A major resurgent caldera in southern Mexico: the source of the late Eocene Tilzapotla ignimbrite

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1 Journal of Volcanology and Geothermal Research 136 (2004) A major resurgent caldera in southern Mexico: the source of the late Eocene Tilzapotla ignimbrite D.J. Morán-Zenteno a, *, L.A. Alba-Aldave a, J. Solé a, A. Iriondo b,1 a Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México D.F , Mexico b Department of Geological Sciences, University of Colorado at Boulder, Campus Box 399, Boulder, CO , USA Accepted 26 April 2004 Abstract The Tilzapotla caldera constitutes the first discovery of a major Tertiary collapse volcanic structure south of the Mexican Volcanic Belt. Although it is spatially associated with silicic ignimbrites in a region relatively distant from the extensive ignimbritic province of the Sierra Madre Occidental (SMO), it is among the largest collapse calderas documented in Mexico. The caldera is defined by a km semi-elliptical structure that encircles the largest exposures of the Tilzapotla ignimbrite and corresponds to the structural margin rather than the topographic rim. A central uplifted block limited by NW-trending faults is the main indication of a resurgent stage. The caldera structural margin is surrounded by extensive exposures of Cretaceous marine sequences that structurally define a broad elliptical dome (45 35 km) originated in the first stage of the caldera evolution. There is evidence showing that the 34 Ma Tilzapotla ignimbrite represents the climatic event of the caldera collapse. It is constituted by a massive sequence of crystal vitric tuff with conspicuous euhedral biotite and abundant quartz. The intra-caldera facies is intercalated with mega- and meso-breccias of limestone and anhydrite fragments derived from the slumping of the caldera wall during the caldera collapse. The overlying sequence includes post-collapse ignimbrites as well as amphibole and pyroxene bearing dacitic to andesitic lava flows. The age (33 to 32 Ma) and isotopic signatures of these lava flows indicate a resurgent event related with the input of more primitive magmas into the magma chamber. The rectilinear northeastern and southwestern segments of the structural margin of the caldera correspond to NW-trending tectonic lineaments that are part of a regional strike-slip system, active at the time of the caldera formation. We interpret that the NW tectonic structures defined zones of weakness that accommodated the caldera collapse in the northeastern and southwestern segments of the caldera structural margin. D 2004 Elsevier B.V. All rights reserved. Keywords: collapse caldera; resurgent caldera; strike-slip tectonics; ignimbrite; mega-breccia; Mexico * Corresponding author. Tel.: ; fax: address: dantez@servidor.unam.mx (D.J. Morán-Zenteno). 1 Present address: Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Queretaro, Qro., Mexico. 1. Introduction Although the Tilzapotla caldera is a volcanic structure with a remarkable semi-elliptical expression in satellite images (Fig. 1), it was not recognized as a major volcanic structure until recently (Morán-Zenteno et al., 1998), probably due to the paucity of studies /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.jvolgeores

2 98 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig. 1. Landsat Thematic Mapper (TM) image of the Tilzapotla caldera area. Tz = Tilzapotla ignimbrite, Tr = Rodarte ignimbrite, Tg = Gallego ignimbrite, Ts = El Salto lava flows, Th = hypabyssal rocks, Gd = Coxcatlán granodiorite intrusion, Km = marine Cretaceous rocks. The caldera structural margin and main tectonic lineaments are indicated. The area where the elliptical dome is recognizable is encircled by a finer dashed line. Satellite image is provided by Industrias Peñoles Mining Company. focused on the Tertiary volcanic rocks in southern Mexico. The caldera is spatially related to a discontinuous silicic volcanic cover distributed in southern Morelos and northern Guerrero states, in the northern Sierra Madre del Sur (Fig. 2A and B). It represents the first discovery of a major collapse caldera south of the Mexican Volcanic Belt and it is among the largest reported in Mexico. Because of its age and depth of erosion, expressed in an inverted relief, this volcanic center clearly displays features of a ring fault zone. The fact that the volcanic zone is surrounded by broad exposures of Cretaceous marine rocks makes the features related with the caldera collapse more prominent (Figs. 1 and 3). Ignimbrites associated with the Tilzapotla caldera are part of a discontinuous dissected belt of Tertiary volcanic rocks that extends for about 600 km from the states of Michoacán to Oaxaca (Morán-Zenteno et al.,

3 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig. 2. (A) Sketch map of the central part of the northern Sierra Madre del Sur showing the distribution of Tertiary volcanic rocks and the main Cenozoic tectonic features in the region. (B) Distribution of the ignimbritic rocks attributed in this study to the Tilzapotla caldera, including the distribution of the outflow sheet remnants of the Tilzapotla ignimbrite. 1999). Volcanic rocks of this belt range in composition from basaltic-andesite to rhyolite. Coeval batholiths are broadly exposed along the exhumed continental margin of southern Mexico. Both, the Tertiary plutonic and volcanic belts represent the wide magmatic arc of the Sierra Madre del Sur. It was originated during subduction episodes along the Pacific margin previous to, and in part contemporary, with the margin truncation attributed to the displacement of the Chortis block (Ross and Scotese, 1988; Pindell et al., 1988; Ratschbacher et al., 1991; Herrmann et al., 1994; Schaaf et al., 1995; Morán-Zenteno et al., 1999). The Tertiary magmatism of the Sierra Madre del Sur is partially coeval with the major episodes of Oligocene ignimbrite volcanism of the northern and southeastern Sierra Madre Occidental (McDowell and Clabaugh, 1979; Nieto-Samaniego et al., 1999, Ferrari et al., 2002; Aranda-Gómez et al., 2003). The region where the Tilzapotla caldera is located is dominated by silicic volcanic rocks that appear to be the southern extension of the flare-up of the Sierra Madre Occidental, where several collapse calderas have been reported (Fig. 2A) (McDowell and Clabaugh, 1979; Swanson and McDowell, 1984). The Tertiary volcanic rocks of the study region were first described by Fries (1960, 1966) and De Cserna and Fries (1981), who described the sequence in terms of Tilzapotla Rhyolite and overlaying Buenavista Andesite or Buenavista Group. They interpreted one of the probable sources of the Tilzapotla Rhyolite as located south of the village of Tilzapotla, without specifying the nature and the precise position of the volcanic center. According to these authors, the Tilzapotla Rhyolite includes a series of pyroclastic flows ranging in composition from dacite to rhyolite (although this unit was identified as a pyroclastic sequence, they used the term rhyolite ). They applied this name even to units

