The segregation and emplacement of granitic magmas
The segregation and emplacement of granitic magmas
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DOI：
10.1144/gsjgs.144.2.0281
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发表时间：
1987-03
影响因子：
2.7
通讯作者：
S. Wickham
中科院分区：
文献类型：
--
作者：
S. Wickham
在低熔体分数下，花岗质岩浆与残余晶体的分离很大程度上取决于熔体的粘度。理论上的考虑表明，在典型的花岗岩熔体粘度范围内（104Pas至1011Pas），在典型的地壳熔融事件持续时间（约106年）内，通过压实机制只能实现非常有限的分离。在混辉岩地体中观察到的小规模（毫米到米）分离可能是由压实作用（可能是由连续变形辅助）或熔体流入伸展裂缝产生的，但由于分离效率有限，低熔体分数的液体不太可能被提取出来形成大型（千米大小）的花岗岩岩体。在较高的熔体分数（bbb30 %）下，部分熔炼过程中强度和有效粘度的迅速下降使其他分离过程得以进行。计算和实验表明，在花岗质体系中，部分熔融岩石的有效黏度（熔体分数范围非常窄，为30-50%）将迅速下降到可以发生千米厚对流翻覆的水平。在无熔体区域内的对流运动能够产生大型（千米大小）均匀的、高结晶分数的、源自地壳的岩浆体，其大小比低熔体分数的岩浆体大几个数量级。在对流不稳定达到之前，小的（厘米到米大小的）花岗质液体舱可以浮起通过，并在这些部分熔融区域的顶部淤积；这一过程与一些麻粒岩似乎是残留岩石的观察结果是一致的，这些岩石的化学成分在最小的熔体成分中被耗尽了。在一定的压力和温度下，地壳中部分熔融区域的有效粘度（以及对对流的敏感性）很大程度上取决于该系统的含水量，因为这既控制着熔体的数量，也控制着熔体的粘度。在典型的地壳温度下，大多数地壳岩性的固有含水量不能促进高百分比的部分熔融或低液体粘度，而这是通过对流均匀化形成大型公里级花岗岩岩体所必需的。这表明，在许多情况下，大的源自地壳的岩浆体的形成所涉及的深熔作用是由外源的含水流体的流入所促进的。这些岩浆分离过程与来自比利牛斯山脉海西期低压变质-厌氧岩地体的三种不同类型花岗类岩体的岩石成因有关。
The segregation of granitic magma from residual crystals at low melt-fraction is strongly dependent on the viscosity of the melt. Theoretical considerations imply that for the typical range of granitic melt viscosities (104Pas to 1011Pas) only very limited separation will be possible by a compaction mechanism over the typical duration of a crustal melting event (c. 106 years). Small-scale segregations (millimetre to metre) of the type observed in migmatite terranes may be generated by compaction (possibly assisted by continuous deformation), or by flow of melt into extensional fractures, but low melt-fraction liquids are unlikely to be extracted to form large (kilometre-size) granitic plutons because of the limited separation efficiency. At higher melt-fractions (>30%) the rapid decrease in strength and effective viscosity during partial melting allows other segregation processes to operate. Calculations and experiments indicate that in granitic systems the effective viscosity of partially melted rocks, having a very narrow melt fraction range of 30–50% will fall rapidly to levels at which convective overturn of kilometre-thick zones can occur. Convective motion within anatectic regions is capable of generating large (kilometre-size) homogeneous, high crystal-fraction, crustally-derived magma bodies, which are orders of magnitude greater in size than low melt-fraction segregates. Before convective instability is reached, small (centimetre- to metre-sized) pods of granitic liquid may rise buoyantly through, and pond at the top of such partly molten zones; such a process is consistent with the observation that some granulites appear to be residue rocks, chemically depleted in a minimum melt component. The effective viscosity (and hence the susceptibility to convection) of a partially melted zone within the crust, is strongly dependent on the water content of the system at a given pressure and temperature, because this controls both the quantity of melt generated and also the viscosity of the melt. The intrinsic water content of most crustal lithologies is incapable of promoting the high percentages of partial melting, or the low liquid viscosities, required to form large kilometre-sized granitic plutons by convective homogenization, at typical crustal temperatures. This suggests that the anatexis involved in the generation of large crustally-derived magma bodies has in many cases been promoted by an influx of externally derived aqueous fluid. These magma segregation processes are illustrated with respect to the petrogenesis of three different types of granitoid pluton from a Hercynian low-pressure, metamorphic-anatectic terrane in the Pyrenees.