A compilation of 290 zircon U–Pb ages of intrusive rocks indicates that the Gangdese Batholith in southern Tibet was emplaced from c. 210 Ma to c. 10 Ma. Two intense magmatic pulses within the batholith occur at: (1) 90 ± 5 Ma, which is restricted to 89–94° E in the eastern segment of the southern Lhasa subterrane; and (2) 50 ± 3 Ma, which is widespread across the entire southern Lhasa subterrane. The latter pulse was followed by a phase of widespread but volumetrically small, dominantly felsic adakitic intrusive rocks at 16 ± 2 Ma. The Linzizong volcanism in the Linzhou Basin was active from 60.2 to 52.3 Ma, rather than 69–44 Ma as previously estimated. During 120–75 Ma, Gangdese Batholith magmatism migrated from south to north, arguing against rollback of the downgoing, north-dipping Neo-Tethyan oceanic lithosphere for the generation of the 90 ± 5 Ma magmatic pulse. Petrological, geochemical and metamorphic data indicate that this pulse was likely to have been generated through subduction of the Neo-Tethyan oceanic ridge lithosphere. Subsequent Gangdese Batholith magmatism propagated both south and north during 70–45 Ma, and finally concentrated at the southern margin of the Lhasa Terrane at 45–30 Ma. The enhanced mafic magmatism since c. 70 Ma, magmatic flare-up with compositional diversity at c. 51 Ma and increased magmatic temperature at 52–50 Ma are interpreted as the consequences of slab rollback from c. 70 Ma and slab breakoff of the Neo-Tethyan oceanic lithosphere that began at c. 53 Ma. The India–Asia convergence was driven by Neo-Tethyan subduction with a normal rate of convergence at 120–95 Ma, ridge subduction at 95–85 Ma, then subduction of a young and buoyant oceanic lithosphere after ridge subduction with rate deceleration at 84–67 Ma, Deccan plume activity and slab rollback with rate acceleration at 67–51 Ma, slab breakoff for sudden drop of the convergence rate at c. 51 Ma, and finally the descent of the high-density Indian continental lithosphere beneath Asia since c. 50 Ma.

The recent discovery of large porphyry copper deposits (PCDs) associated with Miocene (22-12 Ma) granitoid magmas in the eastern section of the Paleocene-Eocene Gangdese magmatic arc in the Himalaya-Tibetan orogenic belt raises new questions about the origin of water-rich (≥4.5 wt.%), oxidized (ΔFMQ 1-3) magmas in continental collisional settings and their mineralization potential. We review the literature and compile available data on whole rock and isotope geochemistry for Cenozoic igneous rocks from Tibet, and add new zircon Ce4+/Ce3+ and Ti-in-zircon thermometry data to better understand variations in oxidation state and thermal evolution of these suites, which are key controls on Cu mineralization. Six distinct Cenozoic igneous suites are defined: Paleocene-Eocene syn-collisional Gangdese magmatic arc rocks (ΔFMQ =-1.2 to +0.8) (suite I), and five broadly contemporaneous Miocene suites. A distinct change in magmatism along the length of the belt occurs at around 88°E in the Miocene suites: to the east, porphyry copper mineralization is associated with a moderately oxidized, high-Sr/Y granitoid suite (suite II, ΔFMQ = +0.8 to +2.9) with minor occurrences of transitional (hybrid) monzonitic (suite III) and trachytic rocks (suite IV; both with zircon Ce4+/Ce3+ > 50-100, EuN/EuN* =~0.5, and ΔFMQ =~+1 to +2). To the west of 88°E, trachytic volcanic rocks (suite V) are more voluminous but more reduced (zircon Ce4+/Ce3+ < 50, ΔFMQ <+1), and are associated with sparse, poorly mineralized high-Sr/Y granitoids (suite VI) which are moderately oxidized (zircon Ce4+/Ce3+ = 20-100, ΔFMQ =~+1 to +3). The Miocene high-Sr/Y granitoids have many compositional and isotopic similarities to the Paleocene-Eocene Gangdese arc rocks, and are interpreted to have been derived by melting of the hydrated arc root, with minor mantle input. In contrast, the highly evolved isotopic signatures of the Miocene trachytic rocks, combined with deep seismic profiles and a xenolith-derived geotherm, suggest their derivation from the underthrust Indian Proterozoic subcontinental lithospheric mantle (SCLM) or old fore-arc Tibetan SCLM during phlogopite breakdown at temperatures of~1100°C. Based on published geophysical data and tectonic reconstructions, we develop a model that explains the origin of the various Miocene magmatic suites, their spatial differences, and the origin of related PCDs. Following the early stages of continental collision (Eocene-Oligocene), shallow underthrusting of the Indian continental lithosphere and subcretion of Tethyan sediments (including oxidized carbonates and possibly evaporites) under eclogite facies conditions promoted the release of aqueous fluids, which hydrated and oxidized the base of the overlying Tibetan plate. This metasomatism rendered the Tibetan lower crust fusible and fertile for metal remobilization. During the mid-Miocene, the Indian slab steepened in the eastern sector (east of~88°E). In this eastern belt, deeply derived trachytic magmas were trapped in melt zones at the base of the Tibetan crust, ...

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