Magmatic responses to Cretaceous subduction and tearing of the paleo-Pacific Plate in SE China: An overview

Abstract

Both Cretaceous arc-type and intraplate rocks are widely distributed in SE China, in association with subduction of the paleo-Pacific Plate. However, it remains unclear whether there exists a genetic link between the intraplate and subduction-related magmatism. Here we conduct a comprehensive geochemical data compilation, sorting and processing of the Cretaceous mafic igneous rocks from SE China, in combination with a 2-D numerical simulation on slab melting, to further decipher the petrogenetic relationship between the arc-type and intraplate magmatism under a unified tectonic framework invoking subduction and rollback-tearing of the paleo-Pacific Ocean.The Cretaceous arc-type mafic rocks (120–80 Ma) include eruptive and intrusive rocks, distributing along the coastal region in a NE-NNE direction. After parent magma compositional recalculation of the mafic intrusions, the results indicate that both the eruptive and intrusive arc-type mafic rocks are hydrous calc-alkaline basalts, with enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but depletions in high field strength elements (HFSEs), crust-like Sr-Nd-Pb-Hf isotopic compositions, and low Lu/Hf ratios. All these features are typical of igneous rocks formed in a continental arc. The combined data suggest their mantle source was mainly metasomatized by melts derived from the subducted terrestrial sediments and reflect a relatively hot subduction zone during Cretaceous.The intraplate mafic rocks (110–70 Ma) distribute in SE China interior and generally show trace element geochemistry similar to oceanic island basalt (OIB, hereafter we term them as OIB-like basalts). They can be further divided into two groups: high-Nb (Nb > 50 ppm) and low-Nb (Nb < 40 ppm) basalts. The high-Nb basalts have depleted asthenospheric mantle-like isotopic compositions. Their mantle source was composed mainly of pyroxenite that was likely formed through interaction between the asthenosphere and melts from the dehydrated subducted slab. Relative to the high-Nb basalts, the low-Nb basalts have lower Nb/REE and Nb/LILE ratios, and more evolved but highly variable isotopic compositions. Their mantle source was also the asthenosphere metasomatized by addition of the slab melts containing a higher proportion of recycled sediment component.From the arc-type to low-Nb and ultimately to high-Nb mafic rocks, increases of Nb, Nb/LILE and Nb/REE ratios are coupled with more depleted Sr-Nd-Pb-Hf isotopic features, reflecting the occurrence of melting from the upper subducted sediment to the lower oceanic mafic crust that had experienced extensive dehydration. Further thermodynamic modelling results show that melting of the dehydrated slab can occur along the torn surface and lower part of the oceanic crust, once the slab is strongly thinned and fractured in response to tearing and fragmentation. In combination with the contemporaneous sedimentary records in SE China, the Cretaceous mafic magmatism provided geochemical records of tectonic transition from early Cretaceous advanced subduction to late Cretaceous rollback of the paleo-Pacific slab, during which slab tearing or fragmentation and the resultant melting of the dehydrated subducted slab acted as a predominant agent for mantle enrichment.