HR: 0800h
AN: T11B-0376    [Abstracts]
TI: Sr-Nd Isotopic Mapping for Intrusions From the Chinese Altai: Implications for Accretionary Orogeny and Continental Growth
AU: * Wang, T
EM: taowang@earth.sinica.edu.tw
AF: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: * Wang, T
EM: taowang@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Academia Sinica Road, Section 2, Taipei, 11529 Taiwan
AU: Kovach, V P
T11B-0376 AF: Institute of Precambrian Geology and Geochronology, RAS, Makarova emb. 2, St. Petersburg, 199034 Russian Federation
AU: Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Academia Sinica Road, Section 2, Taipei, 11529 Taiwan
AU: Tong, Y
T11B-0376 AF: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Hong, D
T11B-0376 AF: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Han, B
T11B-0376 AF: Department of Geology, Peking University, 1, Zhongguangchun Road, Beijing, 100871 China
AB: The Central Asian Orogenic Belt (CAOB) is the world­Ęs largest Paleozoic to Mesozoic accretionary orogen. It is characterized by the occurrence of large amount of juvenile crust. The Altai Mountains in NW China are a type terrane of CAOB. Our new zircon age data, along with the available age information, have established four Paleozoic plutonic events at ca. 460 Ma, 410-400 Ma, 375 Ma and 270-290 Ma. The age framework allows us to conduct an isotopic mapping of the intrusions in the Chinese Altai and to evaluate the proportion of the juvenile crust relative to old Precambrian basement rocks. The results are summarized as follows. (1) Most granitoids of the Altai have initial Sr isotopic ratios of 0.705-0.709 and eNd(T) values of ­V2 to +7. These isotopic signatures indicate that much of the source material for the granitoids is derived from the upper mantle, hence the granitoids contain a large proportion of juvenile component. (2) The eNd(T) values generally increase southward, from near-zero values (-2 to 0) in central Altai to positive values of +1.4 to +5.5 in southern Altai. Their TDM also decrease correspondingly. This feature may be indicative of southward accretion. (3) Isotopic mapping of the intrusions reveals that the central Altai terrane contains a significant volume of continental basement rocks. This probably suggests the existence of an Altai microcontinent. Furthermore, the mapping confirms the existence of a middle Paleozoic accretionary wedge in southern Altai, and provides evidence for incorporation of older continental fragments in the wedge. In conclusion, this study presents an added example of a long-term subduction/accretion of small continental fragments, island arc, and accretionary complex in CAOB. A microcontinent grew laterally by accretion of juvenile terranes and recycled tectonic fragments. (Supported by Major State Basic Research Program of China No. 2001CB409800 and NSC-Taiwan No.93-2116-M-001-025)
DE: 1000 GEOCHEMISTRY
DE: 1020 Composition of the continental crust
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
SC: Tectonophysics [T]
MN: Fall Meeting 2005