Pre-Accretion Metamorphism of the Teloloapan Terrane (Southern Mexico): Example of Burial Metamorphism in an Island-Arc Setting

Journal of South American Earth Sciences 13 (2000) 337±354 www.elsevier.nl/locate/jsames

Pre-accretion metamorphism of the Teloloapan Terrane (southern Mexico): example of burial metamorphism in an island-arc setting

O. Talavera Mendoza*

Escuela Regional de Ciencias de la Tierra, Universidad AutoÂnoma de Guerrero, AP 197, Taxco, Guerrero, Mexico

Abstract Volcanic and interbedded volcaniclastic rocks of the lower Cretaceous island-arc series of the Teloloapan Terrane in southern Mexico contain metamorphic assemblages characteristic of the zeolite, prehnite±pumpellyite and lower greenschist facies produced by burial metamorphism prior to its accretion to nuclear Mexico. Distribution of secondary assemblages throughout the stratigraphic succession, together with the chemical evolution of metamorphic minerals, reveals a depth-controlled metamorphic zonation characterized by the presence of the diagnostic assemblages laumontite 1 pumpellyite 1 epidote and laumontite 1 celadonite 1 pumpellyite ^ epidote (zeolite facies) followed downward by assemblages containing prehnite 1 pumpellyite ^ white mica (prehnite±pumpellyite facies) and ®nally by the presence of the assemblages pumpellyite 1 actinolite 1 epidote and epidote 1 actinolite (greenschist facies). Analysis of assemblages in the Al±Fe31±FM±K system, reveals that facies boundaries are discontinuous, involving the disappearance of at least one phase and the appearance and/or extension of the ®eld of equilibrium of other diagnostic minerals and assemblages. Application of empirically based thermobarometers, phase equilibria, mineral chemistry, and petrogenetic grids indicates that the P±T conditions of metamorphism ranged from 175 to 3428C and P , 3 kbar: The data further indicate high geothermal gradients of about ,558Ckm21. Seawater-derived ¯uids were characterized by high a , high f and low X . q 2000 Elsevier Science B.V.. All rights reserved. K O2 CO2 Keywords: Teloloapan Terrane; Metamorphic zonation; Hydrothermal alteration

1. Introduction nate by interaction of percolating seawater with cooling magma at or near volcanic centers. Far away, hydrothermal It is now accepted that ¯uid circulation through the ocea- transformation is limited. nic crust appears to be the main post-magmatic process Although hydrothermal alteration is considered to be responsible for the major chemical and mineralogical trans- classic of ocean-¯oor environments, identical transforma- formations at low to moderate temperature observed in most tions have been recorded in other tectonic settings, includ- submarine volcanic rocks (e.g. Ito and Anderson, 1983; ing intra-oceanic island arcs and oceanic islands (e.g. Schiffman and Day, 1995), and that transformations Beiersdorfer, 1993; Himmelberg et al., 1995). Despite produced by recrystallization during collision or accretion this, most studies on phase stability in low- to medium- can be more limited than thought. The ¯uid circulation and grade metabasites and patterns of hydrothermal circulation the degree of transformation are essentially controlled by have been essentially carried out in ocean-¯oor suites (Betti- the spatial distribution of hydrothermal vents, fractures, and son-Varga et al., 1995; Manning and Bird, 1995). On the cataclastic zones. The main characteristics of the hydrother- other hand, despite the role of hydrothermal alteration in the mal metamorphism are the absence of penetrative deforma- modi®cation of primary features in volcanic assemblages, tion, conservation of magmatic textures and structures, many workers emphasize the more obvious deformation- destabilization of mostly igneous minerals, and growth of induced metamorphism and, in consequence, the earliest hydrous secondary phases. Detailed studies of the meta- hydrothermal transformations are poorly understood in morphic assemblages, mineral chemistry, ¯uid inclusions, most volcanic terranes. In this paper, we present a detailed and stable isotopes suggest that hydrothermal ¯uids origi- description of low-grade metamorphic assemblages recorded in a 3000 m thick volcanic succession that origi- * Tel.: 152-7-207-41; fax: 152-7-207-41. nated in an intra-oceanic island-arc environment which is E-mail address: [email protected] (O. Talavera Mendoza). now well exposed in cordilleran Mexico. The observed

Fig. 1. Simpli®ed geological map and schematic structural section of the Teloloapan Terrane in the Teloloapan-Arcelia area showing the locations of study samples (based on Campa and RamõÂrez, 1979). Solid circles are sample localities; open circles indicate locations of samples analyzed by microprobe. distribution of critical assemblages and secondary minerals circulation of hydrothermal solutions. Finally, these data throughout the stratigraphic succession together with chem- together with those available from accretion-induced meta- istry of all metamorphic phases allow us to constrain the morphic recrystallization are used to better constrain the variations of physical conditions during increasing meta- metamorphic evolution of the Teloloapan Terrane before morphism as well as the composition and conditions of and during accretion to nuclear Mexico. O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 339

Fig. 2. Stratigraphic locations of analyzed samples showing the distribution of mineral paragenesis throughout the lower volcanic succession.

2. Geological framework et al., 1991; Centeno et al., 1993; Talavera et al., 1995). For detailed data concerning the tectonic setting of the Teloloa- The Teloloapan Terrane is a 350 km long and 80 km wide pan lavas and their spatial and temporal relationships with stratigraphic-structural unit located in the southern portion other arc sequences of western Mexico, the reader is of the cordilleran domain of Mexico (Fig. 1). This terrane is referred to the recent studies by Talavera et al. (1995) and one of the three major tectonostratigraphic sequences Talavera and Guerrero (2000). included in the composite Guerrero Terrane and has been In the Arcelia±Teloloapan area (Fig. 1), the Teloloapan considered to be largely allochthonous relative to the North- Terrane sequence broadly includes two lithostratigraphic American craton (e.g. Campa and Coney, 1983; RamõÂrez et units (Fig. 2). The ®rst (lower) is the volcanic unit formed al., 1991; Talavera et al., 1993). Stratigraphic, sedimento- of approximately 3000 m of pillow lavas, pillow breccias, logic, geochemical, and isotopic studies indicate that Telo- hyaloclastites, and massive ¯ows. At the base, lavas are loapan lavas and related sedimentary rocks originated in a interbedded with Lower Cretaceous radiolarian-rich silic- wholly submarine intra-oceanic island arc during Hauteri- eous sediments and, at the top, with pyroclastic rocks and vian to early Cenomanian times (Guerrero et al., 1990; Ortiz coarse-grained debris ¯ow deposits containing Aptian 340 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 fauna. Discontinuous lenses of clastic and reefal limestone tary unit was also sampled. In total, more than 250 samples are mixed or interbedded with volcaniclastic rocks at the top were studied petrographically, 20 of which were selected for of the volcanic pile. The second (upper) is the 1500 m thick microprobe and whole rock analyses. Spatial locations of sedimentary unit, which includes Upper Aptian volcaniclas- the studied sections are shown in Fig. 1, and the strati- tic turbidites and Albian to Lower Cenomanian reefal and graphic positions of the probed samples are shown in Fig. 2. bioclastic limestones (Guerrero et al., 1990, 1991). Mineral compositions were measured in automated These rocks were deformed during the tectonic emplace- CAMEBAX microprobes at the BRGM-UniversiteÂ d'Or- ment of the Guerrero Terrane onto continental Mexico leÂans common laboratory and at the ENSEEG, UniversiteÂ during Late Cretaceous±Early Tertiary times (Campa et Joseph Fourier-Grenoble using albite (Na), K-feldspar (K), al., 1976; Campa and RamõÂrez, 1979; Campa and Coney, corundum (Al), wollastonite (Si), forsterite (Fe), apatite 1983; Salinas et al., 1993). Accretion produced a very (Ca), chromite (Cr), rutile (Ti), rodonite (Mn), bunsenite heterogeneous deformation pattern characterized by the (Ni) and MgO (Mg) as standards. Analytical conditions presence of east-vergent isoclinal folds and regional thrust were: constant beam current at 10 nA, a 15 kV accelerating faults and a subhorizontal phyllitic cleavage and schistosity potential, and spot size of 3±10 mm. Count times were with an E-W stretching lineation. Despite this deformation, generally 10 s, except for Na for which shorter 6 s count volcanic structures were generally preserved, and the defor- times were used. Under these conditions, concentrations mation obliterated primary volcanic characteristics only below 0.1% are considered below detection limits. near the major tectonic discontinuities. The Teloloapan sequence has been recognized for some time as a low-grade metamorphic terrane, although systema- 4. Pre-accretion metamorphism tic studies concerning its metamorphic evolution are lack- 4.1. Metamorphic zonation of burial assemblages ing. Most workers have emphasized the presence of the greenschist assemblage: chlorite 1 epidote 1 actinolite ^ Metamorphic assemblages are widespread in the lower white mica, which has been accepted as indicative of the volcanic unit and occur in both lavas and interbedded volca- metamorphic conditions in the Teloloapan sequence. This niclastic rocks. Volcaniclastic rocks from the upper sedi- assemblage has been interpreted classically to result from mentary cover are unprovided with static assemblages, synkinematic metamorphism during the tectonic emplace- strongly suggesting that metamorphism occurred immedi- ment of the volcanic series (Campa et al., 1974; De Cserna ately after the extrusion of the lavas and before the deposit Â et al., 1978; Campa and Ramõrez, 1979). of the upper sedimentary unit (Fig. 2). The distribution of Field observations together with fabric, textural, and diagnostic phases throughout the lower volcanic unit mineralogical studies reveal a more complex metamorphic strongly suggests a depth-controlled metamorphic zoning: evolution involving two distinctive, low-grade metamorphic zeolite- and celadonite-bearing assemblages appear only in events: an early, hydrothermal metamorphism that affected the uppermost stratigraphic levels, followed downward by the volcanic rocks immediately after their extrusion, produ- prehnite 1 pumpellyite assemblages; actinolite 1 epidote- cing diagnostic assemblages of the zeolite through bearing assemblages are restricted to the lowermost strati- prehnite±pumpellyite to lower greenschist facies (pre- graphic levels (Fig. 2). These critical minerals, in associa- ); and a later meta- accretion metamorphism, denoted M1 tion with other phases, are diagnostic assemblages of the morphism, related to the tectonic accretion of the sequence, zeolite, prehnite±pumpellyite, and greenschist facies, that affected both volcanic and sedimentary rocks and respectively. Similar patterns recorded in other meta- produced synkinematic recrystallization under greenschist morphic sequences have been interpreted to result from facies conditions (accretion-related metamorphism, hydrothermal- or oceanic-type metamorphism (e.g. Evarts denoted M ). 2 and Schiffman, 1983; Mevel, 1984; Aguirre and Atherton, 1987).

3. Sampling and analytical techniques 4.2. Petrography and metamorphic mineralogy

A number of traverses encompassing both the volcanic Lavas in the Teloloapan area are highly vesicular and unit and the sedimentary cover were carried out in the ®eld. porphyritic. In general, vesicules are greater and more abun- For detailed descriptions and sampling 14 cross-sections dant in the top of ¯ows, although no systematic differences were selected. In all cases, the structure was determined in metamorphic mineralogy or mineral chemistry were carefully and, as much as possible, stratigraphic repetition observed between the core and the top. Conversely, igneous was avoided. Stratigraphic levels were controlled by phases are better developed in the core of lava ¯ows. Lavas combining faunal data, interlayered sedimentary facies, vary in composition from basalt to andesite, with scarce and key lithology (Guerrero et al., 1991, 1993). Systematic rhyolitic ¯ows (for details see Talavera et al., 1995). Pheno- sampling was focused on the core and top of lava ¯ows and crysts in basalts include diopsidic to augitic clinopyroxene, on interbedded volcaniclastics, although the upper sedimen- albitized plagioclase, orthopyroxene, and titanomagnetite O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 341

Table 1 Representative analyses of laumontite, celadonite, and epidote (repartition of sites in pumpellyite formula after Coombs et al. (1976); Plag plagioclase; Amd amygdule; Grm groundmass; Opx orthopyroxene; Amph amphibole; Ox oxide; Zeo zeolite; Pr±Pp prehnite±pumpellyite; Pp±Act pumpellyite±actinolite; Greensch greenschist)

Laumontite Celadonite Sample Tx-33 Z-09B Z-09B Z-09B Z-09B Z-09B Z-18A Z-18A Analysis 1 2 3 4 5 6 7 8 Occurrence Plag Amd Amd Plag Amd Grm Amd Amd Facies Zeo Zeo Zeo Zeo Zeo Zeo Zeo Zeo SiO2 50.68 51.42 51.29 53.02 53.45 56.19 55.07 55.65 Al2O3 22.03 20.89 21.01 22.35 19.82 18.59 18.59 12.60 TiO2 ± ± ± ± 0.02 0.09 0.05 0.05 FeOa 0.03 0.01 0.14 ± 6.72 6.79 7.23 11.78 b Fe2O3 ±±±±±± ± ± MnO ± ± ± ± 0.07 0.03 0.02 0.01 MgO ± ± ± ± 4.50 4.67 4.66 6.15 CaO 12.35 11.85 12.11 11.64 0.20 ± 0.10 0.06 Na2O 0.26 0.32 0.12 0.10 0.17 0.07 0.03 0.10 K2O 0.05 0.09 0.17 0.07 9.56 9.93 9.72 9.36 Total 85.40 84.58 84.84 87.18 94.51 96.35 95.47 95.75 12 Oxygens 22 Oxygens Si 3.954 4.037 4.025 4.023 7.313 7.519 7.461 7.711 Al 2.026 1.933 1.943 1.999 3.190 2.936 2.963 2.053 Ti ± ± ± ± 0.002 0.009 0.005 0.005 Fe21 0.002 0.001 0.009 ± 0.767 0.757 0.816 1.360 Fe31 ±±±±±± ± ± Mn ± ± ± ± 0.008 0.004 0.002 0.001 Mg ± ± ± ± 0.923 0.937 0.947 1.279 Ca 1.032 0.997 1.018 0.946 0.029 ± 0.015 0.009 Na 0.039 0.049 0.018 0.015 0.045 0.018 0.009 0.026 K 0.005 0.009 0.017 0.007 1.670 1.696 1.681 1.655 Total 7.057 7.025 7.025 6.989 13.947 13.866 13.899 14.099 31 XFe ±±±±±± ± ± Fe/Al rock ± ± ± ± ± ± ± ± CeladoniteEpidote Sample Tx-36 Tx-36 Tx-70 Z-18A Tx-65 T-250A T-260 T-254 Analysis 9 10 11 12 13 14 15 16 Occurrence Opx Amd Opx Amd Amd Amd Amph Amd Facies Zeo Zeo Zeo Zeo Zeo Pr±Pp Pp±Act Greensch SiO2 55.55 48.26 56.45 37.55 38.31 38.60 38.20 37.22 Al2O3 6.98 11.90 8.41 23.01 23.05 22.34 27.09 26.38 TiO2 ± ± 0.03 ± 0.11 ± 0.18 0.06 FeOa 14.31 16.32 15.90 ± ± ± ± ± b Fe2O3 ± ± ± 13.65 12.95 10.97 5.88 7.59 MnO 0.01 0.12 0.04 0.01 0.10 0.01 0.07 0.07 MgO 6.19 7.56 6.60 ± 0.04 0.26 ± 0.01 CaO 0.03 0.04 0.01 23.61 22.32 22.32 23.94 23.90 Na2O 0.02 0.03 0.03 ± ± ± ± ± K2O 9.52 7.21 8.62 ± ± ± ± ± Total 92.61 91.45 96.10 97.83 96.88 94.50 95.35 95.23 22 Oxygens 12.5 Oxygens Si 8.106 7.210 7.935 3.000 3.066 3.145 3.043 2.994 Al 1.198 2.092 1.391 2.167 2.174 2.146 2.544 2.501 Ti ± ± 0.003 ± 0.007 ± 0.010 0.004 Fe2 1 1.740 2.033 1.862 ± ± ± ± ± Fe3 1 ± ± ± 0.819 0.778 0.671 0.352 0.459 Mn 0.001 0.015 0.005 0.001 0.007 0.001 0.005 0.005 Mg 1.355 1.695 1.392 ± 0.005 0.032 ± 0.002 Ca 0.005 0.006 0.002 2.020 1.914 1.949 2.044 2.060 Na 0.006 0.008 0.009 ± ± ± ± ± K 1.773 1.376 1.546 ± ± ± ± ± Total 14.184 14.435 14.145 8.007 7.952 7.944 7.998 8.023 31 XFe ± ± ± 0.270 0.26 0.24 0.12 0.16 Fe/Al rock ± ± ± 0.460 0.580 0.560 0.550 0.520

a Total Fe as FeO. b Total Fe as Fe2O3. 342 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 (greenschist facies), amphibole is nearly totally replaced by a combination of chlorite, pumpellyite, epidote, secondary oxides, actinolite, and plagioclase. Clinopyroxene remains unalterated throughout the stratigraphic column. Laumontite. Laumontite is a common phase in the Telo- loapan metabasites. It occurs in amygdules as small (most are ,0.06 mm) but well-de®ned cloudy prisms associated with chlorite, epidote, pumpellyite, celadonite, and quartz, or replacing plagioclase phenocrysts as irregular patches associated with albite, pumpellyite, K-feldspar, and white mica. Individual crystals are very homogeneous in compo-

sition, with only small amounts of Na2O (0.10±0.32%) and negligible FeO and MgO. The calculated (anhydrous)

Fig. 3. MR3-2R3-3R2 ternary plot for Teloloapan celadonites and white formula Ca0.9±1.0 Al1.9±2.0 Si3.9±4.0 O12 closely approaches micas (after Cathelineau and Izquierdo, 1988). Solid circles celadonites; the ideal stoichiometry. Laumontite replacing plagioclase open circles white micas; circled dots chlorite bulk composition. has slightly higher Al2O3 contents (e.g. anal. 1 and 4, Table 1) relative to laumontite found in amygdules (e.g. set in a plagioclase- and glass-rich groundmass. Olivine anal. 2 and 3, Table 1), whereas laumontite coexisting ghosts were observed in some orthopyroxene-free basalts with celadonite, white mica, and/or K-feldspar in any in the lowermost stratigraphic levels, whereas pargasitic domain is slightly richer in K2O (e.g. anal. 3, Table 1). amphibole is a common phase in some chemically evolved Celadonite. Celadonite is a common phase in the upper basalts. Essentially aphyric basalts are less common and levels of the stratigraphic column and constitutes an essen- contain rare microphenocrysts of augitic to salitic clinopyr- tial phase in the diagnostic assemblages of the zeolite facies. oxene and albitized plagioclase in a groundmass containing It is commonly found as deep to apple green, cryptocrystal- salite and plagioclase microlites along with glass and acicu- line aggregates ®lling amygdules, in the groundmass, or as a lar Fe±Ti oxides. Porphyritic andesites contain phenocrysts replacement of orthopyroxene or, more rarely, of olivine of sodic plagioclase, pargasitic to edenitic amphibole, ortho- and plagioclase. Celadonite was also observed in some pyroxene, and Fe±Ti oxides, supported in a groundmass high-variance assemblages associated with chlorite and containing plagioclase microlites and devitri®ed glass. quartz in rocks belonging to the prehnite±pumpellyite Secondary minerals occur in amygdules and veins or facies. In these assemblages, however, it always showed replacing primary minerals and groundmass. Magmatic evidence of instability Ð e.g. the presence of reaction coro- textures and structures are generally well preserved, nas (chlorite and/or white mica?), and was therefore consid- suggesting that alteration might be related to the action of ered to represent metastable equilibrium. The SiO2,Al2O3 hydrothermal ¯uids. In places where the subsequent, dyna- and K2O contents of the Teloloapan celadonites are distinc- mically induced recrystallization severely affected the tively high compared to those found in celadonites from rocks, hydrothermal assemblages were partly destroyed. hydrothermally altered oceanic basalts (Andrews, 1980). However, in some cases they can be distiguished as they They are, however, comparable to compositions reported systematically do not show preferred orientation and in arc-related geothermal systems (Cathelineau and because amygdules and crystals containing hydrothermal Izquierdo, 1988). Celadonite compositions plotted in the assemblages appears in the core of asymmetric structures MR3-2R3-3R2 diagram of Cathelineau and Izquierdo (asymmetric porphyroblasts and a-type structures). Microp- (1988) indicate that Teloloapan celadonites represent vari- robe analyses of metamorphic phases within these structures able proportions of solid substitution of white mica and indicate that they did not undergo signi®cant changes in celadonite end member compositions (Fig. 3). Nevertheless, chemistry. an excess of Fe in some celadonites compared to the ideal Nearly all primary minerals were partly or totally contents in pure celadonite indicate the participation of an replaced by metamorphic assemblages. Plagioclase, olivine, ªextraneousº source of FeO (Table 1). In Fig. 3, these cela- orthopyroxene, and Fe±Ti oxide phenocrysts are trans- donites are displaced toward the 3R2-chlorite corner. This formed completely throughout the lower volcanic unit. fact suggests that the enrichment could be related either to Plagioclase is replaced by a combination of albite, K-feld- some degree of substitution of Fe for Mg in the octahedral spar, prehnite, white mica, pumpellyite, and laumontite; layer under oxidizing conditions (Cathelineau and olivine and orthopyroxene are replaced by a combination Izquierdo, 1988) or, more probably, to a submicroscopic of celadonite, prehnite, pumpellyite, chlorite, and quartz, mixture of celadonite and chlorite with which it is closely whereas Fe±Ti oxides are replaced by hematite 1 titanite 1 associated. epidote/pumpellyite. Magmatic amphibole is preserved only Epidote. Epidote is one of the most abundant secondary in rocks from the upper stratigraphic levels (zeolite and minerals and occurs in almost all rocks throughout the stra- prehnite±pumpellyite facies). At lower stratigraphic levels tigraphic succession. Its shape, size, and composition vary O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 343

31 Fig. 4. Histograms of analyzed XFe ratios of: (a) epidotes; (b) pumpellyites; (c) prehnites, showing the relationships between Fe contents and metamorphic grade. Numbers in parentheses indicate the number of analyses for each facies. as a function of the stratigraphic depth, and therefore with ideal three atoms per formula unit and negligible Ti, Mg, metamorphic grade. In rocks of the zeolite and prehnite± and Mn (Table 1). The Fe31 , Al substitution is restricted tot tot pumpellyite facies, epidote occurs as deep yellow to brown to the range of XFe31 0:11±0:33 XFe31 Fe =Fe 1 radial aggregates (usually 0.05±0.5 mm across) in amyg- Altot; and it systematically decreases with the increasing 31 dules and in the groundmass, or as cloudy masses in plagi- metamorphic grade (Fig. 4a). The XFe values of epidotes oclase phenocrysts and clinopyroxene inclusions. In from zeolite facies rocks (0.17±0.33) and prehnite±pumpel- greenschist facies rocks, it is found as colorless, well- lyite facies rocks (0.17±0.29) are signi®cantly higher than de®ned crystals (0.4±0.8 mm) in amygdules and in the those found in epidotes from greenschist facies rocks, which groundmass, or as radial or fan-like aggregates and isolated range from 0.11 to 0.19. Similar patterns have been found in needles in amphibole phenocrysts. Epidotes from all of the epidotes from many low P±T metamorphic terrains and metamorphic facies have Si contents (2.9±3.1) close to the have been interpreted to re¯ect decreasing f with O2 344 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354

Table 2 Representative analyses of pumpellyite and prehnite (repartition of sites in pumpellyite formula after Coombs et al. (1976). Plag plagioclase; Amd amygdule; Grm groundmass; Opx orthopyroxene; Amph amphibole; Ox oxide; Zeo zeolite; Pr±Pp prehnite±pumpellyite; Pp±Act pumpellyite±actinolite; Greensch greenschist)

Pumpellyite Sample Z-18A Tx-36 Z-18A T-250A T-230 T-265 T-265 Analysis 1 2 3 4 5 6 7 Occurrence Amd Amd Amd Amd Amd Amph Amd Facies Zeo Zeo Zeo Pr±Pp Pr±Pp Pr±Pp Greensch

SiO2 35.86 34.80 35.55 36.70 35.71 35.01 37.73 Al2O3 17.18 21.04 18.59 22.81 25.11 23.45 23.82 TiO2 0.17 ± 0.08 ± 0.03 0.07 0.01 a Fe2O3 11.78 8.61 10.63 6.28 3.87 4.96 4.15 MnO 0.06 0.12 ± ± 0.18 0.15 0.07 MgO 1.99 2.57 1.90 1.77 2.62 2.71 2.49 CaO 21.90 22.35 22.26 22.59 22.60 22.42 22.78

Na2O ±±±± ±±± K2O ±±±± ±±± Total 88.94 89.49 89.04 90.15 90.12 88.77 91.05 24.5 Oxygens and 16 Cations Si 6.266 5.927 6.101 6.172 5.920 5.920 6.144 Al ± 0.073 ± ± 0.080 0.080 ± SUM Z 6.266 6.000 6.101 6.172 6.000 6.000 6.144 Al 3.538 4.000 3.760 4.000 3.996 3.991 4.000 Ti 0.022 ± 0.010 ± 0.004 0.009 ± Fe31 0.440 ± 0.230 ± ± ± ± SUM Y 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Fe31 1.106 1.101 1.140 0.793 0.482 0.630 0.508 Mg 0.518 0.652 0.485 0.444 0.647 0.683 0.605 Al ± 0.151 ± 0.521 0.843 0.657 0.573 SUM X 1.624 1.904 1.625 1.758 1.972 1.970 1.686 Ca 4.100 4.079 4.093 4.070 4.015 4.062 3.975 Mn 0.009 0.017 ± ± 0.025 0.021 0.009 SUM W 4.109 4.096 4.093 4.070 4.040 4.083 3.984 31 XFe 0.30 0.21 0.25 0.15 0.09 0.12 0.10 Fe/Al rock 0.46 0.40 0.46 0.56 ± 0.49 0.55 Prehnite Sample Tx-69 T-268 Tx-69 T-242 Tx-30 T-230 T-230 Analysis 8 9 10 11 12 13 14 Occurrence Plag Vein Amd Amd Plag Amd Amd Facies Zeo Zeo Zeo Pr±Pp Pr±Pp Pr±Pp Pr±Pp

SiO2 42.97 43.34 42.21 42.30 40.76 38.61 39.53 Al2O3 27.00 26.77 28.65 28.85 20.88 31.66 30.42 TiO2 0.01 ± 0.05 ± ± 0.01 ± a Fe2O3 3.84 2.17 4.32 2.49 1.20 1.21 2.14 MnO 0.09 0.06 0.24 ± 0.04 0.06 0.08 MgO 0.34 0.09 0.12 0.01 0.08 0.10 0.05 CaO 19.30 21.86 20.65 20.70 22.85 23.85 23.14

Na2O 1.97 0.04 0.13 1.06 0.04 0.13 ± K2O 0.02 ± 0.06 0.03 0.02 0.01 ± Total 95.53 94.29 96.43 95.43 96.21 95.64 95.36 11 Oxygens Si 2.932 2.978 2.854 2.879 2.759 2.647 2.709 Al 2.171 2.168 2.283 2.314 2.464 2.558 2.457 Ti 0.001 ± 0.003 ± ± 0.001 ± Fe31 0.197 0.112 0.219 0.127 0.61 0.062 0.110 Mg 0.034 0.009 0.012 0.001 0.008 0.010 0.005 Ca 1.411 1.609 1.496 1.509 1.657 0.175 1.699 Mn 0.005 0.003 0.014 ± 0.002 0.003 0.005 Na 0.260 0.005 0.017 0.140 0.510 0.017 ± K 0.002 ± 0.005 0.002 0.002 0.001 ± Total 7.014 6.885 6.903 6.972 7.004 7.052 6.986 31 XFe 0.080 0.05 0.09 0.05 0.02 0.02 0.04 Fe/Al rock 0.51 0.50 0.51 0.62 0.96 ± ±

a Total Fe as Fe2O3. O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 345

Fig. 5. Plot of Teloloapan pumpellyites in the Al±Fetot±Mg diagram of Coombs et al. (1976); the classi®cation schema of pumpellyites by Passaglia and Gottardi (1973) is also shown. The bulk rock trend is denoted by the heavy arrow, indicating chemical compositions of the host rock in which pumpelliyte was recorded. Open symbols pumpellyite within amygdules and veins; solid symbols pumpellyite as replacement of igneous mineral phases and groundmass; crosses whole rock composition. increasing depth or metamorphic grade (Holdaway, 1972; with greenschist paragenesis are comparable to those from Liou, 1973; Aguirre and Atherton, 1987). prehnite±pumpellyite facies. To the Al±Fetot±Mg diagram Pumpellyite. Pumpellyite is a conspicuous phase in rocks of Coombs et al. (1976), we have added the pumpellyite of the prehnite±pumpellyite and zeolite facies. It was addi- classi®cation scheme of Passaglia and Gottardi (1973), tionally recorded in association with actinolite 1 epidote in and bulk rock compositions have also been plotted (Fig. 5). two samples of the greenschist facies. It commonly occurs as radial or spherulitic aggregates or acicular prisms in amygdules and groundmass or as ®ne-grained patches in plagioclase and amphibole, more rarely in clinopyroxene phenocrysts. The Si content of pumpellyites from all metamorphic grades is close to the ideal six atoms, suggesting that Al is absent or present only in very low quantities in the Z-sites. Al is largely in excess and appears as the only cation in Y-sites in most pumpellyites (Table 2). On the other hand, there is a strong negative correlation between Fetot and Al from X-sites. This feature, together with the less evident correlation between Fetot and Mg, suggests that: (1) most Fe in pumpellyites is Fe31, which supports the criteria chosen for calculating the structural formula; and (2) the substitution Fe31 , Al is dominant over the substitution Fe21 , Mg and probably occurs only in the X-sites. The 31 XFe ratios of pumpellyites range from 0.08 to 0.31, and correlate inversely with metamorphic grade as previously observed in other low-grade metamorphic terrains (e.g. Cho et al., 1986; Aguirre and Atherton, 1987). From Fig. 4b, it is evident that pumpellyites from zeolite facies rocks are signi®cantly richer in Fe (X31 ranges from 0.14 to 0.31, Fe Fig. 6. Teloloapan chlorite compositions in the classi®cation diagram of with a maximum at 0.20) compared to those from Hey (1954). Open squares zeolite facies; solid squares prehnite± 31 prehnite±pumpellyite facies (XFe ranges from 0.08 to pumpellyite facies; open circles greenschist facies; Ri ripidolite; Br 0.22, with a maximum at 0.14). Pumpellyites associated brunsvigite; Py pycnochlorite; Di diabantite. 346 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354

Table 3 Representative analyses of chlorite (Plag plagioclase; Amd amygdule; Grm groundmass; Opx orthopyroxene; Amph amphibole; Ox oxide; Zeo zeolite; Pr±Pp prehnite±pumpellyite; Pp±Act pumpellyite±actinolite; Greensch greenschist)

Chlorite Sample Z-09B Z-18B Tx-33 Tx-65 T-206 T-250A Analysis 1 2 3 4 5 6 Occurrence Amd Amd Amd Amd Amd Amd Facies Zeo Zeo Zeo Zeo Pr±Pp Pr±Pp

SiO2 30.85 31.33 28.71 31.28 34.64 31.14 Al2O3 14.30 17.86 18.27 16.05 15.58 14.81 TiO2 ± ± ± ± 0.01 0.04 FeOa 24.60 19.70 24.59 20.82 24.33 23.90 MgO 17.74 17.41 15.09 20.43 13.05 17.87 CaO 0.10 0.35 0.58 0.16 ± 0.22 MnO 0.07 0.22 0.17 0.053 0.38 0.09

Na2O ± 0.08 0.09 ± ± ± K2O ± ± ± 0.04 ± ± Total 87.66 86.94 87.50 89.09 87.98 88.11 28 Oxygens Si 6.438 6.389 6.010 6.289 7.082 6.435 AlIV 1.562 1.611 1.990 1.711 0.918 1.565 AlVI 1.956 2.682 2.518 2.092 2.836 2.043 Ti ± ± ± ± 0.002 0.006 Fe21 4.294 3.360 4.305 3.501 4.160 4.131 Mg 5.519 5.292 4.710 6.123 3.977 5.505 Ca 0.022 0.076 0.104 0.035 ± 0.049 Mn 0.012 0.038 0.039 0.530 0.066 0.016 Na ± 0.032 0.024 ± ± ± K ± ± ± 0.010 ± ± Total 19.803 19.480 19.701 19.814 19.040 19.761 21 XFe 0.44 0.39 0.48 0.36 0.51 0.43 Xsmt 0.12 0.33 0.18 0.13 0.14 0.16 T8C 190 197 257 213 208 190 Chlorite Sample Tx-57 T-230 T-265 T-258 T-254 T-254 Analysis 7 8 9 10 11 12 Occurrence Amd Amd Amd Amd Amd Amd Facies Pr±Pp Pr±Pp Greensch Greensch Greensch Greensch

SiO2 29.34 26.39 26.31 26.07 25.70 25.28 Al2O3 18.12 18.54 19.07 18.74 18.68 19.31 TiO2 0.01 0.03 ± ± ± 0.01 FeOa 24.20 28.63 24.24 25.25 25.09 25.77 MgO 16.23 13.43 14.18 14.55 15.02 14.02 CaO 0.25 0.05 0.03 ± 0.01 0.02 MnO 0.27 0.20 0.17 0.43 0.45 0.36

Na2O ± 0.06 ± 0.03 0.03 0.01 K2O± ±±±±± Total 88.50 87.33 84.00 85.07 84.98 84.80 28 Oxygens Si 6.049 5.691 5.758 5.682 5.591 5.550 AlIV 1.951 2.309 2.242 2.318 2.409 2.450 AlVI 2.454 2.403 2.677 2.496 2.382 2.548 Ti 0.002 0.005 ± ± ± 0.002 Fe21 4.173 5.163 4.436 4.602 4.565 4.732 Mg 4.988 4.317 4.626 4.727 4.871 4.589 Ca 0.055 0.012 0.007 ± 0.002 0.005 Mn 0.047 0.037 0.032 0.079 0.083 0.067 Na ± 0.025 ± 0.013 0.013 0.004 K ± ±±±±± Total 19.744 19.961 19.778 19.917 19.914 19.950 21 XFe 0.46 0.55 0.49 0.50 0.49 51.00 Xsmt 0.18 0.04 0.13 0.05 0.01 0.07 T (8C) 251 310 299 311 326 332

a Total Fe as FeO. O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 347 Although there is an important overlap among ®elds from rocks of the greenschist facies. It generally occurs as trans- the different metamorphic facies, Fe-pumpellyite essentially parent, elongate (0.05±0.8 mm) crystals surrounding relic occurs in zeolite facies rocks whereas Al- and Mg-pumpel- clinopyroxene and amphibole phenocrysts, scattered in the lyites are typical but not restricted to the prehnite±pumpel- groundmass, and ®lling amygdules. It is commonly asso- lyite facies rocks. Pumpellyites associated with greenschist ciated with epidote, white mica, chlorite, hematite, titanite, facies rocks are essentially Mg-pumpellyites. In this way, a albite, and quartz and, less commonly, with pumpellyite. relationship exists between pumpellyite composition and Actinolite has low contents of Al2O3 (,2%), Na2O bulk rock composition. This relationship is marked by the (,0.6%), K2O(,0.07%) and TiO2 (,0.06%; Table 4), tendency of pumpellyites to follow the line referred to as the comparable to actinolites found in other terrains of a similar bulk rock trend. This is particularly true for pumpellyites metamorphic grade (e.g. Coombs et al., 1970; Coombs et from prehnite±pumpellyite facies. al., 1976; Cho and Liou, 1987). Total Al and NaB contents Prehnite. Prehnite was recognized in rocks of the zeolite are remarkably low, suggesting that actinolite crystallized and prehnite±pumpellyite facies, but it is generally more under low-pressure conditions (e.g. Brown, 1977). abundant in the latter. It occurs in amygdules as pale Other secondary phases. Titanite and hematite are also brown radial aggregates made up of micrometric needles secondary minerals in amygdules, in groundmass, and repla- in association with chlorite, celadonite, laumontite, epidote, cing igneous mineral phases (amphibole and Fe±Ti oxides). pumpellyite, titanite, and quartz, or in veins as colorless Titanite generally occurs as deep brown microgranular radial aggregates associated with pumpellyite and/or aggregates and as rhombic-shaped crystals. Hematite epidote, chlorite, white mica, and quartz. It is a product of appears as micrometric, sometimes euhedral, crystals. alteration of both plagioclase phenocrysts and glassy White mica occurs in some domains and was observed in groundmass. Compositionally, prehnite is very heteroge- nearly all studied samples. It appears as aggregates repla- neous and varies even within grains of a similar domain in cing plagioclase or, more rarely, in amygdules. Analyses the same thin section (Table 2). Thus, in sample T-230 with revealed low totals and low interlayer occupancies, indicat- diagnostic assemblages from the prehnite±pumpellyite ing that such sheet silicates are not true micas but rather 31 facies, prehnite occurring in amygdules has a range of XFe illites. In addition, white mice show a wide range of substi- (0.02±0.06) Ð almost as great as the range observed for all tution of the celadonite component (Fig. 3). Moreover, no analyses in this facies (0.01±0.06). Despite this, a small but relationships between white mica composition and meta- signi®cant variation in composition is found between morphic grade were observed (Table 4). Albite, K-feldspar prehnites from different facies (Fig. 4c). Prehnite from the (adularia), and quartz occur in almost all samples, whereas zeolite facies tends to be slightly richer in Fe XFe31 calcite is present only in a few of them. Albite and K-feld- 0:0±0:09; with a maximum at 0.05) than that from the spar largely occur replacing igneous plagioclase, although prehnite±pumpellyite facies XFe31 0:01±0:06; with a they have also been observed in amygdules and veins. maximum at 0.04). Microgranular quartz occurs in amygdules and replacing Chlorite. Chlorite is the most common metamorphic olivine and orthopyroxene phenocrysts. mineral in the Teloloapan metabasites and occurs in all rocks throughout the stratigraphic succession. It appears as 4.3. Mineral assemblages and facies boundaries yellow to pale green, often slightly pleochroic, aggregates ®lling amygdules, in the groundmass, and as replacement of In the Teloloapan metabasites, metamorphic assemblages most igneous minerals. According to the classical classi®- occur within various habitats Ð e.g. in amygdules and cation of Hey (1954), chlorite ranges in composition from veins, replacing igneous minerals and, more rarely, in the ripidolite through pycnochlorite to diabantite (Fig. 6), and groundmass. Metamorphic minerals occurring in similar there seems to be no correlation between chlorite classi®ca- habitats often show signi®cant chemical variations even tion and metamorphic grade. Chlorites show variable but within the same thin section, suggesting that equilibrium signi®cant amounts of CaO and Na2O, suggesting the was only reached within small domains. Minerals within a presence of interlayered smectite within chlorite. Chlorite given habitat are compositionally homogeneous or show proportions calculated from microprobe analyses, using the only little chemical variation. In this study, we assume method proposed by Bettison and Schiffman (1988), indi- that minerals coexisting together (in physical contact or cate that expandable smectites are present in the range of not) within a given habitat are in equilibrium if there is no XSmt 0:0±0:33: It seems that the smectite content does not evidence of reaction between two or more phases, as correlate with increasing metamorphic grade, in contrast to suggested by Zen (1974). Assemblages containing relic the positive correlation recorded by Bevins et al. (1991) in igneous mineral phases are excluded from this assumption. the metabasites of Greenland and Wales (Table 3) although, Metamorphic assemblages recorded from the pre-accre- at the scale of the whole population, a slight decrease in tion, burial metamorphism are listed in order of increasing smectite content exists between most chlorites from greens- grade in Table 5. These assemblages can be represented chist facies relative to those from the zeolite facies. conveniently in an Al±Fe31±FM±K system, assuming Actinolite. Actinolite is a common mineral phase only in that quartz, titanite, and albite are phases in excess and 348 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354

Table 4 Representative analyses of actinolite, titanite, and white mica (Plag plagioclase; Amd amygdule; Grm groundmass; Opx orthopyroxene; Amph amphibole; Ox oxide; Zeo zeolite; Pr±Pp prehnite±pumpellyite; Pp±Act pumpellyite±actinolite; Greensch greenschist)

Actinolite Sample T-260 T-260 T-265 T-265 T-254 T-255 T-255 Analysis 1 2 3 4 5 6 7 Occurrence Grm Amd Amph Amd Amph Grm Amph Facies Greensch Greensch Greensch Greensch Greensch Greensch Greensch

SiO2 53.02 53.85 55.70 56.44 54.90 55.00 55.09 Al2O3 1.24 0.62 2.57 1.16 0.56 1.05 0.81 TiO2 0.06 ± ± 0.06 ± ± ± FeOa 15.40 13.91 12.35 13.44 13.40 12.57 12.67 MnO 0.18 0.07 0.43 0.35 0.15 0.14 0.10 MgO 13.00 13.57 13.41 14.62 14.25 14.20 14.33 CaO 12.78 12.71 11.47 11.89 12.79 12.94 12.70

Na2O 0.17 0.18 1.33 0.43 0.10 0.17 0.04 K2O 0.06 0.06 0.05 0.06 ± 0.03 0.07 Total 95.98 95.04 97.31 98.45 96.15 96.10 95.81 23 Oxygens Si 7.881 8.009 7.994 8.035 8.033 8.021 8.052 Al 0.217 0.109 0.435 0.195 0.097 0.181 0.140 Ti 0.007 ± ± 0.006 ± ± ± Fe21 1.914 1.730 1.482 1.600 1.640 1.533 1.549 Mn 0.023 0.009 0.052 0.042 0.019 0.017 0.012 Mg 2.880 3.008 2.869 3.102 3.108 3.086 3.122 Ca 2.036 2.025 1.764 1.814 2.005 2.022 1.989 Na 0.049 0.052 0.370 0.119 0.028 0.048 0.011 K 0.011 0.011 0.009 0.011 ± 0.006 0.013 Total 15.028 14.963 14.977 14.990 14.931 14.915 14.889 Al/(Al 1 Ti) ± ± ± ± ± ± ± Titanite White Mica Sample Z-18B Tx-65 T-230 T-254 T-258 Z-18A Z-18B T-230 T-254 Analysis 8 9 10 11 12 13 14 15 16 Occurrence Ox Amd Grm Ox Amph Plag Plag Grm Amd Facies Zeo Zeo Pr±Pp Greensch Greensch Zeo Zeo Pr±Pp Greensch

SiO2 30.80 31.61 30.13 29.54 30.30 48.27 46.50 52.37 48.35 Al2O3 6.80 3.22 4.32 3.48 1.99 34.28 32.44 29.93 25.98 TiO2 26.30 31.71 31.95 33.51 38.15 ± ± ± 0.02 FeOa 3.19 2.68 1.52 1.39 0.31 1.53 1.08 2.01 3.24 MnO 0.06 ± ± 0.11 0.03 0.13 ± ± 0.05 MgO 1.57 0.50 0.16 0.53 ± 0.61 0.91 2.34 3.49 CaO 25.72 26.89 28.08 27.24 28.47 0.10 0.11 ± 0.05

Na2O ± 0.08 ± ± ± 0.23 0.24 ± 0.03 K2O ± 0.04 ± ± ± 9.40 8.56 10.35 10.84 Total 94.44 96.73 96.17 95.80 99.25 94.54 89.83 97.00 92.06 20 Oxygens 22 Oxygens Si 4.225 4.155 4.072 4.022 3.979 6.406 6.457 6.799 6.748 Al 1.099 0.498 0.689 0.559 0.308 5.362 5.309 4.581 4.274 Ti 2.714 3.126 3.247 3.432 3.768 ± ± ± 0.002 Fe21 0.366 0.294 0.154 0.158 0.034 0.170 0.126 0.219 0.378 Mn 0.007 ± ± 0.013 0.003 0.014 ± ± 0.005 Mg 0.321 0.099 0.033 0.108 ± 0.120 0.189 0.452 0.727 Ca 3.781 3.787 4.066 3.975 4.007 0.014 0.016 ± 0.008 Na ± 0.020 ± ± ± 0.060 0.064 ± 0.009 K ± 0.007 ± ± ± 1.594 1.519 1.718 1.935 Total 12.512 11.986 12.260 12.267 12.099 13.741 13.680 13.769 14.085 Al/(Al 1 Ti) 0.29 0.14 0.18 0.14 0.08 ± ± ± ±

a Total Fe as FeO. that ¯uid phases were completely mobile (Fig. 7). One K-bearing phases. In contrast, Ca-phases like calcite and the advantage of this model (a modi®ed version of the model Ca component in the Fe±Al silicates and actinolite cannot of Coombs et al., 1976) is that Al±Fe31 substitutions in Fe± be represented. Calcite is a minor phase in the Teloloapan Al silicates can be satisfactorily represented together with metabasites and the fact that the CaO content in the Fe±Al O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 349

Table 5 Metamorphic assemblages in the Teloloapan metabasites (assemblages in bold are diagnostic assemblates de®ning each metamorphic facies; Lm laumontite; Pp pumpellyite; Ep epidote; Ce celadonite; Pr prehnite; Hm hematite; Ti titanite; Wm white mica; Act actinolite)

Metamorphic facies Common assemblages

Zeolite Lm 1 Pp 1 Ep Lm 1 Ce 1 Pp ^ Ep Ce 1 Pp 1 Ep Ce 1 Pr 1 Ep Lm 1 Pp Lm 1 Ep Pp 1 Ep Prehnite±pumpellyite Pr 1 Pp Pr 1 Pp ^ Wm Pr 1 Ep Pr 1 Se Pr 1 Ep Pp 1 Ep Hm 1 Ti 1 Ep Greenschist Act 1 Pp Act 1 Pp 1 Ep Act 1 Ep Act 1 Ep 1 Hm ^ Ti silicates and actinolite shows little or no variation during increasing metamorphism suggests that its in¯uence on the facies boundaries and the phase equilibria is limited. In the Teloloapan metabasites, the zeolite facies is de®ned by the assemblage laumontite 1 pumpellyite 1 epidote 1 albite ^ chlorite ^ quartz (Table 5 and Fig. 7) (Frost, 1980; Cho et al., 1986). Variations of this assemblage such as laumontite 1 pumpellyite or laumontite 1 epidote are common. One of the most interesting aspects of the zeolite facies assemblages is the presence of celadonite in association with laumontite and Fe-rich Fe±Al silicates. Laumontite 1 celadonite-bearing assemblages were observed in amygdules but also replace igneous mineral phases in two samples from different stratigraphic levels (Fig. 2). In one other sample, celadonite is associated with Fig. 7. Metamorphic phase relationships in the Teloloapan rocks in the Al± Fe31±FM±K system. Albite, quartz, titanite, and ¯uid are assumed to be in Fe±Al silicates that display similar chemical compositions excess. Heavy lines connecting open circles represent the critical assem- to those found in association with laumontite 1 celadonite. blages de®ning each facies. Broken lines connect mineral phases forming This fact, together with the absence of celadonite-bearing additional assemblages. For this representation, the total Fe in Fe±Al sili- assemblages in higher metamorphic grades, suggests that cates (epidote, pumpellyite, prehnite) was assumed to be Fe31, whereas in 21 celadonite-bearing, laumontite-free assemblages are also a chlorite, white mica, celadonite, and actinolite was considerated to be Fe . Act actinolite; Ep epidote; Ce celadonite; Chl chlorite; Hm clear indication of the zeolite facies in the Teloloapan meta- hematite; Lm laumonite; Pp pumpellyite; Pr prehnite; Wm white basites, as had been suggested for other low-grade meta- mica. morphic terranes (e.g. Andrews, 1980; Aguirre and Atherton, 1987; Bettison and Schiffman, 1988; Cathelineau pumpellyite 1 epidote 1 chlorite, and hematite 1 and Izquierdo, 1988 and references therein). titanite 1 epidote ^ chlorite. Epidote was seen in associa- With increasing depth and temperature, the zeolite facies tion with both prehnite and pumpellyite, but assemblages assemblages are replaced by assemblages of the prehnite± containing all these three phases were not observed. pumpellyite facies, which is de®ned by the association At increasing depth and temperature, the assemblage prehnite 1 pumpellyite 1 chlorite 1 quartz ^ albite (Table prehnite 1 pumpellyite becomes unstable and is replaced 5 and Fig. 7) (e.g. Coombs et al., 1970; Gassley, 1975; Cho by the assemblage epidote 1 actinolite 1 chlorite 1 albite, et al., 1986). Additional assemblages in this facies involve which is characteristic of the greenschist facies (Hashimoto, prehnite 1 white mica, prehnite 1 epidote 1 albite, 1972; Cho and Liou, 1987; Aguirre and Atherton, 1987). In 350 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354

Fig. 8. AlIV Ð temperature variation of chlorites as a function of metamorphic grade in the Teloloapan metabasites. Temperatures calculated using the thermometer of Cathelineau (1988). a few samples, actinolite or epidote is associated with Al- isotope analyses, Seyfried et al. (1978) and Kastner and rich pumpellyite, suggesting a transitional pass from Gieskes (1976) reported celadonite formed at temperatures prehnite±pumpellyite to the greenschist facies (Table 5 as low as 268C, whereas Andrews (1980) suggested and Fig. 7). Titanite 1 hematite 1 pumpellyite ^ actinolite temperatures around 1008C for celadonites associated with is a common assemblage replacing both Fe±Ti oxides and smectites. However, Cathelineau and Izquierdo (1988) amphibole. found that, in geothermal systems, celadonite in association with chlorite is stable at temperatures between 200 ^ 30 and 4.4. Metamorphic conditions 290 ^ 208C which is in good agreement with temperatures calculated from chlorites of the zeolite facies. A number of constraints are now available for quantita- Prehnite±pumpellyite facies rocks were subjected to tive and semi-quantitative estimates of the P±T conditions higher temperatures. With the exception of one sample in low-grade metabasites (e.g. Brown, 1977; Cathelineau containing chlorites giving uncommonly lower tempera- and Nieva, 1985; Cathelineau and Izquierdo, 1988). tures (186±2308C), all samples indicate temperatures in Although calibrated for geothermal systems, the geotherm- the range 250±3158C(^108C). These temperatures are ometer proposed by Cathelineau (1988) using variations of close to those reported by Cho et al. (1986) in the AlIV with temperature in chlorite seems to be the most prehnite±pumpellyite facies in the metabasites of Karmut- appropriate for the Teloloapan metabasites regarding the sen (190±2958C). However, they are signi®catly lower than presence of chlorite in all metamorphic facies. A similar those experimentally deduced by Nitsch (1971), who approach was successfully applied by Bevins et al. demonstrated that, at Ptotal 2 kbar; the prehnite 1 (1991) in the low-grade regional-metamorphosed meta- pumpellyite 1 chlorite 1 quartz assemblage is stable at basites of Greenland and Wales. Results are summarized temperatures as high as 345 ^ 208C. in Fig. 8. Finally, chlorites from the greenschist facies indicate that Chlorite thermometry indicates that zeolite facies rocks rocks recrystallized at temperatures ranging between 299 recrystallized at temperatures in the range 175±2908C and 3428C(^108C). Similar temperatures have been (^108C). These temperatures are in good agreement with deduced by Cho et al. (1986), who estimated that the transi- data reported by Liou (1979), who suggested that, at low tion between the prehnite±pumpellyite to greenschist facies pressures, the assemblage laumontite 1 Fe-rich pumpellyite in the Karmutsen area probably took place at temperatures is stable between 150 and 2508C. Similar conditions were above 2958C and low pressures (P 1.7 kbar). On the other deduced by Boles and Coombs (1977) in the Hokonui Hill hand, Nitsch (1971) concluded from experimental data that, area, New Zealand, who estimated the ®rst appearance of at Ptotal 2 kbar the assemblage actinolite 1 chlorite 1 Fe-rich pumpellyite at 1908C. However, the presence of epidote 1 quartz exists above 3508C. The experimental celadonite in association with laumontite and Fe-rich Fe± data of Mottl and Holland (1978) indicate that the ®rst

Al silicates in the zeolite facies could imply lower tempera- appearance of actinolite in a basalt±H2O system occurs at tures for the Teloloapan metabasites. Based on oxygen about 3008C, whereas Coombs et al. (1970) estimated the O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 351

Fig. 9. Simpli®ed petrogenetic grid of basaltic systems showing the estimated P±T path for the Teloloapan metabasites (after Liou et al., 1987). disappearance of prehnite in the northern Maine area at titanite 1 hematite in all metamorphic facies, indicates a

4008C. very low XCO2 in the ¯uid phase (Evarts and Schiffman, Evidence suggests that Teloloapan metabasites metamor- 1983; Aguirre and Atherton, 1987 and references therein). phosed under low-pressure conditions. Following Winkler Finally, a high aK in the hydrothermal ¯uid is suggested by (1979), the absence of lawsonite- and/or glaucophane-bear- the presence of celadonite or white mica and/or K-feldspar ing assemblages in the Teloloapan metabasites would indi- in nearly all the rocks examined. cate, at calculated temperatures, that the pressure was lower than 3 kbar. This ®gure is consistent with data reported by Liou et al. (1987), who determined that recognized assem- 5. Accretion-related metamorphism blages in the zeolite and prehnite±pumpellyite facies are stable at pressures around 2 kbar. From the petrogenetic The Teloloapan rocks have been considered as being grid proposed by these authors (Fig. 9), the presence of affected by regional metamorphism related to collisional assemblages containing both pumpellyite and actinolite ^ processes. This event has been described with some detail epidote, together with the absence of assemblages contain- by Campa et al. (1974), De Cserna et al. (1978) and Campa ing only prehnite±actinolite, further suggests that prehnite± and RamõÂrez (1979). General aspects regarding the distribu- pumpellyite and greenschist facies rocks recrystallized at tion of M2 secondary phases and their relationships with pressures near the pumpellyite±actinolite±greenschist deformation structures are described in detail in this issue invariant point Ð i.e. at about 2.5 kbar and 3208C. by Salinas-Prieto et al. (2000). Here, the M2 event is treated There are some constraints to suggest that Teloloapan in a general way. metabasites were subjected to high f : among them, the Throughout the study area, but especially in those areas O2 presence of hematite in almost all metamorphic facies and close to major tectonic discontinuities, rocks from both the 31 domains and the high XFe values in the Fe±Al silicates, lower volcanic unit and the upper sedimentary cover show particularly pumpellyite and epidote. The f seems to clear evidence of deformation-related recrystallization. O2 decrease with increasing depth Ð i.e. with increasing meta- Secondary phases such as chlorite, epidote, white mica, morphic grade as suggested by the systematic decrease in and rare actinolite are observed forming schistosity planes 31 the Fe±Al silicates of the XFe values thought to be (S1) and developing a marked streatching lineation (L1). controlled, in part, by the f (Holdaway, 1972; Liou, Other secondary phases such as calcite, quartz, and even O2 1973; Cho and Liou, 1987). On the other hand, the scarcity chlorite commonly appear within pressure shadows around of calcite throughout the succession, together with the abun- porphyroblast systems. In some rocks from the lower volca- dance of prehnite and pumpellyite and of the assemblage nic unit, M1 metamorphic phases within amygdules behaved 352 O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 as rigid objects during deformation and developed pressure gradients have been reported during hydrothermal meta- shadows containing M2 phases. Locally, some volcanic morphism of island-arc series rocks (Aguirre and Atherton, rocks were severely affected and primary igneous textures 1987) and ophiolites (Evarts and Schiffman, 1983; Bettison and structures were partially to totally obliterate. and Schiffman, 1988).

Although M2 secondary phases are restricted in number, Hydrothermal ¯uids appear to be characterized by high observed assemblages are common in many very low- to concentrations of K ions, as suggested by the presence in the low-grade metamorphic terrains. The assemblage chlorite 1 samples of K-bearing mineral phases. The presence of abun- epidote 1 quartz ^ actinolite ^ white mica ^ calcite dant pumpellyite and prehnite and of the association observed in some volcanic rocks indicates peak meta- hematite 1 titanite indicates low X in the ¯uids (Evarts CO2 morphic conditions within the greenschist facies. and Schiffman, 1983). The overall lack of calcite supports this assumption. Facies boundaries are always de®ned by reactions leading 6. Discussion and conclusions to the disappearance of at least one mineral phase and to the appearance and/or extension of the equilibrium ®eld of a Field observations, rock fabric, textures, and mineralogi- diagnostic mineral or assemblage (Fig. 7). Laumontite and cal data clearly indicate that volcanic and interbedded celadonite disappear and the assemblage prehnite 1 volcaniclastic rocks of the Teloloapan Terrane underwent pumpellyite appears in the transition from zeolite to early recrystallization under zeolite through prehnite± prehnite±pumpellyite facies. Prehnite and pumpellyite pumpellyite to greenschist facies conditions (M1) prior to react to form assemblages containing actinolite 1 epidote its accretion, which produced deformation-related recrystal- of the prehnite±pumpellyite±greenschist boundary. lization (M2). The thermodynamic behavior of most proposed reactions Phases related to M1 occur only within the lower volcanic is known to be largely affected by external (e.g. bulk rock unit and show a clear deep-controlled metamorphic zona- composition and nature of hydrothermal ¯uids) and internal 31 tion, with zeolite facies assemblages in the top through (e.g. XFe in Fe±Al silicates, XFe in chlorites and relic prehnite±pumpellyite assemblages in the middle part to igneous mineral composition) conditions and must therefore greenschist facies assemblages at the base of the strati- be considered as multivariant. Consequently, the facies graphic pile. The nature and the fabric of the assemblages boundaries are also sensitive to these parameters, and are consistent with burial metamorphism produced by certainly the stratigraphic positions of the facies boundaries hydrothermal ¯uids percolating under static conditions. vary from one place to another.

These facts and the preservation of the magmatic textures Phases related to M2 follow the schistosity planes and and structures indicate that observed assemblages cannot be form the streaking lineation or occur within pressure linked to the synkinematic metamorphism produced during shadows around porphyroblast systems. Assemblages indi- collision (Campa et al., 1974; De Cserna et al., 1978; Campa cate that recrystallization took place under the greenschist and RamõÂrez, 1979; SaÂnchez and ElõÂas, 1991). Distribution facies. This recrystallization is related to the deformation and chemical evolution of secondary assemblages through- that accompanied accretion of the Teloloapan Terrane to out the lower volcanic unit indicate that the metamorphic nuclear Mexico. gradients were essentially controlled by depth:

1. the temperature rose from about 1758C in the zeolite Acknowledgements facies rocks in the uppermost stratigraphic levels to about 3428C in the greenschist facies rocks in the lower- The author is grateful to J. Harvey for the critique of an most stratigraphic levels; earlier version of this manuscript. Special thanks are due to 2. f appears to be high throughout the stratigraphic succes- L. Aguirre for careful review and contributions to this O2 sion but with a tendency to decrease with increasing manuscript. Reviews by Sara Roeske and F. Ortega-GutieÂr- depth; rez are greatly appreciated. This research was supported by 3. the chemical composition of some of the secondary a grant from SEP-CONACYT. mineral phases, especially of the Fe±Al silicates (Fig. 4) and chlorite, varies systematically as a function of References the metamorphic grade Ð i.e. of depth; 4. the pressure, although quantitatively less constrained, Aguirre, L., Atherton, M.P., 1987. Low-grade metamorphism and geotec- was less than 3 kbar and probably in the interval 1.0± tonic setting of the Macuchi Formation, Western Cordillera of Ecuador. 2.5 kbar. Journal of Metamorphic Geology 5, 473±494. Andrews, A.J., 1980. Saponite and celadonite in layer 2 basalts, DSDP Leg The temperature interval and the estimated thickness of 37. Contributions to Mineralogy and Petrology 73, 323±340. Beiersdorfer, R.E., 1993. Metamorphism of a Late Jurassic volcano-pluto- the stratigraphic column (3000 m) indicate a relatively high nic arc, northern California, USA. Journal of Metamorphic Geology 11, 21 geothermal gradient, on the order of 558Ckm . Similar 415±428. O. Talavera Mendoza / Journal of South American Earth Sciences 13 (2000) 337±354 353

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