DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF CAMBRIAN TO TRIASSIC MIOGEOCLINAL AND EUGEOCLINAL STRATA OF SONORA, MEXICO

One hundred and eighty two individual detrital zircon grains from Camb rian through Permian miogeoclinal strata, Ordovician eugeoclinal rocks , and Triassic post-orogenic sediments in northwestern Sonora have bee n analyzed. During Cambrian, Devonian, Permian, and Triassic time, mos t zircons accumulating along this part of the Cordilleran margin were shed from 1.40-1.45 and 1.62-1.78 Ga igneous rocks that are widespread in the southwestern United States and northwestern Mexico. Zircons wi th ages of approximately 1.11 Ga are common in Cambrian strata and wer e apparently shed from granite bodies near the sample site. The source s of 225-280 Ma zircons in our Triassic sample are more problematic, a s few igneous rocks of these ages are recognized in northwestern Mexic o: Such sources may be present but unrecognized, or the grains could h ave been derived from igneous rocks of the appropriate ages to the nor thwest in the Mojave Desert region, to the east in Chihuahua and Coahu ila, or to the south in accreted(?) are-type terranes. Because the zir con grains in our Cambrian and Devonian to Triassic samples could have accumulated in proximity to basement rocks near their present positio n or in the Death Valley region of southern California, our data do no t support or refute the existence of the Mojave-Sonora megashear. Ordo vician strata of both miogeoclinal and eugeoclinal affinity are domina ted by >1.77 Ga detrital zircons, which are considerably older than mo st basement rocks in the region. Zircon grains in the miogeoclinal sam ple were apparently derived from the Peace River arch area of northwes tern Canada and transported southward by longshore currents. The eugeo clinal grains may also have come from the Peace River arch region, wit h southward transport by either sedimentary or tectonic processes, or they may have been shed from off-shelf slivers of continents (perhaps Antarctica?) removed from the Cordilleran margin during Neoproterozoic rifting. It is also possible that the Ordovician eugeoclinal strata a re far traveled and exotic to North America.