rock; El Sauz road. Lat. isotropic granite 10081202300W hypidiomorphic Pl in 1885900100N, Long. 0 00 Mex-1 Massive sul®de ore, Kuroko-type Tizapa mine, groundmass of microcrystals of 100814 34 W deposit. Sp, Py, Fre, Ccp, Gn and Zacazonapan. Lat. Pl and Clpx, palagonite, and Apy 1980201000N, Long. amygdules ®lled by Cal, Chl, 10081305500W and/or Ab, Prh and Pmp Mex-2 Massive sul®de ore, Kuroko-type Tizapa mine, ME3-3 Tingambato batholith. La Punta de deposit. Sp, Py, Fre, Ccp, Gn and Zacazonapan. Lat. Hornblende granodiorite; Na-Pl, Tingambato, along Apy 1980201000N, Long. Kfs, Hbl, Qtz, Mag, Spn the Zuluapan-El 10081305500W In®ernillo (Las Mesas) electric plant Mex-3 Massive sul®de ore, Kuroko-type Tizapa mine, road. Lat. deposit. Sp, Py, Fre, Ccp, Gn and Zacazonapan. Lat. 1980404500N, Long. 0 00 Apy 19802 10 N, Long. 10082403800W 10081305500W ME3-1 Temascaltepec granite. Biotite Los Timbres, along ME25-2 Blastomylonitic quartz- Arroyo El Ahogado, granite; Or, Na-Pl, Qtz, Bt theTemascaltepec- muscovite schist. The schist is near Tizapa mine, Zacazonapan strongly foliated and lineated, Zacazonapan. Lat. highway. Lat. 0 00 and gradually changes into 19801 49 N, Long. 1980204500N, Long. 0 00 granitic augen gneiss; it consists 100813 55 W 10080505100W of foliated granoblastic assemblage of Qtz, Ms, grains of EN-179 Temascaltepec granite. Biotite Los Timbres, along Or, Mc and Tur granite; Or, Na-Pl, Qtz, Bt theTemascaltepec- Zacazonapan ME1-1 Phlogopite schist. Phl, Cchl, Ms, RõÂo Temascaltepec, highway. Lat. poikiloblastic Crd, and Rt. Small Los MartõÂnez, 9 km 1980204500N, Long. lenticular bodies intercalated NNW from 10080505100W with greenschist Tejupilco. Lat. 1885901600N, Long. 07-1 Ixtapan del Oro stock. Quartz Ixtapan del Oro, old 10081004800W diorite; Na-Pl, Or, Qtz, Aug, Bt, road to the El Mag. Out of Fig. 2 Bosque dam, ME1-2 Biotite schist. Porphyroclasts of Los MartõÂnez, 9 km ZitaÂcuaro. Lat. Pl, surrounded by foliated NNW from 1981505000N, Long. aggregates of Bt, Chl, Qtz, Ep Tejupilco. Lat. 10081603400W and Ms. Biotite-rich zone in 1885901600N, Long. greenschist 10081100000W ME3-2 Rhyolite plug. Porphyritic Cerro El PenÄon, rhyolite; phenocrysts of Qtz and Tamascaltepec- 01-1 Muscovite schist. Ms, Qtz, Ab, Los MartõÂnez, 9 km Sa in aphanitic groundmass Zacazonapan Bt, and Rt. Small lenticular NNW from containing Qtz, Kfs, Pl and Hem highway. Lat. bodies intercalated with Tejupilco. Lat. 1980301400N, Long. 0 00 greenschist 18858 48 N, Long. 10080604900W 10081005800W M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 359 border of the Guerrero terrane, approximately 130 km 1990). There are, however, indications that it is a pretec- WSW of Mexico City (Fig. 1). On the basis of lithologic, tonic intrusion emplaced into the volcanic-sedimentary structural, and tectonothermal relationships, the Mesozoic rocks, as the granite and its volcanic-sedimentary host rocks in the Tejupilco area are subdivided into two submar- were subsequently deformed and metamorphosed. Sm±Nd ine metavolcanic-sedimentary sequences, both of which isotopic analysis for the Tizapa metagranite (sample AGN1, have island arc af®nities. The lower, here named the Table 1) yielded a present-day eNd ˆ 25:0 and a Precam- Tejupilco metamorphic suite, is an intensely deformed brian Nd model age of 1.266 Ga (Ken Cameron, personal sequence of greenschist-facies metamorphic rocks; the communication, 1997). These isotopic data suggest that the upper, Arcelia-Palmar Chico group, consists of mildly to pluton could have been generated from partial fusion of an moderately deformed sequence of prehnite±pumpellyite- old crustal material without mantle-derived material or from facies metamorphic rocks (Fig. 2). mixing of old continental crust and juvenile material sources. Fragments of molluscan and other fossil material in 2.1. Tejupilco metamorphic suite samples of a limey fossiliferous lens from the Tejupilco metamorphic suite in the San Lucas del MaõÂz area, 6 km The Tejupilco metamorphic suite consists of greenschist NNW from Tejupilco, suggests relatively shallow water facies metamorphic rocks exposed in the Tejupilco-Taxco conditions and an early Mesozoic age for these rocks (R. region; they have previously been described as the Taxco Palmer, personal communication, 1993). The Tejupilco Schist and Taxco Viejo Greenstone, with inferred late metamorphic suite is covered by the parautochthonous Paleozoic and Late Triassic±Early Jurassic ages, respec- Arcelia-Palmar Chico group and is unconformably covered tively (Fries, 1960; de Cserna and Fries, 1981; de Cserna, by the early Tertiary continental clastic Balsas Group, 1982). These metamorphic rocks have also been considered Oligocene pyroclastic ¯ow deposits, Quaternary basaltic to represent an island-arc metavolcanic-sedimentary assem- lava ¯ows, and alluvium. The suite is intruded by the unde- blage of Late Jurassic±Early Cretaceous age (Campa and formed lower Eocene Temascaltepec granite (Fig. 2). RamõÂrez, 1979), and were grouped as the Teloloapan-Ixta- pan subterrane (Campa and Coney, 1983), or more recently, as the Early Cretaceous Teloloapan subterrane (Talavera- 2.2. Tizapa metagranite and its ®eld relationships Mendoza et al., 1993, 1995; Centeno-GarcõÂa et al., 1993a). In the Tejupilco area (Fig. 2), the rocks of this meta- The Tizapa metagranite, which is bounded by a Tertiary morphic suite are largely carbonaceous phyllite, quartzite, normal fault (Fig. 2), shows a conspicuously heterogeneous pelitic sericite schist, greenschist (mainly andesitic and deformation and ranges from isotropic granite in its central dacitic metavolcaniclastics), rhyolitic metatuff, and a mylo- part, to a granitic augen gneiss and blastomylonitic quartz- nitic augen gneiss of granitic composition Ð the Tizapa muscovite schist toward its margins (ElõÂas-Herrera and metagranite. Volcanogenic sul®de mineralization (Kuroko- SaÂnchez-Zavala, 1990). Foliation and lineation (with top- type) is characteristic in the suite. The Tizapa massive sul®de to-the-east sense of shear) in the deformed pluton are paral- deposit (Fig. 2) is the best example of this type of mineraliza- lel to those in the metavolcanic-sedimentary wall rock. The tion, with more than ®ve million tons of polymetallic ore foliation in the pluton is clearly tectonic, and the micro- and (JICA±MMAJ (Japan International Cooperation Agency± mesostructural features as well as the petrographic changes Metal Mining Agency of Japan), 1991). Its Zn±Pb±Cu type in the granite (ElõÂas-Herrera and SaÂnchez-Zavala, 1990) are ore association is characteristic of evolved island arcs (e.g. typical of moderate- to low-temperature solid-state defor- Hutchinson, 1980; Sawkins, 1990), and its Pb-isotopic signa- mation (e.g. Paterson et al., 1989; Miller and Paterson, tures (Table 4) lie between the orogene and upper crust ®elds 1994) indicative of a pretectonic pluton. Although most (Zartman and Doe, 1981), indicating little mantle contribution. criteria for distinguishing between pre-, syn- and posttec- The suite is more than 2000 m thick and is strongly tonic plutons are ambiguous (Paterson and Tobish, 1988; deformed, with tight to isoclinal recumbent folds and a Vernon et al., 1989; Paterson et al., 1987, 1989; Miller remarkable penetrative axial plane foliation developed and Paterson, 1994), the Tizapa metagranite does not under greenschist facies conditions. These structures are show the magmatic foliation that is an important feature refolded into gentle to close folds with moderately to steeply of syntectonic plutons. The foliation in the metagranite is inclined axial planes. Lower structural levels of the suite are de®ned only by relatively low-temperature upper locally exposed near Tizapa mine (Fig. 2), where mylonitic greenschist facies metamorphic minerals (ElõÂas-Herrera deformation in the Tizapa metagranite took place under and SaÂnchez-Zavala, 1990) and it is continuous with the upper greenschist-facies conditions (ElõÂas-Herrera and regional foliation. These features are more consistent with SaÂnchez-Zavala, 1990). Because of its peraluminous char- pretectonic than syntectonic plutons in which magmatic and acter and trace element signatures, the Tizapa metagranite high-temperature solid-state foliations coexist (Paterson et has previously been interpreted to be part of an old pre- al., 1989; Vernon et al., 1989; Miller and Paterson, 1994). Jurassic continental margin over which the arc-related Foliation-porphyroblast relations in the wall rock near the assemblage was thrust (ElõÂas-Herrera and SaÂnchez-Zavala, metagranite are ambiguous and have been modi®ed by the 360 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 thermal effects of the nearby Tertiary Temascaltepec (Delgado-Argote et al., 1992) and paleontologic data granite. (see also DaÂvila-Alcocer and Guerrero-SuaÂstegui, 1990), support an Albian±Cenomanian age for the 2.3. Arcelia-Palmar Chico group Arcelia-Palmar Chico group, although its age range is still not well constrained. In the nearby ZitaÂcuaro and The Arcelia-Palmar Chico group is a parautochthonous Valle de Bravo areas, Aptian ammonites (CantuÂ-Chapa, upper volcanic-sedimentary sequence in the Tejupilco area 1968), Barremian-Hauterivian fossils (Israde-Alcantara that is mildly to moderately deformed and essentially shows and MartõÂnez-Alvarado, 1986), and Berriasian radio- a non-penetrative prehnite±pumpellyite facies submarine larians (Cenosphaera sp., Lithocampe sp., Flustrella metamorphism in the intercalated volcanics. The rocks of sp.) (Contreras-RodrõÂguez et al., 1990) have been the Arcelia-Palmar Chico group (previously described as reported in a volcanic-sedimentrary sequence correlative Amatepec, Xochipala, and Arcelia formations, with inferred with the lower part of the Arcelia-Palmar Chico group. Albian, late Cenomanian-Turonian, and Conacian ages, The Arcelia-Palmar Chico group, according to kinematic respectively; de Cserna, 1982) include Albian ma®c-ultra- indicators in shear zones and fold vergence, is apparently ma®c cumulates, small tectonic wedges of serpentinized segmented by imbricate compressional structures and has an peridotite, and numerous diabase to microgabbroic dikes estimated structural thickness of more than 2500 m. The genetically related to pillow lavas (Delgado-Argote et al., lower part of the Arcelia-Palmar Chico group is thrust 1992; Ortiz-HernaÂndez et al., 1991). over the Tejupilco metamorphic suite (Fig. 2), probably For the purposes of this paper, the Arcelia-Palmar Chico across an erosion surface as indicated by the clastic layers group in the Tejupilco area can be divided into two parts: the with detrital metamorphic components in the hanging wall lower, which is dominantly sedimentary; and the upper, close to this tectonic contact. The upper part is, in turn, which is essentially volcanic. The lower part consists of apparently thrust onto the lower part. The displacements silty and argillaceous limestone, sandstone and on these faults are unknown but, on the basis of geolo- conglomerate, calcareous argillite, a lenticular body of gic mapping (Fig. 2), a displacement of a few tens of pillow lavas, black slate, and radiolarian-siliceous sedi- kilometers can be estimated. These ®eld relationships ments. The sandstone (feldspathic litharenite to lithic indicate a parautochthonous nature for the Arcelia- arkose, and lithic greywacke) and conglomerate contain Palmar Chico group with respect to the Tejupilco meta- sutured polycrystalline quartz, sheared quartz, feldspar morphic suite. The penetrative regional greenschist (plagioclase, perthitic K-feldspar, myrmekite) and mica facies metamorphism is then constrained to at least grains, and rock fragments of carbonaceous slate and pre-Early Cretaceous in age. phyllite, sericite schist, quarzite, granite and metavolca- The Arcelia-Palmar Chico group has been assigned to the nic rock. The underlying Tejupilco metamorphic suite is Arcelia-Palmar Chico subterrane and interpreted as an probably the source of these rock fragments, as indi- Albian±Cenomanian primitive island arc with back-arc cated by the nature of the debris and that the lower oceanic basin (Talavera-Mendoza et al., 1993; Centeno- part of the Arcelia-Palmar Chico group is directly over- GarcõÂa et al., 1993a). However, its age range, lateral lying erosion remnants of the metamorphic suite (Fig. 2). and vertical extension, paleogeographic setting and rela- The upper part of the Arcelia-Palmar Chico group is tionship with the Tejupilco metamorphic suite remain to composed mainly of basaltic-andesite pillow lavas, pillow be determined. The Arcelia-Palmar Chico group is breccias, hyaloclastites, and tuffaceous and siliceous sedi- unconformably overlain by continental clastic rocks ments. In the uppermost portion of the sequence, lenses of and volcanics with Tertiary and Quaternary ages. The fossiliferous reef limestone are intercalated. According to group is intruded by the undeformed Turonian±Conia- their faunal contents Ð Acanthochaetetes, radiolitid and cian (?) Tingambato batholith and by the Tertiary (?) caprinid rudists, Chondrodonta-(Chondrodonta) sp., and Hermiltepec diorite (Fig. 2). Adiozoptyxis coquandiana (d'Orbigny), these limestone lenses are Cretaceous in age (G. Alancaster-Ybarra, perso- 2.4. Other geological elements nal communication, 1994), although a more constrained age was not possible to determine. Other important elements for the geology of the Tejupilco The submarine lavas from the lower and upper parts of region are granulitic gneiss xenoliths (ElõÂas-Herrera et al., the Arcelia-Palmar Chico group show non-penetrative 1996; ElõÂas-Herrera and Ortega-GutieÂrrez, 1997) recently prehnite±pumpellyite facies sea¯oor metamorphism coex- found at Pepechuca in the felsic subvolcanics at the northern isting with palagonitic and fresh holocrystalline and border of the Oligocene La Goleta volcanic ®eld, 15 km microcrystalline sectors in the lavas. From representa- SSE from Tejupilco (Fig. 1). The xenoliths were found in tive fresh holocrystalline samples, the lavas were dated the Pepechuca rhyolitic volcanic plug, which has an intru- by 40Ar/39Ar method and yielded 103:1 ^ 1:3and sive contact with phyllite and sericite schist of the Tejupilco 93:6 ^ 0:6 Ma ages. These 40Ar/39Ar ages, discussed in metamorphic suite. They are gneisses with clearly orogenic detail below, together with other 40Ar/39Ar dates metamorphism and with high-grade mineral assemblages: M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 361

Fig. 3. Schematic lithostratigraphic columnar section of the Guerrero terrane in the Tejupilco area. See text for discussion of isotopic ages. The thicknesses of the different lithologic units are not to scale. quartz 1 K-feldspar 1 plagioclase 1 biotite1sillimanite1 1996). These Sm±Nd isotopic data, typical values of Gren- cordierite 1 spinel 1 corundum (?) in gneissic layering. villian rocks in Mexico, are similar to those of the Tizapa This mineralogy clearly implies that bulk rock and indi- metagranite, which is exposed 30 km NNW from the vidual layers in the xenoliths are peraluminous in compo- Pepechuca xenolith locality. The xenoliths and their isotopic sition and suggest a sedimentary (paragneisses) origin. The Nd signatures indicate the presence of a pre-Mesozoic conti- Pepechuca high-grade gneiss xenoliths (two analyzed xeno- nental crust beneath the Tejupilco metamorphic suite, and liths) have present-day eNd ˆ 26:7and27.3, and Nd they strongly suggest important amounts of recycled older model ages of 1.361 and 1.582 Ga (ElõÂas-Herrera et al., crustal material with a little mantle contribution in the 362 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 igneous units of this arc-related sequence. These peralumi- mesh size fractions, Wil¯ey table, magnetic separation, nous xenoliths may correspond to the type of old crustal heavy liquids) followed by hand picking. Based on morpho- material from which Tizapa-type granitoids could have logical criteria such as shape (subhedral vs. anhedral), derived by partial melting. inclusions, evidence of corrosion or metamict texture and transparency, several zircon populations were obtained. Most were colorless, transparent crystals with well-de®ned 3. Geochronology prismatic and pyramidal faces, and with minor to moderate elongation. From these, 10 fractions were carefully selected 3.1. Previous geochronologic data and analyzed by Samuel A. Bowring at Geochron Labora- tories in Cambridge, MA USA. The results and analytical From two analyzed whole rock samples of Tizapa meta- procedures are displayed in Table 2. granite with very little dispersion in 87Rb/86Sr and a biotite Analyses of the 10 zircon fractions from the Tizapa meta- data point was obtained a 50:3 ^ 1:5 Ma Rb±Sr biotite- granite yielded discordant 207Pb/206Pb ages ranging from whole rock age with a high initial 87Sr/86Sr ratio of 0.7141 254 to 1020 Ma (Table 2). On a concordia diagram (Fig. (R. L. Armstrong, personal communication, 1980). For 4A), they display a complex U±Pb isotopic data pattern Armstrong, these data, coupled with a biotite K±Ar age of which is probably due to the presence of variable amounts 50:7 ^ 1:8Ma; indicated that the metamorphism in that area of isotopically heterogeneous, inherited radiogenic Pb. The ended in Eocene time. With an assumed low initial 87Sr/86Sr discordant data show a fan-shaped pattern, the apex of ratio of 0.7045, a Rb±Sr model age of about 250 Ma for the which intersects the concordia curve near 185 Ma. The metagranite was estimated (R. L. Armstrong, personal pattern of discordant data opens toward older upper inter- communication, 1980). In another geochronological study, cept ages that span from ca. 960±1480 Ma (Fig. 4A). The only a biotite-whole rock two-point isochron age of 53:2 ^ scatter of the data precludes rigorous application of best-®t 0:4 Ma with a high initial 87Sr/86Sr ratio of 0.7152 was procedures for deriving accurate geological ages. obtained; this was interpreted to be related to the ®nal For different combinations of zircon fractions, however, stage of the mylonitic deformation of the Tizapa metagra- the York-®t (unforced) procedure can be used, the regres- nite (Herwig, 1982). Assuming an initial 87Sr/86Sr ratio of sion results of which are shown in Table 3. Although most 0.7060, a Rb±Sr model age of 218 ^ 6 Ma was reported of these results do not form a good linear array (MSWD up (Herwig, 1982). According to the 40Ar/39Ar and K±Ar to 530), all data points (excepting fraction A ) fall within the data discussed below, it is likely that the Rb±Sr isotopic 2 sector bounded by two straight discordia regression lines. system was re-set during the intrusion of the undeformed One of these lines is de®ned by four points, with an upper early Eocene Temascaltepec granite. intercept age of 958 ^ 82 Ma and a lower intercept age of The Japan International Cooperation Agency±Metal 181 ^ 11 Ma MSWD ˆ 43†: The other line is de®ned by Mining Agency of Japan (JICA±MMAJ; 1991) obtained ®ve points, with an upper intercept age of 1481 ^ 35 Ma Pb/Pb isotopic model ages of 129, 114, and 105 Ma for and a lower intercept age of 189 ^ 1:9Ma MSWD ˆ 13† the Tizapa ore deposit, assuming a single-stage Pb isotopic (Fig. 4A). The straight regression line through the least evolution. These Early Cretaceous ages are compared below discordant zircon fractions of the Tizapa metagranite with those obtained using a two-stage Pb isotopic evolution (sample AGN1, fractions C2, B2, A1) yields an upper inter- for the same samples. cept age of 1242 ^ 126 Ma and a lower intercept age of 3.2. New geochronologic data 186:5 ^ 7:4 Ma (Fig. 4A and B). These ages coincide with the mean of upper and lower intercept ages for different The geochronologic data discussed here include U±Pb combinations of zircon fractions (Table 3), and the age of isotopic analyses of zircons, Pb/Pb isotopic model ages of 186:5 ^ 7:4 Ma is indistinguishable from the lower inter- massive sul®de ore, and 40Ar/39Ar age determinations. cept ages within their 2s error. Precisely from the same Seventeen samples were collected for isotopic purposes. sample AGN1, the Nd model age of 1266 Ma was obtained, The description and location of the samples are shown in which is in agreement with the upper intercept age of the Table 1 and Fig. 2. A schematic lithostratigaphic column of less discordant zircon fractions. the Tejupilco area shows the different isotopic ages Common Pb isotopic model ages. Based on the Pb isoto- discussed below (Fig. 3). pic composition of the Tizapa massive sul®de deposit and U±Pb Zircon results. In order to date the Tizapa meta- the values of Doe and Stacey (1974) for a single-stage Pb granite, two samples of this deformed pluton (Table 1: isotopic model, JICA±MMAJ (1991) obtained ages of G102, AGN1), each 40±50 kg, were collected for U±Pb 128.7, 114.2, and 105.4 Ma for three samples of massive isotopic analyses. The samples come from localities where sul®de (Table 4). Given the syngenetic nature of the ore the mylonitic deformation is not intense. From samples deposit, these Early Cretaceous ages would imply a similar G102 and AGN1, zircon concentrations of $90% purity age for the volcanic-sedimentary protolith of the meta- were obtained using standard technical procedures (crush- morphic wall rock. However, a discrepancy between the ing, grinding and sieving to 2200 1 325; 2140 1 200 single-stage Pb age of many conformable ore deposit and M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 363 a ˆ and Pb 1 206 2 / a yr 9 Pb 2 207 10 £ ysis accomplished U 235 / a 0.15513 Pb ˆ 207 U 238 U d: 238 / a Pb 206 ) a 4 493.3 507.3 826.8 13 360.7 465.1 1020.0 3 274.5 305.7 551.4 9 226.8 255.6 528.7 6 252.2 288.2 590.9 5 246.7 305.5 782.9 7 229.7 260.7 549.2 11 200.0 207.7 296.3 6 257.9 285.9 521.4 10 193.7 198.3 253.6 Pb) ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ , a division of Krueger Enterprises Inc., Cambridge MA, 206 Pb/ 207 65 0.06670 163 0.07320 42 0.05860 74 0.05800 52 0.05965 59 0.06527 53 0.05852 66 0.05225 49 0.05778 49 0.05128 U( ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 235 / a Pb 207 6 0.64800 12 0.58100 4 0.35130 7 0.28630 4 0.32816 5 0.35100 5 0.29272 6 0.22700 4 0.32515 4 0.21570 U ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ a 238 Pb a 206 Pb) 206 Pb/ 208 Pb ( con®dence interval. 204

