Evolved Late Mesozoic continental arc: Constraints of detrital zircons from the western East China Sea

ABSTRACT The Late Mesozoic subduction of Izanagi slab beneath East Asia formed large-scale intraplate magmatism in SE South China and subduction mélanges outcropped in SW Japan to eastern Taiwan Island, but the accompanying arc remains uncertain. We conducted LA-ICP-MS U–Pb zircon dating and trace element analyses of proximal sandstones from the SW East China Sea to trace a Jurassic to Cretaceous magmatic arc. Newly acquired data reveal that arc magmatism mostly developed in episodes of 150–124 and 124–102 Ma, exhibiting characteristics of low-T, high Th/U, U/Yb, Sc/Yb, Th/Nb, and low Nb/Yb, Nb/Hf ratios. This magmatic arc, combined with the SE South China intraplate and residual subduction mélanges, spatially forms a Late Mesozoic trench–arc–intraplate architecture responding to the Izanagi subduction. Its identified tectonic scenarios mainly include slab strike-slip subduction (200–170 Ma), slab stagnation and intraplate foundering (170–150 Ma), slab rollback and arc-root removal (150–102 Ma), and arc migration (102–86 Ma). Both the western East China Sea and Cathaysia block share a unified Paleoproterozoic basement that formed at ca. 1.85 Ga, and the Cathaysia-based magmatic arc occurred then.


Introduction
The Izanagi slab subducted beneath East Asia during the Late Mesozoic (Müller et al. 2016;Torsvik et al. 2019;Boschman et al. 2021).Subduction-related tectonic configurations include vast Jurassic to Cretaceous intraplate magmatism getting younger coastward in SE South China (Zhou and Li 2000;Wang et al. 2008), and accretionary complexes scattered from SW Japan, eastern Taiwan Island to central Indonesia (Wakita and Metcalfe 2005;Isozaki et al. 2010;Yui et al. 2012).However, where is the accompanying magmatic arc?The East China Sea (ECS) domain is settled between the trench and intraplate units, in which previous studies have found records of magmatic arc from the Yandang, Yushan, Haijiao, and Hupijiao swells (Yang et al. 2012b;Xu et al. 2017Xu et al. , 2023;;Li et al. 2020b;Si et al. 2021).Some scholars diversely considered the Late Mesozoic magmatism in SE South China as arc-related (Jiang et al. 2009;Yan and Jiang 2019;Xu et al. 2021Xu et al. , 2023)).In general, the tectonic relationship between the ECS and SE South China is uncertain.Whether domains of the ECS and Cathaysia block share a unified basement or the ECS is regarded as an exotic microcontinent (Niu et al. 2015), it is crucial to comprehend arc evolution and slab subduction dynamics.
The utilization of detrital zircons (mostly derived from granitoids) allows for broad arc-related sediment provenances to be covered in this study.A large quantity of detrital zircons offers advantages over the use of a few drilled igneous rocks in revealing regional tectonics.We focused on the magmatic arc, subduction system, and identification of basement, based on investigations of detrital and igneous inherited zircons from the West ECS, taking sediment environment and provenance into account.We have found the Paleoproterozoic basement and Late Mesozoic arc-related records in the West ECS.A Late Mesozoic trench-arc-intraplate architecture that resulted from Izanagi subduction beneath East Asia links the accretionary complexes in SW Japan to eastern Taiwan Island, the magmatic arc in the West ECS, and the intraplate magmatism in SE South China.

Geological outline
The ECS shelf basin (Figure 1a), primarily settled on a pre-Cenozoic basement, comprises the zones of west depression (the Lishui-Jiaojiang, Fuzhou, Qiantang, and Changjiang sags), central swell (the Yandang, Yushan, Haijiao, and Hupijiao swells), and east depression (the Diaobei, Xihu, and Fujiang sags).It adjoins the Zhemin swell in the west and the Diaoyu Island swell in the east.Yang et al. (2020) proposed the ECS shelf basin as a Late Triassic to Late Cretaceous basin in an active continental margin and then evolved into a back-arc strike-slip pull-apart rift after the Late Cretaceous.Garben's growth of the ECS shelf basin migrated eastward from the Paleocene to Miocene due to slab rollback (Suo et al. 2015;Li et al. 2019).Our clastic samples were collected from the Lishui-Jiaojiang sag (LJS), SW ECS shelf basin.The LJS adjoins the Zhemin swell in the northwest and the Yandang swell in the southeast.The tectonic evolution of the Lishui sag mainly includes episodes of syn-rift, post-rift, and regional subsidence, separated by the Oujiang and Huagang events, respectively.Main units of the Lishui syn-rift sag comprise the west subsag (LSWS), Lingfeng barrier, and east subsag (LSES); the sag depositions span from the Late Cretaceous to Paleocene, covering the Shimentan (K 2 s), Yueguifeng (E 1 y), Lingfeng (E 1 l), and Mingyuefeng (E 1 m) Formations (Fms) (Figure 1b and c).Core observations reveal a dominant delta system with littoral to lacustrine facies during this period, and the Lingfeng barrier was entirely covered by the uppermost Mingyuefeng Fm (Liu 2015).When entered the post-rift, the Lishui sag generally exhibited a large half-graben structure; its depositions of the Oujiang (E 2 o) and Wenzhou (E 2 w) Fms formed in environments changing from a fluvial-alluvial plain to an open sea (Zhu et al. 2019).Controlled by the regional subsidence, depositions of the Longjing (N 1 l), Yuquan (N 1 y), and Liulang (N 1 ll) Fms dominated the sag, and the typical graben structure migrated eastward.
