HR: 16:15h
AN: V32H-02    [PDF]
TI: New Geochemical Data on Postcollisional Ultrapotassic Rocks in Western Tibet
AU: * Zhao, Z
EM: zdzhao@cugb.edu.cn
AF: China University of Geosciences, 29 Xueyuan Rd, Beijing, 100083 China
AU: * Zhao, Z
EM: zdzhao@cugb.edu.cn
AF: Center of Isotope Geochemistry, University of California, 475 McCone Hall, MC#4767, Berkeley, CA 94720-4767 United States
AU: Mo, X
EM:
AF: China University of Geosciences, 29 Xueyuan Rd, Beijing, 100083 China
AU: DePaolo, D
EM:
AF: Center of Isotope Geochemistry, University of California, 475 McCone Hall, MC#4767, Berkeley, CA 94720-4767 United States
AU: Owens, T
EM:
AF: Center of Isotope Geochemistry, University of California, 475 McCone Hall, MC#4767, Berkeley, CA 94720-4767 United States
AU: Liao, Z
EM:
AF: Chengdu Institute of Geology and Mineral Resources, Erhuan Rd, Chengdu, 610082 China
AU: Zhu, D
EM:
AF: Chengdu Institute of Geology and Mineral Resources, Erhuan Rd, Chengdu, 610082 China
AB: We studied the postcollisional ultrapotassic volcanic rocks in Gongmutang (N30$\deg$48', E84$\deg$26'), Zabuye salt lake (N31$\deg$26', E84$\deg$20'), Dangreyong lake (N30$\deg$57', E86$\deg$24'), and Xuru lake (N30$\deg$4', E86$\deg$31') in western Lhasa block. These rocks erupted during the time between 27 Ma and 12 Ma ago according to published dating work. They are strongly related to the north-south trending normal faults or valley in tectonic setting. These lavas (SiO$_{2}$=45-62%, K$_{2}$O=6.0-8.8%, K$_{2}$O/Na$_{2}$O=2.0-4.3%, MgO=2.5-10.9%, TiO$_{2}$=0.8-1.6%) are very homogenous in trace element composition, with enriched LREE, Rb, Ba, Th, and U, and depleted Nb, Ta, and Ti. These samples, with $\epsilon_{Nd}$ varies from -11 to -15 and $^{87}$Sr/$^{86}$Sr from 0.718 to 0.736, show much different isotopic features to the postcollisional shoshonitic-dominated rocks in North Tibet ($\epsilon_{Nd}$ varies from -5 to -9 and $^{87}$Sr/$^{86}$Sr from 0.708 to 0.711). But they shift much close or even overlap the Himalayan basement and granitoid Nd-Sr field ($\epsilon_{Nd}$ range from -12 to -25 and $^{87}$Sr/$^{86}$Sr from 0.733 to 0.760). The geochemical characteristics of the rocks in our study are similar to the rocks from Shiquanhe (Turner et al., 1996, J. Petrol.), Xunba (Miller et al., 1999, J Petrol), and Pabbai Zong (Williams et al., 2001, Geology). If we take the widely-accepted origin hypotheses that the ultrapotassic lavas is firstly derived from small degree partial melting of subcontinental lithospheric mantle, we could conclude that an enriched mantle source should be responsible for the lavas in western Tibet. The Nd-Sr isotopic data suggest that at least two possibilities for the structure and composition of the lithosphere in western Tibet: (1) The Himalayan basement affinity of these lavas suggest that the Himalayan crustal materials in the northern margin of Indian plate maybe a reagent that enriched the upper mantle of the western Lhasa block. If this is the fact, the northern part of India would have subducted beneath Lhasa block that suggested by some geophysical result; (2) The western part of Lhasa block are indeed have a similar features to the Himalayan crust. Then, Nd-Sr data do not show any clues for the north trend subduction of India plate.
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting