PERMIAN (~275-258 MA) GRANITIC MAGMATISM IN NW SONORA, MEXICO: THE MISSING LINK TO A LARGE CONTINENTAL ARC EVENT IN SW NORTH AMERICA

The closing of the Rheic Ocean along an ENE-WSW, south-dipping, subduction zone during the Late Paleozoic allowed the final collision of southern Laurentia and Gondwana along the Ouachita-Marathon-Sonora orogenic belt (suture) to achieve the final assembly of Pangea. This south-dipping subduction zone formed the latest Mississippian-Early Permian Las Delicias volcanic arc that went extinct just prior to the final continent-continent collision.

In Permian time, during the last stages of collision, the western part of Pangea experienced approximately east-west convergence that generated a nascent NW-SE, east-dipping, subduction zone that started consuming the proto-Pacific oceanic plate (Mezcalera plate?) allowing the establishment of the East Mexico continental magmatic arc (~300-250 Ma). We hypothesize that the magmatism started initially in southern Mexico, where we find the oldest igneous rocks, and advanced towards the north passing through NW Sonora and going all the way to California and Nevada in the USA. This is a large continental arc event in SW North America that differs in time, and is basically oblique to the older Las Delicias arc found in NE Mexico.

The Permian (~275-258 Ma) granitic rocks, found in NW Sonora, intrude paleoproterozoic (~1.7 Ga) basement rocks of SW Laurentia (Yavapai crust) and represent a change from a passive continental margin, established soon after the rifting of Rodinia, to an active margin in Permian time. These granitic rocks in NW Sonora represent a previously unrecognized local source of Permian zircons that should allow the reinterpretation of U-Pb detrital zircon data for Mesozoic and Cenozoic sedimentary basins allowing for more robust paleogeographic reconstructions of NW Mexico.