新疆阿尔金长沙沟超镁铁质岩锆石U-Pb年龄及其地质意义
Zircon U-Pb Age and Geological Implications of Ultramafic Rocks in Changshagou, Altun Area, Xinjiang Province
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摘要: 通过1:50000地质填图,表明新疆长沙沟(蛇绿)构造混杂岩带超镁铁质岩主要岩石类型为斜长辉石角闪岩、辉石橄榄岩、强蛇纹石化辉石橄榄岩等;总体贫SiO2、CaO、K2O、Na2O,富含MgO、
特征,分异指数DI=0.35～14.45,固结指数SI=47.82～70.05,表明岩浆分异不明显,固结程度较高;里特曼指数σ=0.97,属钙性系列;M/F=1.94～4.39,属镁铁质超基性岩类;其中,辉石橄榄岩TiO2含量接近洋中脊地幔(TiO2=0.10%~0.40%),稀土元素、微量元素特征与地幔橄榄岩近似,认为属原始地幔分离产物,为蛇绿岩套组成部分;斜长辉石角闪岩可能为超镁铁质杂岩.首次获得了辉石橄榄岩206Pb/238U加权平均年龄为(510.60±1.40)Ma(MSWD=0.11),形成时代为寒武纪,其可能代表了南阿尔金洋盆早期的产物. Abstract: During 1:50000 scale regional survey, the author discovered that the ultramafic rocks from Altun ophiolite tectonic mixtite belt are mainly composed of plagioclase pyroxene amphibolite, pyroxene peridotite, and serpentinized peridotite. Geochemistry of the diabase shows that the geochemistry of the major is characterized by low contents of SiO2, CaO, K2O, Na2O and high contents of MgO and FeO. With a low DI value (0.35-14.45), the magmatic differentiation is not obvious; while the solidification degree, with a high SI value (47.82-70.05), is relatively high. The ultramafic rocks belong to calcium series and magnesian ultrabasic rock, with the Rittman index of 0.97 and the M/F index from 1.94 to 4.39. Among the compositions, the TiO2 content in pyroxene peridotite is close to that of mid-ocean ridge mantle(0.10%-0.40%). Characteristics of rare earth element and trace element are similar to those of mantle peridotite. All these characteristics indicate that the ultramafic rock is a part of ophiolite suite derived from the primitive mantle. The plagioclase pyroxene amphibolite may be ultramafic diamictite. Meanwhile, the authors firstly got the U-Pb age (510.60±1.40 Ma) of zircon from pyroxene peridotite and hold that its age is considered to be the Cambrian and it may represent early products of South Altun oceanic basin.-
Key words:
- ultramafic rocks /
- Zircon U-Pb dating /
- geological implications
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