/ s a Pb 206 U spike. All fractions were abraded (Krogh, 1982). Sample weights estimated using a video monitor with a gridded screen, known to within 10%. 235 (ppm) a U± Pb 233 206 Pb± 205 ger, 1977. Common Pb corrections made using the model of Stacey and Kramers (1975) for interpreted crystallization age. For all analyses: U blank È U (ppm) 238 50%. Analyses performed on a VG Sector 54 mass spectrometer in peak-switching mode using the Daly knob and ion counting. Data.reduction and error anal ^ (Steiger and Ja 1 2 3.5 pg yr ˆ 9 2 10 £ 50%; Pb blank 0.98485 ^ ˆ Radiogenic Pb. Zircons analyzed using mixed U a 235 Zircon dissolution followed methods of Krogh (1973) and Parrish (1987). Separation of Pb and U accomplished using HCl chemistry. Decay constants use 1pg Table 2 U±Pb isotopic data for zircons from the Tizapa Metagranite (the different fractions of zircons were run by Samuel A. Bowring at Geochron Laboratories Gl02-1/2 0.0250 527.5 36.8 5654.0 0.085 0.07050 USA) Sample fnd Fractions Wt (mg) Concentrations Isotopic ratios Apparent ages (Ma) using the algorithms of Ludwig (1991); all errors reported in percent at the 2 #1 0.0260 234.5 13.5 1010.9 0.089 0.05750 -2/2 0.0310 484.5 20.7 7353.9 0.086 0.04350 -#3 0.0310 178.7 6.3 3282.9 0.085 0.03580 AGN1-A1 0.0120 771.0 30.4 4654.0 0.089 0.03990 -A2 0.0100 1035.7 39.5 9619.7 0.071 0.03900 -B1 0.0129 537.4 18.9 7771.4 0.067 0.03628 -B2 0.0100 678.0 20.9 3937.1 0.083 0.03151 -C1 0.0150 755.5 29.8 5941.8 0.068 0.04081 -C2 0.0130 1180.3 35. 7 1666.6 0.101 0.03050 364 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375

Fig. 4. (A) Concordia diagram for all zircon fractions from the Tizapa metagranite. The square boxes do not represent the error limits; the error ellipses are too small to be shown. Fraction numbers correspond to those in Table 2; un®lled and ®lled boxes are fractions of samples G102 and AGN1, respectively. (B) The best-®t straight regression line (3 data points) through the discordant zircon fractions of sample AGN1 yields a concordia upper intercept age of 1242 ^ 126 Ma and a lower intercept age of 186:5 ^ 7:4Ma: The enlarged section of the lower intercept area shows the best discordia regression line; ellipses represent error limits (2s). See text for discussion of U±Pb data. their ages determined by other isotopic or geologic methods, 4) for the three Tizapa samples reported by JICA±MMAJ has clearly been demonstrated (e.g. Stacey and Kramer, (1991). These results are more consistent with the U±Pb 1975; Faure, 1986, pp. 320±323). The two-stage model Pb lower intercept ages than are the single-stage Pb dates, evolution (Stacey and Kramer, 1975) has proved consider- and they suggest a pre-Kimmeridgian age for the massive ably more successful at de®ning the age of conformable ore sul®de deposit and its cogenetic wall rock. deposits (e.g. Stacey and Kramer, 1975; Faure, 1986, pp. 40Ar/39Ar age determinations. One sample (ME25-2) 320±323). Using the two-stage model, we obtained dates of from the Tejupilco metamorphic suite and two samples 227.5, 188.3, and 156.3 Ma (average age ˆ 191 Ma, Table (ME26-2 and ME26-1) from the Arcelia-Palmar Chico M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 365

Table 3 Table 5 Regression results of zircon U±Pb data (using the linear regression program 40Ar/39Ar results of Ludwig (1991); MSWD ˆ Mean Square of Weighted Deviate) The sample preparations and 40Ar/39Ar determinations were performed by M. LoÂpez-MartõÂnez, Centro de InvestigacioÂn y de EducacioÂn Superior de Regession parameters U.I. (Ma) L.I. (Ma) MSWD Ensenada (CICESE), Ciencias de la Tierra, Ensenada BC, MeÂxico. T ˆ (unforced) temperature of each step; Ca/K ˆ Ca/K ratio; S 39Ar ˆ cumulative 39Ar released 5 points 1481 ^ 35 189 ^ 1.9 13 5 points 1032 ^ 57 189 ^ 12 270 T (8C) Ca/K S 39Ar Age (Ma) ^1s 5 points 1288 ^ 301 184 ^ 19 440 4 points 958 ^ 82 181 ^ 11 43 ME25-2, muscovite, wt ˆ 68 mg, 177±250 mm grain size 4 points 1466 ^ 119 188.7 ^ 4.1 6 600 ± 0.008 47.6 ^ 6.2 4 points 1282 ^ 461 185 ^ 29 530 700 ± 0.038 48.2 ^ 1.8 3 points 1242 ^ 126 186.5 ^ 7.4 4.3 800 ± 0.128 50.8 ^ 0.6 3 points 1030.7 ^ 2.1 183.19 ^ 0.62 1.7 900 ± 0.268 56.4 ^ 0.4 3 points 991 ^ 114 182.4 ^ 9.9 3.8 1000 ± 0.488 57.8 ^ 0.3 2 points 1114 ^ 123 184.7 ^ 2.1 0.0 1100 ± 0.738 58.4 ^ 0.2 1200 ± 0.928 59.0 ^ 0.2 1300 ± 0.958 56.3 ^ 1.7 goup were dated by the 40Ar/39Ar step-heating method 1450 ± 0.974 52.0 ^ 4.8 (Table 5). Sample ME25-2, a blastomylonitic quartz- 1600 ± 1.000 50.5 ^ 2.1 Total gas age ˆ 56.7 ^ 0.3 Ma muscovite schist collected from a ductile shear zone from the uppermost part of the Tizapa metagranite, was ground ME26-2, whole rock, wt ˆ 286 mg, 500±710 mm grain size and sieved to 260 1 80 and 280 1 120 mesh size fractions. 400 5.0 0.006 80.0 ^ 40.0 500 4.5 0.029 91.0 ^ 12.0 From both fractions, two muscovite concentrates (.99%) 600 11 0.269 105.2 ^ 1.2 were obtained. Samples ME26-2 and ME26-1 are pillow 800 9.5 0.559 104.0 ^ 1.0 lavas collected from the lower and upper parts of Arcelia- 900 3.0 0.759 101.5 ^ 1.5 Palmar Chico group, respectively; they were ground and 1100 3.5 0.899 100.0 ^ 2.0 sieved to 225 1 60 mesh size, washed in HCl (dissolved 1150 13.5 0.929 86.3 ^ 9.1 1200 37.5 0.937 86.1 ^ 31.8 to 10%) for carbonate leaching, and sieved to 225 1 35 1300 106.5 0.951 86.0 ^ 20.5 40 mesh size fractions for Ar/ dating. These two samples 1600 113 1.000 96.2 ^ 4.7 correspond to carefully selected fresh, well-micro- Total gas age ˆ 101 ^ 1.0 Ma crystallized and homogeneous pillow lava fragments. Plateau age ˆ 103.1 ^ 1.3 Ma (600±11008C; 87% of total 39Ar) 40 39 Sample preparation and Ar/ Ar analyses were carried ME26-1, whole rock, wt ˆ 277 mg, 500±710 mm grain size outbyM.LoÂpez-MartõÂnez of the Centro de InvestigacioÂn 400 3.6 0.004 83.2 ^ 23.3 Cientõ®ca y de EducacioÂnSuperiordeEnsenada(CICESE), 500 3.3 0.026 86.4 ^ 5.2 Baja California, MeÂxico. The samples were irradiated in the 600 3.5 0.136 97.0 ^ 1.0 800 4.4 0.197 99.1 ^ 1.6 uranium-enriched research reactor of McMaster University 950 1.5 0.307 96.0 ^ 1.0 40 39 in Hamilton, Ontario, Canada. Ar/ Ar age spectra for 1050 1.6 0.580 94.4 ^ 0.4 these three samples are shown in Fig. 5. The criterion 1300 8.8 0.890 90.0 ^ 0.2 used to de®ne a plateau is that at least three contiguous 1600 6.8 1.000 93.2 ^ 1.0 steps must agree within their 2s errors, together constitut- Total gas age ˆ 93.4 ^ 0.4 Ma ing .50% of the total quantity of 39Ar evolved (McDougall and Harrison, 1988). Muscovite from sample ME25-2 yielded an upward- convex age spectrum with 10 fractions collected between

Table 4 Pb isotopic data of the Tizapa massive sul®de ore deposit (Pb isotopic data after JICA±MMAJ (1991))

Pb isotopic composition (atomic %) Isotopic ratios of Pb mp (slope) Agea (Ma) mpp (slope) Ageb (Ma)

Sample 204Pb 206Pb 207Pb 208Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Mex-1 1.354 25.199 21.216 52.231 18.61 15.67 38.57 0.5779 105.4 0.3583 156.3 Mex-2 1.348 25.187 21.207 52.258 18.68 15.73 38.77 0.5799 128.7 0.3629 227.5 Mex-3 1.351 25.191 21.208 52.250 18.65 15.70 38.67 0.5786 114.2 0.3603 188.3

a Age obtained by JICA±MMAJ (1991) using the values of Doe and Stacey (1974) for the single-stage model, where mp ˆ [(207Pb/204Pb) 2 10.294]/ [(206Pb/204Pb) 2 9.307]. b Age obtained for this paper using the values of Stacey and Kramer (1975) for the two-stage model, where mpp ˆ [(207Pb/204Pb) 2 12.998]/ [(206Pb/204Pb) 2 11.152]. The average age for each case is 116 and 191 Ma, respectively. 366 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375

65 65 600 and 16008C (Fig. 5). During the ®rst two low-temperature ME25-2: quartz-muscovite schist, muscovite 39 (-60+80 mesh) steps around 4% of Ar was released, with an average 60 60 incremental age of 48:1 ^ 0:1Ma: In the next ®ve steps, 50 50 during which 90% of 39Ar was released, the ages rise

