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