The epilog of the western paleo-Pacific subduction: Inferred from spatial and temporal variations and geochemistry of the Late Cretaceous to Early Cenozoic silicic magmatism in coastal South China

Abstract

The Late Cretaceous to Early Cenozoic magmatism in the South China coastal area produced some amounts of rhyolitic rocks in two phases, which may be used to unravel the geohistory of the epilog of the paleo-Pacific plate subduction system. Essence of the Phase I rocks is the high temperature rhyolite (A-type)-trachydacite association in north Fujian (95–91Ma) that was coeval with regional A-type granites. They succeeded the vast rhyolite–dacite–andesite (RDA) associations and I-type granitoids (113.5–96Ma) and preceded the silicic-dominating rhyolite/basalt bimodal suites or monolithologic rhyolite in Zhejiang (89–86Ma). Phase II rocks include (a) the RDA association or rhyolite alone in some drifted continental fragments nearby (83–56Ma) and (b) the following rift-basin related rhyolite–trachyte/basalt bimodal suites in Guangdong and west Taiwan (56–38Ma). The silicic volcanism, spatially changed from a NE–SW to the nearly E–W direction after 83Ma, may reflect tectonic-driven eruptions occurred in the post-orogenic extensional (Phase I), resumed plate subducting (Phase IIa) and continental margin rifting (Phase IIb) stages. Rhyolitic rocks basically are shoshonitic to high-K calc-alkaline affinities while the Phase IIa RDA associations are mostly concentrated in the high-K to medium-K calc-alkaline series. All these rocks generally possess a continental arc character in tectonic discrimination diagrams, except shoshonitic rocks that have within-plate signatures. Based on the trace element and Nd–Pb isotope data, A-type rocks are suggested to have derived from mixing between trachydacitic (or syenitic) magmas and crustal melts of various sources under the high temperature condition (±metasomatism), and the succeeding silicic rocks are derivatives of the contaminated lithospheric mantle melts through crystal fractionation. On the other hand, Phase II silicic rocks are mainly the fractionation products of mafic magmas originated either from the lithospheric or the asthenospheric mantle. In reflection to such magmatotectonics with time, break-off of the subducted slab shortly before 113.5Ma, change of the plate-moving direction between 86 and 83Ma, and termination of plate movement probably at ∼56Ma signify the epilog of the paleo-Pacific plate.