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

For the first time, it has been reported that the characteristic garnet - kyanite - chloritoid assemblage low - temperature - high - pressure (LT - HP) eclogite - facies pelitic rocks were discovered in the Kongling complex Paleoproterozoic Shuiyuesi mélange zone in the northern Huangling dome of the Yangtze Craton. The peak metamorphic mineral assemblage is garnet + kyanite + chloritoid + phengite + rutile + quartz. The phase equilibrium simulation calculation yields a clockwise metamorphic P - T trajectory with near - isothermal decompression, and its peak metamorphic conditions are 571 - 576 °C and 19.2 - 21.8 kbar. The LA - ICP - MS zircon U - Pb chronology study shows that the ages of the cores of detrital zircons in the pelitic rocks are concentrated in 2.1 - 2.2 Ga, and the ages of the metamorphic overgrowth rims are 1991 ± 20 Ma. The original rock formation tectonic environment and peak metamorphic conditions of the Grt - Ky - Cld assemblage eclogite - facies pelitic rocks indicate that they were formed in a cold subduction tectonic environment of an active continental margin under a relatively low geothermal gradient (dT/dP ≈ 300 °C/GPa), further suggesting that the modern plate tectonic regime with the "cold subduction" tectonic characteristics has been initiated since at least the Paleoproterozoic.

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.LA‐ICP‐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.Grt‐Ky‐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（dT/dP≈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 .