大别山金河桥榴辉岩的精细P-T轨迹与多阶段折返

Precise P-T path and multi-stage exhumation of the Jinheqiao eclogite,Dabie Mountains,China

大别山金河桥榴辉岩的岩石学和矿物学资料记录了5个阶段的变质作用,除第一阶段代表峰期变质作用外,其余变质阶段均属退变质过程。根据各变质阶段矿物组合的矿物成分分析,应用适当的温度压力计,估算出各变质阶段的变质温度和压力,第一阶段的变质温压条件为 T=797℃和 P=3.50GPa;第二阶段的变质温压条件为 T=667℃和 P=2.72GPa,第三阶段的变质温压条件为 T=660℃,P=1.50GPa;第四阶段的变质温压条件为 T=616℃和 P=1.10GPa;第五阶段的温压条件为 T=450～550℃,P=0.40～0.70GPa。根据岩石学 P-T 轨迹和最近发表的年代学数据,归纳出一条较精细的 P-T-t 轨迹,包括快速降温降压→极快速等温降压→相对快速降温降压→慢速降温降压等四段,代表了从地下115km 折返出露地表的四阶段折返过程。

Petrological and mineralogical data from the Jinheqiao eclogite recorded a five-stage metamorphism, with the first stage being the peak metamorphism and others the retrograded metamorphism. On the basis of the minerals compositions analysis, appropriate thermobarometries are applied to estimate P-T conditions for different metamorphic stages. The first stage： T = 797℃ and P=3.50GPa; the second stage： T = 667℃ and P = 2.72GPa; the third stage： T = 660℃ and P = 1.50GPa; the fourth stage： T = 616℃ and P = 1.10GPa; the fifth stage： T = 450 -550℃ and P = 0.40- 0.70GPa. Combined our petrological P-T path and the geochronological T-t path published recently, a precise P-T-t path can be established, which is composed of four sections, i. e. , fast decreasing of T and P→very fast decreasing of P isothermally→faster decreasing of T and P→slow decreasing of T and P, and reflects four exhumation progresses form 115 kilometers depth.