The convergence between the Izanagi slab and East Asia in the Late Mesozoic built main tectonic domains such as the West ECS magmatic arc (Xu et al. 2017(Xu et al. , 2023)), forearc deposition (Yang et al. 2017), SE South China intraplate (Wang et al. 2008), and subduction mélanges in SW Japan to eastern Taiwan Island (Wakita and Metcalfe 2005).
The West ECS magmatic arc Pre-Cenozoic igneous and metamorphic rocks are increasingly drilled from the ECS, in which felsic rocks dominated, together with minor basic rocks (Zhang et al. 2019).Guo et al. (2015) reported Late Triassic and Early Jurassic age data  of granitic fragments from borehole WZ27-1 in the Lishui sag.Arc-related granitoids from 10 boreholes provide U-Pb zircon age records of 193-172 and 115-111 Ma in the ECS (Zhang et al. 2019).Granites (U-Pb zircon 174-167 Ma) from borehole MYF-1 display arc-related enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs) in the Lishui sag (Yuan et al. 2018).The arc granodiorite ECS611 (U-Pb zircon ca.187 Ma) from the Jiaojiang sag has high U/Yb and low Th/U, Nb/Hf, and Nb/Yb ratios in zircon (Xu et al. 2017).Xu et al. (2023) reported a series of igneous rocks (200-86 Ma) drilled from the West ECS (Figure 1a).A suggested Early Jurassic arc zone extends from the Yandang swell, Taiwan Talun, to Dongsha Islands in East and South China Seas (Xu et al. 2017).The ECS basement is either considered as an extended South China continent (Li and Li 2007) or as an exotic terrane (Niu et al. 2015).
Late Mesozoic forearc deposition Boreholes 25s and 26s from the Fuzhou sag encountered Jurassic and Cretaceous forearc depositions.The Jurassic Fuzhou (J 1-2 , 538.5 m thick) and Xiamen (J 3 , 439.5 m thick) Fms in 26s deposited in fluvial and lacustrine environments (Zheng et al. 2005;Xu et al. 2023).The Yanshanian uplift led to the absence of some Upper Jurassic strata in this region (Yang et al. 2012a).The drilled Cretaceous Yushan (K 1 , 397.5 m thick), Minjiang (K 2 , 372.5 m thick), and Shimentan (K 2 , 139.0 m thick) Fms belong to the fluvial and delta facies (Zheng et al. 2005).Over 20 boreholes encountered Upper Cretaceous Shimentan Fm in the West ECS, mostly with volcanic layers (e.g.Xu et al. 2023).Equivalents of forearc deposition are also found in the Chaoshan sag, Tainan basin, and Dongsha-Penghu Islands in the South China Sea (Xu et al. 2017).Geophysical data revealed that the residual Mesozoic sequences in the ECS shelf basin are 4,500-6,000 m thick in the south and 1,000-2,500 m thick in the north (Yang et al. 2017).

Subduction-related accretionary complex Jurassic to
Cretaceous accretionary complexes on Japan, intermittently extending to eastern Taiwan Island, imply a Late Mesozoic subduction-related configuration (e.g.Maruyama et al. 1997;Isozaki et al. 2010;Charvet 2013).These exposure in SW Japan mainly consist of the Mino-Tamba (Early to Middle Jurassic; Sano and Kojima 2000), Chichibu (Jurassic to Early Cretaceous; Kato and Saka 2006), and Shimanto (Late Cretaceous; Tatsumi et al. 1998) accretionary complexes.The Mino-Tamba accretionary complex consists of accreted oceanic materials, such as Carboniferous to Permian basalts and limestones, and Permian to Jurassic cherts (Sano and Kojima 2000;Safonova and Santosh 2014).The Chichibu accretionary complex, located south of the Mino-Tamba complex, consists of the northern Chichibu, Kurosegawa, and southern Chichibu terranes.Two Chichibu terranes are dominated by Jurassic to Early Cretaceous accretionary complexes of voluminous Triassic OIB-type basalts, Middle Triassic to Early Jurassic cherts, and siliceous mudstones (Kato and Saka 2006;Safonova and Santosh 2014).The Shimanto accretionary complex consists mainly of Cretaceous to Tertiary thick coarse-grained turbidites with thin mélange intercalations.The exposures of the Late Mesozoic accretionary complexes could extend into the Ryukyu Islands and eastern Taiwan Island.It is described as the Fusaki Fm that overthrust the Late Triassic to Early Jurassic metamorphic complexes in the Ryukyu Islands (Foster 1965;Isozaki et al. 2010).The Late Jurassic to Early Cretaceous Tailuko belt with schist and massive marble, and the Middle to Late Cretaceous Yuli belt with schist and metabasite/serpentinite comprise subduction-related accretionary complexes in eastern Taiwan Island (Yui et al. 2012).

Intraplate magmatism in SE South China
The Late Mesozoic magmatism getting younger coastward is a prominent feature in SE South China.Its lithologies are granite and rhyolite mainly (>95%), together with minor gabbro-basalt and diorite-andesite (Zhou and Li 2000).The four main tectonomagmatic episodes are defined as follows: (1) Early Jurassic suites of basaltrhyolite and granite-syenite-gabbro (200-170 Ma) are restricted to the Ningyuan-Fankeng igneous zone spatially from south Hunan, south Jiangxi to SW Fujian, and this NWW-SEE-trending plutonic-volcanic zone in the SE South China interior is correlated with strike-slip subduction and intraplate reworking (Xu et al. 2017).(2) Middle to Late Jurassic granitic batholiths (165-150 Ma) dominate the SE South China interior, which occurred within a short time interval, accompanied by minor syenite, basalts, and gabbro; they are considered to be the result of intraplate lithospheric extension (Li et al. 2007).(3) Early Cretaceous magmatic suites (145-120 Ma) dominate the SE South China coast: rhyolites mainly in the northeast and granitic rocks mainly in the southwest; they were formed under extensional to rifting settings in response to slab rollback (Xu et al. 2021).(4) The Late Cretaceous magmatic zone (100-85 Ma) trends NNE-SSW along the Zhejiang and Fujian coastal areas, and Iand A-type granites with silicic volcanics feature this zone.This magmatism is associated with crustal extension and slab rollback (Li et al. 2014b;Chen et al. 2016a;Xu et al.;Xu et al. 2023).The main tectonic models to explain this Late Mesozoic tectonomagmatism include slab dip steepening (Zhou and Li 2000), and flat subduction with slab foundering and rollback (Li and Li 2007).

Sample information
Our drilled samples taken from the West ECS include three types: 1) clastic samples for Late Mesozoic magmatism and basement analyses by using detrital zircons, 2) igneous samples for basement analysis by inherited zircons, and 3) sandstone samples for sediment provenance analysis by bulk-rock chemistry.Sample details are listed in Table S1.
Forty-one proximal clastic rocks for detrital zircon analysis are collected from 11 boreholes at depths ranging from 650 to 4,011 m, covering almost all areas of the LJS.They comprise litharenite and arkosic arenite or lithic arkose, mainly coarse-grained and subangular to subrounded in texture.These samples cover the Upper Cretaceous Shimentan, Paleocene Yueguifeng, Lingfeng, and Mingyuefeng, and Eocene Oujiang and Wenzhou Fms, which were dominantly deposited in the delta environment, partly with littoral to lacustrine settings.Analysed zircon grains mostly consisted of a population of euhedral to subhedral, colourless to light yellow prismatic crystals (110-150 μm long, 1-3 length-to-width ratio).Zircon data of the samples 16s18 and 16s20 partly refer to Si et al. (2021).In addition, there are 101 sandstone samples from 16 boreholes selected for bulk-rock chemical analysis.They cover the Lower to Middle Jurassic Fuzhou, Upper Jurassic Xiamen, Lower Cretaceous Yushan, Upper Cretaceous Minjiang and Shimentan, Paleocene Yueguifeng, Lingfeng, and Mingyuefeng, and Eocene Oujiang and Wenzhou Fms, ranging from 650 to 4,086 m at depth.
Twenty igneous samples from 14 boreholes for inherited zircon analysis intermittently cover the areas of the West ECS.Volcanic rocks include andesite, basaltic andesite, dacite, rhyolite, and trachydacite at depth from 1,386 to 4,503 m, stratigraphically including the Lower Cretaceous Yushan, Upper Cretaceous Shimentan, Paleocene Mingyuefeng, and Eocene Pinghu Fms.Intrusive rocks collected from 1,550 to 4,500 m comprise the Early Jurassic diorite, Early Cretaceous granite and granodiorite, and Eocene granodiorite.

Analytical methods
Sample processing included cleaning, handpicking, crushing, pulverizing, mineral separation, mounting, cathodoluminescence imaging, U-Pb isotope and trace element analyses.All the pre-processing experiments were performed at the State Key Laboratory of Marine Geology, Tongji University: zircons were separated from a 75-212 μm fraction of approximately 1.5 kg cleaned rock fragments after conventional crushing, pulverizing, gravimetric and magnetic separation, and handpicking; selected less fractured and inclusion-free zircons were cast; and cathodoluminescence images were taken to reveal zircon structures to select spot placements.
The U-Pb dating analyses were conducted at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan) and the State Key Laboratory of Marine Geology, Tongji University.Following Wuhan lab's procedures, U-Pb isotopes and trace elements were analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), using a 193 nm GeoLas 2005 laser attached to an Agilent 7500a ICP-MS, with helium and argon as carrier gases.According to Hu et al. (2015) and Liu et al. (2010), ICP-MS has an RF power of 1350 W and dual detector mode.The laser properties included ablation type (single spot), energy density (10 J•cm −2 ), repetition rate (5 Hz), and ablation spot size (32 μm).The dwell times were set to 10 ms for Th and U, 15 ms for Pb, and 6 ms for other elements.Each analysis incorporated approximately 20-30 s of background acquisition (gas blank) followed by 40-50 s of zircon data acquisition.For the sample analyses by LA-ICP-MS at the State Key Laboratory of Marine Geology (Tongji University), we used a Resonetics RESOlution M50 193 nm excimer laser system connected to an Agilent 7900 ICP-MS, and the related procedures are described in Huang et al. (2020).Standard materials 91500 (Wiedenbeck et al. 1995) and Plesovice (Sláma et al. 2008) were used for isotope ratio correction and data monitoring.NIST610 (Pearce et al. 1997) was used for elemental determination.We used the ICPMSDataCal10.2software and Isoplot/Ex(4.15)to calculate isotopic ratios, element contents, and weighted mean ages, and plot U-Pb data (Liu et al. 2008(Liu et al. , 2010;;Ludwig 2009).
Bulk-rock element analyses were performed at the Peter Hooper GeoAnalytical Lab, Washington State University (Pullman), U.S.A. and Hubei Geological Research Laboratory, China.The fresh rocks were ground in tungsten carbide ring mills, and low-dilution fusion with dilithium tetraborate was used for element measurements.The concentrations of major and trace elements in each sample were measured using X-ray fluorescence, in which the oxidation state of iron and volatile content were ignored.For the samples analysed at Peter Hooper GeoAnalytical Lab, see Johnson et al. (1999) for associated analytical details.Rare earth element (REE) and trace element abundances were determined using inductively coupled ICP-MS.The precision of this method was better than 5% (RSD) for the REEs and 10% for other trace elements.The analytical procedures are listed at http://soe.wsu.edu/facilities/geolab/technotes/icp-ms_method.html.For the analytical data at Hubei Geological Research Laboratory, major elements were analysed using a Rigaku 3080E1-type XRF spectrometer, with analytical precision better than 5%, and trace elements and REEs were analysed using a Finnigan Triton thermo-ion mass isotope spectrometer.Related analytical parameters refer to Liu et al. (2008).

Paleogene proximal deposition
The main provenances of Paleogene sequences in the LJS include the Zhemin and Yandang swells, which build delta systems in the west, and fan delta systems controlled by synsedimentary faults in the east (Tian et al. 2012).Zhang et al. (2012) concluded that the Lishui sag had been filled up before the Upper Mingyuefeng Fm was deposited on the Lingfeng barrier based on analyses of basin structure, global sea-level change, and sediment supply.Several fan deltas and nearshore subaqueous fans have developed on both sides of the Lingfeng barrier (Tian et al. 2012).Jurassic to Cretaceous sequences are identified from the Fuzhou sag in the West ECS by fossils of sporopollen assemblage and ostracod morphology (Wang et al. 2000), and they involve the Shimentan, Minjiang, Yushan, and Fuzhou Fms.The Shimentan Fm belongs to lake facies sediments deposited in the Late Cretaceous initial rift grabens (Si et al. 2021).
The clastic samples in this study are mostly proximal deposits, which can facilitate sediment provenance evolution.The sample deposition environments are determined through core observations of four boreholes in the LJS.The Yueguifeng Fm drilled in borehole 2s is characterized by subaqueous distributary channels in the prodelta.It has characteristics of interbay deposition between channels shown by dark muds and channel sediments shown by fine sandstones; shells washed down from channels are visible (Figure 2a).The medium sandstones with gravels (1-2.5 cm, Figure 2b) have tabular cross-bedding structures (Figure 2c), implying a strong hydrodynamic force.In borehole 3s, shells and gravels in sandstones are arranged directionally, revealing a strong hydrodynamic force environment of the Lower Mingyuefeng Fm deposition (Figure 2d).The Upper Mingyuefeng Fm from borehole 11s shows inclined bedding structures with slight bioturbation (Figure 2e).Interbedded thin layers of sandstones with mudstones may indicate a transition from distribution channel to interbay facies; the horizontal beddings in sequence reveal a deduced hydrodynamic force (Figure 2f).Cores of the Lower Mingyuefeng Fm from borehole 15s are inequigranular poorly sorted pebbly sandstones with charring plant fragments and mudstone debris (Figure 2g, h and i), reflecting short transport and rapid subsidence of sediments.In general, the Yueguifeng Fm accords with the facies of underwater distributary channels of the prodelta; the Lower Mingyuefeng Fm belongs to delta/fan delta sediments, and the Upper Mingyuefeng Fm is characterized by offshore facies.Gully slope-fan groups within a small scope connect surrounding swells with the LJS (Liu et al. 2004;Cai et al. 2020).Thus, sediment materials of the delta system in the LJS are transported not far from the provenances.Based on the CNOOC Limited-Shanghai reports, Figure 1d and e outline two types of the source-to-sink system for the Upper Lingfeng Fm (with the Lingfeng barrier) and Upper Mingyuefeng Fm (without the Lingfeng barrier), respectively.
