印度新元古代Malani火成岩省成因及地质意义

Felsic large igneous province is a particular geological unit on the Earth. They were formed in special tectonic settings, and contain important information about continental crustal evolution and petrogenesis of granitic rocks. However, felsic large igneous provinces were not well studied in the past few decades. The Neoproterozoic Malani Igneous Province (MIP), located at NW India, is the third largest felsic large igneous province in the world. The rocks are dominated by rhyolites and granites associated with minor basalts and mafic dikes. In addition, the felsic rocks are characterized by I- and A-type affinity geochemical compositions and show low δ18O values. The project will analyses whole rock major and trace elements, Sr-Nd-Pb isotopes, as well as zircon and quartz Hf-O isotope to discuss the following questions: (1) Nature of the mantle sources and mafic melts in formation of the felsic rocks; (2) Petrogenesis of the felsic extrusive and intrusive rocks; (3) Petrogenesis of the low δ18O magmatism and its implications; (4) Tectonic settings and geodynamics of the Malani Igneous Province; (5) Relationships between India and South China blocks in the Rodinia. These results will not reveal the petrogenesis of the felsic large igneous provinces, but also constrain the dynamics of the Rodinia breakup.

酸性火成岩省是地球上独特的地质单元，形成于特殊的构造背景，包含着重要的大陆演化和成岩信息，但长期以来研究非常薄弱。印度西北地区新元古代Malani火成岩省是世界上第三大酸性火成岩省，岩石以流纹岩和花岗岩为主，伴少量玄武岩和基性脉岩。其中酸性岩浆岩显示I-A型岩石组合，具低δ18O特征。本项目拟在详细的野外地质和岩相学工作基础上，运用全岩主量元素、微量元素、Sr-Nd-Pb同位素，锆石和石英Hf-O同位素研究，揭示以下地质问题：(1) 幔源岩浆与酸性岩浆岩之间的关系；(2) 酸性火山岩和花岗岩的成因；(3) 低δ18O岩浆的形成机理与地质意义；(4) Malini火成岩省的构造背景和动力学机制；(5) 印度与华南在Rodinia超大陆中的关系。上述问题的研究，不但能揭示酸性火成岩省的成因，而且对Rodinia超大陆重建和裂解有重要的约束作用。

酸性火成岩省是地球上独特的地质单元，包含着重要的大陆演化和成岩信息，但长期以来研究非常薄弱。印度西北地区新元古代Malani火成岩省是世界上第三大酸性火成岩省，岩石以花岗岩和流纹岩为主，伴少量玄武岩和基性脉岩。本项目在详细的野外地质和岩相学工作基础上，运用全岩主微量元素、Sr-Nd-Pb-Hf同位素和锆石Hf-O同位素研究，揭示了以下地质问题：(1) 火成岩省中灰色花岗岩和火山岩的全岩ƐNd同位素值为+3.31至+6.75，说明是新生地壳部分熔融形成，而粉红色花岗岩及其火山岩ƐNd=-3.68至+0.90，证明有古老地壳物质参与；低δ18O岩浆岩只出现在两个地方，规模有限。(2) 新元古代Delhi Fold Belt早期 I-型花岗岩全岩εNd=+0.41至+1.40，锆石 εHf=+2.34至+9.35，δ18O=6.48‰至7.58‰，由新生地壳部分熔融形成；而晚期A-型花岗岩全岩εNd=+2.28至+0.84，锆石εHf=+2.31至+8.62，δ18O=5.77‰至7.01‰，推测来源于伸展背景下基性地壳物质脱水熔融。(3) 与Malani火成岩省对应的华南新元古代岩浆作用复杂，扬子西缘基性-超基性岩的全岩Nd同位素和锆石Hf-O同位素记录了地幔持续的俯冲流体和熔体交代作用，它们的δ57Fe同位素(+0.07至+0.21‰) 反映了含水玄武质岩浆在深部地壳的分异演化；扬子西缘中酸性侵入岩随着年龄变小，其εNd(t)和εHf(t)值增加，δ18O值减小，这种耦合特征反映了幔源岩浆底侵、地壳加厚、以及深部熔岩相互作用减弱。（4）扬子北缘南秦岭地区早期基性岩显示弧岩浆特征，而晚期出现MORB特点，反映了俯冲到伸展构造体系的转换；而扬子东南缘江南造山带中的SSZ-型蛇绿岩和S型花岗岩分别记录了洋壳俯冲和造山作用。印度和华南的新元古代沉积和岩浆作用显示二者具有密切的时空关系。