Polyphase Deformation in San Miguel Las Minas, Northern
POLYPHASE DEFORMATION IN SAN MIGUEL LAS MINAS, NORTHERN
ACATLAN COMPLEX, SOUTHERN MEXICO
A thesis presented to
the faculty of
the Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Sciences
Brent J. Barley
August 2006 This thesis entitled
POLYPHASE DEFORMATION IN SAN MIGUEL LAS MINAS, NORTHERN
ACATLAN COMPLEX, SOUTHERN MEXICO
by
BRENT J. BARLEY
has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences by
R. Damian Nance
Professor of Geological Sciences
Benjamin M. Ogles
Dean, College of Arts and Sciences Abstract
BARLEY, BRENT J., M.S., August 2006, Geological Sciences
POLYPHASE DEFORMATION IN SAN MIGUEL LAS MINAS, NORTHERN
ACATLAN COMPLEX, SOUTHERN MEXICO (58 pages)
Director of Thesis: R. Damian Nance
Mapping in the northern part of the Acatlán Complex (southern Mexico) has distinguished two lithological units: a high-grade unit assigned to the Piaxtla Suite, and a low-grade unit assigned to the Cosoltepec Formation. Two major Paleozoic tectonothermal events have been identified in these rocks. The first event produced a penetrative deformational fabric (SPS1) parallel to a compositional banding during blueschist and amphibolite facies metamorphism, which has recently been dated as ~346
Ma in a neighboring area, and a greenschist overprint during exhumation. The second event, which is recorded in both the Piaxtla Suite and Cosoltepec Formation, produced two penetrative deformational fabrics under subgreenschist metamorphic conditions. The first, high-grade tectonothermal event accompanied closure of the Rheic Ocean and tectonic juxtapositioning of the two units during exhumation of the high-grade unit in the
Devono-Carboniferous. The second event records convergence along the paleo-Pacific margin of Pangea in the Permo-Triassic.
Approved:
R. Damian Nance
Professor of Geological Sciences 4 Table of Contents
Page
Abstract...... 3
List of Tables ...... 5
List of Figures ...... 6
1. Introduction...... 7
2. Pioneers...... 8
3. Acatlán Geology: Current Models I & II...... 17
4. Geology of San Miguel Las Minas...... 19
5. Implications...... 36
6. Conclusions...... 41
7. References...... 46
Appendix: Petrographic Analysis ...... 52 5
List of Tables
Table Page
1: Polyphase deformation fabrics of Malone et al. (2002) ...... 38
2: Deformational/metamorphic history at San Miguel Las Minas...... 41 6
List of Figures
Figure Page
1: Traditional tectonostratigraphy for the Acatlán Complex...... 9
2: Plate tectonic reconstruction, Middle to Late Ordovician...... 14
3: Plate tectonic reconstruction, Silurian...... 15
4: Plate tectonic reconstruction, ca. 350 Ma...... 16
5: Plate tectonic reconstruction, 300-230 Ma ...... 18
6: Simplified tectonic map of Mexico...... 21
7: Plate tectonic reconstruction, 1350-1100 Ma ...... 22
8: Simplified geologic map of the northern Acatlán Complex...... 23
9: Geologic map of the field area...... 24
10: Photograph of metapsammite from the Cosoltepec Formation ...... 26
11: D3 Structural data ...... 27
12: Photomicrographs of phyllite from the Cosoltepec Formation...... 28
13: D4 Structural data ...... 29
14: Photograph of metapsammite from the Cosoltepec Formation ...... 30
15: Photograph of phyllite from the Piaxtla Suite...... 33
16: Photomicrograph of blueschist from the Piaxtla Suite...... 34
17: D2 Structural data ...... 35
18: Photomicrograph of greenschist from the Piaxtla Suite ...... 36
19: Illustrated structural evolution ...... 43 7 1. Introduction
Two major tectonothermal events have been recognized in rocks of the Acatlán
Complex, which have been assigned the names Acatecan and Mixtecan orogenies (Yanez et al., 1991; Ortega-Guitierrez et al., 1999). Previous work has sought to develop models to explain the tectonic history and origin of the Acatlán Complex and to place it in the context of events occuring during the middle and late Paleozoic. However, many questions about its origin and evolution remain unresolved. Specifically; in which
Paleozoic ocean did the Acatlán Complex form, and of which ocean does it record closure? The Acatlán Complex has recently been interpreted as a vestige of the Rheic
Ocean, recording events along the leading edge of Gondwana on the ocean’s southern margin during the amalgamation of Pangea (Keppie and Ramos, 1999; Nance et al., in press). Alternatively, Talavera-Mendoza et al. (2006) have interpreted the Acatlán complex as a remnant of the Grenville orogen, which contains portions of both Laurentia and Gondwana, and whose units record a complex Paleozoic history. Distinguishing between these opposing models is the next step towards understanding the role of the
Acatlán Complex in the Paleozoic continental assembly process of North America. This study attempts to reconstruct the deformational/thermal history of the two main tectonostratigraphic units of the Acatlán Complex: the Cosoltepec Formation and Piaxtla
Suite. Clarification of the history of these units will provide information essential to the establishment of a plausible tectonic model for the evolution of the Acatlán Complex and, therefore, the Paleozoic evolution of the North American craton. 8 2. Pioneers
Much of our present understanding of the Acatlán Complex is the result of the pioneering work of Ortega Guitiérrez (1978, 1981) whose initial tectonic model explained its history in terms of a suture (expressed as the ophiolitic Xayacatlán
Formation) created by the collision of two continental landmasses following the closure of a pre-Atlantic Ocean during the Cambro-Ordovician. The Acatlán Complex was originally divided into two major tectonostratigraphic units: the Petlalcingo and Acateco groups (Ortega Guitiérrez, 1978). These units were envisioned as upper and lower plates of a west-vergent thrust, on which the allocthonous Acateco Group was emplaced onto the autochtonous Petlalcingo Group during the Acatecan orogeny (Ortega-Guitiérrez et al., 1999) (Figure 1). The Petlalcingo Group, which was interpreted to represent a siliciclastic forearc on the Laurentian margin of Iapetus, consisted of two metasedimentary units: the Cosoltepec Formation and the Chazumba Formation, the base of which is pervasively migmatized and known as the Magdalena migmatite (Ortega-
Guitiérrez et al., 1999). The Acateco Group was interpreted to represent the subducted leading edge of Gondwana, and consisted of mafic-ultramafic and metasedimentary rocks of the Xayacatlán Formation and a suite of megacrystic granites known as the Esperanza granitoids. These two lithologically distinct groups are unconformably overlain by a volcaniclastic metasedimentary sequence known as the Tecomate Formation, which was believed to be of Devonian depositional age (Yanez et al., 1991). 9
Figure 1: Traditional tectonostratigraphy for the Acatlán Complex (after Ortega-Guitiérez et al., 1999), as compared to the revised tectonostratigraphy of Nance et al. (in press) based on recent geochronological data.
