木兰山低温高压微变质体中SiO_2非晶相的特征和意义

Feature and significance of amorphous SiO_2 in the low-temperature and high-pressure micro-metamorphic body from Mulanshan, Hubei province.

木兰山低温高压变质岩中的石英脉及其围岩的矿物组成、变质结构和界面特征的光学显微镜和电子探针研究表明,该石英脉为同构造重结晶脉,经历过低温高压变质作用,其变质温度压力条件与围岩矿物的形成条件相同。透射电镜亚微米纳米尺度分析表明,该石英脉中存在一种已部分转变成α-石英纳米晶的SiO2非晶相,且观察到类似于柯石英假象的胀裂现象。并认为SiO2非晶相的形成及其变化特征可能是大别山高压超高压变质带南缘蓝片岩的一次俯冲和快速折返过程的超显微证据。

Mineral component, metamorphic texture, and interface feature of quartz veins(micro-metamorphic body) and their country rock in the low-temperature and high-pressure metamorphic rocks from Mulanshan, Hubei province have been investigated by optical microscope and electron probe microanalysis. The results show that these quartz veins, having undergone the same low-temperature and high-pressure metamorphism as minerals of country rock, are a kind of paratectonic recrystallization veins. Amorphous SiO2 formed at high-pressure and/or ultrahigh-pressure environment, which had partially been transformed into the nanocrystalline α-quartz, has been found in these quartz veins with transmission electron image, selected area electron diffraction and microarea chemical composition analysis. Furthermore, the decompression expansion phenomenon similar to pseudomorph coesite has been observed in transmission electron image. The appearance and the transforming feature of amorphous SiO2 is probably a super-microscopic evidence of subduction-obduction of blueschist rocks occurring in southern edge of high-pressure and ultrahigh-pressure metamorphic belts in Dabie mountains.