Partial Melting in the Higher Himalayan Crystallines of Eastern Nepal: the Effect of Decompression and Implications for the 'Channel Flow' Model
Partial Melting in the Higher Himalayan Crystallines of Eastern Nepal: the Effect of Decompression and Implications for the 'Channel Flow' Model
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DOI：
10.1093/petrology/egs009
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发表时间：
2012-05-01
影响因子：
3.9
通讯作者：
Indares, Aphrodite
中科院分区：
文献类型：
--
作者：
Groppo, Chiara;Rolfo, Franco;Indares, Aphrodite
深部陆壳的部分熔融既可能发生在地壳加热过程中，也可能发生在地壳减压过程中。虽然温度对地壳熔融的影响已经得到了广泛的研究，但很少有实验研究涉及压力对地壳深熔作用的影响。为了了解部分熔融过程中的压力下降的影响，热力学方法的等化学相图已被应用到石榴石-K-辉长岩-蓝晶石-硅线石深熔片麻岩（巴伦片麻岩）从尼泊尔东部的高喜马拉雅结晶（HHC）。研究了沿着4条以升温或减压为主的理想P-T轨迹的主要熔体生成反应、升温与减压过程中熔体生成量以及熔体损失对矿物组合和成分的影响。基于这些结果，所观察到的显微结构和矿物成分的巴润片麻岩已被解释的熔体生产与熔体消耗反应（如生长的石榴子石与保存的“纳米花岗岩”夹杂物与显微结构之间的固体和熔体的反反应），并用于推导所研究的样品的变质演化。P-T假截面模型预测，在峰值P-T条件下，至少有15-20体积%的熔体是通过白云母和黑云母的脱水熔融产生的，熔体的产生主要是由加热引发的，有或没有减压的综合作用。保存的麻粒岩峰变质组合，然而，是一致的，大部分的熔体的显着损失。从巴润片麻岩不同的、战略性位置的构造层次推断的样品的P-T演化与“通道流”模型的预期一致，包括：（1）P-T路径的顺时针形状;（2）峰值T的估计P（新数据：800摄氏度时10-8千巴;模型：800摄氏度时13-7千巴）;（3）P值在构造上向上递减，为“正常”变质序列;与主中央冲断带最下部的反转变质序列相反;（4）结构最低的样品的近等温折返，反映了被夹带在深部的岩石的渐进折返，高T区域的通道，相对于结构最上面的样品的近等压加热，反映了那些岩石的演变，与下面的通道向外流动。
Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis. To understand the influence of decreasing pressure on partial melting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet-K-feldspar-kyanite-sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC) of eastern Nepal. The main melt-producing reactions, the amount of melt produced during heating vs decompression, and the effects of melt loss on the mineral assemblages and compositions have been investigated along four ideal P-T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have been interpreted in terms of melt-producing vs melt-consuming reactions (e.g. growth of peritectic garnet with preserved 'nanogranite' inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P-T pseudosection modelling predicts that at least 15-20 vol. % of melt was produced at peak P-T conditions through dehydration melting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect of decompression. The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss of most of this melt. The P-T evolution inferred for samples from different, strategically located, structural levels of the Barun Gneiss is consistent with the expectations of a 'channel flow' model, including: (1) the clockwise shape of the P-T paths; (2) the estimated P at peak T (new data: 10-8 kbar at 800 degrees C; model: 13-7 kbar at 800 degrees C); (3) the decreasing P structurally upward, which defines a 'normal' metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel.