东天山平顶山巨眼球状片麻状花岗岩特征及成因
Characteristics and genesis of the Pingdingshan megaaugen gnelssic granite in the eastern Tianshan Mountain areas
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摘要: 摘要本文以东天山平顶山岩体为例，探讨中天山晋宁旋回晚期巨眼球状片麻状花岗岩的特征及其形成机制。野外地质关系、岩相学、岩石化学、稀土元素、微量元素和同位素研究表明，平顶山岩体是岛弧钙碱性火山-沉积岩系经原地改造的产物，其Rb—Sr等时线年龄为927Ma．花岗岩形成作用的动力、热力来源可能与碰撞后天山岩石圈的拆沉有关。拆沉导致的底侵和内侵引发了地壳岩石的韧性剪切，剪切后的岩石有利于部分熔融、熔体-流体运移和化学反应，而熔体和流体的作用反过来又会促进韧性剪切，并将较浅层次的变形变质岩石改造为片麻状花岗岩。平顶山岩体的成岩作用体现了部分熔融、韧性剪切和流体作用的相互反馈，也是这些作用的共同产物。这种原地片麻状花岗岩的成岩过程主要为深源熔体和流体与原岩的相互作用，并使原岩发生不同程度的部分熔融，因而其地球化学特征同时受到原岩和外来熔体-流体的控制。底侵和内侵是造山过程晚期-期后挤压-拉张转折期地壳垂向增生的重要方式，而平顶山这类片麻状花岗岩则是这种垂向增生的产物。Abstract: Taking the Pingdingshan granite as an example, this paper investigates characteristics and genesis of augen gneissic granites of the late Jinning Cycle in the eastern Tianshan Mountain areas. Field relations, petrography, petrochemistry, rare earth elements, trace elements and isotopes indicate that the Pingdingshan granite, with a Rb-Sr isochron age of 927 Ma, was formed essentially by in situ transformation of metamorphosed island arc calc-alkaline volcanosedimentary rocks. Driving factors including heat could be ascribed to post-collisional delamination of the Tianshan lithosphere. Delamination-induced underplating and intraplating caused ductile shearing of the overlying crustal rock, such that partial melting, melt and fluid migration, and chemical reactions were highly enhanced, whereas melts and fluids themselves promoted further ductile shearing and transformed the deformed and metamorphosed rocks at higher levels into the gneissic granite. Formation of the this granite is resultant from a complex feed back of partial melting, ductile shearing and fluid activity. Petrogenesis being dominated by interaction of deep-sourced melts and fluids with protoliths in accompany with minor partial melting, geochemistry of such a kind of in situ gneissic granites is constrained by both protoliths and external melts and fluids. Underplating and intraplating are important manners of vertical accretion of the crust during late- to post-orogenic compresion-exention conversion, and gneissic granites like that at Pingdingshan are produced just by such a type of vertical accretion.
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Key words:
- Augen granite /
- Underplating /
- Intraplating /
- Ductile shear zone /
- Pingdingshan /
- Tianshan Mountains /
- Xinjiang
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