4 100 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig. 3. Geologic map of the Tilzapotla caldera area. Distribution of Mesozoic units was based on the geologic map published by Consejo de Recursos Minerales (Rivera-Carranza et al., 1998). Sections A B and C D presented in Fig. 5 are indicated as solid lines. cropping out in distant volcanic zones (i.e. Taxco region) and displaying some significant lithologic differences. In this paper, we use the informal denomination of Tilzapotla ignimbrite instead of Tilzapotla Rhyolite to avoid confusion. Since the volcanic units overlying the Tilzapotla ignimbrite in the caldera area are more diverse and spatially variable than Fries originally supposed, in this paper different lithostratigraphic units are informally proposed, although Buenavista Group name can be preserved to include most of them. In a regional reconnaissance study of the stratigraphy and petrology of the volcanic rocks in the Taxco-Huautla region, Morán-Zenteno et al. (1998) identified three different major volcanic centers (i.e. Taxco, Tilzapotla-Buenavista and Huautla) (Fig. 2B). They recognized the Tilzapotla and Buenavista volcanic zones as a part of a laterally continuous volcanic cover defining a semicircular structure. The Taxco volcanic field is located about 50 km northwest of the Tilzapotla area and is characterized by a volcanic sequence that includes ignimbrites and rhyolitic lava flows that were originated from ca. 38 to 32 Ma. The Huautla volcanic field is located to the east of the Tilzapotla caldera (Fig. 2B) and is represented by a sequence of lava flows and pyroclastic deposits that overlie outflow volcanic deposits similar to those in the Tilzapotla-Buenavista region (Fries, 1966; Morán- Zenteno et al., 1998). In this paper, we present stratigraphic, structural and geochemical data of the volcanic sequence of the Tilzapotla-Buenavista area that confirm the existence of a large resurgent caldera as the source of the Tilzapotla ignimbrite. These data lead to an interpretation of its volcanic evolution and its relationship with the Tertiary tectonic structures. 2. Caldera structure and related tectonic features The Tilzapotla caldera can be recognized in satellite images as a semi-elliptical structure with a major axis of about 33 km and a minor axis of 24 km (Figs.

5 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) , 3 and 4) that encircles a laterally continuous and thick volcanic sequence. The elongated shape of the caldera is defined by NW-trending lineaments forming its NE and SW boundaries. The most conspicuous lineament is the SW boundary of the caldera that extends from Huitzuco to Tlaxmalac (Los Amates fault). Although less defined, the northeastern limit is also expressed by a NW-trending lineament passing north of Tilzapotla (Fig. 4). The southeastern boundary of the caldera displays a well-defined arcuate segment that coincides with a subvertical contact between beds of Cretaceous marine rocks and the intra-caldera ignimbrite. The western boundary of the caldera, around and north of Buenavista, has a more irregular outline. Cretaceous marine beds surrounding the caldera margin structurally delineate an elliptical dome (45 35 km) whose contour, in the eastern and southeastern segments, is semi-parallel to the caldera margin and oblique to the near north-trending pre-existing Laramide structures (Figs. 1 and 4). The interference between the elliptical dome and the Laramide fold belt can be recognized by nearly opposite plunging folds south and north of the caldera (Fig. 4). There are clear indications that the elliptical shape of the caldera corresponds to the structural margin Fig. 4. Structural sketch map showing the caldera margin, the resurgence block and the related tectonic lineaments. Solid lines indicate those segments with direct evidence of the position of the structural margin. Fault data and striations from selected localities are shown in equal area nets, lower hemisphere. Plunges that portray the structural interference between the dome structure and the Laramide folds are shown in the inset. Solid and open circles indicate plunges measured in marine beds distributed south and north of the caldera, respectively.

6 102 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) rather than the topographic rim. The relatively deep level of erosion allows a steep contact to be observed between the thick ignimbritic sequence and the precaldera rocks, that in some segments of the eastern and southeastern boundary corresponds with vertical and lateral faults (Fig. 4). Additionally, the occurrence of collapse meso- and mega-breccias, as well as lag breccias along the conspicuous rectilinear and arcuate segments of the margin, support this interpretation. Due to the relatively deep level of erosion, there is no evidence of a recognizable topographic rim. In the rectilinear southern segment of the structural margin, east of Huitzuco, the contact between the in-fill ignimbrites and host limestone is a subvertical fault with oblique and subhorizontal striations that suggest reactivation after the collapse (Fig. 4). At the southeastern arcuate segment, near Quetzalapa, lag breccias within the Tilzapotla ignimbrite indicate the proximity of a volcanic vent. The occurrence in this area of hydrothermal alteration zones and sulfide deposits, related to porphyritic sub-volcanic rocks (Rivera- Carranza et al., 1998) (La Mina Au Pb mine), is also suggestive of the nearness of the structural margin. At the southwestern rectilinear margin, there are sulfide vein deposits (Huitzuco Hg District) and hydrothermal alteration zones associated with the caldera volcanic activity. At the northeastern limit of the caldera, the proximity of the structural margin is indicated by the occurrence of lag breccias and collapse megablocks near Tilzapotla. There are also fault segments with left lateral to dip-slip kinematic indicators affecting ignimbrites, collapse breccias and Cretaceous rocks (Fig. 4). The presence of sub-volcanic bodies, north and south of Buenavista (Fig. 3), is also indicative of the structural margin in this area. Differential erosion in the caldera area produced an inverted relief, with higher elevations for the top of volcanic in-fill sequences than for the surrounding Mesozoic rocks. This is also due to the uplift related to the resurgence (Fig. 5). Differences in elevations of the contact between the collapse ignimbrite and overlying volcanic units delineate a resurgent block that occupies more than a half of the caldera area (Figs. 3 and 5). The block is bordered by NW-trending lineaments located north of Huitzuco and south of Tilzapotla, and can be recognized in aerial photo- Fig. 5. Schematic N S and E W trending sections through the Tilzapotla caldera. Vertical scale is exaggerated to enhance resurgence features. The ring fault projection is shown vertical for convenience. Note changes in altitude of Tilzapotla and Rodarte ignimbrites produced by the uplift of the central block.

7 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) graphs and satellite images. The northwestern and southeastern edges of the block seem to coincide with the caldera margin. The southwestern block boundary corresponds to a NW-trending fault zone with oblique to vertical striations and kinematic indicators of a normal component (Fig. 4). A series of dikes and groups of volcanic necks of dacitic composition are intruded along the shear zone. The low position of post-collapse ignimbrites to the south, defines a moat between the caldera margin and the uplifted block. The northeastern bounding fault of the uplifted block is mostly covered by lava flows and related hypabyssal intrusions attributed to the resurgence and can be inferred by the abrupt change in the elevation of the base of the post-collapse sequence (Figs. 3, 4 and 5). The NE and SW rectilinear boundaries of the Tilzapotla caldera are nearly parallel with regional NW-trending tectonic lineaments recognized in the region (Fig. 2A). These lineaments mainly correspond with left-lateral faults active in late Eocene time (Alaniz-Álvarez et al., 2002; Morán-Zenteno et al., 2003). The southwestern rectilinear boundary of the Tilzapotla caldera is collinear with the NW-trending Los Amates fault, located west of the caldera (Fig. 4). This feature is also near collinear with the Tetipac fault (Fig. 2A), which is one of the most striking regional tectonic features and extends more than 50 km northwest of the city of Taxco (Rivera-Carranza et al., 1998; Alaniz-Álvarez et al., 2002). The Tetipac Fault and other near parallel structures (i.e. Chichila and Tuxpan) show evidence of a complex kinematic evolution which includes the reactivation of pre-eocene structures. Los Amates fault zone is characterized by subvertical fault planes with a complex kinematic history with preserved vertical to oblique striations (Figs. 3 and 4). Based on structural observations and age data, Alaniz-Álvarez et al. (2002) concluded that most of the regional NW-trending structures had left lateral displacement in late Eocene time (38 to 33 Ma). Rivera-Carranza et al. (1998) and Fitz-Díaz (2001) reported a NW-trending fault located 10 km southwest of the caldera ring. This fault hosts a wide (20 to 40 m) pyroclastic dike very similar in composition and mineralogy, to the Tilzapotla ignimbrite. The occurrence of left-lateral faults affecting the dike is suggestive of the contemporaneity between the strike-slip tectonics and the volcanic activity of the caldera. Other NW fault segments recognized northwest and south of the caldera display left lateral to oblique kinematic indicators (Fig. 4). These faults also affect the volcanic rocks of the caldera indicating that strike-slip faulting continued after the caldera formation. 3. Volcanic stratigraphy The most extensive volcanic cover related to the Tilzapotla caldera is continuously distributed within the caldera, over an area of 700 km 2. Outcrops of the outflow facies are discontinuously distributed to the northeast and south of the caldera (Figs. 2B and 3). Due to the regional dissection, the outflow facies represent less than 30% of the total area of the volcanic cover and only incomplete sections are preserved. The base of the volcanic sequence is not exposed within the structural margin of the Tilzapotla caldera. A maximum exposed thickness of 1500 m, including the caldera forming ignimbrite and lava flows of the resurgence, has been estimated for the central segment of the caldera. Pyroclastic and lava flows were grouped based on lithological similarities or when contrasting flow units of a continuous sequence could not be separated due to scale restrictions (Figs. 3 and 6). The outflow volcanic sequence has a maximum preserved thickness of 50 m in the proximal facies. Ash fall deposits are preserved only where they lie between ignimbrites. Precollapse pyroclastic deposits were only observed in restricted outcrops near the eastern ring segment (km 147.5, highway 95). They are represented by a 3-m-thick layer of altered ash fall tuff that underlies the extracaldera facies of the Tilzapotla ignimbrite. K Ar, Rb Sr and Ar Ar dates from the volcanic units of the Tilzapotla caldera obtained in this study are listed in Tables 1A, 1B and 1C. K Ar dates for the whole volcanic sequence range from 35.5 to 32.6 Ma, whereas those for the ignimbrites representative of the climatic event range from 35 to 34 Ma. A F 0.09 Ma Ar Ar date was calculated from individual analyses of 21 sanidine grains (Table 1B, Fig. 7) from the middle part of the of the Tilzapotla ignimbrite. There is a previously reported K Ar date of 31.9 F 1 Ma for a biotite concentrate of the intra-caldera Tilzapotla ignimbrite (Alba-Aldave et al., 1996; Morán-Zenteno et al., 1999) (sample SOL5), but replicate analyses of the K content of this sample provided a corrected date of 35.1 F 1 Ma. This corrected date is more compatible

8 104 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig. 6. Generalized composite stratigraphic sections of the central eastern and the western zones of the Tilzapotla caldera, as well as the extracaldera section observed south of Valle de Vázquez. Since the thickness of volcanic units is variable, those indicated are only representative. with the well-defined group of dates obtained in this study from the Tilzapotla ignimbrite and from the overlying volcanic rocks. In the Taxco region, the sequence included within the Tilzapotla unit by De Cserna and Fries (1981) lacks some characteristic features of the Tilzapotla unit in the Buenavista-Tilzapotla area, namely, an abundant crystal content and the presence of conspicuous euhedral biotite. This fact and the presence of lag breccias, as well as lava flows and sub-volcanic rocks in the Taxco area, are suggestive of a source close to this area rather than a distal facies of the Tilzapotla caldera ignimbrite. Age inferences of De Cserna and Fries (1981) for the Tilzapotla Rhyolite were based on dates carried out in the Taxco area (35.5 F1.2 and 36.9 F 1.3 Ma for sanidine and whole rock fractions of the same sample, respectively), but not in the Tilzapotla area. Additionally to the ignimbrites dated by De Cserna and Fries (1981), Alaniz-Álvarez et al. (2002) reported K Ar ages for ignimbrites and rhyolite lavas of the Taxco area, showing that the most voluminous silicic volcanism in this area occurred between 32 and 31 Ma Pre-caldera rocks Volcanic rocks of the Tilzapotla caldera unconformably overlie deformed Cretaceous marine sequences that crop out widely in the region. The most extensive outcrops correspond to the platform limestone beds of the Albian-Cenomanian Morelos Formation and Aptian-Albian evaporitic beds of the Huitzuco Formation (Fries, 1960, 1966; De Cserna et al., 1980; Hernández-Romano et al., 1997; Hernández-Romano, 1999). There are also extensive outcrops of the terrigenous beds of the Turonian- Maestrichtian Mexcala Formation. The most important tectonic structures affecting these units are NNW- to NNE-oriented folds and NW and N S regional lateral and normal-oblique faults (Fig. 2A). Cretaceous rocks are unconformably overlain by Paleocene Eocene fluvial deposits of the Balsas Formation cropping out northwest and southwest of the caldera (Fig. 3) Volcanic rocks associated with the caldera collapse Tilzapotla ignimbrite Intra-caldera facies. The intra-caldera facies of the Tilzapotla ignimbrite is represented by a massive sequence of dacitic, moderately to densely welded tuffs. It includes several pyroclastic flow units with similar petrographic characteristics and poorly defined contacts among them. In the south-

9 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Table 1A K Ar and Ar Ar and Rb Sr dates Sample Location Mineral Rock 40 Ar* (mol/g) K (%) Age (Ma) Tilzpotla ignimbrite Sol 5 99j11V57U biotite ignimbrite F j19V33U Sol 9 99j10V58U biotite ignimbrite F j21V15U Tz j14V54U biotite ignimbrite F j03V24U Tz j24V30U biotite ignimbrite F j33V51U SOL 2 99j10V39W sanidine ignimbrite Single crystal F 0.1 Ar Ar date a 18j22V33W Rodarte ignimbrite Tz j13V34U plg ignimbrite F j26V32U Hypabyssal and El Salto lava flows Tz j72V42U sanidine rhyolite F j43V45U Tz j17V06U plg andesite F j26V37U Tz j17V61U plg andesite F j26V41U Tz j17V10W plg andesite Ar Ar plateau F 0.1 age b 18j27V05W Tz j17V09W plg andesite Ar Ar isochron F 0.1 age b 18j17V09W Sample Location Mineral Rb (ppm) Sr (ppm) 87 Sr/ 86 Sr Age (Ma) Coxcatlán granodiorite Bv 21 99j27V29W biotite Bv 21 18j29V47W WR F 1 K Ar and Ar Ar dates obtained in this study for different volcanic units. K Ar dates were carried out in the Laboratorio Universitario de Geoquímica Isotópica (LUGIS) at the National University of Mexico (UNAM). Ar Ar analytical data of sample Sol 2. a Data in Table 1B. b Data in Table 1C. eastern part of the caldera, an exposed thickness of 600 m has been estimated for this unit. Despite the relatively deep incision of the drainage, the base of the unit is not exposed within the ring fault. Typically, the Tilzapotla ignimbrite is represented by a vitric-crystal tuff with a crypto- to microcrystalline groundmass of quartz and plagioclase that includes ghosts of spherulites and small crystals of Fe Ti oxides and zircon. Although most of the groundmass seems to have been originally vitroclastic, there are no preserved fractions of unaltered glass. The phenocryst fraction includes quartz, broken plagioclase, minor sanidine, and conspicuous euhedral biotite. The lithic fraction is dominated by fragments of crypto-crystalline texture and, in a minor proportion, by porphyritic lava and sub-volcanic fragments. Phenocrysts range from 15 to 50 (vol.) %, being quartz and plagioclase the most

10 106 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Table 1B 49 Ar 39 Ar laser total fusion single-crystal age data for 21 sanidine grains of sample Sol 2, J = F 0.25% Tilzapotla ignimbrite Unit 39 Ar k (mol) Radiogenic yield (%) 40 Ar*/ 39 Ar k K/Ca K/Cl Age (Ma F 1r) Tilzapotla, 1.90e F j V, 3.09e F j V 2.16e F e F e F e F e F e F e F e F e F e F e F e F e F e F e F e F e F e F e F 0.05 Weighted mean age = F 0.09 Ma Analyses in italics are not used to calculate the weighted mean age. Ar Ar analytical data of samples Tz and Tz Ar Ar dates were carried out at the Thermochronology Laboratory of the USGS Denver (see Appendices A and B for analytical procedures). abundant components. The size of phenocrysts of the intra-caldera facies reaches up to 4 mm, while the size of lithic fragments is highly variable, depending on the position with respect to the caldera structural margin. Lag breccias, including vitrophyric fragments altered to zeolites, were observed in Tilzapotla and Quetzalapa areas. Two characteristic lithic components are anhydrite and limestone fragments, derived from the surrounding Cretaceous sequence. The Tilzapotla ignimbrite conformably underlies the pyroclastic rocks of the Rodarte ignimbrite. K Ar and Ar Ar dating of samples collected in three different localities yielded ages ranging from 34.3 to 35.3 Ma (Tables 1A, 1B and 1C, Fig. 7). K Ar dates in biotite concentrates (35.1 F 1 and 34.1 F 1.4 Ma) are undistinguishable, within the error, from Ar Ar dates obtained from individual sanidine crystals (34.3 F 0.09 Ma). Two additional K Ar determinations of the extra-caldera facies yielded 34.3 F 1.5 and 34.7 F 0.9 Ma Extra-caldera facies. Due to the prolonged erosion in the region since the caldera formation, the extra-caldera facies of the Tilzapotla ignimbrite is exposed in relatively isolated outcrops showing incomplete sections. Distal outcrops are distributed east and south of the caldera, reaching distances up to 36 km from the caldera margin (Fig. 2B). Proximal sections of the extra-caldera facies are exposed near Amacuzac, north of Buenavista, and east and south of the caldera margin. South of Amacuzac, ignimbrites are exposed continuously from the margin to a distance of 8 km to the northwest (Fig. 3). In this area, the extra-caldera facies unconformably overlies pre-caldera conglomerates of the Balsas Formation and has a maximum thickness of 50 m, that gradually decreases to the

11 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Table 1C Ar Ar samples Tz and Tz Unit/location T (jc) % 39 Ar of total Radiogenic yield (%) 39 Ar k (mol ) 40 Ar*/ 39 Ar k Apparent K/Ca Apparent K/Cl Apparent age (Ma F 1r) TZ 63-02, plagioclase, J = F 0.25%, wt. = mg El Salto, F j27.090V, F j17.166V F F F 0.14 Total gas % of gas on plateau in 1000 through 1300 steps Plateau age = F 0.11 Isochron age = F 0.36 TZ 62-02, andesite matrix, J = F 0.25%, wt. = mg El Salto, F j27.137V, F j17.144V F F F F 0.06 Total gas No plateau Isochron age = F 0.13 Ages calculated assuming an initial 40 Ar 36 Ar = F 0. Ages of individual steps do not include error in the irradiation parameter J. No error is calculated for the total gas age. north. In the caldera margin, there is an abrupt increase in the ignimbrite thickness to f 600 m. A biotite concentrate from this area yielded a K Ar date of 34.3 F 1.5 Ma. North of Buenavista, the extracaldera facies of the Tilzapotla ignimbrite contains abundant fragments of limestone ( f 30%) and minor volcanic and sub-volcanic fragments in a matrix-supported structure. The best preserved sections of relatively distal facies underlie lava flows of the Huautla volcanic center, east of the Tilzapotla caldera (Fig. 6). South of Valle de Vázquez, 12 km from the caldera margin, the section is constituted by three slightly welded ignimbrite units. An ash fall layer separates the two upper flow units. The three layers are crystal-rich and contain phenocrysts of euhedral biotite. A 34.2 Ma Ar Ar date in biotite was preliminary reported by Campa-Uranga et al. (2002) for an ignimbrite sample collected in this area. The most distal outcrop of the Tilzapotla ignimbrite was found 36 km south of the caldera margin at km 200 of highway 95, leading to Acapulco (Fig 2B). A 34.7 F 1 Ma K Ar age obtained from a biotite concentrate of this ignimbrite (Table 1A) supports its relationship with the caldera collapse event Collapse breccia. Collapse breccia deposits were identified mainly in the eastern and southeastern boundaries of the caldera (Fig. 3). They are represented by discontinuous exposures of breccia accumulations interlayered with the Tilzapotla ignimbrite, as well as relatively isolated mega-blocks. Exposed sections of meso-breccia interlayered with pyroclastic beds, located at the eastern ring fault zone, have a minimum thickness of 100 m. The thickest sections crop out along the deep cuts of highway 95, near the locality of Coaxintlán. At the southeastern segment of the caldera ring, near the village of Quetzalapa, there are accumulations of recrystallized limestone and anhydrite breccia, as well as blocks of marble, several meters in length, embedded in the intra-caldera ignimbrite (Fig. 8). At the northern segment of the ring fault zone, there are also large blocks of limestone and anhydrite, embedded in Tilzapotla ignimbrite. A gypsum quarry in Tilzapotla corresponds to one of the largest collapse blocks contained in the ignimbrite.

12 108 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig. 7. Graphic representations of Ar Ar data obtained from samples: (a) Sol 2 of the Tilzapotla ignimbrite; (b) Tz and (c) Tz of the El Salto lava flows. The Sol 2 age was obtained from the total fusion of 21 single crystals of sanidine. See data in Tables 1B and 1C and analytical procedures in Appendix A.

13 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig. 8. Blocks of limestone embedded in the Tilzapotla ignimbrite of the mega-breccia interval from localities at the southeastern segment of the ring fault Post-collapse volcanic rocks Post-collapse rocks overlying the Tilzapotla ignimbrite are exposed mainly at high elevations in the eastern and south-central parts of the caldera, although there are some preserved outcrops at lower elevations in the southern moat zone (Figs. 3 and 5). They include recognizable ignimbrite flow units and debris flow deposits with irregular contacts among them, probably due to fluvial erosion. Based on recognizable lithological characteristics the sequence was divided in three informal units: Rodarte ignimbrite, Rodeo formation and Las Mesas formation. The Rodarte ignimbrite is represented by a vitroclastic ignimbrite sequence that includes several flow units, ranging from nonindurated to moderately welded, and contains pumice fragments and biotite phenocrysts. La Mesas formation is constituted by a sequence of conglomerate layers and debris flow deposits. The Gallego formation overlies the Rodarte ignimbrite and is formed by a thick sequence of densely welded rheomorphic ignimbrites, vitrophyre flow units and dacitic lava flows and contains phenocrysts of plagioclase, sanidine, biotite and quartz. The presence of some tilted segments of this sequence dipping toward the caldera margin, at the eastern boundary of the uplifted block, is indicative of their emplacement previous to the resurgence. No suitable material was found for dating this unit, but its age is constrained by the overlaying El Salto formation (34 32 Ma) and the underlying Tilzapotla ignimbrite (35 34 Ma) Resurgence-related volcanic rocks Lava flows and hypabyssal rocks overlying and intruding the Tilzapotla ignimbrite and the post-collapse units are distributed in different areas of the caldera margin and in the uplifted central block. Lava flows (El Salto formation) range in composition from hornblende bearing dacites to ortho- and clinopyroxene bearing andesites and comprises subordinated intercalations of debris flow deposits. K Ar and Ar Ar ages of this sequence range from 34.4 to 32.8 Ma (Tables 1A and 1C). Hypabyssal rocks form dikes and volcanic necks emplaced mainly near the caldera margin and along the faults limiting the uplifted block. Although hypabyssal rocks have a wide range in composition, from andesites to rhyolites, their spatial relationships with El Salto formation are, in most cases, suggestive of being their feeding dikes. There are also coarsegrained intrusions of biotite bearing granodiorite located near the western margin of the caldera (Coxcatlán and Buenavista localities), although the Ar Ar age reported for the Buenavista intrusion (35.9 F 0.5Ma; Meza-Figueroa et al., 2003) does not support a connection with the resurgence stage. On the eastern margin of the intra-caldera area, exposures of conglomerates and volcanic debris flow deposits include some thin layers of altered greenish

14 110 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Table 2 Major element chemical analyses Sample SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O P 2 O 5 LOI Total El Salto lava flows Bv Tz Tz Tz Tz Tz Tz Tz Tz Tz Tz Tz Tz Tz17a Tz190b Tz190c Hypabyssal and plutonic rocks Tz Tz Tz Tz Tz Bv Bv Bv Bv Tz Tz Tz46b Tilzapotla ignimbrite SOL SOL Hz Hz Hz Tz Tz Tz Tz Tz112V Tz Rodarte ignimbrite Tz Tz Tz Tz

15 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Table 2 (continued) Sample SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O P 2 O 5 LOI Total Gallego formation Bv Tz Tz Tz17b Tz17c Tz Whole rock chemical composition of representative samples of the volcanic sequence in the study area. Analyses were carried out by XRF at the Laboratorio Universitario de Geoquímica Isotópica (LUGIS) at UNAM. ash fall tuff. The most extensive outcrops are distributed near Coaxintlán (Fig. 3). 4. Major element and isotope geochemistry Representative whole-rock chemical composition of the volcanic rocks of the Tilzapotla caldera is presented in Table 2. SiO 2 content ranges from 57 to 76 (wt.) %, with a dominance of rocks containing more than 60% SiO 2. Because of the lack of unaltered pumice or glass fragments in the Tilzapotla ignimbrite, its composition was estimated from whole rock analyses of samples composed mainly of juvenile material. Hydrothermal alteration zones, as well as accessory and accidental fragments, were avoided to obtain the best approximation of the magma composition. Nonetheless, in the TAS diagram the data of Tilzapotla ignimbrite display some scattering within the dacite field, probably due to some degree of alkali mobilization (Fig. 9). Most differentiated rocks are related to the Tilzapotla ignimbrite and post-collapse pre-resurgence sequences. Intermediate rocks are mainly related to lava flows of the resurgence. Sub-volcanic rocks display a more variable composition ranging from andesite to rhyolite. Normalized alkalis and silica data presented in the TAS diagram (Fig. 9) show the compositional variation of the entire sequence indicating a subalkaline affinity. Biotite is a characteristic accessory mineral in rhyolitic to dacitic rocks, whereas hornblende is common in dacitic and andesitic lava flows of the resurgence. Ortho- and clinopyroxene are common in andesites with the lowest silica content. 87 Sr/ 86 Sr i in the sequence ranges from to (Table 3), with the youngest lava flows having the lowest ratios ( ), and the collapse Fig. 9. TAS diagram showing the composition of representative samples of different volcanic units of the Tilzapotla caldera. See data in Table 2.

