Petrogenesis and tectonic implications of the Triassic volcanic rocks in the northern Yidun Terrane, Eastern Tibet

Abstract

Triassic Paleo-Tethyan subduction-related volcanic rocks are extensively distributed in the Xiangcheng and Changtai regions, northern Yidun Terrane, eastern Tibet. The ~228Ma volcanic rocks in the Xiangcheng region have intermediate-felsic composition (60.1 to 64.7wt.% SiO2) with relatively low Y (14–18ppm) and high Sr (630–1830ppm) yielding high Sr/Y ratios (43–94). They have LREE-enriched and HREE-depleted REE patterns (La/Yb=19–26), typical of adakites. Sr–Nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern Yidun Terrane. The ~231–230Ma volcanic rocks in the Changtai region are composed of basalts, andesites, dacites and rhyolites with SiO2 ranging from 47.7 to 81.0wt.%. Basalts in the Changtai region have initial 87Sr/86Sr ratios from 0.7052 to 0.7058 and εNd(t) from +1.1 to +1.5. They display OIB-like geochemical signatures, and were likely derived from low degrees of partial melting of an OIB-like mantle source with subordinate input of subduction components indicated by increasing Th content. Andesites and dacites show elemental and isotopic compositions that evolved from the basalts, indicative of derivation via crustal contamination and fractional crystallization (AFC) of basaltic magmas. Rhyolites have lower initial 87Sr/86Sr ratios and higher εNd(t) values than andesites/dacites, inconsistent with an AFC process and, are suggested to have been derived from anatexis of crustal materials. Emplacement of volcanic rocks in the Changtai and Xiangcheng regions could be attributed to the subduction of the Ganzi–Litang Ocean, a branch of the Paleo-Tethys Ocean. Volcanic rocks in the Changtai and Xiangchang regions have zircon U–Pb ages about 4–6myr older than that of arc granites in the eastern Yidun Terrane. The spatio-temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern Yidun Terrane in the Middle Triassic and extended to the northern Yidun Terrane at the beginning of the Late Triassic.