45 45 progressively from 50:8 ^ 1:8to59:0 ^ 0:2Ma; although Age (Ma) from ®fth to eighth steps an apparent plateau segment of 40 40 58:37 ^ 0:1 Ma comprising 66% of 39Ar released may be Total gas age = 56.7 0.3 Ma de®ned (Fig. 5). The last three steps of the age spectrum, during which 7% of 39Ar was released, yield a poorly de®ned age of 53:2 ^ 0:2 Ma that is unlikely to have any 110 110 100 100 speci®c geologic signi®cance. The total gas age is 56:7 ^ 0:3Ma; whereas the same muscovite yielded a K±Ar age of 90 103.1 1.3 Ma 90 80 80 59 ^ 3 Ma (Table 6). 70 70 The whole-rock age spectrum of sample ME26-2, Age (Ma) 60 60 obtained by collecting 10 gas fractions between 400 and Total gas age = 101 1.0 Ma 50 50 16008C, yielded a total gas age of 101 ^ 1 Ma (Table 5). ME26-2: pillow lava. Whole rock (-25+30 mesh) The ®rst two fractions represent only 3% of 39Ar released and 110 110 their apparent ages are considered geologically meaningless. 100 ME26-2 100 90 90 The next four fractions, representing 87% of gas released, 80 80 yielded a well-de®ned plateau age of 103:1 ^ 1:3Ma(Fig. 70 70 60 60 5), which is statistically indistinguishable from the total gas Ca/K 50 50 age within the 2s error. The apparent Ca/K ratio for these 40 40 30 30 fractions, which ranges from 2.9 to 10.8 (Fig. 5), indicates a 20 20 homogeneous composition for a whole rock analysis. The 10 10 ®nal four gas fractions coincided with an increase in Ca/K ME26-1: pillow lava. Whole rock (-25+30 mesh) ratio increasing from 13.7 to 110, and their apparent ages are 110 110 considered to have no geologic meaning because they are 100 100 unlikely to represent homogeneous compositions and corre- 39 90 90 spond to only 10% of Ar released. 93.6 0.6 Ma Sample ME26-1, with a whole-rock spectra de®ned by Age (Ma) 80 80 eight fractions collected between 400 and 16008C, yielded a 70 Total gas age = 93.4 0.4 Ma 70 total gas age of 93:4 ^ 0:4Ma: apparent Ca/K ratios vary from 1.4 to 8.6 and suggest an homogeneous whole rock 10 ME26-1 10 composition. The ®rst ®ve fractions, collected up to 9508C, 8 8 rendered 30% of total 39Ar released with the age spectrum 6 6 segment slightly disturbed, whereas the last three fractions, Ca/K 4 4 39 2 2 collected at 1050±16008C and comprising 70% of the Ar 0 0 released, yielded an average age of 92:2 ^ 0:2Ma: Exclud- 0.00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ing the ®rst two steps, a age of 93:6 ^ 0:6 Ma for 98% of Fraction of39 Ar released total 39Ar released was obtained, although this roughly ¯at Fig. 5. Age spectra for samples dated by 40Ar/39Ar incremental release. The age spectrum segment is not a well-de®ned plateau (Fig. 5). gas release pattern for sample ME25-2 is typical of the partial loss of radiogenic argon by a superimposed thermal event. The ®rst two low 3.3. Interpretation of the geochronologic results temperatures increments (4% of total 39Ar) give an age of 48:1 ^ 0:1Ma; which may be a reliable estimation for the reheating event, whereas the apparent plateau age of 58:4 ^ 0:2 Ma (66% of total 39Ar) from steps 5±8 U±Pb zircon data. The complex fan-shaped data pattern may be meaningless; the total gas age is 56:7 ^ 0:3Ma: For sample ME26- for ten fractions of zircon populations from the Tizapa meta- 2, the plateau age is 103:1 ^ 1:3Ma: (87% of total 39Ar), and the total gas granite is typical of peraluminous granites that contain age is 101:0 ^ 1Ma: For sample ME26-1, the average age corresponding to detrital zircons derived from preexisting sources. Resorbed, 39 the roughly ¯at age spectrum segment (98% of total Ar released) is 93:6 ^ corroded and rounded zircon grains in the Tizapa meta- 0:6Ma; and the total gas age is 93:4 ^ 0:4Ma: The plateau age in sample ME26-2 and the corresponding age to the plateau-like age spectrum granite, which were excluded for U±Pb dating, probably segment in sample ME26-1 are interpreted as dating crystallization of the correspond to these detrital components. Similar fan-shaped submarine volcanics. Apparent Ca/K spectra for samples ME26-2 and patterns of discordant zircon data with demonstrated ME26-1 are also shown. See text for discussion of 40Ar/39Ar age spectra. inherited components have been reported from plutonic rocks elsewhere (Gulson and Krogh, 1973; Chen and Moore, 1982; Saleeby et al., 1987; Barth et al., 1989; Wallin, 1990; Hansmann and Oberli, 1991). The regression M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 367

Table 6 K±Ar age determinations for samples of the Guerrero Terrane, Tejupilco area (Analyses performed by D. J. Terrell and M. Escudero-Badilla, Instituto Mexicano del Petroleo, Mexico City

Sample Rock Type and name Dated Material K (wt %) 40Ara (10210 mol/g) 40Ara (%) Age (Ma)

Tejupilco metamorphic suite ME25-2 Quartz-muscovite schist Muscovite 8.58 8.93 93 59 ^ 3 ME1-1 Phlogopite schist Phlogopite 4.43 3.87 75 50 ^ 2 ME1-2 Biotite schist Biotite 6.71 4.83 83 48 ^ 2 01-1 Muscovite schist Muscovite 6.83 6.83 93 57 ^ 3 04-1 Calc-muscovite schist Muscovite 6.42 6.02 91 53 ^ 3 ± Granitic augen gneiss Biotite 7.04 6.28 84.7 50.7 ^ 1.8b Intrusive rocks ME3-3 Tingambato batholith, Hornblende 0.66 1.27 77 107 ^ 5 granodiorite-diorite ME3-1 Temascaltepec granite Biotite 6.80 6.13 88 51 ^ 3 EN-179 Temascaltepec granite Biotite 6.87 6.62 95 48.6 ^ 2 07-1 Quartz diorite Plagioclase 1.21 0.67 67 32 ^ 2 Post-Laramide volcanic rocks ME3-2 Rhyolite Microcrystalline groundmass 7.12 3.87 79 31 ^ 2

