Repeated slab advance–retreat of the Palaeo-Pacific plate underneath SE China

Southeast China, an important part of the circum–Pacific magmatic–metallogenetic belt, was characterized by late Mesozoic extensive magmatism and related metallogenesis. It is now generally accepted that this magmatism was related to subduction of the Palaeo-Pacific plate, and a series of tectonic models such as normal subduction, shallow subduction, and flat-slab subduction have been suggested. Here we propose a new tectonic model involving repeated slab advance–retreat of the Palaeo-Pacific plate on the basis of new geochronological and geochemical data of Late Triassic to Early Jurassic mafic rocks and Early Jurassic A-type granites in southern Jiangxi and western Fujian provinces. The results indicate that Late Triassic (ca. 228 Ma) mafic rocks are shoshonitic, formed in a post-collisional regime of the Tethyan tectonic domain. Early Jurassic (ca. 197–191 Ma) mafic rocks are sodic, emplaced in a continental arc setting coupled with the subduction of the Palaeo-Pacific plate. Early Jurassic (ca. 189 Ma) granites, occurring as a NNE-trending belt, belong to the A₂ group and formed in an extension setting caused by slab break-off. There are an other four A-type granite belts in southeast China, i.e. the Late Triassic, Late Jurassic, Early Cretaceous, and Late Cretaceous A-type granite belts, respectively. Late Triassic (229–221 Ma) A-type granites occur as an ENE-trending belt and were coincident with the Late Triassic mafic magmatism. Late Jurassic (163–153 Ma), Early Cretaceous (136–124 Ma), and Late Cretaceous (101–91 Ma) A-type granite belts, together with the Early Jurassic (189 Ma) A-type granite belt, are all NNE-trending, parallel to the present coastline. The Late Jurassic belt is located further inland, on the west side of the Early Jurassic belt. The Early Cretaceous belt almost overlaps the Early Jurassic belt and the Late Cretaceous belt is located at the coastal area of southeast China. Integrating these observations, we propose a repeated slab-advance–retreat model for the late Mesozoic magmatic evolution of southeast China. Palaeo-Pacific plate subduction underneath southeast China initiated in the Late Triassic Rhaetian and reached southern Jiangxi by ca. 197 Ma, followed by slab rollback during 197–191 Ma and by slab break-off at ca. 189 Ma. Then slab advance was reestablished with the northwestward subduction approaching southern Hunan at ca. 178 Ma. From ca. 174 Ma, slab rollback reinitiated and gradually migrated from inland to the coastal area. This repeated slab-advance–retreat model is helpful to further understand the geodynamic mechanism of the late Mesozoic tectono-magmatism and related metallogenesis of southeast China.

中国华南地区是环太平洋岩浆成矿带（circum–Pacific magmatic–metallogenetic belt）的重要组成部分，以晚中生代大规模发育的岩浆作用及相关成矿作用为典型特征。目前学界普遍认为，该岩浆作用与古太平洋板块（Palaeo-Pacific plate）的俯冲作用密切相关，此前已先后提出正常俯冲、浅俯冲、平板俯冲等一系列构造模型。基于赣南与闽西地区晚三叠世至早侏罗世镁铁质岩石（mafic rocks）以及早侏罗世A型花岗岩（A-type granites）的最新年代学与地球化学数据，本文提出了一套全新的构造模型：古太平洋板块发生周期性的板片前进-后撤运动。研究结果表明：晚三叠世（约228 Ma）镁铁质岩石属于钾玄岩系列，形成于特提斯构造域（Tethyan tectonic domain）的后碰撞体制环境；早侏罗世（约197~191 Ma）镁铁质岩石为富钠质，形成于与古太平洋板块俯冲作用耦合的大陆弧环境；早侏罗世（约189 Ma）花岗岩呈北北东向展布，归属于A₂型花岗岩类，形成于板片断离引发的伸展构造环境。中国华南地区还发育另外4条A型花岗岩带，分别为晚三叠世、晚侏罗世、早白垩世及晚白垩世A型花岗岩带。其中，晚三叠世（229~221 Ma）A型花岗岩带呈北东东向展布，与晚三叠世镁铁质岩浆作用同期形成。晚侏罗世（163~153 Ma）、早白垩世（136~124 Ma）、晚白垩世（101~91 Ma）A型花岗岩带，与早侏罗世（189 Ma）A型花岗岩带一致，均呈北北东向展布，与现今海岸线平行。晚侏罗世花岗岩带位于更靠内陆的位置，处于早侏罗世花岗岩带西侧；早白垩世花岗岩带几乎与早侏罗世花岗岩带完全重叠，而晚白垩世花岗岩带则分布于中国华南沿海地区。综合上述观测结果，本文提出了一套适用于中国华南晚中生代岩浆演化的周期性板片前进-后撤模型。古太平洋板块向中国华南地区的俯冲作用起始于晚三叠世瑞替期，约197 Ma时抵达赣南地区；随后在197~191 Ma期间发生板片回撤，并于约189 Ma出现板片断离。此后板块俯冲再次恢复为北向俯冲，约178 Ma时抵达湘南地区。约174 Ma起，板片回撤作用再度启动，并逐渐从内陆向沿海地区迁移。这套周期性板片前进-后撤模型，有助于进一步厘清中国华南晚中生代构造岩浆作用及相关成矿作用的地球动力学机制。