Recently, U-Pb zircon age, geochemical data, and structural/kinematic constraints have been reported that describe a fundamentally different tectonostratigtraphy for the
Acatlán Complex than that previously envisioned (Figure 1). Elias-Herrera and Ortega
Gutiérrez (2002) have since showed that the final tectonic juxtapositioning of the Acatlán and Oaxacan complexes did not occur until the Early Permian. The NNW- trending 10 Caltepec fault zone (CFZ) that defines the terrane boundary was shown to be a ductile, dextral transpressional structure, which they interpreted to be a component of the
Marathon-Ouachita suture developed during oblique impingement of the leading edge of
Gondwana onto the southern margin of Laurentia during the formation of Pangea. A neosome of syntectonic migmatitic rock found along the suture yielded a concordant U-
Pb zircon age of 275.6 ± 1 Ma, which they interpreted to date peak tectonothermal activity.
Malone et al. (2002) conducted a detailed analysis of polyphase deformation in two key units of the Acatlán Complex, the Cosoltepec and Tecomate formations, and likewise concluded that much of it was of Early Permian. They demonstrated that whereas three phases of penetrative deformation affect the Cosoltepec Formation, only two are recorded in the Tecomate Formation. The first phase, which produced a bedding parallel schistosity axial planar to tight to isoclinal folds, was not observed in the
Tecomate Formation, and was attributed to obduction at ca. 440 Ma associated with the
Acatecan orogeny. However, the second and third phases of deformation, which respectively record north-south dextral transpression and north-south upright folding, were observed in both units, as well as the Totoltepec granite, and were attributed to a tectonothermal event at ca. 290 Ma that also juxtaposed the Acatlán and Oaxacan complexes along the Caltepec fault zone.
Keppie et al. (2004) report conodont fossils from marble horizons in the
Tecomate Formation, as well as granite pebbles from a conglomerate with U-Pb
SHRIMP zircon ages of ~320-264 Ma, which revealed the unit to be of Permo-
Carboniferous rather than of Devonian depositional age. Keppie et al. (2006) reported 11 xenocrystic and detrital zircon data from the Magdalena Migmatite and Chazumba
Formation that showed them to be no older than Permo-Triassic. They interpreted the two units to be clastic wedge deposits developed in front of south-vergent thrust sheets that formed during convergence along the paleo-Pacific margin of Pangea.
In contrast, deposition of the Cosoltepec Formation was constrained to be between 455 ± 4 Ma (youngest concordant detrital zircon age) and ~ 305 Ma (maximum depositional age of unconformably overlying Tecomate Formation) (Keppie et al., 2004,
2006). On the basis of the large disparity between the ages of the Magdalena and
Chazumba units and that of the Cosoltepec Formation, Keppie et al. (2006) suggested that the Cosoltepec Formation be excluded from the Petlalcingo Group and that the
Magdalena and Chazumba units be defined as lithodemes of the Petlalcingo Suite. Based on the presence of tectonically interleaved basalts (Keppie, in press) and its continental derived turbiditic nature, Keppie et al. (2006) further suggested that the Cosoltepec
Formation represents a continental rise prism deposited on oceanic lithosphere.
Owing to its structural complexity and high-grade metamorphic/metaigneous composition, Keppie et al. (2006) and Middleton et al. (in press) suggest that, in accordance with the American stratigraphic code (1975), the Acateco Group renamed the
Piaxtla Group by Ramirez-Espinoza (2001), be referred to as the Piaxla Suite. Murphy et al. (in press) bracketed the depositional age of metasedimentary rocks of the Piaxtla Suite
(Asis Lithodeme) between ~700 Ma (U-Pb detrital zircon data) and the intrusion of a quartz augen granite at ~470 Ma (U-Pb SHRIMP zircon age). Murphy et al. (in press) further report, based on geochemical analysis of the Asis lithodeme, that the metasedimentary rocks are consistent with deposition in an immature rift-related to 12 mature passive margin setting with a continental source. Geochemical analysis of amphibolites layers in the Asis lithodeme show there to be continental rift tholeiites
(Murphy et al., in press). Keppie et al. (2004) and Middleton et al. (2004) report a concordant U-Pb zircon age of 346 ± 3 Ma from a mafic lens containing aligned omphacite needles in the Asis Lithodeme, which they interpret to date the high-pressure metamorphism of this unit.
When combined with the mafic-metaigneous units of the Piaxtla Suite, the
Esperanza granitoids are interpreted as part of a major bimodal suite (Miller et al., 2006).
The Esperanza granitoids are calc/alkaline with mixed arc/within-plate affinities, and favored to have formed during rifting along the formerly active southern Gondwanan margin of the Rheic Ocean (Miller et al., in press). Based on inherited xenocrystic zircon
U-Pb ages that closely match protolith ages within the Oaxacan Complex and detrital zircon age populations in its Paleozoic sedimentary cover (900-1300 Ma), Miller et al.
(2006) further suggest that the Esperanza granitoids were either derived from, or assimilated, the Oaxacan Complex and/or its Paleozoic cover.
These data suggest that emplacement of the Piaxtla Suite over the Cosoltepec
Formation did not occur until the Mississippian, while suturing of the Acatlán Complex to Laurentia did not occur until the Early Permian. If so, the Acatlán Complex is unlikely to record the closure of the Iapetus (Figure 2), which was accomplished by the Silurian, but, instead, is more likely to record closure of its immediate successor, the Rheic Ocean.
