Melting of Crustal Rocks During Continental Collision and Subduction
Melting of Crustal Rocks During Continental Collision and Subduction
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DOI：
10.1007/978-94-015-9050-1_2
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
A. Douce;T. C. McCarthy
中科院分区：
文献类型：
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作者：
A. Douce;T. C. McCarthy
大陆地壳可以部分熔融生成硅质岩浆。这些岩浆组成的多样性是由熔融压力、自由含水流体的可用性和原岩的组成决定的。由于地壳岩石在大陆碰撞过程中受到极端压力和温度条件的影响，碰撞造山带可能是岩浆产生的环境。本章讨论可能在大陆碰撞中形成的熔体和固体残余物的性质。在缺乏富氢流体的情况下，变质岩的融化是由含水矿物的分解引起的。这些不一致的脱水-熔化反应产生含氢熔体和无水（或少水）固体残留物。除了压力小于1 GPa的富含白云母的变质岩外，普通地壳岩石的脱水熔融需要850℃以上的温度。只有当岩石圈地幔与地壳分离，地壳受到地幔源岩浆的侵入，或者俯冲作用将岩石转移到上地幔时，碰撞造山带中的地壳岩石才会加热到这样的温度。在850-11 OOC的温度下，大量石英辉石岩脱水熔融产生的熔体为花岗岩，随着压力的增加，镁铁含量减少，总碱和氧化铝含量增加。固体残留物变化较大，取决于来源成分，但在P $1 GPa处一般为粒状，在P~ 2 GPa处一般为榴辉质，过渡区间为石榴石麻粒岩。在岩石圈地幔盖下的加厚大陆地壳中，以及由于俯冲而被掩埋并迅速挖出的大陆地壳薄片中，温度不太可能超过800-850℃。在这种条件下，只有富含白云母的变质长岩片岩才能经历脱水熔融，产生过铝浅花岗岩。白云母脱水-熔融反应的dP/dT斜率相当平坦，这意味着熔融最有可能发生在相对较浅的深度（0.8 GPa），这是由深部岩石的构造挖掘引起的减压过程。在这些相对“寒冷”的环境中，任何石英岩的深层融化都需要富氢流体的涌入。以这种方式形成的熔体
The continental crust can partially melt to generate silicic magmas. Compositional diversity among these magmas is determined by the pressure of melting, by the availability of free aqueous fluids, and by the composition of the protolith. Because crustal rocks are subject to extreme pressure and temperature conditions during continental collisions, collisional orogens potentially are environments for magma generation. This chapter discusses the nature of the melts and solid residues likely to be formed in response to continental collision. In the absence of H20-rich fluids, melting of metamorphic rocks is triggered by the breakdown of hydrous minerals. These incongruent dehydration-melting reactions give rise to H20-bearing melts and to anhydrous (or less hydrous) solid residues. With the exception of muscovite-rich metapelitic rocks at pressures of less than-I GPa, dehydration melting of common crustal rocks requires temperatures in excess of 850 C. Heating of crustal rocks to such temperatures in collisional orogens takes place only if the lithospheric mantle becomes detached from the crust, if the crust is invaded by mantle-derived magmas, or if subduction transports rocks to the upper mantle. Melts generated by dehydration melting of a wide range of quartzofeldspathic rocks at temperatures of 850-11 OOC are granitic, and become less ferromagnesian and richer in total alkalis and alumina with increasing pressure. The solid residues are more variable and depend on source composition, but are generally granulitic at P $1 GPa and eclogitic at P~ 2 GPa, with a transitional interval of garnet granulite. In thickened continental crust underlain by a lid of lithospheric mantle, an, d in slices of continental crust that are buried by subduction and exhumed rapidly, temperatures are unlikely to exceed 800-850 C. Under these conditions only muscovite-rich metapelitic schists can undergo dehydration melting, yielding peraluminous leucogranites. The rather flat dP/dT slope of the muscovite dehydration-melting reaction means that melting most likely takes place at relatively shallow depth «0.8 GPa), during decompression caused by tectonic exhumation of deep-seated rocks. Deeper melting of any quartzofeldspathic rock in these relatively" cold" environments requires influx of H20-rich fluids. Melts formed in this manner