We use the Quartz-Feldspar-Rock fragment (QFR) scheme (Pettijohn 1975, Figure 3) to estimate the distance of sediment transport from the provenances, based on analyses of our 43 rock sections and 465 rock identification data from CNOOC Limited-Shanghai in LJS (10 boreholes).With regard to lower sequences (K 2 s-E 1 l 2 , see stratum symbols in Figure 1c), the results of 173 rock sections from the LSWS belong to the type of litharenite; the analyses of 27 sections from the LSES vary between litharenite and arkosic arenite.Furthermore, analyses of 300 rock sections of upper sequences (E 1 m 1 -E 2 w) from the whole Lishui sag accord with the litharenite type.In general, 54% of all rock sections yields relatively low compositional maturities (<0.5), and 33% of rock sections has low to medium compositional maturities (0.5-1.0).They are medium to poor clastic particle sorting, and subangular to subcircular psephicity.
It is known that unstable components are eliminated, while stable components (quartz detritus) gradually accumulate during sediment transport.The more rock fragments left in the rocks, the closer their distance from the provenances.The existence of Lingfeng barrier (K 2 s-E 1 l 2 ) subdivides the Lishui sag into two parts: LSWS and LSES.LSWS, which is adjacent to the huge magmatic provenance of the Zhemin swell, has enough sediments to be deposited.Whereas, sediments in the LSES are from a small magmatic provenance of the Yandang swell, which possibly contains recycled sediments of the Shimentan Fm.In context, sediments from the LSES fall between litharenite and arkosic arenite because they have more stable components than those from the LSWS (litharenite).Eventually, when the Lingfeng barrier disappeared, the Lishui sag developed as a full basin (E 1 m 1 -E 2 w), accepting proximal sediments mainly from the Zhemin and Yandang swells, forming the litharenite type.Both delta facies and litharenite type support that the samples in the LJS belong to proximal deposits, and the Zhemin and Yandang swells are their main provenances, partly with the Lingfeng barrier.Detrital zircon analysis of proximal sediments in the LJS enable to provide robust constraints on tectonomagmatic evolution that dominates these provenances.

Characteristics of sediment provenances
Bulk-rock elements of the rocks from sedimentary basins are commonly used to trace characteristics of the sediment provenances (Floyd et al. 1990;van Staden et al. 2006;Chakrabarti et al. 2009;Robertson and Palamakumbura 2019).Although Bhatia (1983) and Roser and Korsch (1986) designed major element schemes for provenance and tectonic discrimination, some uncertainties remain (Armstrong-Altrin and Verma 2005).Compositions of Al 2 O 3 and TiO 2 of parental rocks are retained in sediments owing to the low solubility of their oxides in aqueous solutions; thus, the covariation of TiO 2 and Al 2 O 3 can distinguish felsic, intermediate, and mafic sources (Hayashi et al. 1997).Trace elements are relatively less mobile than major elements, so they can yield more reliable discriminations (Ryan and Williams 2007).Floyd and Leveridge (1987) stressed the validity of Hf versus low and uniform La/Th ratios in distinguishing arc sources, as sediments that derived from igneous rocks in arc and active continental margin yield variable LILEs.La and Th are commonly high in silicic igneous rocks; they together with the immobile element Sc are used to reveal characteristics of the average source rocks (Cullers 1995).Bhatia and Crook (1986) proposed that La/Sc versus Ti/Zr can effectively distinguish tectonic settings owing to a systematic increase in light REEs in greywackes from oceanic island arc, continental island arc, and active continental margin to passive rifted margin.These discriminating diagrams are in general useable, despite some controversies (Ryan and Williams 2007).
Usually, the lower strata would accept upper materials from the provenances.Cretaceous felsic magmatism rose through the SE South China coastal areas, but a magmatic lull occurred at 86-50 Ma (Li et al. 2014a;Niu et al. 2015).Accordingly, Paleogene clastic samples from SW ECS could largely derivate from Cretaceous igneous suites (mainly 110-86 Ma; Li et al. 2014b).In this study, bulk-rock major elements (Si, Ti, and Al), trace elements (Th, Hf, and Zr), and REEs (La and Sc) from analyses of 101 sandstone samples were used to reveal provenance features (Table S2).
