Abstract

ARVIZU, Harim E.  and  IRIONDO, Alexander. Time control and geology of the Permo-Triassic magmatism at Sierra Los Tanques, NW Sonora, Mexico: Evidence for the beginning of the Cordilleran arc magmatism in SW Laurentia. Bol. Soc. Geol. Mex [online]. 2015, vol.67, n.3, pp.545-586. ISSN 1405-3322.

Sierra Los Tanques is located in NW Sonora, Mexico, and represents one of the main outcrops of Permo-Triassic granitic rocks reported in the region. U-Pb zircon geochronology conducted in two groups of granitoids from various locations in NW Sonora provide a range of ages between 284 - 221 Ma. Rock types are dominated by granodiorite, followed by quartzmonzodiorites, and monzogranites. There are two main granitic suites-leucocratic and melanocratic granitoids. Field relationships between the two types suggests that the melanocratic granitoids are older as they are clearly intruded by the leucocratic suite. This field evidence is supported, in most cases, by the U-Pb zircon ages obtained in samples of both granitic suites. Most of the zircons from these Permo-Triassic granitoids have high U concentrations (~ 47 - 9508 ppm), suggesting significant Pb loss in their crystal structure. The Permo-Triassic zircons show highly variable Th/U ratios (0.01 - 0.73). These zircons have an igneous origin not only because of their prismatic morphologies, but also due to their oscillatory zoning characteristic of magmatic growth as observed in cathodoluminescence studies. The presence of inherited cores, as shown in cathodoluminescence images, can be associated with the ages of meta-igneous Proterozoic basement present in Sierra Los Tanques (~ 1.7 - 1.6 Ga, ~ 1.4 Ga and ~ 1.1 Ga) that could have been incorporated during magma-genesis in Permo-Triassic time. These granitoids associated with subduction give evidence for the beginning of cordilleran arc magmatism in SW North America (Laurentia) established along the western margin of Pangea immediately following the latest stages of the collision between Laurentia and Gondwana. This Permo-Triassic magmatism is important for understanding the tectonic evolution of NW Mexico. Tentatively, the occurrence of this magmatism is associated with a zone of crustal weakness spatially associated with the Mexican Yavapai basement province in NW Sonora. Understanding this magmatic pulse is also of particular importance as it represents a regional source of detrital Permo-Triassic zircons previously unrecognized for Mesozoic and Cenozoic sedimentary basins in Sonora and southern Arizona. Lastly, this Permo-Triassic subduction-related magmatic pulse present in NW Sonora is part of a larger magmatic arc event along the American Cordillera that extends from western-southwestern United States, passing through northern, central, and southern Mexico and reaching northern South America.

Keywords : Sierra Los Tanques; Permo-Triassic; continental magmatic arc; subduction; Laurentia.