The Late Paleozoic magmatic evolution of the Aqishan-Yamansu belt, Eastern Tianshan: Constraints from geochronology, geochemistry and Sr–Nd–Pb–Hf isotopes of igneous rocks

Abstract

The Aqishan-Yamansu belt in the Eastern Tianshan (Xinjiang, NW China) is an important mineralization belt. The belt mainly comprises Carboniferous volcanic, volcaniclastic and clastic rocks, and hosts many intermediate–felsic intrusions and Fe (–Cu) deposits. The biotite diorite, felsic brecciated tuff, granodiorite and syenite from the western Aqishan-Yamansu belt are newly zircon U–Pb dated to be 316.7±1.4Ma, 315.6±2.6Ma, 305.8±1.9Ma and 252.5±1.4Ma, respectively. The mafic rocks (mafic brecciated tuff and diabase porphyry) are tholeiitic to calc-alkaline series, LILE-rich (e.g., Rb, Ba and Pb), HFSE-depleted (e.g., Nb and Ta), and have high Mg#(44–60), Nb/Ta (15.0–20.0), Ba/La (>30) and Ba/Nb (>57) values/ratios, and low Th/Yb ratios (<1), probably originating from mantle wedge metasomatized by slab-derived fluids. The intermediate–felsic igneous rocks are LILE-rich, HFSE-depleted, with high Sr and Y contents showing typical of normal arc magma affinity. Moreover, the depleted εHf(t) (>2.10) and positive εNd(t) (>5.7), combined with variable Nb/Ta ratios (9.52–21.4), Y/Nb ratios (1.47–39.7) and Pb isotopes (206Pb/204Pb=16.225–17.640, 207Pb/204Pb=15.454–15.520, 208Pb/204Pb=37.097–38.025) suggest that these rocks were magma mixing products between juvenile crustal-derived magmas and minor mantle-derived magmas. Combined published works with our new ages, geochemical and isotopic data, we propose that the Aqishan-Yamansu belt was an Early Carboniferous fore-arc basin during the southward subduction of the Kangguer oceanic slab beneath the Yili-Central Tianshan block. With the continuing southward subduction, the Aqishan-Yamansu fore-arc basin initiated to close, which generated the mafic and intensive intermediate–felsic magmatism associated with regional Fe (–Cu) mineralization.