As shown in Figure 4a and b, almost all samples between J 1-2 f and E 1 l 2 are plotted on the felsic igneous provenance space.They yield high Al 2 O 3 /TiO 2 (16.7 to 76.1), low La/Th (0.78 to 4.5), and variable Hf contents (1.8 to 8.9 ppm).Meanwhile, these samples have affinities to granite gneiss sources, except for some from the LSES (E 1 y to E 1 l 2 ) belonging to granite gneiss and metabasite mixed sources (Figure 4c).They yield high La (6.6 to 39.7 ppm) and Th (3.0 to 14.7 ppm), but some from mixed sources are with low Sc contents (4.1 to 12.8 ppm).With disappearance of the Lingfeng barrier after the Early Paleocene, the entire LJS accepted sediment depositions (E 1 m 1 -E 2 w) sourced from a wide region.These clastic samples fall within a felsic field (Figure 4a) by Al 2 O 3 /TiO 2 (19.5 to 77.6), between felsic and mixed sources (Figure 4b) by variable Hf (1.1 to 12.0 ppm) versus low La/Th (0.73 to 4.2), and between granite gneiss and mixed sources (Figure 4c) by relatively high La (5.0 to 33.0 ppm), Th (2.2 to 12.9 ppm), and Sc (0.7 to 10.4 ppm); Jurassic to Cretaceous felsic suites within the Zhemin and Yushan swells are speculated to be major provenances for them, and the metabasite materials would come from Cretaceous basaltic rocks commonly occurring in coastal areas (Li et al. 2014b).
Based on La/Sc (1.7 to 12.4) versus Ti/Zr (9.1 to 32.1), Jurassic to Cretaceous samples (J 1-2 f-K 2 s) are dominantly plotted in the active continental margin setting (Figure 4d), which corresponds to the Izanagi subduction beneath East Asia (Wang et al. 2005;Li and Li 2007).Paleocene to Eocene samples (E 1 y-E 2 w) easterly from the Fuzhou sag and LSES have active continental margin and continental island arc affinities according to La/Sc (2.1 to 9.6) versus Ti/Zr (7.6 to 43.6) scheme, whereas those westerly from the LSWS are inversely plotted in the passive rifted margin shown by La/Sc (1.2 to 14.1) versus Ti/Zr (4.1 to 23.5) scheme.Both reveal a west-toeast tectonic difference in sediment provenances; Cretaceous extensional magmatism (110-85 Ma) dominates the Zhemin swell owing to slab rollback, while eastern sediment provenances are mostly correlated with arc and active continental margins.We also made sample discrimination by major elements following Roser and Korsch (1988) and Verma and Armstrong-Altrin ( 2013), but the results appear to be implausible.

Basement affinity of East China Sea with Cathaysia
The concept of Cathaysia Old Land was first presented by Grabau (1924).Guo et al. (1983) further considered the Cathaysian basement as a Hercynian to Indosinian geosynclinal fold belt.The term Cathaysia block is now commonly used.The Cathaysia block amalgamated with the Yangtze block, building the South China block by Neoproterozoic Jiangnan orogeny.Part of the landmass is assumed to be sunk into the East and South China Seas (Yu et al. 2006).The crustal thickness thinned eastward from the South China mainland to the Ryukyu Islands (Wei et al. 1990).Niu et al. (2015) considered the ECS region to be exotic that collided with the South China block at ca. 100 Ma.
Basements of the Cathaysia block principally contain Paleoproterozoic and Neoproterozoic rocks exposed in SW Zhejiang and north Fujian, while Mesoproterozoic rocks are limited to Hainan Island (e.g.Zhao and Cawood 2012).The Badu Complex outcropped in SW Zhejiang is the oldest suite found in the Cathaysia block, consisting of schist, paragneiss, and migmatite with minor amphibolite (Zhao and Cawood 2012).The amphibolite is dated at 1.9-1.8Ga (U-Pb zircon age); some granitoids in the Badu Complex yield ca. 1.9 Ga (Yu et al. 2008(Yu et al. , 2012;;Zhao and Cawood 2012).The Paleoproterozoic metamorphic and magmatic records of 1.9-1.8Ga are related to the growth of the Columbia supercontinent (Li et al. 2022).There is no Archean rock found in the Cathaysia block, but there are Archean detrital and inherited zircons (e.g.Yu et al. 2006;Zhao and Cawood 2012), in which the age spectrum of ca.2.5 Ga is prominent.The oldest rocks in Hainan include the amphibolite-facies Baoban Complex and greenschist-facies Shilu Group (ca.1.43 Ga; e.g.Yao et al. 2017); they formed a continental rift in relation to the break-up of the Nuna supercontinent (e.g.Yao et al. 2017).
The oldest gneiss from the West ECS was encountered by two boreholes, dated at ca. 1.85 Ga (Xu et al. 2023).It represents the Paleoproterozoic basement offshore, termed the Wendong Group, fairly comparable to the Badu Complex in the Cathaysia block.Detrital zircon age spectra of proximal deposition enable to provide robust constraints on basement formation and reworking.A total of 441 U-Pb age data of detrital zircons and igneous inherited zircons are plotted in the age spectra (Figure 5; see data in Table S3).These age data from the West ECS cover a wide range from the Paleoproterozoic to Triassic, and their five major tectonomagmatic events are ca.2.44 Ga (4.4% of all analyses), 1.85 Ga (25.1%), 780 Ma (10.6%), 442 Ma (5.9%), and 240 Ma (42.3%).All of them can find their sites in the age spectra of the Cathaysia block (Xu et al. 2023).
