Polyphase Deformation in San Miguel Las Minas, Northern
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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