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 K ₂ O ( > 8.5wt %) and MgO ( > 8.8wt %) 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 eHf(t) values of -2.8 to 1.3, δ ¹⁸ O values of 6.5 to 7.4‰, and enriched bulk-rock Sr-Nd-Pb isotope compositions (( ⁸⁷ Sr/ ⁸⁶ Sr)i=0.73134, εNd(t)=-13.7, ( ²⁰⁶ Pb/ ²⁰⁴ Pb)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 ( ⁸⁷ Sr/ ⁸⁶ Sr)i of 0.710993 to 0.711139, εNd(t) of -12.3 to -12.4, and ( ²⁰⁶ Pb/ ²⁰⁴ Pb)i of 18.59 to 18.72. Taken together with observations of a negative trend between εNd(t) and MgO content; positive trends between ( ⁸⁷ Sr/ ⁸⁶ Sr)i, ( ²⁰⁶ Pb/ ²⁰⁴ Pb)i and MgO content from ultrapotassic lamprophyres to potassic lamprophyres; the existence of abundant Miocene inherited zircons showing similar ages and eHf(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 adakitelike 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 SiO ₂ , and positive correlations between Sr, Sr/Y and SiO ₂ , 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 ( ⁸⁷ Sr/ ⁸⁶ Sr)i of 0.709401 to 0.710362, εNd(t) of -11.1 to -9.9, and ( ²⁰⁶ Pb/ ²⁰⁴ Pb)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, adakitelike 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 ( ⁸⁷ Sr/ ⁸⁶ Sr)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 subductionmodified 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.