The age spectra of 1.93-1.75Ga (peak at ca. 1.85 Ga) in the West ECS are prominent, overlapping the age record of drilled Paleoproterozoic gneiss (Wendong Group).They represent the age of Paleoproterozoic basements in the West ECS and are comparable to the Paleoproterozoic Badu Complex, which is typical of Cathaysian basements.This is linked to the Columbia supercontinent assembly (Yu et al. 2012).Another Paleoproterozoic component from the West ECS represented by 2.50-2.38Ga also has its equivalent within Cathaysian basements.Zircon εHf(t) data of Wendong Group gneiss (1.85 Ga) and Paleoproterozoic detrital zircons (2.55-1.80Ga) from the West ECS (Xu et al. 2023) range from −13.4 to +7.2; comparably, the Paleoproterozoic Badu Complex gneiss (Yu et al. 2012;Zhao et al. 2015), granite (Liu et al. 2014), and detrital zircons (Xu et al. 2007;Lin et al. 2018) have a zircon εHf(t) range from −17.3 to +9.82 in the Cathaysia block.Both of them point to a unified Cathaysian basement; Hf model ages for them (3.50 to 2.42 Ga) correlate with crustal protolith extraction nominally from the depleted mantle (Xu et al. 2023).The age absence of 1.13-0.83Ga in the West ECS implies less influence by Jiangnan orogeny than the Cathaysia.
Reworking of the West ECS basements is recorded by three age spectra.Records of 830-740 Ma (peak at ca. 780 Ma) commonly occur in Cathaysia respond to a continental rift owing to the South China break-up after the Cathaysia-Yangtze collision (Xia et al. 2018).The 485-400 Ma age spectra (peak at ca. 442 Ma) in the West ECS accords with the Early Paleozoic Wuyi-Yunkai orogeny in association with the convergence between the Australia and South China blocks (Zhang et al. 2017).The 275-200 Ma spectra are shown most remarkably in the West ECS; they are the result of the Indosinian orogeny in the West ECS and Cathaysia block synchronously.This may be due to the south-directed subduction of the South China block (Faure et al. 2016).
In general, the West ECS and Cathaysia block have a unified basement, with the Paleoproterozoic Wendong Group and Badu Complex as the oldest units, dated at ca. 1.85 Ga together with 2.5 Ga age components.They consistently underwent tectonomagmatic reworkings by the break-up of South China (805-750 Ma), Wuyi-Yunkai orogeny (480-400 Ma), and Indosinian orogeny .The West ECS should be a seaward continuation of the Cathaysia block, not exotic.

Magmatic episodes and arc signatures
Mesozoic magmatic rocks have been increasingly drilled from the West ECS.Triassic granites (223- 206 Ma) and Jurassic and Cretaceous igneous rocks  from the Lishui sag were dated by U-Pb zircon (Guo et al. 2015;Yuan et al. 2018;Xu et al. 2023).Cretaceous deposition was drilled by borehole Ei4 (Xu et al. 2023) from the Lishui sag, and by boreholes TB13, 16s, 25s and 26s from the Fuzhou sag (Li et al. 2000;Si et al. 2021).The detrital zircons from the Lishui sag mainly provided U-Pb age components of 194-184, 148-133, and 119-98 Ma in association with magmatic episodes (Xu et al. 2017).Li et al. (2020b) proposed a Late Mesozoic continental magmatic arc in the ECS based on detrital zircon data from the Xihu sag.S3).The results demonstrate that the magmatism occurred abundantly in the Early Cretaceous but relatively less in the Jurassic.Jurassic zircons have lower Th/U ratios than Cretaceous zircons (Figure 6c).The probabilistic histogram of detrital zircon data covers two major magmatic episodes of 150-124 Ma (peak at 132 Ma) and 124-102 Ma (peak at 112 Ma), followed by minor episodes of 102-86 and 200-150 Ma, which can be subdivided into 200-170 and 170-150 Ma (Xu et al. 2017).
The magmatic records of two subsags seem to be comparable but not identical in terms of episodes, as their provenances sit on different sides.We contrast differences of Late Mesozoic detrital zircon age distribution from two subsags to reveal their magmatic episodes (Figure 7).Before the Lingfeng barrier was immerged, the lower sequences (K 2 s-E 1 l 2 ) provide 472 age data from the LSWS (5 boreholes) and 246 age data from the LSES (3 boreholes), as plotted in Figure 7b.They indicate different magmatic episodes between two subsags during 200-124 Ma, but the episode of 124-86 Ma remains comparable.Owing to existence of the Lingfeng barrier, the magmatic records of 200-150 Ma in the LSES were sourced dominantly from the Yandang swell in the east, while the magmatic zircons of 150-124 Ma abundantly in the LSWS were supplied from the Zhemin swell in the west.After disappearance of the Lingfeng barrier, the upper sequences (E 1 m 1 -E 2 w) provide 955 age analyses from the LSWS (5 boreholes) and 430 age analyses from the LSES (3 boreholes), as plotted in Figure 7a.Overall, they demonstrate comparable magmatic episodes between two subsags, including 200-150, 150-124, 124-102, and 102-86 Ma; but the first and last components are relatively minor.The disappearance of the Lingfeng barrier causes mixed sediment provenances from the Yandang and Zhemin swells, making the component of 170-124 Ma from different in lower sequences to be unified in the upper sequences.In short, five magmatic episodes  for all sequences feature both the LSWS and LSES.The zircon components of 200-170 Ma may primarily come from the Yandang swell, but the components of 150-124 Ma may mainly come from the Zhemin swell.

