Miocene Ultrapotassic, High-Mg Dioritic, and Adakite-like Rocks from Zhunuo in Southern Tibet: Implications for Mantle Metasomatism and Porphyry Copper Mineralization in Collisional Orogens

High-Mg diorites and/or ultrapotassic volcanic rocks are generally associated with postcollisional porphyry copper deposits, but their contribution to the formation of the mineralization remains unclear. A suite of Miocene postcollisional ultrapotassic–potassic lamprophyres, high-Mg diorites, and adakite-like intrusions have been recognized in the Zhunuo porphyry Cu deposit, located in a continental collisional zone within the Gangdese belt, southern Tibet. The post-mineralization ultrapotassic–potassic lamprophyres have zircon U–Pb ages of 12·2 ± 0·1 Ma and contain abundant Proterozoic to Miocene inherited zircons. The ultrapotassic lamprophyres have high K2O (>8·5 wt %) and MgO (>8·8 wt %) contents, are enriched in light rare earth elements (LREE; La = 123 ppm) and large ion lithophile elements (LILE; e.g. Ba = 3102 ppm, Th = 116·6 ppm, and Pb = 140 ppm), and display high Th/Yb and Rb/Sr, and low Ba/Rb and Hf/Sm ratios. They have zircon εHf(t) values of -2·8 to 1·3, δ18O values of 6·5 to 7·4‰, and enriched bulk-rock Sr–Nd–Pb isotope compositions ((87Sr/86Sr)i =0·73134, εNd(t) = -13·7, (206Pb/204Pb)i = 19·20). Their parental magmas were derived from partial melting of an enriched mantle source that had been metasomatized by fluids and sediment-derived melts associated with Neo-Tethyan oceanic subduction and subsequent Indian continental lithosphere subduction. The potassic lamprophyres have lower contents of K2O, MgO, REE and LILE than the ultrapotassic lamprophyres and (87Sr/86Sr)i of 0·710993 to 0·711139, εNd(t) of -12·3 to -12·4, and (206Pb/204Pb)i of 18·59 to 18·72. Taken together with observations of a negative trend between εNd(t) and MgO content; positive trends between (87Sr/86Sr)i, (206Pb/204Pb)i and MgO content from ultrapotassic lamprophyres to potassic lamprophyres; the existence of abundant Miocene inherited zircons showing similar ages and εHf(t) values to the adakite-like intrusions; and variable Hf/Sm ratios with some Hf/Sm ratios similar to adakite-like intrusions, we propose that the potassic lamprophyres were formed by mixing of ultrapotassic lamprophyre magmas with adakite-like magmas. The syn-mineralization high-Mg diorites including diorite porphyry and enclaves hosted by the adakite-like intrusions at Zhunuo have zircon U–Pb ages of 13·0 ± 0·2 Ma and 13·1 ± 0·2 Ma. They show negative correlations between Y, Yb, Dy/Yb and SiO2, and positive correlations between Sr, Sr/Y and SiO2, among which some more evolved samples (such as diorite porphyry) show adakite-like geochemical signatures. The high-Mg diorites are enriched in LREE and LILE, depleted in high-field-strength elements (HFSE), and have (87Sr/86Sr)i of 0·709401 to 0·710362, εNd(t) of -11·1 to -9·9, and (206Pb/204Pb)i of 18·62 to 18·71. Taken together with petrographic observations that show magma mixing, we argue that the high-Mg diorites were derived from previously subduction-modified Tibetan lithospheric mantle with little or no input from Indian continental sediment. Mixing with adakite-like magmas and fractional crystallization of hornblende and/or titanite are also responsible for the differentiation of the high-Mg diorites. The ore-hosting, adakite-like granitic rocks at Zhunuo with zircon U–Pb ages of 14·7 ± 0·3 Ma and 14·6 ± 0·2 Ma have lower concentrations of REE, LILE and HFSE, much higher εNd(t) (-6·1 to -6·9) and lower (87Sr/86Sr)i (0·707325–0·707663) values than the ultrapotassic lamprophyres and the high-Mg diorites. They were derived from remelting of previously subduction-modified Tibetan lower crust with some involvement of hydrous high-Mg dioritic magmas during magma mixing. The postcollisional adakite-like intrusions in the Gangdese belt could be generated by remelting of previously subduction-modified lower crust and mixing with hydrous high-Mg dioritic magmas in a lower crustal MASH zone and/or in an upper-crustal adakite-like magma chamber. The metallogenic potential of postcollisional adakite-like intrusions largely depends on rejuvenation of subduction-modified lower crust by previous arc magmas, differentiation of hydrous high-Mg dioritic magmas, and magma mixing of high-Mg dioritic magmas with lower crustal magmas.