Carbonate Metasomatism was Found in the CCSDPP3 Ultramafic Rocks in Ganyu County, Sulu Area
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Nanjing Center, China Geological Survey,CARMA, State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, CAGS,CARMA, State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, CAGS

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    Abstract:

    The ultramafic body sampled in Hole PP3 is located in the eastern part of the Dabie-Sulu UHP metamorphic belt near Donghai County. It consists chiefly of garnet peridotite, dunite and serpentinite. MgO content of the ultramafic body varies from 46.2wt.% to 48.6wt.% and CaO from 0.005% to 0.34%, and Al2O3 varies from 0.23wt.% to 1.19wt.% with the average of 0.41wt.%; Mg# is generally high (92 to 93) and Cr# is also high (30 to 45), and variable in a small scale. Garnet-bearing peridotites have the same Mg# and Mg/Si (~1.6) as garnet-free peridotites, which shows enrichment of Mg, depleted of Ca and Al, peridotites are extremely depleted in the east of China. The principal minerals include olivine, chromium spinel, diopside, augite (and/or garnet), orthopyroxene, amphibole and phlogopite. Individual olivine grains are compositionally uniform and have forsterite contents ranging from 88.7 to 94.5, with up to 0.79% NiO and 0.95% Cr2O3. olivine grains within a given sample are uniform in composition but olivine shows multistage, original olivines are typically more magnesian than later grains. Olivine in a sample of amphibolitized phlogopite peridotite has the lowest forsterite contents (88.7). Composition of clinopyroxenes is attributed to two generations. The later generation which is called as endiopside( Wo27.61En68.78Fs2.27Ac1.34) with higher MgO often rims the early diopside ( Wo45.98En47.89Fs2.73Ac3.39). Chromite in PP3 ultramafic rocks is compositionally variable, Cr#(molar100Cr/(Cr+Al)) varies from 51 to 89, and TiO2 and MnO2 are lower than 0.26wt% and 0.46wt% respectively. Both the olivine and orthopyroxene are highly magnesian. Olivines, chromites, and clinopyroxenes show multistage.PP3 ultramafic body is extremely depleted, but geochemistry shows that the rocks have high Sr and Ba content and depleted HSFE with a characteristic REE pattern, all show the ultramafic body was metasomatised by carbonate in mantle and indeed it is found that diopside was metasomatised by endiopside and amphibole and phlogopite were formed in part and even magnesite and dolomite was formed in olivines. The metasomatism is limited and fast and out of balance.

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CHEN Shizhong), YANG Jingsui), XU Zhiqin), ZHU Xiaoting ).2016. Carbonate Metasomatism was Found in the CCSDPP3 Ultramafic Rocks in Ganyu County, Sulu Area[J]. Acta Geologica Sinica,90(10):2721-2732

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History
  • Received:March 06,2016
  • Revised:April 16,2016
  • Adopted:April 18,2016
  • Online: October 25,2016
  • Published: