Petrogenesis and Tectonic Implications of Late Carboniferous Intrusions in the Tuwu-Yandong Porphyry Cu Belt (NW China): Constraints from Geochronology, Geochemistry and Sr–Nd–Hf Isotopes

The Tuwu-Yandong porphyry Cu belt is located on the southern margin of the Dananhu island arc in eastern Tianshan, constituting the largest Cu metallogenic belt in Northwest China. Two episodes (~334 Ma and ~317 Ma) of porphyry Cu-Mo mineralization in the belt have been recognized, associated with Early and Late Carboniferous felsic intrusions, respectively. The Carboniferous intrusions, therefore, provide a unique opportunity to investigate tectono-magmatic-metallogenic evolution of the belt. New LA–ICP–MS zircon U–Pb dating indicates that the mineralization-related and post-mineralization intrusions (granodiorite porphyry, gabbro, and granite porphyry) were formed at 321.8 ± 3.1 Ma, 313.5 ± 1.2 Ma, and 309.8 ± 2.5 Ma, respectively. The zircon trace element shows that the granodiorite porphyry (Ce4+/Ce3+ ratios, avg. 129, median = 112, n = 15) was likely derived from a more oxidized (and hydrous) magma source than that of the gabbro (Ce4+/Ce3+ ratios, avg. 74, median = 40, n = 15) and granite porphyry (Ce4+/Ce3+ ratios, avg. 100, median = 91, n = 15), being favorable for porphyry copper mineralization. The granodiorite porphyry shows an adakitic affinity (e.g., high Sr/Y ratios and low Y contents) and has high εNd(t) (6.4–6.7), εHf(t) (11.4–14.3), and Mg# values (47.4–58.1) and low (87Sr/86Sr)i (0.703804–0.703953), suggesting that the melt was derived from partial melting of a subducted oceanic slab followed by mantle peridotite interaction. The gabbro exhibits higher Al2O3 (16.5–17.4 wt.%), Cr (107–172 ppm), and Ni (37–77 ppm) contents and εNd(t) (6.6–7.2), εHf(t) (11.6–15.9), and Mg # (53.3–59.9) values, while it has lower (87Sr/86Sr)i values (0.703681–0.703882) than the granodiorite porphyry, indicating a depleted mantle source. The granite porphyry exhibits an affinity with non-fractionated I-type granites and possesses higher SiO2 (71.1–72.0 wt.%) contents, lower but positive εNd(t) (4.8–5.2), εHf(t) (10.3–13.0), and Mg # (38.7–41.0) values, and higher (87Sr/86Sr)i (0.704544–0.704998) than the granodiorite porphyry and gabbro, together with young Nd and Hf model ages, suggesting that the parental magmas originated from the partial melting of a juvenile lower crust. The enrichment in LREEs and LILEs (e.g., Ba, U, K and Sr) and depletion in HFSEs (e.g., Nb, Ta, and Ti) indicate that these intrusive rocks formed in the subduction zone. With the integration of previous studies, it can be inferred that the northward flat subduction of the Kangguer ocean slab at ca. 335–315 Ma caused the formation of the adakites and associated porphyry Cu mineralization in the Tuwu-Yandong belt. After the prolonged flat subduction, slab rollback may have occurred at ca. 314–310 Ma, followed by a “quiet period” before the final closure of the ancient Tianshan Ocean along the Kangguer Fault in this belt.