This scenario favors the continental reconstruction of Keppie and Ramos (1999) in which the Acatlán and Oaxacan complexes remain attatched to Amazonia throughout the
Paleozoic (Figure 3), colliding with Laurentia only with the closure of the Rheic Ocean 13 sometime in the Carboniferous (Figure 4). In this reconstruction, Oaxaquia was “trapped” between North and South America during the formation of Pangea, and complete transfer of the Acatlán Complex from Gondwana to Laurentia did not occur until the break-up of
Pangea in the Mesozoic. Support for this view is provided by Sanchez-Zavala et al.
(1999) who showed that the Early-to-Mid Paleozoic faunas of Oaxaquia are similar to those of Gondwana and different from those in North America, suggesting that the
Oaxaquia microcontinent was isolated from Laurentia at this time. 14
Figure 2: Plate tectonic reconstruction based upon the tectonic model for the evolution of the Acatlán Complex as a vestige of the Iapetus Ocean, in the Middle to Late Ordovician
(modified from Ortega-Guitierrez, 1999); O = Oaxaquia. 15
Figure 3: Plate tectonic reconstruction based upon the tectonic model for the evolution of the Acatlán Complex as a vestige of the Rheic Ocean, showing Oaxaquia attatched to
Gondwana during Silurian time (modified from Keppie et al., 2006); O = Oaxaquia, A =
Acatlán Complex, SM = Sierra Madre. 16
Figure 4: Plate tectonic reconstruction based upon the tectonic model for the evolution of the Acatlán Complex as a vestige of the Rheic Ocean, showing closure of the Rheic
Ocean ca. 350 Ma (modified from Keppie et al., 2006); O = Oaxaquia, A = Acatlán
Complex, SM = Sierra Madre. 17 3. Acatlán Geology: Current Models I & II
Model I
Based upon these new age and structural/kinematic constraints, Nance et al. (in press) proposed a revised tectonic interpretation of the Acatlán Complex (Figure 1), in which the complex records the following Paleozoic tectonic history: (1) ca. 345 Ma subduction of the Piaxtla Suite and associated high-pressure metamorphism/deformation
(Acatecan orogeny), (2) rapid exhumation of the Piaxtla Suite and emplacement over the
Cosoltepec Formation under greenschist facies metamorphic conditions, (3) deposition and deformation of the Tecomate Formation (290-275 Ma) during N-S dextral transpression (greenschist facies), and (4) Permo-Triassic regional deformation which emplaced the Cosoltepec Formation over the Chazumba and Magdalena lithodemes.
Nance et al. (in press) further suggest that the Cosoltepec Formation represents a continental rise prism on the Gondwanan margin of the Rheic Ocean, and that the Piaxtla
Suite represents the leading edge of this margin, which was subducted during closure of the Rheic Ocean in the Late Devonian-Early Carboniferous. Nance et al. (in press) also suggest that exhumation of the Piaxtla Suite and subsequent deformation of the Acatlán
Complex under greenschist facies metamorphic conditions was followed by multiple thrusting events related to convergent tectonics along the paleo-Pacific margin of Pangea
(Figure 5). 18
Figure 5: Plate tectonic reconstruction showing position of the Acatlán Complex, the
Oaxacan Complex, and the Permo-Triassic arc, at 300-230 Ma (modified from Keppie et al., 2006); O = Oaxaquia, A = Acatlán Complex.
Model II
In marked contrast, the entire geologic community has not embraced this revised
Paleozoic tectonic history of southern Mexico. Talavera-Mendoza et al. (2006) propose 19 an alternate view, based upon single-crystal laser ablation U-Pb zircon geochronology, in which the Acatlán complex is regarded as a remnant of the Grenville orogen, whose units record at least five tectonothermal events during the Paleozoic. They interpreted the complex to contain portions of both Laurentia and Gondwana and to record a complex tectonothermal history. Laurentian units are believed to record Late Cambrian-Early
Ordovician rifting and arc-construction, based upon magmatic zircon ages from metaigneous units, as well as Late Ordovician-Early Silurian blueschist facies metamorphism during closure of the Iapetus Ocean (Acatecan orogeny), based upon U-
Pb magmatic zircon ages interpreted to date high-pressure metamorphism of metasedimentary units.
Distinguishing between these opposing models is essential if the role of Oaxaquia in the Paleozoic tectonics of North America is to be understood. This study attempts to reconstruct the deformational/thermal history of the two main tectonostratigraphic units of the Acatlán Complex: the Cosoltepec Formation and Piaxtla Suite. Clarification of the history of these units will provide information essential to the establishment of a plausible tectonic model for the evolution of the Acatlán Complex and, therefore, the
Paleozoic evolution of the North American craton.
4. Geology of Las Minas
Southern Mexico is divided into a number of tectonostratigraphic terranes (Campa and Coney, 1982; Sedlock et al., 1993; Keppie, 2004), two of which, the Mixteca and
Zapoteca terranes, expose basement rocks of Paleozoic age or older (Figure 6). The
Oaxacan Complex of the Zapoteca terrane is the largest exposed portion of the Grenville- 20 age microcontinent, Oaxaquia, which underlies much of present-day Mexico (Ortega-
Guitierrez et al., 1995). The complex exposes ca. 1.0 Ga granulite facies gneisses overlain by unmetamorphosed Early Paleozoic sedimentary rocks bearing Gondwana fossil taxa (Robison and Pantoja-Alor, 1968). The basement gneisses of the Zapoteca terrane are believed to have formed as a ~1.2 Ga volcanic arc adjacent to the eastern margin of Amazonia (Keppie et al., 2003; Gillis et al., 2005) (Figure 7). The Mixteca terrane, to the west, contains Paleozoic basement rocks that display multiple phases of metamorphism and deformation, and are overlain by unmetamorphosed Jurassic and
Cretaceous sediments. The Paleozoic basement of the Mixteca terrane is known as the
Acatlán Complex (Ortega-Guitiérrez, 1978), and is believed to record the closure of a
Paleozoic ocean.
