Himalayan-like crustal melting and differentiation in the southern North American Cordilleran anatectic belt during the Laramide orogeny: Coyote Mountains, Arizona

Abstract

Abstract The southern U.S. and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (ca. 80-40 Ma). The metamorphic sources of melt are not exposed at the surface, however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62-42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon REE concentrations and indicate prograde heating, mineral breakdown, and melt generation took place from 62-53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53-51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51-42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution-reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, LILE, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High silica, muscovite ± garnet leucogranite dikes that crosscut two-mica granite represent more evolved, residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of 1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, 2) heat and fluid transfer related to the Laramide continental arc, and 3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.