From sediment to granite: timescales of anatexis in the upper crust
From sediment to granite: timescales of anatexis in the upper crust
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DOI：
10.1016/s0009-2541(99)00121-7
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发表时间：
2000-01
期刊：
影响因子：
3.9
通讯作者：
N. Harris;D. Vance;M. Ayres
中科院分区：
文献类型：
--
作者：
N. Harris;D. Vance;M. Ayres
花岗岩的形成是由原岩的逐渐加热引发的一系列事件的最终结果，随后形成晶界熔体，熔体偏析成脉状网络，熔体通过网络上升，最后熔体结晶。对白云母不一致熔化形成地壳熔体的实验限制，结合喜马拉雅高山深熔作用的地球化学研究，可以评估每个过程所需的时间尺度。根据喜马拉雅深熔花岗岩的独居石和锆石测温确定的不一致温度表明，至少对于某些侵入岩来说，熔体中轻稀土元素不饱和，这意味着熔体可能在不到 10 ka 的时间内被提取。实验研究表明，一些喜马拉雅熔体的锆含量也不饱和，这意味着偏析可能在 100 年内发生。如此短的时间尺度证实了变形驱动机制对于从源头提取这些熔体非常重要。喜马拉雅花岗岩熔体的输送距离约为 10 公里，岩浆在大约 1 天内通过岩脉上升即可实现。按照这样的速度，理论上即使是最大的花岗岩也可以在 10 年内就位。喜马拉雅熔体的结晶需要更长的时间。如果以薄片形式（∼100 m 宽）放置，则需要 >500 年的时间尺度，而较大的虫石的单阶段侵入则需要 >30 ka 的时间尺度。对于复合材料片复合体，岩浆结晶而不是熔体上升构成了侵入物就位的速率决定步骤。因此，许多造山花岗岩的熔体偏析和侵位的总体时间尺度小于 10 ka，甚至可能小于 1 ka。相比之下，原岩逐渐加热所需的时间尺度为≫1 Ma。由于熔体产生速率是由进入原岩的热流决定的，而不是由反应动力学（对于任何具有重要地质意义的时期）决定的，因此我们得出结论，由加热机制和地壳岩石的热扩散率决定的热流提供了地壳深熔作用的总体速率决定步骤。
Granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain-boundary melt, melt segregation into a vein network, ascent of the melt through the network and, finally, crystallisation of the melt. Experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the High Himalaya allow the timescales required for each of these processes to be assessed. Discordant temperatures determined from monazite and zircon thermometry for Himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in LREE implying that melts have probably been extracted in less than 10 ka. Experimental studies suggest that some Himalayan melts are also undersaturated in Zr, implying segregation may have occurred within 100 years. Such short timescales confirm that deformation-driven mechanisms are important in extracting these melts from their source. The transport distances of Himalayan granitic melts of ∼10 km may be achieved by the ascent of magma through dykes in about 1 day. At such rates even the largest granite could theoretically be emplaced in ∼10 years. Crystallisation of Himalayan melts involves much longer periods. If emplaced as thin sheets (∼100 m wide) a timescale of >500 years is required compared with >30 ka for single stage intrusion of the larger laccoliths. For composite sheet complexes magma crystallisation, rather than melt ascent, comprises the rate-determining step on the emplacement of the intrusion. The overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka, and possibly less than 1 ka. In contrast, the timescale required for prograde heating of the protolith is ≫1 Ma. Since the melt production rate is determined by heat flow into the protolith, and not by reaction kinetics (for any geologically significant period) we conclude that heat flow, determined by both the mechanism of heating and the thermal diffusivities of crustal rocks, provides the overall rate-determining step of crustal anatexis.