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

&NA; 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 &egr;Hf(t) values of ‐2·8 to 1·3, &dgr;18O values of 6·5 to 7·4‰, and enriched bulk‐rock Sr‐Nd‐Pb isotope compositions ((87Sr/86Sr)i =0·73134, &egr;Nd(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, &egr;Nd(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 &egr;Nd(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 &egr;Hf(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, &egr;Nd(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 &egr;Nd(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.