Timing and origin of silicic volcanism in northwestern Mexico: Insights from zircon U-Pb geochronology, Hf isotopes and geochemistry of rhyolite ignimbrites from Palmarejo and Guazapares in Southwest Chihuahua

Abstract

Precise timing and constraints for magma source(s) are critical to understand the post-subduction tectonomagmatic evolution of the world's largest Cenozoic Silicic Large Igneous Province in western Mexico. We provide in-situ zircon U–Pb ages and Hf isotope compositions of rhyolitic ignimbrites exposed in the Palmarejo and Guazapares at the northwestern extremity of Sierra Madre Occidental. Four rhyolite ignimbrite samples from Palmarejo yielded zircon weighted mean U–Pb ages between 23 and 24 Ma. However, five individual analyses from one sample yielded the oldest age of 25.09 ± 0.41 Ma (MSWD = 1.7, n = 5). A rhyolitic tuff from the Guazapares district yielded older individual dates varying from 25.7 to 28.6 Ma with a main population yielding a weighted mean age of 27.31 ± 0.36 Ma. Zircon U–Pb ages suggest that the ignimbrite flare-up in southwestern Chihuahua remained restricted to the Early Middle Oligocene to Early Miocene. We suggest that the zircon U–Pb dates >24 Ma (particularly from Guazapares) are the antecrystic zircons included from the underlying older ignimbrites. The zircon U–Pb dates of 25–28 Ma are related to the culmination of syn-extensional first ignimbrite flare-up and bimodal volcanism started as early as 35 Ma. The first ignimbrite flare-up is possibly resulted from the detachment of the leading edge of the Farallon slab around 35–28 Ma. The younger age of 24–23 Ma from Palmarejo can be correlated to the second episode of ignimbrite flare-up erupted during the pervasive regional extension, normal faulting and onset of formation of metamorphic core complexes. This second flare-up is resulted by the subsequent events of break off/rollback of the Farallon plate. The positive Hf isotope composition (εHf (t) = +1.0 to +5.8) and concomitant mafic volcanism suggest partial melting of subcontinental lithospheric mantle and the overlying Laramide crust by upwelling of decompressing asthenosphere during post-Laramide regional extension. This interpretation is consistent with the successive detachments, fragmental removal and/or rollback of the leading edges of the Farallon slab beneath the western continental margin of North America. We did not find any xenocrystic zircon inheritance neither from the Laramide plutonic basement/roots of the Jurassic Guerrero terrane nor from the Proterozoic North American crust. However, zircon inheritance of Laramide age has been reported from the Guazapares district. Partial melting of Laramide basement was possibly facilitated by the heat advection and ponding of subcontinental lithospheric mantle-related magmas in the crust during the Cenozoic regional extension.