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The Mesozoic geology of SE China has been widely considered to relate to subduction of the Paleo-Pacific Plate beneath the Eurasia, accompanied with extensive felsic magmatism. However, the relationship between the origin of the voluminous crust-derived magmatism and the Paleo-Pacific slab subduction remains unclear. Here we present petrology, zircon U-Pb-Hf isotope data, and whole-rock geochemical and isotopic compositions of two episodes of Cretaceous silicic volcanic rocks (~114–113 Ma for Group 1 and ~ 100–93 Ma for Group 2) from southeastern Zhejiang Province in SE China, to provide new constraints on the geodynamic transition of the Paleo-Pacific slab subduction. Both volcanic groups show similarly unradiogenic whole-rock ε Nd (t) and zircon ε Hf (t) values (ε Nd (t) = −8.6 to −6.4 and ε Hf (t) = −8.5 to −0.7 for Group 1, and ε Nd (t) = −7.1 to −6.1 and ε Hf (t) = −7.8 to −2.2 for Group 2). The limited emplacement of coeval mafic magmas and absence of mafic microgranular enclave and intermediate lavas suggest an insignificant role of mantle-derived melt either as a parental magma for differentiation or a mafic component during magma mixing. Instead, the low MgO and high concentrations of SiO 2 and incompatible elements and the evolved Nd and Hf isotopic compositions indicate derivation of these two groups of silicic magmas from similar hybrid crustal sources containing the metasedimentary rocks of Cathaysia basement and the newly accreted arc crust. Group 1 rocks contain a higher proportion of hydrous minerals (e.g., biotite) and Sr/Y and La/Yb CN ratios but lower zircon saturation temperature than Group 2. Such variations in mineral and geochemical compositions could be mainly attributed to the change of melting conditions from a relatively deeper, colder and water-fluxed crust to a shallower, hotter and less hydrous state. By combining our new results and previous studies of the late Mesozoic volcanic rocks in SE China, we propose that a geodynamic transition from subduction to slab rollback of the Paleo-Pacific Plate was responsible for such P-T-H 2 O variations of the crustal source. • 1. Two episodes of Cretaceous felsic volcanic rocks were identified in SE China. • 2. Both of them were formed by crustal melting at different P-T-H 2 O conditions. • 3. Transition from subduction to slab rollback occurred during ca. 110–90 Ma. [ABSTRACT FROM AUTHOR]

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. • Geochemistry of Cretaceous arc-type magmas indicate an active continental margin in SE China. • Both high-Nb and low-Nb intraplate basalts were derived from the asthenosphere metasomatized by slab melts. • Geodynamic simulation suggests melting of dehydrated oceanic crust only in strongly thinned slab. • Cretaceous mafic magmatism in SE China recorded the subduction and tearing of the paleo-Pacific slab. [ABSTRACT FROM AUTHOR]