Stagnated eclogitic slab-related shoshonitic series volcanic rocks in West Tianshan, Xinjiang, China: Insights from Li-B-Sr-Nd-Hf-Pb isotope and trace element compositions

Abstract

This study presents major, trace element and Li-B-Sr–Nd–Pb-Hf isotope compositions of post-collisional (Permian) shoshonitic volcanic rocks (SVRs) from West Tianshan, NW China. Zircon U Pb dating indicates that the West Tianshan SVRs were mainly formed in the late Early Permian (272–279 Ma). They include absarokites, shoshonites and banakites, which have variable SiO 2 (46–63 wt%) and K 2 O (2.12–8.12 wt%) contents. They display enrichment in light rare earth elements (LREE) and depletion in heavy rare earth elements. They are enriched in large ion lithophile elements (e.g., La, Rb and Ba) and depleted in high field strength elements (e.g., Nb, Ta and Ti) with slightly negative to negligible Eu anomalies, pronounced positive Pb anomalies and low Nb/U (9.32 ± 6.54), Ce/Pb (0.14–10.0, average 5.03 ± 3.02), relatively high Nb/Ta (16.74 ± 2.72) and Zr/Hf (39.56 ± 6.06) ratios. They have relatively high Li contents (26.0–93.0 ppm) and B contents (6.19–84.7 ppm), and low but variable δ 7 Li (−4.0‰ to +2.4‰). They are characterized by low ( 87 Sr/ 86 Sr) i (0.7045–0.7068), positive ε Nd (t) values (+0.31 − +5.53), relatively low 206 Pb/ 204 Pb (18.16–18.65), 207 Pb/ 204 Pb (15.51–15.63) and 208 Pb/ 204 Pb (37.97–39.22) and positive ε Hf (t) (+5.4 − +12.5). These features consistently suggest a metasomatized enriched mantle source having affinities with an EM I-like reservoir for the SVRs. Source enrichment was generated dominantly by subducted slab related aqueous fluid and altered oceanic crust (AOC) melts. Dehydration modeling of Li and B isotopes and Li, Y, B, Nb trace element fractionation collectively show that the flat, deeply subducted and stagnated, eclogitic slab suffered progressive dehydration(~5–12%)and was distinctly 7 Li-depleted. Based on these compositional features and model calculations, we suggest that West Tianshan SVRs had an enriched mantle source metasomatised by aqueous fluid and hydrous slab melt, and their parental magmas were derived from variable mixing of spinel and garnet peridotite (60Gt:40Sp − 80Gt:20Sp). Taking into account petrogenetic and regional tectonic data, we suggest that the West Tianshan SVRs developed in an intraplate setting during the late Early Permian. It is most probable that the Early Permian Tarim mantle plume, located in the south side of the study area, promoted Early Permian asthenospheric upwelling that resulted in melting of a fossil subduction-enriched source to produce shoshonitic volcanic rocks in the West Tianshan. • Lower δ7Li and wider range are evidently lighter than most volcanic suites world-wide. • A deeply subducted and stagnated eclogitic slab is one end-member of enriched source. • Parental magmas were from mixing of garnet and spinel peridotite (60/40–80/20). • SVRs formed in post-collision transition from subduction-related to intraplate setting.