大陆科学钻探CCSD-PP3钻孔超镁铁岩岩石学和矿物学特征及其意义
Petrology and mineralogy of PP3 ultramafic rocks in Sulu UHP belt and its significance
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摘要: PP3超镁铁岩在钻孔中出露厚度达480米,包括纯橄岩、石榴橄榄岩和闪石化金云母橄榄岩等岩石类型。岩石主要组成矿物橄榄石、铬尖晶石、石榴子石、单斜辉石和斜方辉石等。橄榄石为镁橄榄石(Fo在88.7-93.1之间),SiO2含量在38.8wt.%-42.4wt.%;石榴子石为钙饱和的镁铝榴石(Py65.945Alm18.095Uv7.518Gr4.695And3.605Sp0.142),MgO含量在16.77wt.%-20.04wt.%,CaO含量在5.4到6.4wt.%之间,Cr2O3含量在0.3到3.3wt.%,FeO含量在2.48wt.%-11.78wt.%,TiO2含量小于0.04wt.%,代表早期矿物组合;两类斜方辉石的成分都为顽火辉石(OpxⅠ:En91.86Fs7.80Wo0.34,OpxⅡ:En91.83Fs7.91Wo0.21Ac0.06)。斜方辉石Mg#(Mg/(Mg Fe)×100)在91.7-92.5间,Cr2O3含量低于0.32wt.%,CaO含量在0.102-0.199wt.%间,Al2O3含量为0.32-1.06wt.%,均值在0.72wt.%;单斜辉石可分为早期透辉石(Wo45.98En47.89Fs2.73Ac3.39)和晚期顽透辉石(Wo27.61En68.78Fs2.27Ac1.34)两种。铬尖晶石的Cr#(Cr/(Cr Mg)×100)从51到89变化,TiO2和MnO2含量分别低于0.26wt%和0.46wt%。橄榄石、铬尖晶石和单斜辉石等矿物组合表现为3-4期次的特点。Abstract: Ultramafic rocks about 480 metres sandwiching eclogite are revealed, which include garnet peridotite, dunite, serpentinite and few hargzburgite and amphibole phlogopite peridotite, contain olivine, chromium spinel, diopside, endiopside, or/and garnet, orthopyroxene, amphibole and phlogopite. Olivines have forsterite contents ranging from 88.7 to 93. 1, SiO2 ranging from 38. 8wt. % to 42. 4wt. % . All garnets ( Py65.945 Alml8.095 Uv7.518 Gr4.695 And3.605 Sp0.142 ) are pyropic, with CaO between 5.4 to 6.4 wt. % , and Cr2 O3 from 0. 3 to 3. 3 wt. % , represent early stage of mineral assemblage. Orthopyroxenes ( En91.8 Fs7. 9 Wo0.25 ) are unzoned, Cr2O3 ranges from 0 to 0. 32wt. % , and CaO ranges from near detection limit to 0. 20wt. %. Most of the Orthopyroxenes (the average is 0. 72wt. % ) have Al2O3 < 1. 1wt. %. Composition of clinopyroxenes in PP3 ultramafic rocks is attributed to two generations. the early generation is called diopside ( Wo45. 98 En47.89 Fs2.73 Ac3.39 ) and the later is endiopside(Wo27.61 En68.78 Fs2.27 Ac1.34) with higher MgO. Chromite in PP3 ultramafic rocks is compositionally variable, Cr#( molarlOOCr/( Cr Al) ) varies from 51 to 89, and TiO2 and MnO2 are lower than 0. 26wt. % and 0. 46wt. % respectively. Olivines, chromites, and clinopyroxenes show multistage, which were associated with partial melting, subduction and exhumation.
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Key words:
- Garnet /
- Peridotites /
- Diopside /
- Chromium spinel
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