Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications

Abstract

Limited geochronological and geochemical data from Early Cretaceous igneous rocks of the Gangdese Belt have resulted in a dispute regarding the subduction history of Neo-Tethyan Ocean. To approach this issue, we performed detailed in-situ zircon U–Pb and Hf isotopic, whole-rock elemental and Sr–Nd isotopic analyses on Late Mesozoic volcanic rocks exposed in the Liqiongda area, southern Lhasa terrane. These volcanic rocks are calc-alkaline series, dominated by basalts, basaltic andesites, and subordinate rhyolites, with a bimodal suite. The LA–ICPMS zircon U–Pb dating results of the basaltic andesites and rhyolites indicate that these volcanic rocks erupted during the Early Cretaceous (137–130Ma). The basaltic rocks are high-alumina (average>17wt.%), enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), showing subduction-related characteristics. They display highly positive zircon εHf(t) values (+10.0 to +16.3) and whole-rock εNd(t) values (+5.38 to +7.47). The silicic suite is characterized by low Al2O3 (<15.4wt.%), Mg# (<40), and TiO2 (<0.3wt.%) abundances; enriched and variable concentrations of LILEs and REEs; and strongly negative Eu anomalies (Eu/Eu*=0.08–0.19), as well as depleted Hf isotopic compositions (εHf(t)=+4.9 to +16.4) and Nd isotopic compositions (εNd(t)=+5.26 to +6.71). Consequently, we envision a process of basaltic magmas similar to that of MORB extracted from a source metasomatized by slab-derived components for the petrogenesis of mafic rocks, whereas the subsequent mafic magma underplating triggered partial melting of the juvenile crust to generate acidic magma. Our results confirm the presence of Early Cretaceous volcanism in the southern Lhasa terrane. Combined with the distribution of the contemporary magmatism, deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo-Tethyan oceanic lithosphere was flat-lying beneath the Lhasa terrane during the Early Cretaceous.