PERMO-TRIASSIC (~284–221 MA) INITIATION OF THE CORDILLERAN MAGMATIC ARC OF SW NORTH AMERICA: PETROLOGICAL CONSTRAINTS AND INSIGHTS FROM NW SONORA, MEXICO

Permo-Triassic granitic rocks crops out in NW Mexico, and have been considered as part of a nascent cordilleran magmatic arc of Late Paleozoic to Early Mesozoic age that extends from SW North America, across Mexico and into the northern part of South America.

Initiation of the Cordilleran magmatic arc in SW North America is marked by intrusion of granitoid plutons along the west coast during Permo-Triassic time. The Permo-Triassic granitoid rocks (~284–221 Ma), found in NW Sonora, Mexico, intrude Paleoproterozoic basement rocks (~1.7–1.6 Ga) of SW Laurentia (mostly Yavapai-type crust). All these rocks belong to the medium- to high-K calc-alkaline series and present typical characteristics of subduction type rocks, with enrichment in LILE and LREE over HFSE and HREE with pronounced negative Nb-Ta and positive K and Pb anomalies. The granitoid rocks show heterogeneous and enriched zircon Hf-isotope compositions with negative εHf(t) values of -25 to –9 and old TCDM ages of 1.59–2.39 Ga. All available data, including field and petrographic relationships, mineral chemistry, bulk major and trace element compositions, and zircon Hf-isotopes suggest that granitoids were sourced primarily from Paleoproterozoic basement rocks (~1.7–1.6 Ga) of SW Laurentia (Yavapai-type crust) through processes of assimilation and/or magma mixing (MASH processes).

We propose that Permo-Triassic magmas were formed in the early stage of an active continental margin established in SW North America during the northeastward subduction of a paleo–Pacific oceanic plate beneath the North America craton. Specifically, Permo-Triassic subduction-related magmatism in NW Sonora was triggered by partial melting of different crustal protoliths driven by basaltic underplating. Accordingly, evolved isotope and geochemical signatures were recorded in the early stage of the nascent Cordilleran magmatic arc of Late Paleozoic in the SW Laurentia margin.

Research supported by projects CONACYT 82518 & 129370 and PAPIIT 113906 & 116709.