16 112 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Table 3 Sr, Nd and Pb isotopic data of selected samples of the Tilzapotla ignimbrite, lava flows of the resurgence and hypabyssal rocks of the study area Sample Rb Sr Rb/Sr 87 Rb/ 87 Sr 87 Sr/ 86 Sr 2r mean 87 Sr/ Sm Nd 86 Sr i 147 Sm/ 144 Nd Lava flows of the resurgence Tz F F Tz F F Tz F F Ignimbrites Sol F F Tz F F Tz F F Hypabyssal rocks Bv F F Bv F F Tz F F Tz F F Tz F F Nd/ 144 Nd 2r mean 143 Nd/ 144 Nd i Sample 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb Lava flows of the resurgence Tz-17 plag Tz-17 WR Tz-18 plag Tz-18 WR Tz-20 plag Tz-20 WR Hypabyssal and Rodarte ignimbrite Bv-17 plag Bv-17 WR Tz-53 WR Tz-4 sanidine Tz-4 WR The Sr, Nd and Sm isotope ratios were determined using a Finnigan MAT262 mass spectrometer equipped with eight faraday collectors, installed at the Isotope Geochemistry Laboratory (LUGIS) of the National University of Mexico. The isotopic measurements were made in a static collection mode. Analysis of Rb were carried out using a single collector NBS mass spectrometer. Rb, Sr, Sm and Nd, were loaded as chlorides and measured as metallic ions. Values of 2r (m) (2r (m) =2r abs Mn) were calculated from 60 individual isotopic determinations for Rb, Sr and Nd and 20 for Sm. The measured 87 Sr/ 86 Sr values were normalized to an 86 Sr/ 88 Sr value of , and those of 143 Nd/ 144 Nd to an 146 Nd/ 144 Nd ratio of The 87 Sr/ 86 Sr of the NIST-SRM987 and 143 Nd/ 144 Nd of the La Jolla-standard throughout this study were F 18 ( F 1r abs, n = 237) and F 21 ( F 1r abs, n = 134). Pb samples were loaded with a mixture of silica gel and phosphoric acid and runs consisted of 100 individual measurements. Laboratory mean values of standard NIST-NBS981 (Pb) ( 206 Pb/ 204 Pb = %, 207 Pb/ 204 Pb = %, 208 Pb/ 204 Pb = %, n = 23) (0.13% fractionation per mass unit). Pb isotope ratios presented in table are present-day values. Relative uncertainties for 87 Rb/ 86 Sr was F 2% and for 147 Sm/ 144 Nd F 1.5% (1r). Relative reproducibility (1r) for Rb, Sr, Sm and Nd abundances was F 4.5%, F 1.8%, F 3.2%, and F 2.7%, respectively. Total procedure blanks were ng for Rb, ng for Sr, ng for Sm, ng for Nd. Chemical blank for Pb was 94 pg. and post-collapse ignimbrites the highest ratios ( ). Hypabyssal rocks display more variable 87 Sr/ 86 Sr i values ( ). Although Tilzapotla ignimbrite samples were carefully selected for isotopic analyses, avoiding accessory lithic fragments of limestone and secondary calcite precipitates, the possible influence of carbonate impurities in the Sr isotopic signatures cannot be completely discarded. Given the 87 Sr/ 86 Sr obtained ( ) and the Sr abundance range ( ppm) for Creta-

17 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) Fig Sr/ 86 Sr i 143 Nd/ 144 Nd i diagram showing initial isotopic ratios of representative samples of volcanic rocks of the Tilzapotla caldera. Initial ratios were calculated for t = 34 Ma. Analyses were carried out in the Laboratorio Universitario de Geoquímica Isotópica (LUGIS), UNAM. ceous limestone, the differences observed between the Tilzapotla ignimbrite and the resurgence lava flows are greater than expected from the sole influence of the small amount of carbonate impurities acquired during the eruption and later remobilizations. Differences were also observed between Nd isotopic signatures of the collapse and post-collapse ignimbrites ( 143 Nd/ 144 Nd i = ) with respect to those of the lava flows of the resurgence ( 143 Nd/ 144 Nd i = ; Table 3, Fig. 10). Analyzed hypabyssal rocks display two groups of 87 Sr/ 86 Sr i and 143 Nd/ 144 Nd i values indicating isotopic affinity with both ignimbrites and lava flows of the resurgence. Although common Pb isotopic signature in feldspars is less sensitive to the influence of limestone impurities during the eruption and later fluid remobilization, Pb isotopic ratios carried out in plagioclase concentrates of hypabyssal rocks ( 206 Pb/ 204 Pb = ) and lava flows ( 206 Pb/ 204 Pb = ) display recognizable differences (Table 3). This variability suggests the input into the magma chamber of more primitive magmas related to the resurgence stage. 5. Discussion 5.1. Regional stratigraphic and tectonic implications The geochronology and distinctive features in the petrography of the Tilzapotla ignimbrite allow to differentiate it from other silicic ignimbrites in the region. Although there is an overlap in the ages of the volcanic sequences of the Tilzapotla and the Taxco regions, they display significant mineralogic differences. These differences, as well as the occurrence of lava flows and sub-volcanic rocks and some diachronism in the peaks of volcanism in both areas (35 34 Ma in Tilzapotla and Ma in the Taxco), confirm the existence of two different volcanic centers. Stratigraphic relationships of the extra-caldera Tilzapotla ignimbrite, with overlying extensive andesitic lava flows and pyroclastic deposits, on the eastern flank of the Huautla range (Valle de Vázquez-Chinameca sector) indicate a younger volcanic center located to the east, as was previously recognized by Fries (1960), although, at present, there are no available geochronologic data to constrain the age range of the volcanic sequences of the Huautla volcanic center. Ages obtained from the Tilzapotla ignimbrite together with geochronologic data from the Taxco volcanic field indicate that silicic volcanism in the northern Sierra Madre del Sur is partially coeval with the ignimbrite volcanism in the northern Sierra Madre Occidental (SMO), where main episodes occurred between 38 and 28 Ma (McDowell and Clabaugh, 1979; Aranda-Gómez et al., 2003). In the southern Sierra Madre Occidental and the Mesa Central regions, north of the Mexican Volcanic Belt, the Tertiary silicic volcanism is slightly younger, ranging in age from 30 to 21 Ma (Nieto-Samaniego et al., 1999; Ferrari et al., 2002). The Tilzapotla caldera represents a remarkable volcanic feature, not only for its size but also for its tectonic framework. The rectilinear northeastern and

