OAXAQUIA, A PROTEROZOIC MICROCONTINENT ACCRETED TO NORTH-AMERICA DURING THE LATE PALEOZOIC

Grenville-age granulite facies rocks in southern, central, and northea stern Mexico have distinctive geologic features that suggest a common tectonic evolution. The similarities include northwest-trending struct ural grain from Oaxaca to Tamaulipas, massif-type anorthosite-chamocki te complexes, protoliths rich in sedimentary rocks of shallow-marine p latform or continental rift-related facies devoid of calc-alkaline vol canic rocks, common metamorphism under granulite facies conditions, U- Pb zircon ages of about 1.0 Ga., and an apparently common history of u plift and cooling. Altogether, this evidence strongly suggests a coher ent geologic history for this block. Paleontologic data from the overl ying sedimentary sequences indicate that Oaxaquia was not part of Laur entia during most of the Paleozoic. This precludes emplacement of Oaxa quia in its present position by simple lateral displacement from the s outhern United States as well as a Taconic time of emplacement. Oaxaqu ia was probably emplaced to its present position during late Paleozoic time. The concept of a Mesoproterozoic ''Oaxaquia'' microcontinent ex tending for about 1000 km in Mexico needs to be considered in the reco nstruction of the Grenville orogen as a whole and for the Paleozoic te ctonic interactions between eastern Laurentia and western Gondwana.