A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7million-year-old ultrapotassic dyke intruding Xigaze flysch immediately to the north of the Yarlung-Tsangpo suture zone in southern Tibet. Garnet-clinopyroxene-plagioclase-quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130-1330 °C and pressures between 22-26 kbar indicating equilibration in the high-pressure and ultrahightemperature granulite field and defining a geotherm of ca. 16 °C/km. Ultramafic xenoliths consist mainly of hornblende and biotite, probably of restitic crustal rather than mantle origin, and attained peak metamorphic conditions of 920-1130 °C and 17-24 kbar, whereas felsic granulites equilibrated at 870-900 °C at an inferred pressure of 17 kbar. In-situ U-(Th)-Pb LA-ICP-MS dating of zircons shows that protoliths may include Proterozoic basement rocks, Late Cretaceous calc-alkaline tonalites of the Gangdese batholith root and/or remnants of a Neo-Tethyan oceanic arc. Certain zircons from a felsic granulite and an ultramafic xenolith have mean 206 Pb/ 238 U ages of 16.8 ± 0.9 and 15.6 ± 0.6 Ma respectively, and monazites from a micaceous xenolith yielded a mean 208 Pb/ 232 Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached ca. 80 km thickness by 17-14 Ma at the latest and that the crust remained as thick to the present day.

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Models that support a single collision scenario for India and Eurasia are incompatible with the evidence that an intra-oceanic island arc (IOIA) existed within the Neotethyan Ocean. Understanding the spatial and temporal extent of any IOIA is crucial for India-Eurasia collision studies as the entire ocean, including any intraoceanic features, must have been consumed or emplaced prior to continental collision.Here, we review what is known about the Neotethyan IOIA and report evidence from sedimentary successions in NW India and southern Tibet to constrain when and where it was emplaced. We use detrital mineral geochemistry and supporting provenance 2 and age data to identify the source of the sediments and compare the timing of erosion of IOIA-derived material in both regions. Detrital chrome spinels, extracted from distinct sedimentary horizons in southern Tibet (Sangdanlin) and NW India (Ladakh), exhibit similar average geochemical values (TiO 2 = 0.09 and 0.24%, Cr# = 0.66 and 0.68 and Mg# = 0.45 and 0.53, respectively) and supra-subduction zone (SSZ), forearc peridotite signatures. Furthermore, they overlap with in-situ chrome spinels reported from the Spongtang Ophiolite in NW India and the Sangsang Ophiolite in southern Tibet. As with many of the ophiolitic remnants that crop out in and adjacent to the Yarlung-Tsangpo and Indus suture zones (YTSZ and ISZ respectively), the Spongtang and Sangsang ophiolites formed in an IOIA setting. Linking the source of the detrital chrome spinels to those analysed from remnant IOIA massifs in the YTSZ and ISZ is strong evidence for the emplacement of the IOIA onto the Indian margin.The timing of the IOIA collision with India is constrained by the depositional ages of the chrome spinel-bearing sediments to the end of the Paleocene (Thanetian) in southern Tibet and the Early Eocene in NW India. This indirectly provides a maximum age constraint of Late Paleocene-Early Eocene for intercontinental collision between India and Eurasia. Additionally, this study highlights the importance of targeting distinct sedimentary horizons in collision zones to find evidence for discrete tectonic events that may be obfuscated by later collisions.

Mesozoic granitic plutons are found throughout the Indochina terrane of eastern Cambodia and southern Vietnam. The granitic rocks range in age from Early Triassic (240 Ma) to Late Cretaceous (80 Ma) and record distinct tectonomagmatic periods associated with subduction of the Paleotethys and Paleo-Pacific oceans. Samples collected from the Snoul pluton, eastern Cambodia are composed of silicic and intermediate dioritic rocks, and basalt. The quartz diorites and diorites are magnesian, metaluminous, calcic to calc-alkalic, and similar to volcanic-arc granitoids whereas the basaltic rocks are compositionally similar to within-plate basalt. Zircon U-Pb geochronology and Lu-Hf isotopes and whole rock Sr-Nd isotopes show that the silicic rocks are Albian and isotopically juvenile (107.5 ± 0.3 Ma, 109.1 ± 0.4 Ma; εHf(t) = +7.0–+17.0; 87Sr/86Sri = 0.704313–0.707681; εNd(t) = +3.1–+4.9). Fractional crystallization modeling using a dioritic composition as the parental magma demonstrates that it is possible to generate the quartz diorite compositions under oxidizing (ΔFMQ +1) and hydrous (H2O = 2 wt%) conditions suggesting that they are consanguineous. The isotopically juvenile nature of the dioritic rocks and their compositional similarity (SiO2 ≥ 56 wt%, Al2O3 ≥ 15 wt%, Sr ≥ 400 ppm, Y ≤ 18 ppm, Yb ≤ 1.9 ppm) to adakitic rocks indicates that the parental magmas of the Snoul pluton were likely derived by partial melting of juvenile mafic basement rocks of the Indochina terrane. Moreover, Early Cretaceous plutonic rocks of Cambodia are isotopically distinct from plutonic rocks of similar age and tectonic setting from Vietnam suggesting that there could be a lithotectonic domain boundary within the Southern Indochina terrane. In contrast, the basaltic rocks likely record a temporally distinct period of magmatism associated with Late Cenozoic tensional plate stress.