GEOLOGY, GEOCHEMISTRY AND STRUCTURAL GEOLOGY OF THE MYLONITIC COMPLEXES OF THE TEZIUTLAN BASEMENT, PUEBLA STATE, IN EASTERN MEXICO

The La Soledad and Xucayucan Mylonitic complexes are located in Teziutlan in Puebla State and belong to the Sierra Madre terrane. The La Soledad Mylonitic Complex is made up of quartz-feldspar mylonitic gneiss and schist, whose major- and trace-element chemistries suggests continental-arc affinities. The one sample analyzed for Nd gave a model age, TDM, of 971 Ma, suggesting evidence of Precambrian crustal contamination. Population and single zircon U-Pb geochronology indicates that the La Soledad mylonitic complex has some inherited Precambrian zircons (1054 to 837 Ma), and Paleozoic (magmatic?) zircons between 327 to 345 Ma. The quartz-feldspar gneiss has interbedded chlorite + plagioclase + leucoxene schist, possibly derived from basic rocks. The Xucayucan mylonitic complex is a volcanosedimentary sequence made up of massive and pillowed metabasalt and metaandesitic basalt, interbedded with quartzfeldspar- and mica mylonitic schist and quartzite, which protolith was sandstone and shale. The lavas show flat REE patterns with slightly enriched or depleted LREE. Th, Ti, Ta, Ni and Yb concentrations suggest a depleted mantle source for the parental magmas. The Xucayucan mylonitic complex appears to have originated in a back-arc or arc in an extensional setting. Textures within the two mylonitic complexes suggest at least four phases of deformation. Banded textures within the porphyroclasts suggest an older phase of deformation, D1, previous to mylonitization. D2 is a regional mylonitic event, characterized by low-angle shear zones with kinematic indicators that suggest a NE to SW direction of transport. This deformational event is associated with isoclinal and kink band folds, mineral lineation and a tectonic interlayering of the different lithologies. This mylonitic event could represent a tectonic contact associated to the amalgamation of Pangea during Permo-Triassic time. D3 is characterized by open folds and thrusthing in more brittle conditions. Since D3 indicators are found in the Mesozoic units as well, it probably originated during the Laramide Orogeny. Younger strike-slip and normal faults, D4, cut all the units apparently in the Cenozioc, and is the youngest deformational event of the area.