The Silurian to devonian magmatic evolution of the Eastern Tianshan Terrane: New insights from geochemistry, geochronology, and Sr–Nd–Hf isotopes of new-discovered Sidingheishan porphyry Cu–Mo deposit, NW China

Abstract

The Sidingheishan porphyry Cu–Mo deposit is located in the northeastern margin of the Kanggur–Huangshan Shear Zone of the Eastern Tianshan Terrane (ETT) in the Xinjiang Province of northwestern China. Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA–ICP–MS) zircon U–Pb dating of rocks in the area yielded an emplacement age of 427 ± 3 Ma for the ore-bearing granodiorite, which is intruded by 406 ± 3 to 397 ± 4 Ma gabbro, and a 382 ± 4 Ma monzogranite. These rocks have a tholeiitic to calc–alkaline and metaluminous geochemical affinities, with A/CNK values ranging from 0.65 to 1.14. The Silurian granodiorite and Carboniferous monzogranite are enriched in LREEs and LILEs (i.e. K, Ba, Pb and Sr), depleted in HREEs and HFSEs (i.e. Nb, Ta and Ti), and slightly positive Eu anomalies (Eu/Eu* = 0.94–1.39) indicative of crystal fractionation. The granodiorite and monzogranite have elevated MgO assays (0.36–1.85 wt%) and Mg# values (34–52), low (87Sr/86Sr)i ratios of 0.70320–0.70435, high εNd(t) values (5.9–6.9), TDM2 Nd model ages between 687 and 568 Ma, and εHf(t) values between + 8.37 and + 17.44. The geochemistry of the Early Devonian gabbro is characterised by its low SiO2 assays of 47.69–51.34 wt% and high Al2O3 assays of 18.07–22.91 wt% and MgO assays of 2.29–7.09 wt% (with Mg# values of 38–64), εNd(t) values of 4.7–6.9, εHf(t) of 8.04–17.97, and (87Sr/86Sr)i values of 0.70371–0.70420. The gabbro samples are enriched in LILEs, depleted in HFSEs, and have positive Eu anomalies. Combined with the regional geological history and above mentioned data, we suggest that the Sidingheishan granodiorite and monzogranite were formed during partial melting of subducted oceanic crust interacting with the lithospheric mantle. We also proposed that the Devonian gabbro developed during an extensional tectonic setting involving subduction–related magmatism in a continental arc margin rather than during mantle plume activity in the post–collisional extensional tectonic setting. The Sidingheishan granodiorite and associated porphyry Cu–Mo mineralisation were generated in an arc setting, associated with the N-dipping subduction of the Palaeo–Tianshan oceanic plate beneath the Dananhu–Tousuquan island arc.