Geochronology and genetic model for early Cretaceous volcanic rocks from the southern Qiangtang Terrane, northern Tibet, China: constraints from U-Pb zircon dating, whole-rock geochemical and Sr-Nd isotopic data

Abstract

Post‐collisional volcanic rocks of Mesozoic age occur in the regions adjacent to Gerze, part of the southern Qiangtang Terrane of northern Tibet, China. Geochronological, geochemical, and whole‐rock Sr‐Nd isotopic analyses were performed on the volcanic rocks to better characterize their emplacement age and models for their origin. Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U‐Pb zircon analyses yielded consistent ages ranging from 123.1±0.94 Ma to 124.5±0.89 Ma for six volcanic rocks from the study area. The intermediate volcanic rocks belong to the alkaline and sub‐alkaline magma series in terms of K₂O+Na₂O contents (5.9%–9.0%), and to the shoshonitic and calc‐alkaline series on the basis of their high K₂O contents (1.4%–3.3%). The Gerze volcanic rocks are characterized by the enrichment of light rare earth elements [(La/Yb)N=34.9–49.5] and large–ion lithophile elements (e.g., Rb, Ba, Th, U, K, Pb, and Sr), slightly negative Eu anomalies (Eu/Eu*=0.19–0.24), and negative anomalies in high field strength elements (e.g., Nb, Ta, Hf 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 εNd(t) values from –0.89 to –2.89. These results suggest that the volcanic rocks studied derived from a compositionally heterogeneous mantle source and that their parent magmas were basaltic. The more mafic, parental magmas to the Gerze volcanic rocks likely underwent fractional crystallization of clinopyroxene, hornblende, biotite, and potassium feldspar, during ascent, with little to no crustal contamination, prior to their eruption/emplacement. While these volcanic rocks exhibit geochemical signatures typical of magmas formed in a destructive plate‐margin setting, it is plausible that their mantle source might also have acquired such characteristics in an earlier episode of subduction.