Carboniferous magmatic activity in the Aqishan–Caixiashan polymetallic ore cluster, eastern Tianshan, NW China: implications for tectonic evolution and regional metallogeny

Abstract

The Aqishan-Caixiashan polymetallic ore cluster, located in the eastern Tianshan Orogenic Belt, is part of the southern Central Asian Orogenic Belt (CAOB) and experienced extensive tectono-magmatic events with polymetallic mineralization during the Carboniferous. However, the Carboniferous tectonic affinity and magmatic evolution of the ore cluster are still controversial, which limits further understanding of the tectonic evolution and regional metallogeny. With the aim of solving these problems, we have carried out geochronological and geochemical studies on Carboniferous intrusive rocks and volcanic rocks from the ore cluster. The samples of the Carboniferous magmatic rocks are enriched in LREEs and LILEs but depleted in HFSEs, which attest to the I-type affinity and are similar to those formed in a continental arc. The early Carboniferous granodiorites and granites (350–343 Ma) in the Central Tianshan Block (CTB), with high Mg# values of 37–66, low Y values of 2.3–17.2 ppm, and high Sr/Y ratios (34–85), show characteristics of adakites that are formed by partial melting of delaminated oceanic crust and/or lower crust. Additionally, they have a wide range of εHf(t) values (5.4–15.9), high εNd (t) values (1.78–6.31) and low I_Sr values (0.705–0.706), indicating a mixed source of depleted mantle and lower crust for its origin. The Late Carboniferous volcanic rocks (314–306 Ma) in the Aqishan-Yamansu Tectonic Belt (AYTB) north of the CTB have obviously negative Nb–Ta-Ti anomalies, as well as high Y contents (12–20 ppm) and low Sr/Y ratios (5.6–29.9) compared to primitive mantle, which has a relatively flat chondrite-normalized REE pattern, resembling arc-related magmas. Additionaly, they have positive values of εHf(t) values (8.7–14.7), high εNd(t) values (3.04–4.45) and low I_Sr ratios (0.704–0.707), showing a depleted mantle source, which is weakly affected by the contamination of ancient crustal material. Combining this study and previous works, we suggest that the continuous southwards subduction of the North Tianshan Ocean during the Carboniferous generated the continental arc and forearc basin in the northern part of the CTB. The expansion of the Aqishan-Yamansu forearc basin formed intrusions and volcanic formations along the arc belt. The early Carboniferous intermediate-felsic mantle-derived adakitic magmas in the CTB are probably related to regional Zn-Pb enrichment and mineralization, and the Late Carboniferous intrusions and volcanic formations in the AYTB are related to large-scale Fe-Cu-Zn-Pb-Ag mineralization.

Graphical Abstract