北祁连南缘右行韧性走滑剪切带糜棱岩矿物的化学成分及其对形成环境的响应

MINERAL CHEMISTRY RESPONSE TO THE FORMATION OF MYLO-NITE FOR THE DEXTRAL DUCTILE STRIKE-SLIPE SHEAR ZONEIN SOUTHERN MARGIN OF NORTHERN QILIAN MOUNTAINS

长英质糜棱岩是北祁连南缘右行韧性走滑剪切带内的主要构造岩类型,其中主要矿物都发生不同程度的塑性变形。石榴石内矿物包裹体的痕迹、长英质矿物的定向分布及矿物间的接触关系等表明糜棱岩中的矿物为同构造生长的产物。石榴石、黑云母、白云母和斜长石的矿物化学成分特点表明糜棱岩的变质程度达角闪岩相,形成于高温环境。石榴石微区成分存在两种类型:一种为环带型,从核部到边缘,MnO含量从中心向边缘逐渐降低,FeO和MgO含量逐渐增加,反映石榴石形成于递进变质环境;另一种为均一型,显示剪切带内局部变质作用的强度继续增大,温度进一步升高(>600℃),组分扩散能力的提高使石榴石内的环带部分或全部均一化。矿物化学成分温压计的计算结果表明矿物形成温度为651～763℃,压力为6 2×108～8 4×108Pa。据此,推测该韧性剪切带形成于地下17～19km,后随祁连加里东造山带一起隆升出露地表。

The felsic mylonite is a main type of tectonites in the dextral ductile strike-slipe shear zone in the southern margin of northern Qilain Mountains,the major minerals in it had been deformed plastically in various degree. The minerals in mylonite are of syntectonic growth that is showed by mineral inclusions in garnet,directional felsic minerals and the contact relation between minerals,The mineral chemistry of garnets,biotites,muscovites and plagioclases have showed that the metamorphic degree of mylonite is in amphibolite facies,and was formed in high temperature,There are two types of garnet microdomain compositions: one is zonal that MnO content decreases,and FeO,MnO contents increase gradually from core to rim,which shows that the garnet was formed in prograde metamorphism; the other is homogeneous,which was caused by component diffusion in garnet crystals during high temperature metamorphism (t>600℃). The forming temperature is 651℃ to 763℃,and forming pressure is 6.2×10~8 Pa to 8.4×10~8 Pa for the mylonite which are obtained by mineral geothemobarometer.Hereby,it is speculated that the ductile shear zone was formed in the depth of 17 to 19 km,and then exhumated with Qilian Caledonian orogeny.