18 114 D.J. Morán-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) southwestern boundaries are coincident with Tertiary strike-slip faults active in late Eocene time. It has been recognized that the shape and structure of the collapses and resurgences are often controlled by pre-existing normal faults (i.e. Lipman, 1984; Acocella et al., 2004), but there are few reports of collapses accommodated by vertical displacement along segments of coeval strikeslip faults (i.e. Acocella et al., 2002). According to Alaniz-Álvarez et al. (2002), extension in overstep zones in this region favored the emplacement of voluminous silicic magma. In the Taxco area, the initial stages of magmatism occurred synchronous with left lateral displacement of regional NW-trending faults that induced NNW extension and subsidence (Alaniz- Álvarez et al., 2002). Left-lateral displacement in faults of the northeastern and southwestern segments of the Tilzapotla caldera structural margin (Fig. 4) could have produced extension that favored magma ascent to the upper crust in the caldera area. Extension could have occurred in two possible scenarios. In the first case, extension is restricted to the termination of the two faults. In the second case, extension is produced in the releasing overstep by interaction of the two faults. Due to the younger volcanic cover in the Huautla area, it is not possible to determine the length of the northeastern lineament toward the east to support the second scenario. There is no preserved evidence of subsidence associated with extension in the Tilzapotla caldera area, previous to the tumescence stage. The volume of ascending magma could have accommodated the extension inhibiting subsidence, as has been documented in some extension zones (Parsons et al., 1998). Collapse calderas in extension zones, associated with strike-slip systems, have also been documented in the central Andes (Riller et al., 2001). Eocene strike-slip tectonics, in this region and in a great part of southern Mexico (Morán-Zenteno et al., 1999), contrasts with the regional Oligocene Miocene coaxial extension associated with ignimbrites and large collapse calderas in the Sierra Madre Occidental and with other major calderas in the world (Lipman, 1984, 1997; Aguirre-Díaz and McDowell, 1993; Aguirre- Díaz and Labarthe-Hernández, 2003). In the Sierra Alquitrán region, 150 km farther south, there is also a volcanic collapse structure that seems to have evolved in a similar tectonic scenario (Morán-Zenteno et al., 2003). Other silicic volcanic centers such as Taxco and Huautla evolved in step-overs of left lateral faults segments, but no collapse calderas have been identified yet (Alaniz-Álvarez et al., 2002) Volcanic evolution Initial tumescence stage The distribution of the Cretaceous Morelos/Huitzuco beds, bordering the caldera in a higher position with respect to the surrounding areas, is the main indication of a tumescence stage related to the caldera evolution. Due to the pre-existing folding, at the east and southeast of the caldera, the structural attitude of Cretaceous beds does not clearly portray the flanks of an antiform. Nevertheless, fold plunges define a structural interference produced by the doming process (Fig. 4). The northwest side of the elliptical dome extends about 10 km from the caldera margin, indicating an incomplete collapse with respect to the doming produced by ascending magma (Fig. 1). This also is suggested by the presence, in this area, of the Coxcatlán intrusion (Fig. 3), which probably was an apophysis of the magma chamber. Evidence of the first stages of volcanism of the Tilzapotla caldera previous to the collapse is fragmentary. In several areas, the direct contact of the outflow facies of the collapse ignimbrite over the pre-volcanic marine and fluvial sequence is suggestive of the removal of initial ash fall deposits by active erosion. At only few localities, east of the caldera, a thin ( < 3 m) altered layer of ash fall pyroclastic material was preserved below the first ignimbrite accumulations. The small 35 Ma granodiorite intrusion close to Buenavista could also be a manifestation of pre-collapse magmatism. Northeast and south of the caldera the Tilzapotla ignimbrite unconformably overlies Cretaceous limestone and anhydrite, as well as tilted beds of the Balsas Formation Collapse stage The collapse stage and the first episodes of voluminous ash flows can be documented in the massive intra-caldera sequence of the Tilzapotla ignimbrite and by the presence of large blocks of marine limestone and anhydrite, embedded in the ignimbrite near the margin. Outflow facies of the Tilzapotla ignimbrite extended a minimum distance of 36 km from the caldera margin, but the general distribution area of the ignimbrite and ash fall deposits cannot be determined given the erosion of most of the extra-caldera