扬子克拉通古元古代冷俯冲低温-高压榴辉岩相变泥质岩的发现及其大地构造意义

Discovery and Tectonic Implications of Paleoproterozoic Cold Subduction Low-Temperature/High-Pressure Eclogitic Metapelites,Yangtze Craton

首次报道了扬子克拉通黄陵穹隆北部崆岭杂岩古元古代水月寺混杂岩带中发现特征性石榴石-蓝晶石-硬绿泥石组合低温-高压(LT-HP)榴辉岩相变泥质岩,其变质峰期矿物组合为石榴石+蓝晶石+硬绿泥石+多硅白云母+金红石+石英.相平衡模拟计算得到一条近等温减压的顺时针型变质P-T轨迹,其峰期变质条件为571~576℃,19.2~21.8 kbar.LAICP-MS锆石U-Pb年代学研究获得变泥质岩中碎屑锆石核部年龄集中于2.1~2.2 Ga,变质增生边年龄为1 991±20 Ma.GrtKy-Cld组合榴辉岩相变泥质岩原岩形成构造环境和变质峰期条件指示,其形成于较低地温梯度(d T/d P≈300°C/GPa)下的活动大陆边缘冷俯冲构造环境,进一步表明至少从古元古代开始具有"冷俯冲"构造特征的现代板块构造体制已经启动.

In this paper, it documents for the first time the presence of the distinctive Grt-Ky-Cld assemblage low-temperature/high-pressure(LT-HP) eclogitic metapelites from the Paleoproterozoic Shuiyuesi mélange belt in the Kongling complex, Yangtze craton. The high-pressure metapelites display peak metamorphic mineral assemblages of garnet+kyanite+chloritoid+phengite+rutile+quartz. A near-isothermal decompression(ITD) clockwise P-T path with peak conditions of 571-576 ℃ and 19.2-21.8 kbar were estimated by phase equilibria modelling. The LA-ICP-MS zircon U-Pb dating shows that the metapelite has a metamorphic age of 1 991±20 Ma recorded by zircon rims and ages ranges from 2.1 to 2.2 Ga recorded detrital zircon cores. The sedimentary tectonic setting and metamorphic peak conditions of the Grt-Ky-Cld assemblage in eclogitic metapelites indicate that it has formed in a cold subduction zone with low thermal gradient(d T/d P≈300 ℃/GPa) at the active continental margin. Our study further implies that the modern plate tectonics with"cold subduction"tectonic characteristics has been initiated at least since the Paleoproterozoic.