a 40 ±24 Radiogenic Ar. All K±Ar ages standarized to the decay constants recommended by Steiger and JaÈger (1977). Abundance of K/Ktotal ˆ 1.167 £ 10 mol/ mol. b Dr. R. L. Armstrong (personal communication, 1980). results of U±Pb zircon data of the Tizapa metagranite yield of the emplacement of the granite. In the ®rst case, albeit the upper intercept ages ranging from 958 ^ 82 to 1481 ^ uncertainty on the upper intercepts ages is quite large 35 Ma (Table 3). These Middle Proterozoic ages are inter- because of the long extrapolations, a Precambrian age for preted as old inherited Pb components, probably located in the pluton is implied and metamorphic zircon rims or impor- xenocrystic cores that we were unable to identify by optical tant Pb-loss as a result of recrystallization during the meta- methods. morphism must have occurred. On the basis of geological The upper intercept ages are in good agreement with relationships, a Precambrian age for the granitic protolith is other Grenville U±Pb zircon ages for the Oaxacan Complex untenable. On the other side, the mylonitization, according (Anderson and Silver, 1971; Ortega-Gutierrez et al., 1977; to the involved mineralogy, occurred under low-grade Robinson, 1991; Robinson et al., 1989), and for the Gren- metamorphic conditions (#5008C; ElõÂas-Herrera and villian Huiznopala Gneiss in eastern Mexico (Ortega- SaÂnchez-Zavala, 1990) Ð well below the estimated closure GutieÂrrez et al., 1995). They are also consistent with the temperature range for the U±Pb system in zircon from Precambrian crustal-residence ages for certain plutonic plutonic rocks (650±7508C, Mattinson, 1978), and the rocks from southwestern Mexico (Shaaf, 1990), as well as temperature range for the recrystallization (Pb-loss event) with those in the Varales Formation of the Arteaga complex of zircons in metamorphic rocks (600±6508C, Mezger and (Centeno-GarcõÂa et al., 1993b; Centeno-GarcõÂa, 1994) and Krogstad, 1997). Under greenschist-facies mylonitic defor- the Taxco Schist (Centeno-GarcõÂa, 1994). This suggests that mation, metamorphic overgrowth of zircon is unlikely and, at the inherited zircon components originated from older least in the moderately deformed parts of the granite where Precambrian crystalline basement rocks. Furthermore, a the samples for dating were collected, the zircon U±Pb Precambrian Nd model age for a granulitic xenolith TDM ˆ system probably remained closed (e.g. Wayne and Sinha, 1:5Ga† at the Neogene Valle de Santiago, Guanajuato, volca- 1992). At low temperature, only metamict zircons may lose nic ®eld (Urrutia-Fucugauchi and Uribe-Cifuentes 1999), and Pb by chemical alteration, leaching and diffusion (Mezger the recently found Pepechuca high-grade gneiss xenoliths and Krogstad, 1997). For this U±Pb zircon dating, grains (ElõÂas-Herrera et al., 1996; ElõÂas-Herrera and Ortega-GutieÂr- with corrosion or metamict features were carefully excluded. rez, 1997), support the presence of old continental crust in Alternatively, we consider that the lower intercept ages south-central Mexico. The Grenvillian Nd isotopic data of the provide a good approximation for the emplacement of the Pepechuca high-grade gneiss xenoliths (ElõÂas-Herrera et al., granite. The grains of the analyzed zircon fractions, as 1996) are similar to those of the Tizapa metagranite and its mentioned above, probably had discrete and submicro- Precambrian recycled zircon components. scopic old inherited cores overgrown by magmatic rims. On the other hand, the lower intercept ages, which Most of these zircons correspond to colorless and elongated converge about 185 Ma (Fig. 4A), can be interpreted in euhedral crystals, and they have high U-concentrations, two ways: as the age of metamorphism (mylonitization) specially those of sample AGN1 (Table 2). These features during which primary zircons recrystallized, or as the age suggest growth at magmatic temperatures (e.g. Mezger and 368 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 Krogstad, 1997) rather than partial recrystallization at Table 6). The remainder of the spectrum yields ages that greenschist facies conditions for the analyzed zircon grains. have been reduced due to 40Ar loss, and they might re¯ect The zircon fractions of sample AGN1, with their minor old the T±t history, which is not the objective of this paper. inherited components, justly constrain the lower intercept Thus, the original age of crystallization of the ME25-2 ages (Fig. 4A and B). Thus, we interpret the U±Pb lower muscovite remain undetermined. intercept age of 186:5 ^ 7:4Ma; which corresponds to the The apparent plateau of 58:4 ^ 0:2 Ma in the spectrum, regression line through the least discordant zircon fractions which is similar to the 59 ^ 3 Ma K±Ar age for the mica of sample AGN1 (Fig. 4B), to be the best estimation for the from the same sample, could alternatively be interpreted as magmatic crystallization age of the Tizapa metagranite. the age of the mylonitization. However, the geological This lower intercept age is indistinguishable (within the evidence does not support this interpretation. The mylonitic 2s error) from the other lower intercept ages (Table 3). deformation of the Tizapa metagranite is clearly coeval with The corresponding upper intercept age, 1242 ^ 126 Ma; is the greenschist regional metamorphism of the Tejupilco in good agreement with the Precambrian Nd model ages of metamorphic suite, which in several places is unconformably 1266 Ma for the Tizapa metagranite, and with those of Nd overlain by undeformed and nonmetamorphosed continen- model ages of the Pepechuca high-grade gneiss xenoliths. tal clastic rocks of the Balsas Formation and a latest Cretac- An Early-Middle Jurassic age for the intrusion of the Tizapa eous±early Eocene range age has been deduced by its metagranite is consistent with the Late Triassic±Early stratigraphic relationship (de Cserna, 1982). Furthermore, Jurassic age range for the surrounding metavolcanic-sedi- in the Tejupilco area, the sandstone and conglomerate of mentary rocks, deduced from the two-stage Pb model ages the lower part of the Arcelia-Palmar Chico group, with an for the syngenetic Tizapa massive sul®de deposit. Early Cretaceous age, contain detritus of deformed grani- 40Ar/39Ar and K±Ar. Upward-convex age spectra similar toids and other metamorphic rocks whose provenance could to that of muscovite sample ME25-2 have been reported in be the Tizapa metagranite and related metamorphic rocks. micas containing excess 40Ar (Pankhurst et al., 1973; K±Ar mica ages from various metamorphic rocks of the Hammerschmidt, 1983; Foland, 1983), although no Tejupilco metamorphic suite range from 59 ^ 3to48^ mechanism for the generation of such age spectra was 2 Ma (Table 6), whereas Tizapa metagranite samples have proposed. Wijbrans and McDougall (1986) attributed yielded Rb±Sr biotite-whole rock isochron ages of 53:2 ^ upward-convex age spectra to the mixing of two generations 0:4 Ma (Herwig, 1982) and 50:3 ^ 1:5 Ma (R.L. Armstrong, of white mica, each with a different age spectrum and argon personal communication, 1980). These are surprisingly release pattern. In this model, it is possible to obtain good consistent with the K±Ar biotite ages of the Temascaltepec intermediate-temperature plateaus of meaningless age, granite and may similarly imply re-setting of K±Ar and Rb± whereas the ages of the high temperature segment provides Sr systems by thermal effects associated with the emplace- an estimate for the age of the younger mica component. ment of this stock. If so, the local static metamorphic event According to this interpretation, the 58:4 ^ 0:2 Ma apparent reported by ElõÂas-Herrera (1989) in the San Lucas del MaõÂz plateau age of sample ME25-2 (Fig. 5, Table 5) may be a area might correspond to a superimposed early Eocene meaningless, whereas the age of 50:5 ^ 2:1 Ma for the last thermal metamorphism related to the Temascaltepec highest temperature step may represent the age of neocrys- granite. Mineral assemblages that include cordierite 1 tallized muscovite. This age is in good agreement with the muscovite 1 biotite in black phyllite, cordierite 1 K±Ar biotite ages (51 ^ 3 Ma and 48:6 ^ 2Ma; samples phlogopite 1 rutile 1 clinochlore 1 muscovite in pelitic ME3-1 and EN-179, Table 6) of the nearby undeformed schist lenses (with apparent metasomatic Mg-enrichment), Temascaltepec granite. This would suggest that the K±Ar and clinopyroxene 1 plagioclase 1 green hornblende 1 system of the blastomylonitic quartz-muscovite schist was actinolite/tremolite 1 clinozoisite 1 sphene 1 rutile over- re-set during the emplacement of this stock and that the printing mylonitic metabasite would be consistent with original age of the muscovite generated during mylonitiza- this interpretation. tion cannot be determined from its 40Ar/39Ar age spectrum. In summary, it is apparent that the thermal effects of the If the last three high temperature fractions of the age undeformed early Eocene Temascaltepec granite caused spectrum of the ME25-2 muscovite are considered to be signi®cant re-setting of argon isotopic ages and of K±Ar geologically meaningless steps, the spectrum may indicate and Rb±Sr isotopic systems. Although the age of this ther- partial radiogenic argon loss related to reheating events or mal event may be deduced from the 40Ar/39Ar age spectrum slow cooling (e.g. Harrison, 1983; Harrison and McDougall, of the ME25-2 muscovite, this argon age spectrum and the 1980, 1982; McDougall and Harrison, 1988). In the case of K±Ar dates in micas from metamorphic rocks do not shed diffusion loss of 40Ar by a superimposed thermal event, the any light on the age of mylonitic deformation of the Tizapa ®rst two steps (600±7008C), with the lowest initial ages of metagranite or of regional metamorphism. the spectrum and an average incremental age of 48:1 ^ Although 40Ar/39Ar release patterns of submarine 0:1Ma; yield a good estimate for this reheating event. volcanic whole-rock samples are in general complex and This age is also consistent with the K±Ar biotite ages of dif®cult to interpret because of sea¯oor alteration and the Temascaltepec granite (samples ME3-1 and EN-179, their polymineralic features, with a number of phases of M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 369 different argon retentivity (McDougall and Harrison, 1988), summary of all geochronologic results discussed here is samples ME26-2 and ME26-1 yielded little disturbed given in Table 7), previous regional interpretations of the 40Ar/39Ar age spectra from which reliable estimates of the Tejupilco metamorphic suite as a Late Triassic±Early Juras- crystallization age could be obtained. The lavas of the sic basement unit of the easternmost part of the Guerrero Arcelia-Palmar Chico group effectively show non-penetra- terrane can be evaluated. A U±Pb zircon magmatic cristal- tive, very low-grade submarine metamorphism, and palago- lization age of 186 Ma for the Tizapa metagranite is key to nitic material is common. However, the samples for dating the resolution of several important geological relations in were carefully selected and correspond to fresh holocrystal- the Guerrero terrane. line plagioclase-rich fragments from the interiors of pillows. The Tizapa metagranite intruded the volcanic-sedimen- In the interpretation of their age spectra, the release of argon tary sequence assimilating pelitic sediments during the later essentially from the plagioclase is therefore assumed. In stages of its magmatic evolution, following which the these cases, the crystallization and cooling of the lavas sequence and the Tizapa metagranite were deformed and must have been almost coeval with the sea¯oor metamorph- metamorphosed. The local character of the Tizapa meta- ism. Thus, the plateau ages of 103:1 ^ 1:3 Ma (87% of gas granite and its local content of partially assimilated released) in sample ME26-2 and the average incremental garnetiferous-pelitic schist xenoliths, which were probably age of 93:6 ^ 0:6 Ma corresponding to the plateau-like incorporated as sediments (ElõÂas-Herrera and SaÂnchez- age spectrum segment (98% of total 39Ar released) in Zavala, 1990), support this interpretation and also explain sample ME26-1 (Fig. 5, Table 5) are interpreted to date the peraluminous character of the metagranite. The tectonic the crystallization of the submarine volcanics of the lower foliation passes through these partially assimilated xeno- and upper part of the Arcelia-Palmar Chico group in the liths, indicating that both granite and xenoliths were solid Tejupilco area, respectively. These ages are indistinguish- during their regional deformation (e.g. Paterson et al., able from their respective 40Ar/39Ar total gas ages, which in 1989). Similar relations may also hold for the granite's similar cases have been considered as reliable estimates of contact with the surrounding volcanic-sedimentary rocks, the crystallization age for partly altered deep-sea basalts as the mylonitic foliation in the granite is continuous with (Dalrymple et al., 1980; Walker and McDougall, 1982). the regional foliation. The strong ductile shear zone along The ages of 103 and 93 Ma for, respectively, the submar- the margins of the Tizapa metagranite is a common feature ine volcanics of the lower and upper part of the Arcelia- of pretectonic plutons (e.g. Lamouroux et al., 1980; Pater- Palmar Chico group in the Tejupilco area are consistent with son and Tobish, 1988; Paterson et al., 1989) and can be their stratigraphic relationships and with a ca. 105 Ma attributed to differences in the rheological behavior of 40Ar/39Ar mineral age reported for hornblende-rich cumu- the rocks involved during regional deformation whereby the lates of the San Pedro LimoÂn stock that are apparently Tizapa metagranite behaved as a rigid object. Hence, the related to pillow lavas in the Palmar Chico-San Pedro ductile shear zone need not imply signi®cant regional LimoÂn area (Delgado-Argote et al., 1992). They are also displacement as previously assumed (ElõÂas-Herrera and in agreement with the Albian±Cenomanian age obtained SaÂnchez-Zavala, 1990). The pretectonic Early±Middle from radiolaria in siliceous sediments interlayered with Jurassic emplacement of the Tizapa metagranite therefore pillow lavas in the Arcelia area (DaÂvila-Alcocer and Guer- postdates the protoliths of the metavolcanic-sedimentary rero-SuaÂstegui, 1990). sequence. In an alternative interpretation with a possible The K±Ar hornblende age of the undeformed Tingam- syntectonic nature for the pluton, its emplacement would bato batholith (107 ^ 5Ma; Table 6) is similar to the date the penetrative regional deformation and greenschist Albian±Cenomanian 40Ar/39Ar ages of the intruded submar- facies metamorphism and would once again postdate the ine volcanics of the Arcelia-Palmar Chico group. However, protoliths of the surrounding metavolcanic-sedimentary the clear intrusive relationship of this batholith with the rocks. At present, the Pb/Pb Late Triassic±Early Jurassic Arcelia-Palmar Chico group requires a post-Cenomanian model age range is the best estimate of the age of the age for the batholith. It is therefore likely that the batholith Tejupilco metamorphic suite sequence. was contaminated with 40Ar during its intrusion into these The paleogeographic relationship between the Tejupilco volcanics, as suggested by the many partially assimilated metamorphic suite and the Arteaga and Placeres complexes, xenoliths of volcanics in the pluton. The batholith is both of which have poorly constrained early Mesozoic ages nonconformably covered by continental clastic rocks of (Centeno-GarcõÂa et al., 1993a,b, 1994), is currently the pre-Eocene Balsas Formation. Thus, only a poorly unknown. According to our data, the Tejupilco meta- constrained Late Cretaceous age for Tingambato batholith morphic suite is also a poorly constrained early Mesozoic can be offered at this time. basement unit composed of a complex, evolved volcanic arc assemblage over which at least part of the Cretaceous Arce- lia-Palmar Chico group and Morelos-Guerrero platform 4. Discussion and regional implications were deposited. The Arteaga and Placeres complexes are apparently pre-Upper Jurassic basement units with oceanic From the geochronologic data discussed above (a crust af®nities, over which Jurassic-Cretaceous arc 370 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 zircon 0.7141 0.7152 0.7223 ˆ 4.3) ˆ ˆ ˆ i i i ˆ Sr) Sr) Sr) 86 86 86 Sr/ Sr/ Sr/ 87 87 87 ( ( (MSWD age, respectively. Ductile shear zone in the Tizapa metagranite age for the ma®c-ultrama®c stock age, respectively respectively Tizapa metagranite sample avg) avg) Model age Isochron, 2 pts., ( whole rock, Zrn ˆ plagioclase, WR ˆ communication, 1980) communication, 1980) communication, 1980) phlogopite, Pl ˆ 1.86 R. L. Armstrong (personal 0.4 Herwig (1982)2.5 Herwig (1982)7.4 Herwig (1982) This paper Model age Isochron, 2 pts., Isochron, 2 pts., Lower intercept age, 3pts. 0.3 This paper Integrated age and plateau-like 31 Delgado-Argote et al. (1992)6 Delgado-Argote et al. (1992)30.4 Plateau age Delgado-Argote Plateau et age. al. The (1992) best estimated Delgado-Argote et This al. paper (1992)1.0 Plateau age Integrated age This paper Integrated age and plateau-like Integrated age and plateau age, 0.1 3 This paper Ductile shear zone in the 0.6 1.3 2 This paper 2 This paper 3 This paper 3 This paper 1.5 R. L. Armstrong (personal ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 93.6 58.37 103.1 microcrystalline groundmass, Phl ˆ Ar Ms 56.7 ArAr Hbl HblArArAr Hbl Hbl WRAr 114 105 WR 104 100 93.4 101 39 39 39 39 39 39 39 Ar/ Ar/ Ar/ Ar/ Ar/ Ar/ Ar/ 40 40 40 40 40 40 40 muscovite, MX ˆ hornblende, Ms n-El Sauz  ˆ n area n area n area n area     nez, Tejupilco K±Ar Phl 50 nez, Tejupilco K±Ar Bt 48 nez, Tejupilco K±Ar Ms 57    n, Acatitla  galena, Hbl ˆ Tizapa areaTizapa areaTizapa area K±Ar Bt Rb±Sr U±Pb Ms, Bt 50.7 Zrn 48.2 186.5 Tizapa areaTizapa area Rb±Sr Rb±Sr WR WR, Bt 218 53.2 Tizapa area Tizapa area K±Ar Ms 59 Tizapa area Rb±Sr WR, Bt 50.3 Tizapa area Rb±Sr WR ca. 250 R. L. Armstrong (personal biotite, Gn ˆ Mineral abbreviations: Bt Augen gneiss granite (Tizapa metagranite) Augen gneiss granite (Tizapa metagranite) Augen gneiss granite (Tizapa metagranite) Augen gneiss granite (Tizapa metagranite) Augen gneiss granite (Tizapa metagranite) Blastomylonitic quartz- muscovite schist (Tizapa metagranite) HornblenditeHornblenditeBasaltic pillow lava San Pedro Limo Los Epazotes, Tejupilco San Pedro Limo Hornblendite San Pedro Limo Massive sul®de oreMassive sul®de orePhlogopite schist Tizapa mine Tizapa mine Los Martõ Pb±Pb Pb±Pb Gn Gn 116.1 190.7 JICA±MMAJ (1991) This paper Single-stage model age (3 Two-stage model age (3 sample Blastomylonitic quartz- muscovite schist (Tizapa metagranite) Basaltic pillow lava El Limo Biotite schist Los Martõ Muscovite schist Los Martõ Calc-muscovite schist Cerro de la Pila, Zacazonapan K±Ar Ms 53 Arcelia-Palmar Chico group Hornblendite San Pedro Limo Augen gneiss granite (Tizapa metagranite) Rock type and nameTejupilco metamorphic suite Augen gneiss granite (Tizapa metagranite) Locality Method Dated material Age (Ma) Ref Comments Table 7 Summary of geochronology results M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 371 5 This paper 3 This paper 2 This paper 3 This paper 2 2 This paper ^ ^ ^ ^ ^ ^ 48.6 n, Temascaltepec K±Ar MX 31  o Ä La Punta de Tingambato K±Ar Hbl 107 Ixtapan del Oro K±Ar Pl 32 ) continued Rock type and nameIntrusive rocks Granodiorite-diorite (Tingambato batholith) Locality Method Dated material Age (Ma) Ref Comments Table 7 ( Granite (Temascaltepec granite) Los Timbres, Temascaltepec K±Ar Bt 51 Rhyodacitic ignimbrite Otzoloapan K±Ar WR 28.8 Quartz diorite (Ixtapan del Oro stock) Post-Laramide volcanic rocks Rhyolite (volcanic plug) Cerro El Pen 372 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 assemblages were deposited (Centeno-GarcõÂa et al., (Centeno-GarcõÂa et al., 1993a; Talavera-Mendoza et al., 1993a,b, 1994). These similar geologic relationships 1995). In this interpretation, deformation, metamorphism, suggest a heterogeneous basement and a complex tectonic and accretion of the Guerrero terrane would be Late Cretac- history for southwestern Mexico, involving the tectonic eous or Paleocene in age. juxtaposition of those successive terranes during multiple On the other hand, the inherited Precambrian zircon accretion events. components in the Tizapa metagranite, the Grenvillian Nd The intense deformation and regional metamorphism of isotopic data for this pluton and for the Pepechuca high- the Tejupilco metamorphic suite may be related to initial grade xenoliths, together with the Precambrian Nd model accretion in Middle to early Late Jurassic time. The AcatlaÂn ages and e Nd values with continental af®nity of rhyolitic Complex, which is the pre-Mississippian basement of the metatuff from the Taxco Schist (Ruiz et al., 1991; Mixteco terrane (Sedlock et al., 1993; Ortega-GutieÂrrez et Centeno-GarcõÂa, 1994), which for us is clearly correlative al., 1994), may have been the continental framework onto with Tejupilco metamorphic suite, indicate involvement of which the easternmost part of the Guerrero terrane was old continental crust in the magmatic evolution of the Guer- accreted. The original Guerrero terrane (NaÂhuatl)-Mixteco rero terrane in the Tejupilco-Taxco region. This precludes terrane boundary must be overlapped by the Cretaceous generation of Tejupilco metamorphic suite magmas, or carbonates. The apparent accretionary boundary of the those of the easternmost Guerrero terrane, from simple Guerrero terrane in the Teloloapan-Taxco area (Campa differentiation of a Phanerozoic mantle melt or from melt and Coney, 1983 (Fig. 1) and the Papalutla thrust fault (de derived from Mesozoic oceanic lithosphere. The presence of Cserna et al., 1980), which has also been suggested as the a pre-Mesozoic sialic crust beneath the Tejupilco meta- eastern boundary of the Guerrero terrane (NaÂhuatl, Sedlock morphic suite in the region is strongly supported by the et al., 1993) (Fig. 1), are likely to be post-accretion struc- Pepechuca high-grade gneiss xenoliths, with their clear tures. continental af®nity, and by the Nd isotopic signatures. The relationship between the accretion of the Tejupilco These xenoliths, together with granulitic xenoliths with metamorphic suite and the mildly deformed early Mesozoic Precambrian Nd model age in Neogene volcanics from the cover of the AcatlaÂn Complex in the western part of the Guanajuato volcanic ®eld (Urrutia-Fucugauchi and Uribe- Mixteco terrane remains to be examined. Deposition of Cifuentes, 1999), indicate the presence of old continental Triassic (?) ignimbrite and Middle Jurassic epicontinental crust beneath at least the eastern parts the Guerrero terrane. strata over the AcatlaÂn Complex and overlying Permian This has also been suggested from regional gravity data strata in the Olinala area (Corona-Esquivel, 1981), 200 km (Urrutia-Fucugauchi and Molina-Garza, 1992; Urrutia- SE from the study area, was apparently coeval with the Fucugauchi et al., 1993; GarcõÂa-PeÂrez, 1995). deformation of the Tejupilco metamorphic suite. These Old continental crust beneath at least the eastern parts the cover strata may have accumulated under anorogenic condi- Guerrero terrane proves the heterogeneous nature of this tions far from the collision zone where the tectonic effects of tectonostratigraphic terrane and is the other new crucial the Middle±early Late Jurassic accretion was not geologic element whose implications for the crustal struc- pronounced. In paleogeographic interpretations (e.g. ture and tectonic evolution of southern Mexico must be Sedlock et al., 1993; Ortega-GutieÂrrez et al., 1994), the evaluated. Thus, the current tectonic interpretations consid- Jurassic geodynamic evolution between the Guerrero ering that the island arc assemblages of the eastern part of terrane (NaÂhuatl) and Mixteco terranes is, however, very the Guerrero terrane were developed over Mesozoic oceanic uncertain. lithosphere (Campa, 1978; Campa and RamõÂrez, 1979; The deformation and imbricated thrust faults in the Arce- Ortiz-HernaÂndez et al., 1991; Tardy et al., 1991, 1992, lia-Palmar Chico group may be related to a second tectonic 1994; Lapierre et al., 1992; Talavera-Mendoza, 1993; event during the Late Cretaceous. These structures are Talavera-Mendoza et al., 1995) have to be re-examined. related to an early phase of Laramide orogenesis whose age in the Tejupilco area could be constrained by the age of the undated Tingambato batholith. This batholith, which 5. Conclusions intrudes the Arcelia-Palmar Chico group and is noncon- formably overlain by the Balsas Formation, is probably a Although further geologic mapping as well as strati- late synorogenic Laramide pluton. graphic, geochronologic, geochemical and isotopic studies An alternative interpretation for the eastern part of the are required to understand the paleogeographic setting and Guerrero terrane has been proposed (Centeno-GarcõÂaet tectonic evolution of the region, the data discussed here al., 1993a; Talavera-Mendoza et al., 1995). These authors support the presence of a intensely deformed Late Triassic- consider the Taxco Schist to be part of the Mixteco terrane Early Jurassic basement unit, the Tejupilco metamorphic thrust over the Morelos-Guerrero platform as part of the suite, in the easternmost part of the Guerrero terrane. The same terrane, whereas the Lower Cretaceous Teloloapan geologic and geochronologic data of the Tejupilco meta- sequence of the easternmost part of the Guerrero terrane is morphic suite, which may represent an evolved volcanic also thrust over the calcareous Morelos-Guerrero platform arc system, implies deformation, metamorphism and M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 373 erosion during Middle to early Late Jurassic time that was de Tumbiscatio, MichoacaÂn. In: Sociedad GeoloÂgica Mexicana, 6th probably related to the accretion and consolidation of the ConvencioÂn Nacional (MeÂxico DF), Programa y Resumenes, p. 48. easternmost Guerrero terrane with the AcatlaÂn Complex. Campa, M.F., Coney, P.J., 1983. Tectono-stratigraphic terranes and mineral resource distribution in Mexico. Canadian Journal of Earth Sciences 20, U±Pb zircon data for the Tizapa metagranite and Gren- 1040±1051. villian Nd isotopic signatures for this pluton and for the CantuÂ-Chapa, 1968. Las rocas eocretaÂcicas de ZitaÂcuaro, MichoacaÂn. Insti- Pepechuca high-grade xenoliths and other igneous units tuto Mexicano del PetroÂleo, TecnologõÂa de la ExploracioÂn, SeccioÂn indicate inherited Precambrian components in the Tejupilco GeologõÂa, MonografõÂa 2, 3±18. metamorphic suite. This precludes the simple generation of Centeno-GarcõÂa, E., 1994. Tectonic Evolution of the Guerrero Terrane, Tejupilco metamorphic suite magmas from a Phanerozoic Western Mexico. PhD thesis, University of Arizona, 96p. Centeno-GarcõÂa, E., GarcõÂa, J.L., Guerrero-SuaÂstegui, M., RamõÂrez-Espi- mantle or Mesozoic oceanic lithosphere sources. These data noza, J., Salinas-Prieto, J.C., Talavera-Mendoza, O., 1993a. Geology of and high-grade xenoliths in the Cenozoic volcanics indicate the southern part of the Guerrero terrane, Ciudad Altamirano-Teloloa- the presence of old continental crust beneath at least the pan area. In: Ortega-GutieÂrrez, F., Centeno-GarcõÂa, E., MoraÂn-Zenteno, eastern part of the Guerrero terrane. Hence, earlier tectonic D.J., GoÂmez-Caballero, A. (Eds.). Terrane Geology of Southern models involving the accretion of a Late Jurassic±Early Mexico: Guidebook of Field Trip B: First Circum-Paci®c and Circum-Atlantic Terrane Conference (Guanajuato, Mexico), Universi- Cretaceous intra-oceanic island arc onto the North America dad Nacional AutoÂnoma de MeÂxico, Instituto de GeologõÂa, Mexico, pp. plate during late-Early Cretaceous or Late Cretaceous time 22±33. will need to be substantially modi®ed. The paleotectonic Centeno-GarcõÂa, E., RuõÂz, J., Coney, P.J., Patchett, P.J., Ortega-GutieÂrrez, setting of the eastern part of the Guerrero terrane is conse- F., 1993b. Guerrero terrane of Mexico: its role in the Southern Cordil- quently open to reinterpretation, and further studies will be lera from new geochemical data. Geology 21, 419±422. needed to determine its role in the tectonic evolution of Chen, J.H., Moore, J.G., 1982. Uranium±lead isotopic ages from the Sierra Nevada batholith, California. Journal of Geophysical Research 87, southern Mexico. 4761±4784. Coney, P.J., Campa-Uranga, M.F., 1987. Lithotectonic Terrane Map of Mexico (West of the 91st Meridian). US Geological Survey, Miscella- Acknowledgements neous Field Studies Map MF-1874-D, Scale 1:2,500,000. Contreras-RodrõÂguez, L., Miranda-Molina, J.J., Vargas-MontanÄo, M., 1990. We are especially grateful to Zoltan de Cserna and Estudio geoloÂgico del aÂrea de Valle de Bravo-Nvo. Sto. TomaÂsdelos Fernando Ortega-GutieÂrrez for their constructive reviews Platanos, Estado de MeÂxico. In: 10th ConvencioÂn GeoloÂgica Nacional   of an earlier version of this manuscript. The manuscritp (Mexico DF), Memoria de ResuÂmenes, p. 35. Sociedad Geologica Mexicana. was also improved by the critical reviews by R. D. Nance Corona-Esquivel, R., 1981/1983. EstratigrafõÂa de la regioÂn de OlinalaÂ- and three anonymous reviewers. We would like to thank Tecocoyunca, noreste del Estado de Guerrero. Universidad Nacional Samuel A. Bowring for U±Pb zircon analyses; M LoÂpez- AutoÂnoma de MeÂxico, Instituto de GeologõÂa, Revista 5, 17±24. MartõÂnez for 40Ar/39Ar data; D.J. Terrell for K±Ar analyses; Cserna, Z., 1971. Mesozoic sedimentation, magmatic activity and deforma- Kenneth L. Cameron for Sm±Nd analyses; Richard L. tion in northern Mexico. In: Seewald, K., Sundeen, D. (Eds.). The Geologic Framework of the Chihuahua Tectonic Belt, West Texas Armstrong for providing, via Zoltan de Cserna, unpublished Geological Society, Midlands, TX, pp. 99±117. Rb±Sr and K±Ar data; and Antonio Quintino Mora for de Cserna, Z., 1982/1983. Hoja Tejupilco 14Q-g (9), con Resumen de la assistance with crushing, grinding, sieving the samples Hoja Tejupilco, Estados de Guerrero, MeÂxico y MichoacaÂn. Universi- discussed in this study and the Wil¯ey table procedures dad Nacional AutoÂnoma de MeÂxico, Instituto de GeologõÂa, Carta GeoloÂ- used. Jose de JesuÂs Vega Carrillo and Luis Burgos-Peraita gica de MeÂxico, Serie de 1:100,000. are thanked for their help in preparating the illustrations, de Cserna, Z., Fries, C., 1981. Hoja Taxco 14Q-h(7), con Resumen de la geologõÂa de la Hoja Taxco, Sstados de Guerrero, MeÂxico y Morelos. and we are grateful to Barbara Martiny for improving the Universidad Nacional AutoÂnoma de MeÂxico, Instituto de GeologõÂa, English. Carta GeoloÂgica de MeÂxico, Serie de 1:100,000. de Cserna, Z., Ortega-GutieÂrrez, F., Palacios-Nieto, M., 1980. Reconoci- miento geoloÂgico de la parte central de la cuenca del alto RõÂo Balsas, References Estados de Guerrero y Puebla. In: Libro GuõÂa de la ExcursioÂn GeoloÂgica a la Parte Central de la Cuenca del Alto RõÂo Balsas, Estados de Guerrero Anderson, T.H., Silver, L.T., 1971. Age of granulite metamorphism during y Puebla, pp. 1±33. Sociedad Geologica Mexicana. the Oaxaca orogeny, Mexico. Geological Society of America, Abstracts Dalrymple, G.B., Lanphere, M.A., Clague, D.A., 1980. Conventional and 40 39 with Programs 3, 492. Ar/ Ar, K±Ar ages of volcanic rocks from the Ojin (site 430), Barth, S., Oberli, F., Meier, M., 1989. U±Th±Pb systematics of morpho- Nintoku (site 432), and Suiko (site 433) seamounts and the chronology logically characterized zircon and allanite: a high-resolution isotopic of volcanic propagation along the Hawaiian-Emperor chain. Initial study of the Alpine Rensen pluton (northern Italy). Earth and Planetary Reports of the Deep Sea Drilling Project, 55, 659±676. Science Letters 95, 235±254. DaÂvila-Alcocer, V.M., Guerrero-SuaÂstegui, M., 1990. Una edad basada en Campa, M.F., 1978. La evolucioÂn tectoÂnica de Tierra Caliente, Guerrero. radiolarios para la secuencia volcanosedimentaria al oriente de Arcelia, BoletõÂn de la Sociedad GeoloÂgica Mexicana 39 (2), 52±64. Estado de Guerrero. In: 10th ConvencioÂn GeoloÂgica Nacional (MeÂxico Campa, M.F., RamõÂrez, J., 1979. La EvolucioÂn GeoloÂgica y la MetalogeÂn- DF), Memoria de ResuÂmenes. Sociedad GeoloÂgica Mexicana, p. 83. esis del Noroccidente de Guerrero. Universidad AutoÂnoma de Guerrero, Delgado-Argote, L.A., LoÂpez-MartõÂnez, M., York, D., Hall, C.M., 1992. Serie TeÂcnico-Cientõ®ca, 101 p. Geologic framework and geochronology of ultrama®c complexes of Campa, M.F., RamõÂrez, J., Bloome, C., 1982. La secuencia volcano-sedi- southern Mexico. Canadian Journal Earth Science 29, 1590±1604. mentaria metamor®zada del Triasico (Ladiniano-Carnico) de la regioÂn Doe, B.R., Stacey, J.S., 1974. The application of lead isotopes to the 374 M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375

problems of ore genesis and ore prospect evaluation: a review. Israde-Alcantara, I., Martinez-Alvarado, M.L., 1986. ContribucioÂn al Estu- Economic Geology 69, 757±776. dio GeoloÂgico de la TransicioÂn Pacõ®co-Tethis en el A rea de Zitacuaro, ElõÂas-Herrera, M., 1989. GeologõÂa MetamoÂr®ca del A rea de San Lucas del MichaocaÂn. Tesis de Licenciatura, ESIA, Instituto Politecnico Nacio- MaõÂz, Estado de MeÂxico. Universidad Nacional AutoÂnoma de MeÂxico, nal, 128p. Instituto de GeologõÂa, BoletõÂn 105, 79p. JICA-MMAJ Informe de la ExploracioÂn Cooperativa de Mineral en la ElõÂas-Herrera, M., 1993. Geology of the Valle de Bravo and Zacazonapan RegioÂn de Arcelia, Estados Unidos de MeÂxico, Fase IV. Japan Inter- areas, south-central Mexico: Field trip. In: Ortega-GutieÂrrez, F., national Cooperation Agency-Metal Mining Agency of Japan, 155p.; Centeno-GarcõÂa, E., MoraÂn-Zenteno, D.J., GoÂmez-Caballero, A. with Appendix, 97p. (Eds.). Terrane Geology of Southern Mexico: Guidebook of Field Krogh, T.E., 1973. A low-contamination method for hydrothermal decom- Trip B: First Circum-Paci®c and Circum-Atlantic Terrane Conference position of zircon and extraction of U and Pb for isotopic age determi- (Guanajuato, Mexico), Universidad Nacional AutoÂnoma de MeÂxico, nation. Geochimica et Cosmochimica Acta 37, 485±494. Instituto de GeologõÂa, pp. 12±21. Krogh, T.E., 1982. Improved accuracy of U±Pb zircon ages by the creation ElõÂas-Herrera, M., SaÂnchez-Zavala, J.L., 1990/1992. Tectonic implications of more concordant systems using an air abrasion technique. Geochi- of a mylonitic granite in the lower structural levels of the Tierra mica et Cosmochimica Acta 46, 637±649. Caliente complex (Guerrero terrane), southern Mexico. Universidad Lamouroux, C., Soula, J.C., Deramond, J., Debut, P., 1980. Shear zones in Nacional AutoÂnoma de MeÂxico, Instituto de GeologõÂa, Revista 9, the granodioritic massifs of the central Pyrenees and the behavior of 113±125. these massifs during Alpine orogenesis. Journal of Structural Geology ElõÂas-Herrera, M., Ortega-GutieÂrrez, F., Cameron, K., 1996. Pre-Mesozoic 2, 49±53. continental crust beneath the southern Guerrero terrane: xenolith Lapierre, H., Tardy, M., Coulon, C., Ortiz, H.E., Bourdier, J.-L., MartõÂnez, evidence. GEOS, UnioÂn GeofõÂsica Mexicana, ResuÂmenes y Programas R.J., Freydier, C., 1992. CaracteÂrisation, geneÂse et eÂvolution geÂodyna- de la ReunioÂn Anual, 16, 234. mique du terrain de Guerrero (Mexique occidental). Canadian Journal ElõÂas-Herrera, M., Ortega-GutieÂrrez, F., 1997. Petrology of high-grade peli- Earth Science 29, 2478±2489. tic xenoliths in an Oligocene rhyodacitic plug: precambrian crust Ludwig, K.R., 1991. ISOPLOT: A Plotting and Regression Program for beneath the southern Guerrero terrane? Revista Mexicana de Ciencias Radiogenic-Isotope Data, Version 2.50. US Geological Survey Open- GeoloÂgicas 14, 101±109. File Report 91, 40 p. Faure, G., 1986. Principles of Isotope Geology. 2nd ed.Wiley, New York, Mattinson, J.M., 1978. Age, origin and thermal histories of some plutonic NY (589p.). rocks from the Salinian Block of California. Contributions to Mineral- Foland, K.A., 1983. 40Ar/39Ar incremental heating pleteaus for biotites with ogy and Petrology 67, 233±245. excess argon. Isotope Goesciences 1, 3±21. McDougall, I., Harrison, T.M., 1988. Geochronology and Thermo- Fries, Carl, Jr., 1960. GeologõÂa del Estado de Morelos y de Partes Adya- chronology by the 40Ar/39Ar Method. Oxford University Press, New centes de MeÂxico y Guerrero, RegioÂn Central Meridional de MeÂxico. York, NY (212p.). Universidad Nacional AutoÂnoma de MeÂxico, Instituto de GeologõÂa, Mezger, K., Krogstad, E.J., 1997. Interpretation of discordant U±Pb zircon BoletõÂn 60, 236p. ages: an evaluation. Journal of Metamorphic Geology 15, 127±140. GarcõÂa-PeÂrez, F., 1995. CaracterizacioÂn GeofõÂsica de la RegioÂn de Tierra Miller, R.B., Paterson, S.R., 1994. The transition from magmatic to high- Caliente y A reas Colindantes de los Estados de Guerrero, MeÂxico y temperature solid-state deformation: implications from the Mount Morelos. Tesis de MaestrõÂa en SismologõÂayFõÂsica del Interior de la Stuart batholith, Washington. Journal of Structural Geology 16, 853± Tierra, Universidad Nacional AutoÂnoma de MeÂxico, 55p. 865. GonzaÂlez-Arreola, C., VillasenÄor-MartõÂnez, A.B., Corona-Esquivel, R., Ortega-GutieÂrrez, F., 1978. EstratigrafõÂa del Complejo AcatlaÂnenla 1994. Permian fauna of the Los Arcos Formation Municipality of Mixteca Baja. Estados de Puebla y Oaxaca. Universidad Nacional OlinalaÂ, State of Guerrero, Mexico. Revista Mexicana de Ciencias AutoÂnoma de MeÂxico, Instituto de GeologõÂa, Revista 2, pp. 112±131. GeoloÂgicas 11, 214±221. Ortega-GutieÂrrez, F., 1981. Metamorphic belts of southern Mexico and Gulson, B.L., Krogh, T.E., 1973. Old lead components in the young Bergell their tectonic signi®cance. Geo®sica Internacional 20 (3), 177±202. massif, south-east Swiss Alps. Contributions to Mineralogy and Petrol- Ortega-GutieÂrrez, F., Anderson, T.H., Silver, L.T., 1977. Lithologies and ogy 40, 239±252. geochronology of the Precambrian craton of southern Mexico. Geolo- Hammerschmidt, K., 1983. Hump shaped 40Ar/39Ar age spectra. Indication gical Society of America, Abstracts with Program 9, 1121±1122. of excess argon in white micas from the Swiss Alps. Fortschritte der Ortega-GutieÂrrez, F., Sedlock, R.L., Speed, R.C., 1994. Phanerozoic Mineralogie 1, 61±78. tectonic evolution of Mexico. In: Speed, R.C. (Ed.). Phanerozoic Evolu- Hansmann, W., Oberli, F., 1991. Zircon inheritance in an igneous rocks tion of the North American Continent Ð Ocean Transitions [DNAG suite from the southern Adamello batholith (Italian Alps): implications Continent-Ocean Transect Volume, Geological Society of America, for petrogenesis. Contributions to Mineralogy and Petrology 107, 501± Boulder, CO, pp. 265±306. 518. Ortega-GutieÂrrez, F., LoÂpez, R., Cameron, K.L., Ochoa-Camarillo, H., Harrison, T.M., 1983. Some observations on the interpretation of 40Ar/39Ar SaÂnchez-Zavala, J.L., 1995. Grenvillian Huiznopala Gneiss and its age spectra Isotope. Geosciences 1, 319±338. correlation with other Grenville-age high-grade terranes in eastern Harrison, T.M., McDougall, I., 1980. Investigations of an intrusive contact Mexico. Geological Society of America, Abstracts with Program 27, northwest Nelson, N.Z., II. Diffusion of radiogenic and excess 40Ar in 398. hornblende from 40Ar/39Ar age spectra. Geochimica et Cosmochimica Ortiz-HernaÂndez, L.E., Yta, M., Talavera, O., Lapierre, H., Monod, O., Acta 44, 2004±2020. Tardy, M., 1991. Origine intra-oceÂanique des formations volano-pluto- Harrison, T.M., McDougall, I., 1982. The thermal signi®cance of potassium niques d'arc du Jurassique SupeÂrieur-CreÂtace InfeÂrieur du Mexique feldspar K-Ar ages inferred from 40Ar/39Ar age spectrum results. centro-meÂridional. Compte Rendu, AcadeÂmie des Sciences Paris, Geochimica et Cosmochimica Acta 46, 1811±1820. SeÂrie II 312, 399±406. Herwig, J.C., 1982. Stratigraphy, Structure, and Massive Sul®de Potential Pankhurst, R.J., Moorbath, S., Rex, D.C., Turner, G., 1973. Mineral age of Late Mesozoic Metamorphic Rocks Near El Cirian, Mexico State, patterns in ca. 3700 m.y. old rocks from west Greenland. Earth and Mexico. MA thesis, University of Texas at Austin, 107p. Planetary Science Letters 20, 157±170. Hutchinson, R.W., 1980. Massive base metal sulphide deposits as guides to Pantoja-Alor, J., 1990. Rede®nicioÂn de las unidades estratigra®cas de la tectonic evolution. In: Strangway, D.W. (Ed.). The Continental Crust secuencia mesozoica de la regioÂn de Huetamo-Altamirano, estados de and Its Mineral Deposits, Geological Association of Canada Special MichoacaÂn y Guerrero. In: 10th ConvencioÂn Nacional (MeÂxico DF), Paper 20 Geological Association of Canada, Canada, pp. 659±684. Memoria de Resumenes. Sociedad GeoloÂgica Mexicana, p. 66. M. ElõÂas-Herrera et al. / Journal of South American Earth Sciences 13 (2000) 355±375 375

