High-Mg Diorite from Qulong in Southern Tibet: Implications for the Genesis of Adakite-like Intrusions and Associated Porphyry Cu Deposits in Collisional Orogens

作者：Zhiming Yang, Yongjun Lu, Zengqian Hou, Zhaoshan Chang · 发表于：Journal of Petrology · 年份：2015 · 被引用次数：308 · 研究领域：Geological and Geochemical Analysis、High-pressure geophysics and materials、Geochemistry and Geologic Mapping

We have investigated a suite of Miocene high-Mg diorite porphyries from Qulong in southern Tibet, the largest porphyry Cu–Mo deposit in China. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating shows that the high-Mg diorite porphyry was emplaced at 15·7 ± 0·2 Ma, which is slightly younger than the Qulong adakite-like Rongmucuola pluton (c. 19·5–16·4 Ma). The Qulong high-Mg diorites have phenocrysts showing disequilibrium textures and include high-Mg# clinopyroxene (0·91–0·97). These porphyry rocks exhibit both ultrapotassic and adakite-like features, and are characterized by high contents of MgO (4·2–5·1 wt %), K 2 O (3·2–3·6 wt %), and compatible trace elements (e.g. Ni: 115–142 ppm; Cr: 214–291 ppm), as well as by high Sr/Y and La/Yb ratios. The rocks have highly radiogenic isotopic compositions with ( 87 Sr/ 86 Sr) i = 0·707004–0·707198, ε Nd(t) = −5·1 to −5·5, 207 Pb/ 204 Pb = 15·697–15·704, and 208 Pb/ 204 Pb = 39·082–39·116, as well as variable zircon ε Hf values of −3·0 to +5·9. Petrographic, elemental, and isotopic evidence suggests that the Qulong high-Mg diorites were formed by mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively. In contrast, the Qulong pre-ore Rongmucuola pluton is characterized by high SiO 2 (66·3–68·9 wt %) and Al 2 O 3 (16·4–17·0 wt %) contents, high Sr/Y ratios (121–151), low compatible element contents (e.g. Ni = 16·0–17·...