内蒙古乌拉山地区石榴子石花岗岩中石榴子石转熔成因的显微结构及矿物学证据

Microstructure and mineralogical evidence for peritectic garnet in thegarnet granite in the Wulashan area, Inner Mongolia

内蒙古乌拉山地区石榴子石花岗岩内发育大量石榴子石,这些石榴子石中保留着寄主岩石的变质演化历程的重要信息,故明确其成因类型至关重要。前人虽提出其为转熔成因,但未能提供充分证据证明。对此,本文对研究区深熔石榴黑云片麻岩以及石榴子石花岗岩内的石榴子石进行了岩相学以及矿物学研究。结果显示,在野外露头上,低度深熔石榴黑云片麻岩→中度深熔石榴黑云片麻岩的脉体/石榴子石花岗岩,石榴子石粒度逐渐增大,并且当岩石内的脉体小而少时不见粗粒石榴子石,脉体大而多时粗粒石榴子石发育,而石榴子石花岗岩内发育大量的粗粒石榴子石,此外脉体内的石榴子石粒度整体大于基体内的石榴子石粒度。在显微特征上,中度深熔石榴黑云片麻岩的脉体内的石榴子石和石榴子石花岗岩内的石榴子石晶形较差,形态各异,边界呈锯齿状,发育筛状变晶结构,并部分具有核部发育边部不发育的特点,基质内还具有逆反应结构以及斜长石堆晶体。矿物化学特征上,中度深熔石榴黑云片麻岩的脉体内的石榴子石和石榴子石花岗岩内的石榴子石以富Fe、Mg贫Mn、Ca为特点,不发育生长环带,与黑云母接触的石榴子石发育扩散环带,表现为TFeO曲线由中心向两边升高,MgO曲线由中心向两边降低,二者呈负相关。综合分析得出石榴子石花岗岩内的石榴子石是在变质深熔过程中,由固相变质生长至处于深熔熔体包裹状态下继续生长而成,这种石榴子石是转熔成因,为转熔矿物相(相当于残留矿物相),且中度深熔石榴黑云片麻岩的脉体内的石榴子石也具有相同成因,说明二者关系密切,为石榴子石花岗岩是石榴黑云片麻岩高度深熔的产物提供了有力依据。同时大量转熔型石榴子石保留在花岗岩内的现象说明研究区内的花岗岩并未经历长距离运移,故而未达到彻底的固液两相分离,进一步证明了其原地—半原地成因。

A large number of garnets are developed in the garnet granites in the Wulashan area, Inner Mongolia. As these garnets contain important information about the metamorphic evolution history of the host rock, it is very important to clarify their genetic type. Although the predecessors suggested that it is peritectic garnet, they failed to provide sufficient evidence to prove it. Towards this end, this article has conducted petrographic and mineral chemistry studies on the anatase garnet biotite gneiss and garnet in the garnet granite.The results show that, on the outcrop, from low anatectic garnet-biotite gneiss to garnet granite or veins of moderate anatectic garnet-biotite gneiss, the size of garnet gradually increases. When the veins in the rock are small and few, there are no coarse-grained garnets;when the veins are large and many, the coarse-grained garnets develop in the rock, and a large number of coarse-grained garnets develop in the garnet granite. In addition, the grain size of garnet in the vein is larger than that in the matrix as a whole. On the microscopic features, the garnets in the garnet granite or veins of moderate anatectic garnet-biotite gneiss have poor crystal shape and different shapes, with a jagged boundary. They also have sieve texture, and some have the characteristics of developing at the core but not at the edge. The reverse reaction structure and plagioclase pile crystals can be seen in the matrix of the garnet granite or veins of moderate anatectic garnet-biotite gneiss. In terms of mineral chemistry, the garnets in the garnet or veins of moderate anatectic garnet-biotite gneiss are characterized by rich Fe and Mg and poor Mn and Ca, with no growth zone. The garnet in contact with biotite develops diffusion zones, which shows that the FeOTcurve increases from the center to the sides, and the MgO curve decreases from the center to the sides, and the two are negatively correlated. Comprehensive analysis shows that the garnet in the garnet granite grows from solid phase metamorphism to a state of continuous growth in a state where an anatomic melt is encased in the metamorphic anatex process. It is believed that this kind of garnet is the cause of remelting, and it is a remelted mineral phase(equivalent to the residual mineral phase). In addition, the garnets in the veins in the moderate anatectic garnet-biotite gneiss also have the same genesis, indicating that the two are closely related, and provide a strong basis for the garnet granite to be a highly anatectic product of the garnet-biotite gneiss. At the same time, the phenomenon that a large number of transformation-type garnets remain in the granite indicates that the granite in the study area has not undergone long-distance migration to completely separate the solid and liquid phases, which further proves its in-situ-semi-situ origin.