Parrish, R.R., 1987. An improved microcapsule for zircon dissolution in U± 1995. Petrology and geochemistry of the Teloloapan subterrane: a Pb geochronology. Chemical Geology 66, 99±102. lower Cretaceous evolved intra-oceanic island-arc. GeofõÂsica Interna- Paterson, S.R., Tobish, O.T., 1988. Using pluton ages to date regional cional 34, 3±22. deformations: problems with commonly used criteria. Geology 16, Tardy, M., Lapierre, H., Boudier, J.L., Yta, M., Coulon, C., 1991. The Late 1108±1111. Jurassic-Early Cretaceous arc of western Mexico (Guerrero terrane); Paterson, S.R., Tobish, O.T., Saleeby, J., 1987. Recognition of pretectonic origin and geodynamic evolution. In: 1st ConvencioÂn sobre la Evolu- intrusives: implications for the dating of structural events. Geological cioÂn GeoloÂgica de MeÂxico, and Congreso Mexicano de MineralogõÂa Society of America, Abstracts with Programs 19, 800. (Pachuca, Hidalgo, MeÂxico), Memoria, pp. 213±215. Paterson, S.R., Vernon, R.H., Tobish, O.T., 1989. A review of criteria for Tardy, M., Lapierre, H., Boudier, J.L., Coulon, C., Ortiz-HernaÂndez, L.E., the identi®cation of magmatic and tectonic foliations in granitoids. Yta, M., 1992. Intraoceanic setting of the western Mexico Guerrero Journal of Structural Geology 11, 349±363. terrane Ð implications for the Paci®c-Tethys geodynamic relationships Robinson, K.L., 1991. U±Pb Zircon Geochronology of Basement Terranes during the Cretaceous. Universidad Nacional AutoÂnoma de MeÂxico, and the Tectonic Evolution of Southwestern Mainland Mexico. MS Instituto de GeologõÂa, Revista 10, 118±128. thesis, San Diego State University, 114p. Tardy, M., Lapierre, H., Freydier, C., Coulon, C., Gill, J.B., Mercier de Robinson, K.L., Gastil, R.G., Campa-Uranga, M.F., RamõÂrez-Espinoza, J., Lepinay, B., Beck, C., MartõÂnez-Reyes, J., Talavera-Mendoza, O., 1989. Geochronology of basement and metasedimentary rocks in south- Ortiz-HernaÂndez, L.E., Stein, G., Boudier, G., Yta, M., 1994. The Guer- ern Mexico and their relation to metasedimentary rocks in peninsular rero suspect terrane (western Mexico) and coeval arc terranes (the California. Geological Society of America, Abstracts with Programs 21, Greater Antilles and the Western Cordillera of Colombia): a late Meso- 135. zoic intra-oceanic arc accreted to cratonal America during the Cretac- Ruiz, J., Centeno-GarcõÂa, E., Coney, P.J., Patchett, P.J., Ortega-GutieÂrrez, eous. Tectonophysics 230, 49±73. F., 1991. El terreno Guerrero y su posible correlacioÂn con el basamento Urrutia-Fucugauchi, J., Uribe-Cifuentes, R.S., 1999. Lower-crustal xeno- de la regioÂn del Caribe. In: 1st ConvencioÂn sobre la EvolucioÂn GeoloÂ- liths from the Valle de Santiago maar ®eld, Michoacan-Guanajuato gica de MeÂxico, and Congreso Mexicano de MineralogõÂa (Pachuca, Volcanic Field, central Mexico. International Geology Review 41, Hidalgo, MeÂxico), Memoria, pp. 192±193. 1067±1081. Saleeby, J.B., Scams, D.B., Kistler, R.W., 1987. U/Pb zircon, strontium, Urrutia-Fucugauchi, J., Molina-Garza, R.S., 1992. Gravity modeling of and oxygen isotopic and geochronological study of the southernmost regional crustal and upper mantle structure of the Guerrero terrane. 1. Sierra Nevada batholith, California. Journal of Geophysical Research Colima graben and southern Sierra Madre Occidental, western Mexico. 92, 10 443±10 466. Geo®sica Internacional 31 (4), 493±507. Sawkins, F.J., 1990. Integrated tectonic-genetic model for volcanic-hosted Urrutia-Fucugauchi, J., Mena, M., Flores-Ruiz, J.H., Molina-Garza, R.S., massive sul®de deposits. Geology 18, 1061±1064. 1993. Gravity models of regional crustal and upper mantle structure of Sedlock, R.L., Ortega-GutieÂrrez, F., Speed, R.C., 1993. Tectonostrati- the Guerrero terrane, southwestern Mexico. In: 1st Circum-Paci®c and graphic Terranes and Tectonic Evolution of Mexico. Geological Circum-Atlantic Terrane Conference (Guanajuato, Mexico), Proceed- Society of America, Special Paper 278, 153p. ings. Universidad Nacional AutoÂnoma de MeÂxico, Instituto de Geolo- Shaaf, P., 1990. Isotopengeochemische Untersuchungen an Granitoiden gõÂa, pp. 160±162. Gesteinen eines Aktiven Kontinentaalrandes. Alter und Herkunft der Vernon, R.H., Paterson, S.R., Geary, E.E., 1989. Evidence for syntectonic Tiefengesteinskomplexe an der Pazi®kkuste Mexikos Zwischen Puerto intrusion of plutons in the Bear Mountains fault zone, California. Geol- Vallarta und Acapulco. PhD thesis, Ludwig-Maximilians UniversitaÈtde ogy 17, 723±726. MuÈnchen, 202p. VillasenÄor-MartõÂnez, A.B., 1987. BioestratigrafõÂa del Paleozoico superior Stacey, J.S., Kramer, J.D., 1975. Approximation of terrestrial lead isotope de San Salvador Patlanoaya, Puebla, Mexico. Revista de la Sociedad evolution by a two-stage model. Earth and Planetary Science Letters 26, Mexicana de PaleontologõÂa 1, 396±417. 207±221. Walker, D.A., McDougall, I., 1982. 40Ar/39Ar and K-Ar dating of altered Steiger, R.H., JaÈger, E., 1977. Subcommission on geochronology: conven- glassy volcanic rocks: the Dabi Volcanics P.N.G. Geochimica et tion on the use of decay constants in geo- and cosmochronology. Earth Cosmochimica Acta 46, 2181±2190. and Planetary Science Letters 36, 359±362. Wallin, E.T., 1990. Petrogenesis and tectonic signi®cance of xenocrystic Talavera-Mendoza, O., 1993. Les Formationes OrogeÂniques MeÂsozoõÈques Precambrian zircon in Lower Cambrian tonalite, eastern Klamath du Guerrero (Mexique MeÂridional); Contribution a la Connaissance de Mountains, California. Geology 18, 1057±1060. l'E volution GeÂodynamique des CordilleÂres Mexicaines. TheÂse d'E tat, Wayne, D.M., Sinha, A.K., 1992. Stability of zircon U±Pb systematics in a Universite Joseph Fourier-Grenoble I, 462p. greenschist-grade mylonite: an example from the Rock®sh Valley Fault Talavera-Mendoza O., RamõÂrez-Espinosa, J., Guerrero-SuaÂstegui, M., Zone, central Virginia, USA. Journal of Geology 100, 593±603. 1993. Geochemical evolution of the Guerrero terrane Ð example of Wijbrans, J.R., McDougall, I., 1986. 40Ar/39Ar dating of white micas from a late Mesozoic multi-arc system. In: 1st Circum-Paci®c and Circum- an Alpine high-pressure metamorphic belt on Naxos (Greece): the reset- Atlantic Terrane Conference (Guanajuato, Mexico), Proceedings, pp. ting of the argon isotopic system. Contributions to Mineralogy and 150±152. Universidad Nacional AutoÂnoma de MeÂxico, Instituto de Petrology 93, 187±194. GeologõÂa. Zartman, R.E., Doe, B.R., 1981. Plumbotectonics-the model. Tectonophy- Talavera-Mendoza, O., RamõÂrez-Espinosa, J., Guerrero-SuaÂstegui, M., sics 75, 135±162.