A large portion of the Acatlán Complex is exposed in the state of Puebla, southern
Mexico. The field area for this study, San Miguel Las Minas, is located approximately fifteen kilometers southeast of Izucar de Matamoras, Puebla, Mexico (Figure 8). This field area exposes two major tectonostratigraphic units of the Acatlán Complex, the
Cosoltepec Formation (PzC) and Piaxtla Suite (PzPS) (Figure 9), which are juxtaposed along a tectonic contact. Basement units in this field area are unconformably overlain by sedimentary units of inferred Jurassic depositional age, and are intruded by volcanic necks, interpreted to be associated with the Tertiary Sierra Madre del Sur magmatic province of the Trans-Mexican volcanic belt (Torres-Alvaredo et al., 2000). Basement units in San Miguel Las Minas show evidence of multiple phases of deformation and metamorphism. Units of the Piaxtla Suite record four phases of deformation at both high- and low-metamorphic grade, and rocks of the Cosoltepec Formation record three phases 21 of deformation under low metamorphic grade. Rocks of the Cosoltepec Formation are exposed in the northwestern portion of the field area, and rocks of the Piaxtla Suite comprise the remaining portion. The contact between the Cosoltepec Formation and
Piaxtla Suite is a vertical fault with horizontal slickensides on the fault surface, indicating latest motion was strike slip.
Figure 6: Simplified tectonic map of Mexico showing pre-Mesozoic tectonostratigraphic basement terranes and Tertiary volcanics. 22
Figure 7: Plate tectonic reconstruction outlining the formation of the Oaxacan Complex,
1350-1100 Ma (Keppie, 2004). 23
Figure 8: Simplified geologic map of the northern Acatlán Complex, southern Mexico
(modified from Ortega-Guitiérrez et al., 1999); A = field area of San Miguel Las Minas
(this study), B = field area of Asis Lithodeme (Middleton, 2006), C = field area near
Acatlán (Malone, 2004). 24
Figure 9: Geologic map of the field area: San Miguel Las Minas, Puebla, Mexico.
Cosoltepec Formation
Rocks of the Cosoltepec Formation in San Miguel Las Minas consist of interlayered metapsammite to quartzite and phyllite units (refer to Appendix A for detailed descriptions). Petrographically, the Cosoltepec units contain mostly quartz and muscovite. Three phases of penetrative deformation were observed in all units of the
Cosoltepec Formation (Figure 10), all of which occurred under greenschist- subgreenschist facies metamorphic conditions. Original bedding was not observed at 25 outcrop scale, but is visible in thin-section. The first phase of deformation (DC1) is characterized by isoclinal folding of bedding, which produced an axial-planar foliation of
aligned muscovites and recrystallized quartz (SC1) coplanar to original bedding. DC1 occurred under greenschist facies metamorphic conditions and involved the
crystallization of quartz veins parallel to SC1.
Sheath folds of the SC1 fabric and quartz veins, producing an axial-planar spaced crenulation cleavage (SC2) characterize the second phase of deformation (DC2) (Figures
11, 12). Measurement of sheath folds axis was difficult due to the mostly metapsammitic lithology of the Cosoltepec Formation in the field area, however, sheath folds are interpreted to be south-vergent due to observed double closures of folded quartz veins which trended N-S, and the south–vergent kinematics of sheath folds observed by
Malone et al. (2004) in the Cosoltepec Formation exposed outside of the city of Acatlán
(Figure 8). DC2 is interpreted to be the result of deformation under subgreenschist facies
conditions because there is no evidence of new mineral growth following DC1.
The third phase of deformation (DC3) involved NE-SW upright folding of SC2,
which produced an axial-planar solution cleavage (SC3) (Figure 13). The asymmetry of
FC3 upright folds indicates an E-SE vergence for DC3 kinematics (Figures 13, 14). DC3 is interpreted to be the result of deformation in the subgreenschist facies because, once
again, there is no evidence of new mineral growth following DC1 or DC2. 26
Figure 10: Photograph of metapsammite from the Cosoltepec Formation showing penetrative, planar deformation fabrics. 27
Figure 11: Lower hemisphere stereographic projections of structural data from the
Acatlán Complex units exposed in San Miguel Las Minas.
a) Poles to spaced crenulation cleavage planes (SC2) from the Cosoltepec Formation; mean vector length: 0.8793/1, n = 26.
b) Sheath fold axes (FPS3) from the metasedimentary units of the Piaxtla Suite. Filled circles represent “s” fold axes, and empty circles represent “z” fold axes. Sheath fold
geometry indicates south-vergent kinematics during this phase of deformation termed D3. 28
Figure 12: Photomicrographs of phyllite from the Cosoltepec Formation showing penetrative, planar deformation fabrics (field of view is 2 mm). 29
Figure 13: Lower hemisphere stereographic projections of structural data from the
Acatlán Complex units exposed in San Miguel Las Minas.
a) Upright fold axes (FC3) from the Cosoltepec Formation. Asymmetry of the upright
folds indicate southeast-vergent kinematics during this phase of deformation termed D4. b) Chevron fold axes (FPS4) from the metasedimentary units of the Piaxtla Suite. 30
Figure 14: Photograph of metapsammite from the Cosoltepec Formation showing SE- vergence of FC3 folds (looking down plunge).
Piaxtla Suite
Rocks of the Piaxtla Suite in San Miguel Las Minas can be subdivided into three units: the Los Lobos unit, the Las Minas unit, and the Esperanza granitoids (Figure 9).
The contact between the Los Lobos and Las Minas units is interpreted to have been originally depositional and is affected by large-scale NE-SW folds, and their contact with the Esperanza granitoid is thought to be intrusive. Rocks of the Los Lobos unit consist of interlayered garnet schist and metabasite units, with blueschists preserved in tectonic lenses (see to Appendix A for a detailed description). The metabasites are interpreted as 31 continental rift tholeiites based upon unpublished geochemical data (J. D. Keppie, pers. comm.). Petrographically, the garnet schists contain retrogressed garnets (and chlorite pseudomorphs after garnet), muscovite, and quartz. Metabasite units contain intermediate-plagioclase, hornblende retrogressed to actinolite, epidote, muscovite, and quartz. Blueschists preserved in tectonic lenses consist of calcic-plagioclase, glaucophane retrogressed to actinolite, lawsonite, garnet retrogressed to chlorite, epidote, muscovite, and quartz. Original bedding was not observed in outcrop, nor was it distinguished in thin-section.
Rocks of the Las Minas unit consist mostly of garnet metapsammite with local garnet schist, which are intruded by metabasite dikes interpreted to be continental rift tholeiites (J. D. Keppie, pers. comm.) on the basis of unpublished geochemical data (see
Appendix A for a detailed description). Petrographically, garnet metapsammites contain retrogressed garnets (and chlorite pseudomorphs after garnet), muscovite, and quartz.
Metabasites contain hornblende retrogressed to actinolite, epidote, chlorite, and quartz.
Original bedding was not observed either in outcrop or thin-section.
The Esperanza granitoid, is exposed in the southeastern portion of the field area where it is thought to intrude both the Las Minas and Los Lobos units. Petrographically, the granitoid contains albite, orthoclase feldspar, quartz, garnet, muscovite, chlorite, and sericite. Deformation of this granitoid body is characterized by a pervasive greenschist facies foliation, which overprints a potassium feldspar megacryst and garnet porphyroblast fabric. The greenschist facies foliation is affected by later folding phases developed under subgreenschist metamorphic conditions. 32 Evidence of four phases of penetrative deformation can be found in the metasedimentary/metaigneous units of the Piaxtla Suite in San Miguel Las Minas (Figure
15). Evidence of the first phase of deformation (DP1) is found in all lithological units of the Piaxtla Suite. DP1 is characterized by a foliation (SP1) of glaucophane, lawsonite, and garnet in blueschist units (Figure 16), hornblende porphyroblasts in metabsite units, garnet porphyroblasts in metasedimentary units, and garnet porphyroblasts and potassium felspar porphyroclasts in granitoid units. When viewed as a lithological package, the assemblage of minerals present during the first phase of deformation in the Piaxtla Suite
(hornblende/glaucophane, potassium feldspar, and garnet), indicate minimum amphibolite facies metamorphic conditions during DP1.
The second phase of deformation to affect rocks of the Piaxtla Suite (DP2) is expressed as a greenschist foliation (SP2) produced by the isoclinal folding of SP1 (Figure
17). DP2 is interpreted to have occured under greenschist metamorphic conditions because
SP1 garnets and amphiboles were retrogressed to chlorite and actinolite during the
formation of SP2, and there is growth of new muscovite parallel to SP2 (Figure 18). The kinematics for the DP2 event are indicated by a stretching mineral lineation of quartz and
chlorite on SP2 surfaces in the Esperanza granitoids, and by nonrotational augen tails of recrystallized quartz around porphyroclasts of potassium feldspar, both of which indicate a top-to-the-east sense of shear (Figure 17).
The third phase of deformation recorded in rocks of the Piaxtla Suite resulted in
the formation of a crenulation cleavage (SP3) axial-planar to south-vergent sheath folds
(Figures 11). DP3 is interpreted to be the result deformation under subgreenschist facies conditions because there is no evidence of new mineral growth following DP2. 33
The fourth phase of deformation (DP4) involved NE-SW chevron folding of SP3 to produce kink bands (SP4). Fold axes of kink bands plunge NE-SW (Figure 13). DP4 is interpreted to be the result of progressive deformation in the subgreenschist facies
because, again, there is no evidence of new mineral growth following DP2 or DP3.
Figure 15: Photograph of phyllite from the Piaxtla Suite showing penetrative, planar deformation fabrics. 34
Figure 16: Photomicrograph of blueschist (preserved in tectonic lense) from the Piaxtla
Suite showing penetrative, planar deformation fabric (SPS1) (field of view is 2 mm). 35
Figure 17: Lower hemisphere stereographic projections of structural data from the
Acatlán Complex units exposed in San Miguel Las Minas (a-c).
a) Poles to greenschist facies foliation planes (SPS2) from the metasedimentary units of the
Piaxtla Suite; mean vector length: 0.8793/1, n = 54.
b) Poles to greenschist facies foliation planes (SPS2) from the Esperanza granitoid of the
Piaxtla Suite; mean vector length: 0.8775/1, n = 18.
c) Stretching mineral lineation of quartz and chlorite (LPS2) formed on greenschist facies foliation planes (SPS2) in the Esperanza granitoids of the Piaxtla Suite; mean vector length: 0.9014/1, n = 9. d) Photograph of nonrotational augen tail of recrystallized quartz around porphyroclast of
potassium feldspar indicating a top-to-the-east sense of shear during D2 in the Esperanza granitoid of the Piaxtla Suite. 36
Figure 18: Photomicrograph of greenschist from the Piaxtla Suite showing penetrative,
planar deformation fabrics SPS2 (field of view is 2 mm).
5. Implications
Based on the results of this study, an attempt will be made to reconstruct the thermal and deformational history, as it is recorded in San Miguel Las Minas, of two
major tectonostratigraphic units of the Acatlán Complex: the Cosoltepec Formation (PzC) and the Piaxtla Suite (PzPS). In summary, deformation in the Cosoltepec Formation is characterized by three penetrative foliations (Figure 10): a greenschist facies foliation, a 37 spaced crenulation cleavage, and a crenulation cleavage. The greenschist foliation (SC1) is axial-planar to isoclinal folds, the spaced crenulation cleavage (SC2) is axial-planar to sheath folds (Figure 11, 12), and the crenulation cleavage (SC3) is axial-planar to upright folds (Figures 13, 14).
This sequence is consistent with the findings of Malone et al. (2002) (Table 1). In the same manner that Malone et al. used comparisons between the Cosoltepec and
Tecomate formations and the Totoltepec pluton to constrain the relative timing of deformation, known information about the Cosoltepec Formation is here used to clarify the relative timing of deformation in the Piaxtla Suite.
Deformation in the Piaxtla Suite is characterized by four penetrative foliations
(Figure 15): a blueschist/amphibolite facies foliation (SP1) (Figure 16), a greenschist
facies foliation (SP2) (Figures 17, 18), a crenulation cleavage axial planar to sheath folds
(SP3) (Figures 11), and kink bands (SP4) (Figure 13). In order to constrain the relative timing of these phases of deformation, a link needs to be established between the sequences of the Cosoltepec Formation and the Piaxtla Suite.
Upright folds (F C3) in the Cosoltepec Formation (Figures 13, 14) and kink bands
(F P4) in the Piaxtla Suite (Figure 13) are interpreted to record the same phase of deformation. This links the third phase of deformation in the Cosoltepec Formation to the fourth phase of deformation in the Piaxtla Suite. As mentioned before, asymmetry of the
upright folds (F C3) in the Cosoltepec Formation indicates a SE-vergence during this phase of deformation, termed D4. With this link, it is now possible to work backwards through time, comparing the sequence recorded in the Piaxtla Suite with that of the Cosoltepec
Formation. 38
Table 1: Table showing polyphase deformational fabrics recorded in the Cosoltepec
Formation, Tecomate Formation, and Totoltepec Pluton exposed east of the town of
Acatlán, southern Mexico (Malone et al., 2002).
The third phase of deformation in the Piaxtla Suite is characterized by south- vergent, sheath folds (Figures 11). These folds are interpreted to record south-vergent thrusting during N-S dextral transpression at ca. 276 Ma. This is based on the U-Pb zircon age of syntectonic neosome developed under the same conditions of N-S dextral shear along the Caltepec Fault Zone approximately 70 kilometers east of San Miguel Las
Minas (Elias-Herrera and Ortega Guitierrez, 2002). As mentioned above, sheath folds 39 were also developed in the Cosoltepec Formation during its second phase of deformation
(Figures 11, 12). Therefore, this links the second phase of deformation in the Cosoltepec
Formation with the third phase of deformation in the Piaxtla Suite, here designated D3.
Likewise, the second phase of deformation in the Piaxtla Suite is characterized by a subhorizontal, greenschist facies foliation (Figure 17, 18) akin to the greenschist facies metamorphic fabric that developed during the first phase of deformation in the
Cosoltepec Formation (Figure 12). Therefore, this links the first phase of deformation in the Cosoltepec Formation with the second phase of deformation in the Piaxtla Suite, here
designated D2. Kinematics for the D2 event are indicated by a stretching mineral lineation
of quartz and chlorite on SP2 surfaces in the Esperanza granitoids, and by nonrotational augen tails of recrystallized quartz formed around porphyroclasts of potassium feldspar, both of which indicate a top-to-the-east sense of shear (Figures 17).
Evidence for rapid exhumation of the Piaxtla Suite is provided by the unconformably overlying, unmetamorphosed, Otates Formation exposed 10 km north of the Asis Lithodeme (Figure 8). This unit contains Upper Fammenian conodonts (Vachard et al., 2000; Sánchez-Zavala et al., 2004), which require the Piaxtla Suite in this area to
have been exhumed prior to the Carboniferous (Middleton et al., in press). Therefore, D2 is interpreted to record rapid exhumation of the Piaxtla Suite over the Cosoltepec
Formation in the Devonian.
The first planar deformational fabric recorded in the Piaxtla Suite (SP1) is characterized by a foliation (SP1) of glaucophane, lawsonite, and garnet in blueschist units
(Figure 16), hornblende porphyroblasts in metabsite units, garnet porphyroblasts in metasedimentary units, and garnet porphyroblasts and potassium felspar porphyroclasts 40 in granitoid units. When viewed as a lithological package, the assemblage of minerals present during the first phase of deformation in the Piaxtla Suite, hornblende/glaucophane, potassium feldspar, and garnet, indicate minimum amphibolite
facies metamorphic conditions during D1. Middleton et al. (in press) report a concordant
U-Pb zircon age of 346 ± 3 Ma from an eclogitic amphibolite in the Asis Lithodeme of the Piaxtla Suite. The Asis Lithodeme is exposed in proximity to, and along strike from
San Miguel Las Minas (Figure 8), and this age is interpreted to date the timing of the high-grade metamorphism of the units of the Piaxtla Suite in the field area. The timing of this high-grade metamorphism is contemporaneous with closure of the Rheic Ocean
(Middleton et al., in press). There is no corresponding phase of deformation recorded in the Cosoltepec Formation, so it is interpreted to have been uninvolved in this phase of
deformation/metamorphism, which is designated D1.
Four phases of deformation are consequently recorded in this paired history: (i)
D1 was produced under blueschist/amphibolite metamorphic conditions in the Piaxtla
Suite and is not recorded in the Cosoltepec Formation, (ii) D2 developed under
greenschist facies conditions and is recorded in both units, (iii) D3 records south-vergent thrusting during N-S dextral transpression, and (iv) D4 records NW/SE shortening (Table
2). D1 is interpreted to record a tectonothermal event characterized by deformation under blueschist/amphibolite facies metamorphic conditions. Owing to the presence of retrograde assemblages indicating much lower metamorphic grade, and the absence of an
equivalent event in the Early Permian Tecomate Formation (Malone et al., 2001), D2 is interpreted to record rapid exhumation of the high-grade unit in the Devonian. D3 and D4 are interpreted to record progressive deformation under subgreenschist facies 41 metamorphic conditions, during a second tectonic event that is likely to be Permo-
Triassic based on available age constraints and the presence of equivalent events in the
Tecomate Formation (Nance et al., in press).
Table 2: Table summarizing the deformational/metamorphic history of the Piaxtla Suite and Cosoltepec Formation units exposed in San Miguel Las Minas.
6. Conclusions
Four phases of deformation/metamorphism are recorded in rocks of the Acatlán
Complex in San Miguel Las Minas. During the first phase of deformation, the Piaxtla
Suite was partially subducted under Laurentia during the closure of the Rheic Ocean in the Late Devonian/Early Mississippian (Acatecan orogeny) (Figure 4), resulting in the development of a penetrative deformation fabric under blueschist/amphibolite facies metamorphic conditions (Figure 16). The Cosoltepec Formation is unaffected by this event, and so maintains its original bedding (Figure 19a). During the second phase of 42 deformation, which is also considered to be Devono-Carboniferous, the Piaxtla Suite was exhumed against the Cosoltepec Formation under greenschist facies metamorphic conditions, resulting in a penetrative foliation common to both units (Figures 19b, 12,
18). During the third phase of deformation, the Piaxtla Suite and the Cosoltepec
Formation record south-vergent thrusting presumed to be synchronous with N-S dextral transpression along the Caltepec Fault Zone in the Early Permian (Figures 19c, 11, 12).
During the fourth, and final, phase of penetrative deformation, the Piaxtla Suite and
Cosoltepec Formation are compressed in response to NW-SE shortening (Figures 19d,
13, 14). D1 and D2 are considered to be progressive deformational events associated with the exhumation of the Piaxtla Suite against the Cosoltepec Formation in the Devono-
Carboniferous. D3 and D4 occur in both the Piaxtla Suite and Cosoltepec Formation, and record progressive deformation under subgreenschist metamorphic conditions during convergence along the paleo-Pacific margin of Pangea in the Permo-Triassic (Mixtecan orogeny) (Figure 5). 43 44 Figure 19: continued 45 Figure 19: Illustrated structural evolution of the Acatlán Complex at San Miguel Las
Minas (not to scale).
(a) During the first phase of deformation recorded in San Miguel Las Minas, the Piaxtla
Suite is partially subducted under Laurentia during the closure of the Rheic Ocean in the
Late Devonian/Early Mississippian (Acatecan orogeny), resulting in the development of a penetrative deformation fabric under blueschist/amphibolite metamorphic conditions. The
Cosoltepec Formation is unaffected by this event and so maintains its original bedding.
(b) During the second phase of deformation, the Piaxtla Suite is exhumed against the
Cosoltepec Formation under greenschist facies metamorphic conditions, in the Devonian, resulting in a penetrative foliation common to both units.
(c) During the third phase of deformation, the Piaxtla Suite and the Cosoltepec Formation record south-vergent thrusting, presumably at the same time as N-S dextral transpression along the Caltepec Fault Zone in the Early Permian.
(d) During the fourth, and final, phase of penetrative deformation, the Piaxtla Suite and
Cosoltepec Formation are compressed in response to NW-SE shortening. 46 7. References
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Samples from the Piaxtla Suite
Las Minas Unit
#09 garnet psammite N 18° 31.264’ W 098° 20.261’
Sample contains albite, garnet, muscovite, chlorite, quartz, serecite, and opaques (pyrite,
graphite). The thin section slab was cut parallel to azimuth 166, and perpendicular to S3PS in order to display F3PS folds. In thin section, this sample showcases F3PS folds and the S2PS
fabric. The S2PS spaced-crenulation cleavage contains quartz, albite, and muscovite, which wraps around garnet porphyroblasts of the SPS1 fabric. Sheath folds (F3PS) of S2PS and quartz veins are visible in thin section, and result in the development of a weak axial
planar penetrative fabric of oriented fine-grained muscovite growth (S3PS). During retrograde metamorphism, chlorite crystallized as pseudomorphs of garnet and muscovite. Deuteric alteration of muscovite and chlorite has resulted in serecite crystallization.
#11 garnet psammite N 18° 31.604’ W 098° 21.209’
Sample contains quartz, albite, garnet, muscovite, chlorite, calcite, and opaques (pyrite,
graphite). The thin section slab was cut parallel to azimuth 266, and perpendicular to S3PS in order to display F3PS folds. In thin section, this sample showcases F3PS folds and the S3PS fabric. The S2PS spaced-crenulation cleavage contains quartz, albite, and muscovite, which wraps around garnet porphyroblasts of the SPS1 fabric. Sheath folds (F3PS) of S2PS and quartz veins are visible in thin section, and result in the development of a weak axial 53 planar penetrative fabric of oriented fine-grained muscovite growth (S3PS). During retrograde metamorphism, chlorite crystallized as pseudomorphs of garnet and muscovite. Deuteric alteration of muscovite and chlorite has resulted in serecite crystallization. Calcite occurs as crack filling veins.
#13 garnet schist N 18° 31.604’ W 098° 21.209’
Sample contains albite, quartz, garnet, muscovite, chlorite, and opaques (pyrite, graphite).
The thin section slab was cut perpendicular to S2PS, and parallel to azimuth 310. In thin section, this sample showcases the S2PS fabric. The S2PS spaced-crenulation cleavage of
quartz, albite, and muscovite, wraps around garnet porphyroblasts of the SPS1 fabric.
During retrograde metamorphism, chlorite crystallized as pseudomorphs of garnet and
muscovite. Kink bands (S4PS) are visible in this thin section.
#10 epidote-amphibolite N 18° 31.367’ W 098° 20.899’
Sample contains albite, quartz, actinolite, epidote, clinozoisite, chlorite, and opaques
(graphite). The thin section slab was cut parallel to azimuth 105, and perpendicular to
S3PS. In thin section this sample showcases the S2PS fabric. The S2PS fabric consists mostly of actinolite pseudomorphs of relict horneblende porphyroblasts of S1PS, epidote, clinozoisite, and chlorite. Kink bands (S4PS) are visible in this thin section, and correspond with quartz and albite rich zones.
#12 epidote-amphibolite N 18° 31.604’ W 098° 21.209’ 54 Sample contains albite, quartz, actinolite, epidote, clinozoisite, chlorite, and opaques
(graphite). The thin section slab was cut parallel to L2PS along azimuth 310 and perpendicular to S2PS in order to determine kinematics. In thin section, this sample showcases the S2PS fabric. The S2PS fabric consists mostly of actinolite pseudomorphs of relict horneblende porphyroblasts of S1PS, epidote, clinozoisite, and chlorite. Kink bands
(S4PS) are visible in this thin section, and correspond with quartz and albite rich zones.
Los Lobos Unit
#06 glaucophane schist/epidote-amphibolite N 18° 32.639’ W 098° 21.441’
Sample contains intermediate to calcic plagioclase, quartz, glaucophane, lawsonite, actinolite, garnet, muscovite, epidote, clinozoisite, and chlorite. The thin section slab was
cut parallel to azimuth 185 and perpendicular to S2PS. In thin section, this sample
showcases the S1PS and S2PS fabrics. The S1PS fabric is characterized by porphyroblasts of glaucophane and garnet, and aligned lawsonite crystals. The plagioclase is interpreted as
a part of the high pressure S1PS metamorphic assemblage. The S2PS fabric is characterized by muscovite, which wraps around S1PS porphyroblasts, epidote, and clinozoisite. During retrograde metamorphism, actinolite crystallized as pseudomorphs of glaucophane; and chlorite crystallized as pseudomorphs of garnet.
