DEFORMATIONAL HISTORY OF THE GRANJENO SCHIST NEAR CIUDAD VICTORIA, MEXICO A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science David S. Dowe June 2004 This thesis entitled DEFORMATIONAL HISTORY OF THE GRANJENO SCHIST NEAR CIUDAD VICTORIA, MEXICO BY DAVID S. DOWE has been approved for the Department of Geological Sciences and the College of Arts and Sciences by R. Damian Nance Professor of Geological Sciences Leslie A. Flemming Dean, College of Arts and Sciences Dowe, David S. M.S. June 2004. Geological Sciences Deformational History of the Granjeno Schist Near Ciudad Victoria, Mexico (pp. 109) Director of Thesis: R. Damian Nance Exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt of northeastern Mexico, the Paleozoic Granjeno Schist comprises a polydeformed assemblage of metasedimentary and metavolcaniclastic rocks and serpentinized mafic- ultramafic units. Deformation of the Granjeno Schist has produced at least four sets of structures. The earliest deformation (D1) predates emplacement of a leucogranite at 351±54 Ma and may record obduction of this oceanic unit. Subsequent deformations (D2-D4) record the tectonic juxtapositioning of the Granjeno Schist against the Grenville- aged Novillo Gneiss by NNW-directed dextral shear under conditions of decreasing temperature. Mica cooling ages of 313±13 Ma and 300±4 Ma are considered to date the onset of dextral motion, which continued into the Permian. These events are linked to the Late Paleozoic closing of the Rheic Ocean. Approved: R. Damian Nance Professor of Geological Sciences Acknowledgements I would like to thank the members of my advising committee – Drs. Damian Nance, Greg Nadon and David Schneider, for without their encouragement, advice, and revisions, this project could not have been completed. More thanks to Damian Nance for providing the opportunity to work in Mexico, and to Drs. Duncan Keppie and Fernando Ortega- Gutiérrez for their keen ideas and geological knowledge that they willingly shared in the field. Many thanks to Victoria Tong, for her vast technical knowledge that helped carry this thesis through many hardships. A great thank you to my wife, Loretta Ransom, whose time, energy, and personal sacrifices, have ultimately made this thesis a completed project. 5 Table of Contents Page Abstract............................................................................................................................... 3 Acknowledgements............................................................................................................. 4 List of Tables ...................................................................................................................... 7 List of Figures..................................................................................................................... 8 List of Plates ....................................................................................................................... 9 I. Introduction .................................................................................................................. 11 II. Regional Setting .......................................................................................................... 15 Geology of Mexico ....................................................................................................... 15 Guachichil Terrane.................................................................................................... 15 Huizachal-Peregrina Anticlinorium.......................................................................... 18 Stratigraphy........................................................................................................... 18 III. Field Relations and Petrography................................................................................ 25 Introduction................................................................................................................... 25 Granjeno Schist............................................................................................................. 26 Metasedimentary units.............................................................................................. 26 Pelitic Schist.......................................................................................................... 26 Interbedded Pelitic and Psammitic Schist............................................................. 33 Metachert .............................................................................................................. 35 Graphitic Schist..................................................................................................... 35 Metavolcaniclastic Schist...................................................................................... 37 Metaigneous Units .................................................................................................... 45 Metabasalt............................................................................................................. 45 Serpentinite and Metagabbro........................................................................................ 47 Serpentinite ........................................................................................................... 47 Metagabbro ........................................................................................................... 49 IV. Structural Geology..................................................................................................... 52 Introduction................................................................................................................... 52 D1 Structures................................................................................................................. 52 D2 Structures................................................................................................................. 61 D3 Structures................................................................................................................. 66 D4 Structures................................................................................................................. 69 Structural Analysis.................................................................................................... 71 Relationship to faulting............................................................................................. 75 V. Geochronology............................................................................................................ 78 VI. Deformation and Metamorphism............................................................................... 83 Introduction................................................................................................................... 83 D1 Tectonothermal Event.............................................................................................. 83 D2 Tectonothermal Event.............................................................................................. 85 D3 Tectonothermal Event.............................................................................................. 91 D4 Tectonothermal Event.............................................................................................. 92 VII. Tectonic History of the Granjeno Schist .................................................................. 94 VIII. Potential linkages to the Acatlán Complex of southern Mexico............................. 97 6 References....................................................................................................................... 101 Appendix......................................................................................................................... 109 7 List of Tables Table 5.1 U-Pb data for zircon in leucogranite ................................................................ 80 Table 5.2 40Ar/39Ar data for (a) muscovite in leucogranite and (b) phengite in quartz keratophyre ............................................................................................................... 82 8 List of Figures Figure 1.1 Location of the Huizachal-Peregrina Anticlinorium ...................................... 12 Figure 1.2 General geology of the Huizachal-Peregrina Anticlinorium.......................... 13 Figure 2.1 The tectonostratigraphic terranes of Mexico.................................................. 16 Figure 2.2 The Guachichil Terrane of northeastern Mexico............................................ 17 Figure 2.3 Generalized stratigraphic column of rocks that overlie the Novillo Gneiss within the Huizachal-Peregrina Anticlinorium......................................................... 21 Figure 2.4 Timing of deformations in sedimentary rocks overlying the Novillo ............ 23 Figure 4.1 D1/D2 in Subarea 1.......................................................................................... 53 Figure 4.2 D3 in Subarea 1............................................................................................... 53 Figure 4.3 D4 type locality in Subarea 1 .......................................................................... 54 Figure 4.4 Structural relationship in Subarea 1 on limbs and hinges of F1/F2................. 55 Figure 4.5 D1/D2 in Subarea 2.......................................................................................... 56 Figure 4.6 D3 in Subarea 2..............................................................................................