大别山南部高压-超高压变质地体中云母片岩和片麻岩的相平衡研究

Phase equilibria of mica-schists and gneisses in the HP-UHP Belt of Southern Dabie Shan.Acta Petrologica

对于大别山高压-超高压变质带中大面积出露的云母片岩和片麻岩的变质条件和演化历史有不同的认识:如中低压绿帘角闪岩相,高压角闪岩相和超高压榴辉岩相。通过对大别山南部超高压榴辉岩区、高压榴辉岩区和宿松变质杂岩带中的云母片岩和片麻岩进行详细的岩相学及相平衡研究表明:它们的变质条件分别为610℃～690℃,1.2～1.4Ga;590～600℃、1.3～1.5GPa;520～590℃,0.9～1.4GPa,相当于高压绿帘角闪岩相、高压角闪岩相与榴辉岩相之间的过渡。依据超高压榴辉岩区绿帘石黑云片麻岩中的石榴石环带特征推测,它们很可能经过超高压变质阶段,现有矿物组合记录了抬升阶段的条件。但是高压榴辉岩区的石榴石绿帘石二云片麻岩和宿松变质杂岩带中石榴石云母片岩中的石榴石环带特征表明,其现有矿物组合记录了变质峰期条件,不应该经历更高压变质阶段。在 KFMASH 体系的 PT 视剖面图上计算多硅白云母 Si 含量等值线表明,在多数矿物组合中白云母的 Si 含量可作为地质压力计,但 Si 等值线的斜率受到矿物组合的控制。

There have been controversial opinions on the extensively distributed mica-schists and gneisses in the Dabieshan HP-UHP terranes ： Some geologists believe that most of the mica-schists and gneisses are only metamorphosed to epidote-amphibolite facies; and the others suggest that they have been subjected to HP amphibolite facies or UHP metamorphism together with the eclogites, On the basis of study on the petrography and phase equilibria for the mica-schists and gneisses from the UHP eclogite terrane, HP eclogite terrane and the Susong metamorphic complexes, the P-T conditions of these mica-schists and gneisses were calculated as 610 -690℃ at 1,2 - 1.4GPa; 590 - 600℃ at 1.3 - 1.5GPa and 520 - 590℃ at 0.9 - 1.4GPa, respectively, equivalent to the transitions among HP epidote-amphibolite, HP amphibolite and eclogite facies, According to the garnet zonal characters in the epidote-biotite gneisses from the UHP eclogite terrane, it is inferred that these gneisses may have experienced UHP metamorphism, and the present mineral assemblage has recorded a PT condition of uplifting. However, the garnet zonations from the garnet-epidote mica gneiss in the HP eclogite terrane and the garnet-mica schists in the Susong complexes indicate that the existing mineral assemblages in these rocks have recorded the peak metamorphic PT conditions and it is impossible for them to have experienced UHP metamorphism. In addition, contours of the Si content in phengite in the KFMSH pseudosections show that the phengite Si contents could be used as a geobarometer in most mineral assemblages, but both Si contents and slopes of Si isopleths are dependent of mineral assemblages.