Ages and origins of granitoids from the Kalatag Cu cluster in Eastern Tianshan, NW China: Constraints on Ordovician–Devonian arc evolution and porphyry Cu fertility in the Southern Central Asian orogenic belt

Abstract

The pre-Carboniferous evolution of the Eastern Tianshan orogen, Xinjiang, NW China, an important Paleozoic tectonic component and Cu metallogenic belt in the southern Central Asian Orogenic Belt (CAOB), is controversial. New zircon UPb ages, whole-rock geochemistry, and zircon LuHf isotope data for three major granitic complexes (i.e., Kalatag, Dongerqu, Xierqu) from the Kalatag Cu cluster of Eastern Tianshan were combined with previous data to evaluate Ordovician–Devonian arc evolution and porphyry Cu fertility in the southern CAOB. The zircon UPb results reveal that the granitic intrusions and porphyry Cu mineralization in the Kalatag area formed during 453–427 Ma and 387–379 Ma, suggesting that along with Carboniferous intrusions, Ordovician–Silurian and Devonian intrusions were also very important targets for porphyry Cu mineralization in Eastern Tianshan. All intrusions are calc-alkaline I-type rocks with low K2O/Na2O ratios (mostly <1), moderate Mg# values (33.3–58.8), listric-shaped chondrite-normalized rare earth element (REE) patterns, typical arc magmatic trace element features (i.e., Ba, K and U enrichments relative to Nb, Ta and Th), and moderate Nb/Ta ratios (12.5–21.6). They have similar depleted source features (i.e., εHf(t) = 11.8–15.3) and likely formed by deep crustal melting, assimilation, storage, and homogenization (MASH) processes followed by minor variable degrees of fractional crystallization in an island arc environment (underlain by minor old relict crust) related to the northward subduction of the Kanggur Ocean. The mineralized porphyries likely derived from a hydrous pyroxene+amphibole±garnet magma source, as shown by adakitic features (e.g., Sr/Y > 40, Yb < 1.9 ppm), and were strongly metasomatized by slab fluids (with high Ba/Th = 200–800 and Ba/La = 20–100); these elements and ratios are thus important chemical indicators of the porphyry Cu fertility magma in Kalatag. The dry and barren arc intrusions are characterized by low Sr/Y ratios and show increased alkalinity and Sm/Yb and Sr/Y ratios with time, revealing crustal evolution from Ordovician–Silurian ca. 30–35-km-thick crust (plagioclase+pyroxene-dominant; Sm/Yb = 1–2) to Devonian thicker (<40 km; pyroxene+plagioclase±amphibole-dominant; Sm/Yb = 2–4) and mature crust.