拉萨地块林芝杂岩体含十字石石榴角闪岩的岩石学和变质过程研究

摘要

在拉萨地块林芝杂岩体中新发现的石榴角闪岩矿物组合为石榴子石、角闪石、十字石、绿泥石、斜长石、钠云母以及少量的钛铁矿和磷灰石。石榴角闪岩中石榴石核部富锰（Xsps=0.12~0.15）贫铁（Xalm=0.45~0.50）而石榴子石边部相对贫锰（Xsps=0.01~0.03）富铁（Xalm=0.60~0.65），表明石榴子石的核部和边部分别形成于变质作用两个不同阶段。从核部到边部，镁铝榴石升高而钙铝榴石降低，表现为进变质环带特征，这表明石榴子石核部形成于进变质过程。生长在不同的变质阶段的角闪石具有不同的成分特征，作为变质基性岩中罕见的富铝矿物，十字石的结构特征记录了不同变质阶段的信息，结合石榴石的成分和结构特征，为相平衡模拟研究其P−T演化过程提供了可能。我们利用Perplex相图模拟软件在Mn−NCKMASHO体系中模拟出该石榴角闪岩的视剖面图，利用石榴子石边部镁铝榴石和钙铝榴石含量等值线确定出石榴角闪岩峰期温压为：610~630ºC，12×105~13×105 kPa，对应峰期矿物组合为石榴子石，角闪石，十字石和白云母。同时结合十字石保存的退变信息得到该石榴角闪岩经历了一个顺时针的变质演化轨迹。%The newly found staurolite-bearing garnet amphibolites in Nyingchi complex of the Lhasa Block have the mineral assemblage of garnet, amphibole, staurolite, chlorite, plagioclase, mica and minor ilmenite and apatite. The cores of the garnet in the garnet amphibolite are extremely rich in Mn (XSps=0.12-0.15) and poor in Fe (XAlm=0.45-0.50), whereas their rims are relatively Mn poor (XSps=0.01-0.03) and Fe rich (XAlm=0.60-0.65), implying that the core and the rim of the garnets belong to two metamorphic generations. Xpy increases and Xgr decreases from the garnet core to the rim, indicating characteristics of prograde metamorphic zonation. The amphibole growing at different metamorphic stages have obvious composition differences. Staurolite, as an uncommen aluminium-rich mineral in the metabasite, has also recorded different metamorphic processes in their microstructures. In combination with the garnet compositional profile, it is possible to calculate the P-T evolution by phase diagram. The authors modeled the pseudosection of the staurolite-bearing garnet amphibolite under the model system of Mn-NCKMASHO, using Perple-X program with the help of Xpy and Xgr isopleths of the garnet, and obtained the peak metamorphic condition of 620ºC and 12kbar. The peak mineral assemblages are garnet, amphibole, staurolite and mica. In the meanwhile, the clockwise P-T path has also been demonstrated by the microstructures in the staurolites, suggesting that the staurolite-bearing garnet amphibolites have experienced three stages of metamorphism.