HR: 1330h
AN: V22D-0616    [PDF]
TI: Chemistry and its implication of Ultrahigh-Pressure Peridotites from Pre-pilot hole of Chinese Continental Scientific Drilling Project (CCSDP)
AU: * Li, T
EM: litianfu@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Zhang, R
EM: zhang@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, Stanford, CA 94305 United States
AB: The Chinese pre-pilot hole (PP1) is located at Zhimafang village, Donghai of the Sulu UHP terrane, east China. Peridotite of 115 m thick within gneiss from the PP1 is composed of abundant lherzolite, harzburgite, and minor wehrlite and dunite. Peridotite at the contacts with gneiss is strongly serpentinized. More than 90 vol% peridotites contain garnet and phlogopite; some contain magnesite and Ti-clinohumite. The PP1 peridotites were recrystallized at 780-950oC and 4.5-7.0 +/- 0.2 GPa in the subduction zone environment. Major, REE, trace elements and O, Sr and Nd isotope compositions of peridotites were analyzed by XRF, ICPMS and mass spectrum. Mineral oxygen isotope has the mantle value of +4e - +6e, indicating that O isotope equilibrium between minerals has been reached. All peridotites contain lower "fertile element" concentrations than those of the primitive mantle. Mg number ranges from 90.29 to 92.66 (three down to 81.93-86.61). MgO content (41-48 wt%) has negative correlation with Na2O (0.01-0.24 wt%), Al2O3 (0.42-3.55, most $<2$ wt%) and CaO (0.23-2.41wt%, one up to 3.15 wt%) contents, and moderately positive correlation with compatible element Ni and Co. These major element data suggest that the PP1 peridotites were derived from a depleted upper mantle resulting from partial melting of primitive mantle and melt removement. Among them, the most depleted rock is Grt-free dunite. The PP1 peridotites show slight to moderately fractionated REE pattern with (La/Yb)N ratios of 0.66-12.36 with a few exceptions. In spidergrams, conspicuous positive Ba and Sm, and negative Nb, Zr and Ti anomalies are observed. Whole rock has high present 87Sr/86Sr ratio of 0.7104-0.7227 and 143Nd/144Nd ratio of 0.51209-0.51254. These trace element and isotope data suggest that these rocks were probably derived from long-term enriched mantle or were subjected to crustal contamination. These characteristics described above and lack of correlation between refractory degree and enrichment of incompatible trace elements are attributed to metasomatism by various melts or H2O-CO2 fluid derived from both mantle and subducting slab.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting