矿床地球化学国家重点实验室知识库

The 1600 km-long Gangdese magmatic belt features extensive Paleocene–Eocene I-type intrusive rocks and coeval volcanic successions, abundant but more localized Oligo-Miocene calc-alkaline to alkaline plutons, and Miocene potassic to ultrapotassic volcanic rocks. These Cenozoic igneous rocks record geodynamic changes related to the India–Asia collision which began at ~ 55–50 Ma. New and published lithogeochemical and multiple isotopic (Os–Sr–Nd–O–Hf) analyses of these Cenozoic igneous rocks reveal that the Paleocene–Eocene magmas have similar compositions to continental arc rocks throughout the belt, but later Miocene magmas show sharp longitudinal contrasts in geochemical and isotopic compositions, which are also correlated with the occurrence of porphyry-type mineralization.

Sparse Miocene high-K calc-alkaline to shoshonitic volcanic rocks in the eastern Gangdese belt have low to moderate (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7057–0.7121), moderately negative εNd_i values (− 9.4 to − 3.4), low (¹⁸⁷Os/¹⁸⁸Os)_i ratios (0.154–0.210), highly variable εHf_i values (− 5.9 to + 10.1), and low zircon δ¹⁸O values (+ 5.0–+ 6.7‰), which are interpreted to reflect derivation by partial melting of subduction-modified Tibetan sub-continental lithospheric mantle (SCLM). In contrast, Miocene high-K calc-alkaline to shoshonitic volcanic rocks in the western Gangdese belt have higher (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7069–0.7263), more negative εNd_i values (− 17.5 to − 6.0) and εHf_i values (− 15.2 to + 0.7), and crust-like zircon δ¹⁸O values (+ 6.2–+ 8.8‰), but mantle-like (¹⁸⁷Os/¹⁸⁸Os)_i values (0.156–0.182), and high Ni and Cr contents. These features suggest that potassic to ultrapotassic magmas in the western Gangdese belt were also derived from partial melting of Tibetan SCLM but with ~ 3–25% input of melts ± fluids from the underthrust Indian plate (⁸⁷Sr/⁸⁶Sr = 0.74–0.76, εNd = − 18 to − 10, δ¹⁸O = + 10 − + 14‰). In contrast, Miocene alkaline magmas to the east were unaffected by this source.

Oligo-Miocene calc-alkaline to high-K calc-alkaline granitoids related to large porphyry Cu–Mo deposits in the eastern Gangdese belt (east of ∼ 89° E) are geochemically broadly similar to the early Paleocene–Eocene rocks. They are thought to be derived from partial melting of subduction-modified lower crust with mixing of alkaline melts from partial melting of SCLM, and have relatively low (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7047–0.7076), high εNd_i values (− 6.1 to + 5.5) and εHf_i values (1.4–8.7), moderate (¹⁸⁷Os/¹⁸⁸Os)_i ratios (0.224–0.835), and low zircon δ¹⁸O_VSMOWvalues (+ 5.5–+ 6.6‰). These magmas also had high water contents (weak Dy/Yb enrichment, characterized with amphibole fractionation) and oxidation states (ΔFMQ 0.8–2.9), which explain their unique association with porphyry Cu–Mo mineralization. In contrast, Miocene high-K calc-alkaline to shoshonitic granitoids in the western Gangdese belt (west of ∼ 89° E) show differences in geochemical and isotopic compositions to the earlier Paleocene–Eocene magmatism, and are characterized by crust-like zircon δ¹⁸O values (+ 6.2–+ 8.8‰), high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7147–0.7165), negative εNd_i values (− 11.3 to − 7.9), crust-like (¹⁸⁷Os/¹⁸⁸Os)_i values (0.550–1.035), and low εHf_i values (− 13.0 to 3.9). These magmas are interpreted to reflect involvement of melts ± fluids from the underthrust Indian plate and high degrees of crustal contamination upon emplacement. Only one small porphyry Cu–Mo deposit is known to be associated with these western granitoids.

We suggest that this difference reflects the variable extent of underthrusting of the Indian plate continental lithosphere beneath Tibet in the Oligo-Miocene, and diachronous breakoff of the Greater India slab. In the absence of underthrust Indian lithosphere to the east of ~ 89° E in the Oligo-Miocene, slab breakoff triggered asthenospheric upwelling and partial melting of previously subduction-modified Tibetan lithosphere, generating hydrous, oxidized calc-alkaline magmas with the potential to generate porphyry Cu–Mo deposits. In contrast, underthrusting of the Indian plate to the west at this time limited the involvement of asthenospheric melts and the extent of partial melting of subduction-modified lithosphere, with the result that melts ± fluids derived from the underthrust lithosphere were infertile.