Zircon U–Pb geochronology and Sr–Nd–Hf–O isotope geochemistry of Late Jurassic granodiorites in the southern Qiangtang block, Tibet: Remelting of ancient mafic lower crust in an arc setting?

Abstract

A Late Jurassic–Late Cretaceous magmatic belt extends for >800 km along the southern margin of the southern Qiangtang Block (SQB) in central Tibet. However, the Mesozoic tectonic setting of these igneous rocks remains uncertain. Here, we report new in situ zircon U–Pb ages and Hf–O isotopes as well as whole-rock major- and trace-element compositions and Sr–Nd isotopes for the Jiacuo granodiorites and dikes in the SQB. Zircon laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating indicates that these rocks were emplaced during the Late Jurassic (ca. 153–151 Ma). All of the rocks have high SiO2 (64.7–68.4 wt%) and K2O (4.2–5.2 wt%) contents and are characterized by enrichment in light rare-earth elements ((La/Yb)N = 16–19) and large-ion lithophile elements, and depletion in high-field-strength elements. The granodiorites and dikes have variable zircon Hf–O isotope values (εHf(t) = −15.7 to −10.1, δ18O = +5.32‰ to +7.88‰), but uniform and high whole-rock initial 87Sr/86Sr isotopic (0.7079 to 0.7080) and low εNd(t) (−7.9 to −7.6) values. We suggest that these granodiorites were originated by partial melting of ancient mafic lower crust of the SQB. Combining our new data with the temporal and spatial distributions of Mesozoic magmatic rocks in the SQB, we propose that Late Jurassic magmatism in the SQB was resulted from northward subduction of the Bangong–Nujiang Tethyan oceanic slab during this period.