HR: 1340h
AN: V13B-1465    [Abstracts]
TI: Discovery of Late Cretaceous Granodiorites with Adakitic Geochemical Signatures from Southern Tibet: Petrogenesis and Tectonic Implications
AU: * Wen, D
EM: d89224006@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei, 106 Taiwan
AU: Song, B
EM: songbiao@cags.net.cn
AF: Institute of Geology, Chinese Academy of Geological Sciences, No.26 Baiwanzhung Road, Beijing, 100037 China
AU: Iizuka, Y
EM: yiizuka@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128 Academia Road Sec. 2, Nankang, Taipei, 115 Taiwan
AU: Ji, J
EM: grsange@pku.edu.cn
AF: School of Earth and Space Sciences, Peking University, Building YiFu No.2, Peking University, Beijing, 100871 China
AU: Chung, S
EM: sunlin@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei, 106 Taiwan
AU: Liu, D
EM: dunyiliu@cags.net.cn
AF: Institute of Geology, Chinese Academy of Geological Sciences, No.26 Baiwanzhung Road, Beijing, 100037 China
AU: Yang, H
EM: hjyang@mail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng Kung University, No.1, Ta-Hsueh Road, Tainan, 701 Taiwan
AU: Zhang, Q
EM: zhangqi1218@vip.sina.com
AF: Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O.BOX 9825 , Beijing, 100029 China
AB: The Transhimalayan magmatism and resultant Gangdese Batholith emplaced from the Cretaceous to Eocene in southern Tibet have been widely considered as products related to northward subduction of the Neo-Tethyan slab under Asia. Therefore, the Batholith has been conventionally thought as composed typically of calc-alkaline rocks. Here we report the discovery of granodioritic intrusions from the eastern Gangdese belt that instead show adakitic geochemical features, marked by apparently higher Sr/Y and La/Yb, coupled with lower Y and HREE, than the other Gangdese and common arc magmas. SHRIMP U-Pb zircon dating analyses of two such granodiorite samples yielded $^{206}$Pb/$^{238}$U dates of 79.9 \pm 0.7 and 83.8 \pm 1.5 (2\sigma) Ma, respectively, constraining their emplacement ages. The overall elemental characteristics of the intrusions suggest an origin by partial melting of garnet amphibolites, which we interpret on the basis of Sr and Nd isotope data [I$_{Sr} = 0.7044 \sim 0.7045; \epsilon$_{Nd}(T) = +1 \sim +3] to have been the juvenile lower crust that was produced by Cretaceous basaltic underplating above the mantle wedge and later thickened by tectonic contraction due to flattening of the Neo-Tethyan subduction, a process that eventually led to generation of the granodiorites. This interpretation is compatible with petrographic data, in particular the occurrence of magmatic epidote in the granodiorites, which imply the plutonism to have formed under moderately high pressures corresponding to middle to lower crustal depths in a fairly oxidizing environment. These furthermore allow us to infer that during the early Late Cretaceous southern Tibet was situated in an accretionary convergent margin where orogenic processes involving arc magma accretion, crustal thickening and rapid tectonic uplift may have been operating actively.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 1035 Geochronology
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting