麻粒岩相变质的岩石学成因模式

REVIEW OF PETROLOGIC GENETIC MODEL FOR GRANULITE FACIES METAMORPHISM

大多数麻粒区高峰期变质温度、压力与许多地壳岩类部分熔融的p-T条件重叠,代表地温梯度被扰动的深部地壳环境,并且普遍涉及附加热源。麻粒岩类形成干水活度降低的流体条件下,以往更多地强调CO_2流体的稀释作用,即碳酸变质模式,但碳、氧同位素的研究并不普遍支持这一理论,它仅适用于少数麻粒岩区。大量的野外与实验研究带来麻粒岩类与重熔花岗岩之间具有成因联系的有力证据,即麻粒岩类很可能为无流体条件下部分熔融的残留岩,或无水岩浆结晶的产物。

A majority of regional granulite facies terrains preserve the p-T records for peak condition, which overlaps with partial melting condition for common crustal rock types. It represents the deep crustal setting with geotherm perturbed,associated with additional heat source during the metamorphism. In addition,the granulites are characterized by reduction of H_2O activity, interpreted as the result of influx of CO_2-rich fluid,i, e. carbonic metamorphism, which seems to be locally applicable to several terrains, but not uniformly supported by more and more available C,O isotopic evidence. According to results from the melting experiment under high p-T conditions and field geological constraints,the genetic connection between granulites and anatectic granitic melt is accepted widely,which indicated that granulites might be restites left behind by dehydration associated with fluid-absent partial melting, and extraction of hydrous melt from the system. Eventually,the third possible model for granulites formation,especially with charnockites,is related to cumulates crystallized from anhydrous magma.