Late Jurassic To Early Eocene Magmatic Events In The Southeast Gangdese Batholith, Southern Tibetan Plateau

Granite is a major constituent of the continental crust. The formation, growth and evolution is the foudamental problem of earth science. The petrogenesis of the granite is, therefore, vital to studying the growth and evolution of the continents. Granites are widely distributed in Tibetan Plateau, especially along the north of the Yarlung Zangbo suture zone with a giant magmatic belt. One of the biggest is Gangdese batholith in the southern Tibet. In this research we take western Zedong subterrane, eastern Zedong granodiorite, Langxian early Cretaceous and early Carboniferous granodiorite, Linzhi biotite granite, Gengzhang biotite granite and Bayi biotite granite as objets. Through the research, we got the following: (1)Our data indicate that Zedong granodiorite formed at 157.5±1.4 Ma, the granitic rocks within the Zedong terrane represent the re-melting products of juvenile mafic lower crust derived from the mantle wedge. The Zedong subterrane and possibly the Dazhuka subterrane to the west, represent a series of remnant Jurassic intro-oceanic system within the Neo-Tethyan Ocean, which could condition the sub-arc lithospheric mantle for the later arc magmatism as represented by the Cretaceous to Eocene Gangdese batholithic rocks; (2)Langxian terrane was consisted by the early Cretaceous Diorite, early Carboniferous granodiorite and augen granitic gneiss. The early Cretaceous Diorite intruded into the early Carboniferous granodiorite, the two both intruded into the augen granitic gneiss. The early Cretaceous diorite formed at 121.8±1.5 Ma; the early Carboniferous granodiorite formed at 360.8±3.5Ma, and it experienced metamorphism at 311.7±4.8 Ma; the augen granitic gneiss formed at 474.2±2.0 Ma, and it experienced metamorphism at 27.0±0.4 Ma; they were hydrous magma derived from the partial melting of fluid metasomatised mantle wedge during the northward subduction of the Neo-Tethyan slab. Emplace depths for the early Cretaceous as well as the early Carboniferous granodiorite estimated by Al-in-hornblende geobarometer indicate that they were emplaced at?13 km and?21 km, respectively, which in turn yields a long term exhumation rate of?0.04 mm/yr; (3)eastern Zedong granodiorite formed at 61.5±1.0 Ma, it maybe represent the re-melting products of juvenile mafic lower crust, emplace depths for the eastern Zedong granodiorite estimated by Al-in-hornblende geobarometer indicate that they were emplaced at?10 km, it indicates that in the early Paleocene the south Lahsa terrane experienced a long term exhumation rate which is the same as the early Cretaceous; (4)Linzhi biotite granite formed at 61.5±1.0 Ma, maybe they were derived from the juvenile crust, and there was some involvement of abundant old mature materials; (5)Bayi biotite granite formed at 66.9±0.7 Ma, they were derived from partial melting of ancient mature materials resulted from underplating by basaltic magma; (6)Gengzhang biotite granite formed at 54.9±0.7 Ma, they were derived from ancient mature materials, or contaminated by crust in the process of magma evolution.