HR: 14:12h
AN: V13C-03    [Abstracts]
TI: The Chinese Continental Drilling Program: Geochemical and Isotopic (O, Sr, Nd, Pb), Investigations on a Vertical Sequence of UHP metamorphism
AU: * Hoefs, J
EM: jhoefs@gwdg.de
AF: Geowissenschaftliches Zentrum, Uni. Goettingen Goldschmidtstr. 1, Goettingen, D-37077 Germany
AU: Xiao, Y
EM: yxiao@gwdg.de
AF: Geowissenschaftliches Zentrum, Uni. Goettingen Goldschmidtstr. 1, Goettingen, D-37077 Germany
AU: Zhang, Z
EM: zzm@ccsd.org.cn
AF: Institute of Geology, CAGS, Baiwanzhung Road 26 , Beijing, 100037 China
AU: Romer, R L
EM: romer@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AB: Various UHP metamorphic rocks - orthogneiss, paragneiss, eclogite, amphibolite, ultra-mafic rocks, and rare schist and quartzite - have been drilled within the main hole of the Chinese Continental Drilling Program (CCSD) in Donghai, Sulu. Petrographic studies indicate that all the rocks have been subjected to UHP metamorphism. R-mode factor analysis vs. depth, combined with rock associations and lithologies, indicate that the 2050-m-thick slab can be divided into 6 units in order of increasing depth. Geochemical characteristics of eclogites from different units suggest that their protoliths may be of diverse origins: eclogites from unit 1(0-530m) are probably of cumulate origin being similar to Bixiling and Maowu in Dabie Shan; the protolith of the eclogite from unit 2 (530-600m) is a Fe-Ti-rich gabbroic rock-body, whereas that of the eclogite within ultramafic rocks from unit 3 (600-690m) is of mantle origin; eclogites fom unit 4 (690-1160m) and unit 6 (1600-2050m) that interlayered with paragneiss are metamorphic supracrustal rocks. Geochemical characteristics of orthogneiss (unit 5, 1160-1600m) and paragneiss suggest that their protoliths are probably of granitic and of supracrustal sedimentary origins, respectively. Oxygen isotope data indicate that eclogites and gneiss from depths above 1600m of the main hole have depleted bulk d18O values of -5.3 to 4.5%, indicating significant meteoric water/rock interaction; by contrast, samples from depths between 1600-2050m show bulk d18O values greater than 5.8%, without any indication of a meteoric water/rock interaction. This suggests an "in situ" origin and structural coherence throughout subduction and exhumation. The lowest d18O values occur at depths of 1100-1200m, which corresponds to the boundary between eclogites and the about 400m-thick ortho- (granitic) gneiss layer (boundary between unit 4 and unit 5; this implies that the intrusion of the granitic body could be a heat source for initiating meteoric water/rock interaction. Correlations in Sr-Nd-Pb isotopic ratios among the UHP drill cores, Mesozoic and Cenozoic volcanics from the North China craton, and depleted MORB mantle under eastern China suggest interactions between the UHP rocks (formerly subducted continental crust) and the eastern China mantle. This conclusion from radiogenic isotopes contrasts sharply with the oxygen isotope data, which suggest very limited crust/mantle interaction.
DE: 3660 Metamorphic petrology
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting