U–Pb zircon age, geochemical and Sr–Nd isotopic data as constraints on the petrogenesis and emplacement time of andesites from Gerze, southern Qiangtang Block, northern Tibet

Abstract

Mesozoic post-collisional andesites occur in the Gerze area within the Qiangtang Terrane of northern Tibet, China. Geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses were performed on this suite of volcanic rocks to characterize their petrogenesis. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded consistent ages ranging from 122.4 ± 0.4 Ma to 124.4 ± 0.4 Ma for two andesite samples (JC01 and GZ01). The andesites belong to the alkaline and sub-alkaline magma series in terms of K 2O + Na 2O contents (4.7–10 wt.%), and belong to the high-K calc-alkaline and calc-alkaline series on the basis of their high K 2O contents (1.0–3.3 wt.%). The volcanic rocks are further characterized by the enrichment of light rare earth elements [(La/Yb) N = 7.4–11.5] and large-ion lithophile elements (e.g., Rb), slightly negative Eu anomalies (Eu/Eu * = 0.79–0.94), and negative anomalies in high field strength elements (e.g., Nb, Ta, and Ti) relative to primitive mantle. The samples show slightly elevated ( 87Sr/ 86Sr) i values that range from 0.7049 to 0.7057, and low ε Nd( t) values of −3.4 to 0.7. These results suggest that the rocks were derived from a compositionally heterogeneous mantle source and that their parent magmas are basalt magma. The parent mafic magmas may have undergone fractional crystallization of olivine, pyroxene, hornblende, and to a lesser extent, plagioclase during ascent through the thickened crustal pile, with minor or no crustal contamination, before the finally erupting as andesites.