Isotope geochemistry of the 1985 Tibet Geotraverse, Lhasa to Golmud

Abstract

Geochronological data from the Golmud —Lhasa section across the Tibetan Plateau indicate progressively younger periods of magmatism from north to south associated with successively younger ocean closures. Pre -collision Eocene magmatism (50—4 0 Ma) exposed along the southern margin of the Lhasa Terrane in the Gangdise Belt resulted from anatexis of mid -Proterozoic crust (~ 1000 Ma) at depths greater than 10 km, but at higher crustal levels subduction-related intrusions were predominantly mantle-derived with ~ 30 % crustal assimilation . Intrusions from the northern Lhasa Terrane are early Cretaceous in age (130 —110 Ma). These form a bimodal suite comprised of two-mica granites derived from anatex is of Mid -Proterozoic crust and of biotite -hornblende granodiorites from about 60 % crustal assimilation by mantle magmas above a post-collision subduction zone. They place a minimum constraint on collision between the Lhasa and Qiangtang Terranes of 130 Ma . Granite magmatism from the Kunlun Mountains is late P ermian -early Jurassic in age (260—190 Ma). The Kunlun batholith represents reworked mid-Proterozoic crust (1400 —1000 Ma) at an active continental margin from 260 —2 4 0 M a . Post-tectonic granites were emplaced in a post-collision setting (200 -190M a). Collision between the Qiangtang and Kunlun Terranes is dated as end -Triassic. Nd model ages of sediments from across the plateau record up lift and erosion of young source regions throughout the Phanerozoic confirming that the Tibetan Plateau is the site of multiple continental collision through time. Phanerozoic magmagenesis throughout the plateau requires considerable crustal reworking and limited crustal growth which suggests thickened continental crust in the region may predate the most recent Eocene collision.

Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications

“…1a), indicating that the Songpan-Gangzi oceanic basin subducted beneath the southern Kunlun Block at that time ( Fig. 10) (Harris et al, 1988;Xiao et al, 2002a,b;Yin and Harrison, 2000;Yuan et al, 2009).…”

“…The data for marine sediments are from Plank and Langmuir (1998). The Nd-Sr isotope data for Triassic magmatic rock in the central-northern Tibetan Plateau are from these references (Fu et al, 2010;Harris et al, 1988;Roger et al, 2003Roger et al, , 2004Wang et al, 2008b;Xiao et al, 2007;Yuan et al, 2009Yuan et al, , 2010Zhang et al, 2006aZhang et al, , 2007a, and references therein). displays considerable geological variation along its length.…”

“…They occur as minor interbeds within the Norian (Late (2000)). Ages references: (Fu et al, 2010;Harris et al, 1988;Reid et al, 2007;Roger et al, 2003Roger et al, , 2004Wang et al, 2008b;Weislogel, 2008;Xiao et al, 2007;Yuan et al, 2009Yuan et al, , 2010Zhang et al, 2006aZhang et al, , 2007a, and references therein. Main suture zones between major blocks: AKMS-Anyimaqen-Kunlun-Muztagh; JSJinshajiang; BS-Bangong; IS-Indus; GS-Ganzi-Litang.…”

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

“…Chung et al (2009) suggested that the Kuday dykes were derived from the lower crust. As discussed above, the Kuday and Mayum rocks have geochemical characteristics similar to the adakitic (2008) and Yue and Ding (2006); Yarlung MORB is from Miller et al (2003), Mahoney et al (1998), Xu and Castillo (2004), and Zhang et al (2005); lower crust in the western Lhasa terrane is from Miller et al (1999); Shoshonitic dykes are from Zhao et al (2009); adakites related to slab melting are from Kay (1978), Kay et al (1993), and Stern and Kilian (1996); Early Cretaceous granite and rhyolites in the western Lhasa terrane are from Zhu et al (2009); Northern Himalayan gneiss and granites are from Zhang et al (2004); High Himalayan Crystalline Series is from Harris et al (1988); mafic granulite in the east Himalayan syntaxis is from Xu et al (2010) (other data sources and symbols are as in Figs. 1 and 3).…”

“…In addition, based on the Sr-Nd isotope composition of adakitic rocks in the Lhasa terrane, Gao et al (2010) considered that the mafic granulite in the Himalaya terrane is unlikely to be a non-radiogenic end-member for the mixing array in the Sr-Nd plot. Finally, the adakitic rocks in the Lhasa terrane have different Sr-Nd isotopic characteristics to the gneiss and granites of the North Himalayan (Zhang et al, 2004) and High Himalayan Crystalline Series (Harris et al, 1988), which have anomalously enriched Sr-Nd isotopic compositions relative to the adakitic rocks (Fig. 8).…”