胶北地块高压与低压泥质麻粒岩的相平衡关系与p-T演化轨迹

利用最新的内洽性热力学数据库和THERMOCALC3.21程序对胶北地块高压与低压泥质麻粒岩的相平衡关系进行了定量分析。计算了胶北地块高压泥质麻粒岩、低压泥质麻粒岩和夕线石榴黑云片岩等代表性富铝岩石KFMASH(K2O-FeO-MgO-Al2O3-SiO2-H2O)体系的p-T视剖面图,再现了这些岩石随温压条件变化可能出现的各种矿物组合与矿物成分变化,发现原岩成分不同的变质岩石,尽管变质演化过程有所差异,但在麻粒岩相变质条件下所形成的矿物组合一致。通过计算泥质岩石在高压(p=1.0GPa)和低压(p=0.5GPa)条件下的T-X视剖面图,发现极度富铁、贫镁的岩石,在高压麻粒岩相条件下并不会生成含蓝晶石的特征矿物组合,在低压麻粒岩相条件下也不会生成含堇青石的特征矿物组合。将样品实际观测结果与p-T视剖面图的计算结果对比,确定胶北地块高压泥质麻粒岩变质峰期的温压条件为830~860℃,1.25~1.4GPa,峰期后呈现顺时针样式的p-T演化轨迹,反映陆壳先碰撞增厚、后又快速减薄的地质动力学过程;确定胶北地块低压泥质麻粒岩变质峰期的温压条件为790~820℃,0.62~0.68GPa,峰期后呈现近等压冷却的p-T演化轨迹。

Phase equilibria and p-T path of high- and low-pressure pelitic granulites from the Jiaobei massif

The phase equilibria of high-and low-pressure pelitic granulites from the Jiaobei massif are quantitatively studied by using the internally consistent thermodynamic dataset and the software THERMOCALC 3.21. p-T pseudosections are calculated for the KFMASH systems that are representative Al-rich rocks of high-and low-pressure pelitic granulite and sillimanite-garnet-biotite schist, which show all possible mineral assemblages and mineral compositions with changes of p-T conditions. The p-T pseudosections show that pelitic rocks with different bulk compositions evolve with different mineral assemblages with changing temperature and pressure. However, they may produce similar mineral assemblages under granulite p-T conditions. Two T-X pseudosections are also calculated for the KFMASH system at p=1.0 and 0.5 GPa, respectively. Based on the T-X pseudosections, we find that the typical high-pressure pelitic granulite with kyanite-bearing assemblage and the low-pressure pelitic granulite with cordierite-bearing assemblage cannot be produced in Fe-rich and Mg-poor rocks.The metamorphic peak p-T conditions for the high-pressure pelitic granulite from the Jiaobei massif are estimated as 830~860 ℃ and 1.25~1.4 GPa for the peak stage. A post-peak p-T path of clockwise type is related to a geodynamic process of rapid thinning in a thickened continental crust region. The metamorphic peak p-T conditions for low-pressure pelitic granulite from the Jiaobei massif are determined as 790~820 ℃ and 0.62~0.68 GPa,and a post peak p-T path is characteristic of isobaric cooling.