Early Cretaceous Post-Collisional Collapse of the Yidun Terrane: Geochronological and Geochemical Constraints from Calc-alkaline to Alkaline Basalts in Xiqiu Area, Southwest China

Abstract

Several Cretaceous Carlin-like or hydrothermal gold deposits along the Garze-Litang suture zone and Early Cretaceous hydrothermal copper mineralization along the southeastern margin of the Songpan-Garze fold belt were presumed to have a magmatic heat source. However, no actual coeval magmatic events nearby were discovered. Here, we report zircon SIMS U-Pb age, whole-rock geochemical and Sr-Nd isotopic data of the Xiqiu basalts in the southern end of the Yidun terrane, eastern Tibetan Plateau. New zircon U-Pb ages yield weighted mean 206Pb/238U age of 117.7±1.6 Ma. The basalts are classified as calc-alkaline to alkaline and have relatively high MgO (4.77 wt.%–10.84 wt.%) and Mg number values (Mg#=(100×Mg/(Mg+Fe2+)); 45.35–67.28) and positive eNd(t) (t=118 Ma) values (+1.86 to +3.2), suggesting a OIB-like mantle source that is consistent with the normalized patterns of trace elements and rare earth elements (REEs). Geochemical data suggest that the primary basaltic magma was generated by low degree partial melting of a peridotite-dominated mantle source with a minor component of garnet-eclogite or pyroxenite and experienced olivine+clinopyroxene dominated fractional crystallization. The primary melt compositions calculated from the high MgO samples, in turn, suggest that the Xiqiu basalts were generated at 1.6–2.9 GPa with abnormally hot mantle potential temperatures from 1 465 to 1 540 oC. The melting temperatures are similar to the abnormally hot mantle underneath the Colorado Plateau and hotter than the mid-ocean range basalt (MORB) mantle and normal intra-continental mantle. Combined with previous studies, the Cretaceous Xiqiu basalts allow us to reconstruct a tectonic and geodynamic evolutionary model responsible for the Late Jurassic to Late Cretaceous geological records (magmatism, ore deposits and enhanced exhumation) in the Yidun terrane and southern Songpan-Garze fold belt.