Different Origins of Garnet in High to Ultrahigh Pressure Metamorphic Rocks
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摘要: 石榴石是高压-超高压变质岩石中最重要的变质矿物之一，是研究俯冲带深部变质和熔融过程的理想研究对象.通过对俯冲带内不同条件下形成的石榴石进行详细研究，确定了岩浆成因、变质成因和转熔成因石榴石.岩浆石榴石是岩浆熔体在冷却过程中结晶形成，成分主要为锰铝榴石-铁铝榴石，通常含有石英、长石、磷灰石等晶体包裹体.变质石榴石是在亚固相条件下通过变质反应形成，包裹体为参与变质反应的矿物组合；进变质生长的石榴石通常显示核部到边部锰铝榴石降低的特征.转熔石榴石是在超固相条件下通过转熔反应形成，通常含有晶体包裹体，其中既有从转熔熔体结晶的矿物包裹体，也有转熔反应残留的矿物包裹体.对超高压变质岩石中转熔石榴石的识别，可以为深俯冲陆壳岩石的部分熔融提供重要的岩石学证据，是大陆俯冲带部分熔融研究的重要进展之一.Abstract: Garnet is one of the most important mineral in high pressure (HP) to ultrahigh pressure (UHP) metamorphic rocks. It is an ideal phase to constrain the P-T-t conditions of metamorphic and anatectic processes during continental subduction zone metamorphism. Garnets from subduction zone metamorphic rocks can be classified into metamorphic,magmatic and peritecitc garnets based on their typical features. Magmatic garnet crystallizes from magmatic melts,shows almandine-spessartine in compositions and contains crystal inclusions such as quartz,feldspar and apatite. Metamorphic garnet forms through metamorphic reactions at subsolidus conditions,shows decreasing spessartine from core to rim,and contains crystal inclusions mainly composed of metamorphic reactants. Peritectic garnet forms through peritectic reactions at supersolidus conditions,and contains crystal inclusions consisting of not only minerals crystallized from peritectic melts but also residual minerals from peritectic reactants. The identification of peritecitc garnet in UHP metamorphic rocks provides unique evidence for partial melting of the deeply subducted continental crust,which is an important progress in crustal anatexis of collisional orogens.
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Key words:
- peritectic garnet /
- metamorphic garnet /
- magmatic garnet /
- high to ultrahigh pressure /
- partial melting /
- petrology
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图 1 大别山双河地区超高压变质片麻岩中变质成因和转熔成因石榴石
Fig. 1. Metamorphic and peritectic garnets in anatectic UHP metamorphic gneiss at Shuanghe in the Dabie orogen
图 2 喜马拉雅东构造结高压混合岩和脉状花岗岩岩中转熔石榴石和岩浆石榴石
Fig. 2. Peritectic and magmatic garnets in HP migmatites and vein granites from eastern Himalayan syntaxis
图 3 超高压变质花岗岩中岩浆石榴石残斑核和变质生长石榴石幔部和边部
a.背散射图像; b.Mn元素分布图示; c.主量元素剖面; d.稀土元素剖面; 修改自Xia et al.(2012)
Fig. 3. The residual of magmatic garnet in the core and new growth of metamorphic garnet in the mantle and rims from the UHP metamorphosed granites
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