HR: 0800h
AN: V11A-1408    [Abstracts]
TI: Geochemical Characteristics of Ultramafic Rocks From Main Hole of CCSDP, Sulu UHPM Belt
AU: Li, T
EM: litianfu@ccsd.org.cn
AF: Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AF: Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Xu, Z
EM: xzq@ccsd.org.cn
AF: Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: * Chen, S
EM: chenshizhong
AF: Key Laboratory for Contiental Dynamics of MLR, Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AB: In the main hole of Chinese Continental Scientific Drilling Project (CCSDP), the ultramafic rocks, hosted by rutile eclogite, occur at the depth between 603.2$\sim$.683.5 m. Ultramafic rocks are composed of mainly wehrlite and minor lherzolite. Most of these rocks contain abundant garnet and Ti-clinohumite, but minor rocks have no garnet and Ti-clinohumite. Serpentinization varies in different degrees ,through the depth and minor talc and prehnite, present mainly at the lowermost part of the profile. Besides, there is a 10 m thick layer and many thin lens of rutile phengite eclogite, as well as small blocks of eclogite and thin layers of phlogopite orthopyroxenite and megacrystic amphibolite in the ultramfic sequence. The contacts between the ultramafic rocks and hosting eclogite and lens in them are sharp. The uppermost ultramafic rock possibly has a tectonic contact, with the host eclogite, characterized by mixed agglomerates of them. Major element concentrations of wehrlite and its altered product serpentinite range between SiO$_{2}$ 40.60%$\sim$.43.42%, TiO$_{2}$$<$0.31%, Al$_{2}$O$_{3}$ 3.16%$\sim$.8.96%, total FeO 12.27%$\sim$.16.77%, MnO 0.18%$\sim$.0.27%, MgO 27.65%$\sim$.39.20%, CaO 1.02%$\sim$.5.32%, Na$_{2}$O 0.01%$\sim$.0.74%, K$_{2}$O 0.01%$\sim$.0.81% and P$_{2}$O$_{5}$ 0.01%$\sim$.0.12%. The average fusible compositions are fairly high(TiO$_{2}$ 0.21%­›Al$_{2}$O$_{3}$ 5.06%­›CaO 2.53% and Na$_{2}$O 0.25%) and closing to or exceeding the corresponding compositions of primary mantle .Representative refractory composition MgO is lower than that of primary mantle. Mg$^{\#}$ is lower and varies narrowly between 77.26$\sim$.85.07. When compared with A type Zhimafang peridotite(Zhang et al, 2000), the wehrlite in borehole has relatively lower SiO$_{2}$ and MgO and higher TiO$_{2}$, Al$_{2}$O$_{3}$, CaO and total FeO. In a word, wehrlite shows more fertile characteristics. Total REE concentrations range 4.38$\times$10$^{-6}$$\sim$.43.26$\times$10$^{-6}$, most of which are higher than that of primary mantle(PM, 6.86$\times$10$^{-6}$). Most of the samples show moderate LREE enriched characteristics with œ"La/Ybœc$_{N}$ ratios of 1.02$\sim$21.03 and slight Eu positive anomaly in PM normalized partition pattern diargram. Trace element spider diagram shows no evident incline with œ"Rb/Ybœc$_{N}$ ratios of 0.25$\sim$.2.98, but has U, La, Zr, Hf and Sr positive anomalies and Ba, Nb and Ta negative anomalies. Tatol content of PGE is very low ranging between 1.14$\times$10$^{-9}$$\sim$.2.72$\times$10$^{-9}$ which are fairly lower than that of primary mantle(23.5$\times$10$^{-9}$), that of mantle xenoliths in Cenozoic basalt in East China(16.5$\times$10$^{-9}$$\sim$.50.7$\times$10$^{-9}$) and that of Ronda peridotite. All the data above indicate that the protolith of wehrlite was possibly not a residual mantle block but a ultrmafic body in the crustal depth, i. e. , B type ultramafic body in UHP belt.
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting