EVIDENCE FOR LATE PALEOZOIC MAGMATISM IN WESTERN PANGEA (MEXICO)

Upper Paleozoic rocks exposed in northeast and east Mexico were previously interpreted as an orogenic flysch associated with the collision of South and North America during the formation of Pangea. However, recent mapping, petrological and geochemical analysis have shown that those rocks originated in a continental arc that extended along eastern Mexico, from Molango in Hidalgo State to Ciudad Victoria Tamaulipas and Delicias, Coahuila. The arc developed on Oaxaquia, the Proterozoic block that constitutes the basement of eastern Mexico. Rocks at Molango comprise submarine andesitic-basaltic lava flows, siliciclastic and volcaniclastic turbidites, calcareous debris-flow and conglomerate, and contains Late Pennsylvanian, but mostly Permian (Wolfcampian-Leonardian), fossils. Rocks at Ciudad Victoria are Upper Pennsylvanian to Lower Permian sandstone and shale turbidites, and conglomerates with abundant volcanic clasts. In this locality, Mississippian ignimbrites suggest that the magmatism might have started in the Carboniferous. Volcanic and volcaniclastic rocks of Pennsylvanian to Permian reported from Delicicas, Coahuila, show similar REE composition with rocks from Molango. The lavas and volcaniclastics are andesitic to basaltic in composition and related to subduction. Their REE patterns have slight enrichment in LREE, flat HREE and no Eu anomaly. Field evidence indicates that the arc was built upon Precambrian continental crust. The rocks at Molango are interpreted to have been deposited in an intra-arc basin and rocks of Delicias in Coahuila and of Ciudad Victoria to be part of a forearc basin. Although some authors have suggested that the volcanic arc was separated from North America by a subducting ocean basin located between Laurentia and Gondwana, paleontological evidence suggest that Oaxaquia must had approached North America before the Mississippian, since Brachiopods from Carboniferous rocks from Ciudad Victoria, Molango and other southern localities have strong biogeographic affinities with the Mid-continent province of North America. Apparently the arc migrated eastward by Permo-Triassic time, but by Late Triassic subduction-related magmatism was shut off. REE element geochemistry suggest that Late Paleozoic tuffs from New Mexico were probably associated with the arc of central-eastern Mexico.