#07 garnet schist N 18° 32.225’ W 098° 19.984’
Sample contains albite, quartz, garnet, muscovite, chlorite, and opaques (graphite). The
thin section slab was cut parallel to azimuth 007, and perpendicular to S3PS in order to display F3PS folds. In thin section, this sample showcases F3PS folds and the S2PS fabric. 55
The S2PS spaced-crenulation cleavage contains quartz, albite, and muscovite, which wraps around garnet porphyroblasts of the SPS1 fabric. Sheath folds (F3PS) of S2PS and quartz veins are visible in thin section, and result in the development of a weak axial planar
penetrative fabric of oriented fine-grained muscovite growth (S3PS). During retrograde metamorphism, chlorite crystallized as pseudomorphs of garnet and muscovite.
#08 epidote-amphibolite N 18° 32.207’ W 098° 19.963’
Sample contains albite, quartz, actinolite, muscovite, epidote, clinozoisite, calcite, and opaques (pyrite, graphite). The thin section slab was cut parallel to azimuth 330 and
perpendicular to S3PS in order to display F3PS. In thin section, this sample showcases the
S2PS fabric and F3PS sheath folds. The S2PS fabric consists mostly of actinolite
pseudomorphs of relict horneblende porphyroblasts of S1PS, epidote, clinozoisite, and chlorite. Sheath folds (F3PS) of S2PS are visible in thin section, and result in the
development of a weak axial planar penetrative crenulation cleavage (S3PS). Calcite occurs as crack filling veins.
Esperanza granitoids
#01 metagranitoid N 18° 31.055’ W 098° 19.670’
Sample contains albite, orthoclase feldspar, quartz, garnet, muscovite, chlorite, serecite,
calcite, and opaques. The thin section slab was cut parallel to L2PS along azimuth 078 and perpendicular to S2PS in order to determine kinematics. In thin section, this sample showcases the S2PS fabric. The S2PS fabric is characterized by albite, quartz, and aligned muscovite, which wrap around porphyroclasts of orthoclase and porphyroblasts of garnet. 56
Orthoclase porphyroclasts and garnet porphyroblasts are interpreted to represent S1PS.
Asymmetric porphyroclasts of orthoclase indicate a top to the east sense of shear.
Deuteric alteration of orthoclase feldspar has resulted in the crystallization of serecite.
Calcite occurs as crack-filling veins.
#15 metagranitoid N 18° 31.602’ W 098° 21.253’
Sample contains albite, orthoclase feldspar, quartz, muscovite, chlorite, and serecite. The
thin section slab was cut parallel to azimuth 236 and perpendicular to S2PS. In thin section, this sample showcases the S2PS fabric. The S2PS fabric is characterized by albite, quartz, and aligned muscovite, which wrap around porphyroclasts of orthoclase. Orthoclase
porphyroclasts are interpreted to represent S1PS. Asymmetric porphyroclasts of orthoclase indicate a top to the east sense of shear. Deuteric alteration of orthoclase feldspar has resulted in the crystallization of serecite.
#16 metagranitoid N 18° 31.491’ W 098° 21.473’
Sample contains albite, orthoclase feldspar, quartz, garnet, muscovite, chlorite, serecite, and calcite. The thin section slab was cut parallel to azimuth 045 and perpendicular to
S2PS, in order to determine kinematics. In thin section, this sample showcases the S2PS fabric. The S2PS fabric is characterized by albite, quartz, and aligned muscovite.
Orthoclase porphyroclasts and garnet porphyroblasts are interpreted to represent S1PS.
Chlorite has crystallized as pseudomorphs of garnet porphyroblasts. Asymmetric porphyroclasts of orthoclase indicate a top to the east sense of shear. Deuteric alteration 57 of orthoclase feldspar has resulted in the crystallization of serecite. Calcite occurs as crack-filling veins.
#26 chloritoid schist xenolith N 18° 31.336’ W 098° 19.636’
Sample contains albite, quartz, chloritoid, muscovite, and chlorite. The thin section slab was cut perpendicular to the S2PS fabric for petrographic analysis. In thin section, this sample showcases the S2PS fabric. The S2PS fabric is characterized by albite, quartz, and aligned muscovite. Chlorite has crystallized as pseudomorphs of chloritoid porphyroblasts.
Samples From the Cosoltepec Formation
#03 phyllite N 18° 32.934’ W 098° 21.824’
Sample contains albite, quartz, and muscovite. The thin section slab was cut parallel to
azimuth 270 and perpendicular to the S2C fabric in order to display F3C folds. In thin section, this sample showcases all three phases of deformation, as well as original bedding. Primary muscovite growth, and development of quartz veins occurred during the first phase of deformation: axial planar to isoclinal folds of original bedding.
#04 phyllite N 18° 32.827’ W 098° 21.724’
Sample contains albite, quartz, and muscovite. The thin section slab was cut parallel to
azimuth 005 and perpendicular to the S2C fabric for petrographic analysis. In thin section, this sample showcases all three phases of deformation, as well as original bedding. 58 Primary muscovite growth, and development of quartz veins occurred during the first phase of deformation: axial planar to isoclinal folds of original bedding.
#02 quartzite N 18° 32.934’ W 098° 21.824’
Sample contains albite, quartz, and muscovite. The thin section slab was cut parallel to
azimuth 243 and perpendicular to the S2C fabric in order to display F3C folds. In thin section, this sample showcases all three phases of deformation, as well as original bedding. Primary muscovite growth, and development of quartz veins occurred during the first phase of deformation: axial planar to isoclinal folds of original bedding.
#05 quartzite N 18° 32.761’ W 098° 21.690’
Sample contains albite, quartz, muscovite, and calcite. The thin section slab was cut
parallel to azimuth 008 and perpendicular to the S2C fabric for petrographic analysis. In thin section, this sample showcases all three phases of deformation, as well as original bedding. Primary muscovite growth, and development of quartz veins occurred during the first phase of deformation: axial planar to isoclinal folds of original bedding.
#05b quartzite N 18° 32.761’ W 098° 21.690’
Sample contains albite, quartz, muscovite, chlorite, and calcite. The thin section slab was
cut parallel to azimuth 262 and perpendicular to the S2C fabric in order to display F3C folds. In thin section, this sample showcases all three phases of deformation, as well as original bedding. Primary muscovite growth, and development of quartz veins occurred during the first phase of deformation: axial planar to isoclinal folds of original bedding.