Zircon signatures of magmatic arc
The elements of arc magma are inseparably connected to subduction-dehydrated fluids in the subduction factory (Rüpke et al. 2004;Grove et al. 2012).Slab-dehydrated fluids are key components that form arc magmas in subduction zones.Dehydration of oceanic slab in the ocean-to-land subduction zone enters the mantle wedge causing fluid-present partial melting of the mantle, followed by formation of arc basaltic magma.Much of basaltic magma ascends, and underplates at the Moho level (Fyfe 1992).Basaltic magma underplating leads to extensive partial melting of the crust, hybridization of mafic and felsic magmas, generating abundant arc granitic melts (Annen et al. 2006).Arc-related melts are enriched in fluid-mobile LILEs and light REEs compared to fluid-immobile HFSEs (Ionov and Hofmann 1995).Enrichments in LILEs reflect addition of slabderived components; however, HFSE depletion is attributed to residual Ti minerals in the slab (Kelemen et al. 2014).As detrital zircons for our samples are dominantly derived from the granitoids, they have compositional characteristics that potentially indicate the environment of granitic magmatism.
Zircon trace element geochemistry is sensitive to changes in melt composition based on the control of various melt fractionations on zircon growth (Barth et al. 2013).Owing to zircon affinity and element substitution, HFSEs such as Hf, Y, and P, REEs, and LILEs such as U and Th, are mostly abundant in zircons (Hoskin and Schaltegger 2003).To constrain Late Mesozoic magmatism in the West ECS, 10 trace elements and 14 REEs were measured from 2,103 dated detrital igneous zircons in this study (Figure 6d, Table S4).Mostly enriched elements in zircons are Hf (6,450 to 14,797 ppm), Y (146.7 to 2,625 ppm), and P (44.4 to 1,088 ppm), whereas REEs have wide ranges of content.LILEs such as Th (25.2 to 541.6 ppm) and U (21.0 to 610.7 ppm) are also enriched.However, HFSEs, such as Ti (0.14 to 16.8 ppm) and Nb (0.33 to 8.8 ppm), are relatively depleted.The enrichment of LILEs (i.e.U and Th) and depletion of HFSEs (i.e.Nb and Ti) are the characteristics of magmatic arcrelated zircons.
Trace elements in zircon occur as part of melt composition; thus, they can be used to discriminate tectonomagmatic settings.Grimes et al. (2015) globally combined U, Yb, Nb, and Sc ratios of zircons for discrimination of continental arcs from ocean island and midocean ridge environments.Detrital igneous zircons from the SW ECS fall into the continental arc field in plots of log 10 (U/Yb), log 10 (Sc/Yb), and log 10 (Nb/Yb) (Figure 8 a and b).They have the arc-related characteristics of high U/Yb (0.11 to 7.31), low Nb/Yb (0.002 to 0.064), and high Sc/Yb (0.21 to 5.87) ratios.The decrease in log 10 (U/Yb) from 200-150 to 150-86 Ma is distinct.Compositions of low Nb/Hf and high Th/Nb ratios in zircons from arcrelated magmas distinguished from those in withinplate setting; continental contamination increases Th/ Nb ratios and decreases Hf/Th ratios of zircons from within-plate magmas (Yang et al. 2012c).Zircons from the SW ECS are plotted in arc-related/orogenic tectonic fields, as shown in the diagram of Th/U versus Nb/Hf (Figure 8c); they are characterized by depleted HFSE Nb (0.33 to 8.84 ppm), low Nb/Hf (2.98 × 10 −5 to 3.06 × 10 −3 ), and high Th/U (0.10 to 4.05).The distinct increase in Th/U from 200-150 Ma to 150-86 Ma is shown.The zircons from the SW ECS also have high Th/Nb (5.71 to 817.6), indicating increasing continental contamination in magmas.Grimes et al. (2007) found that plots of Hf or Y versus U/Yb could effectively discriminate continental zircons from oceanic type.Almost all zircons from the SW ECS are continental zircons (Figure 8d).U/Yb in zircons decrease with an increase in Y (146.7 to 2625 ppm) through the Late Mesozoic.Overall, it has a trend of migration from continental zircon to oceanic type from the Jurassic to Cretaceous.
Dehydration of subduction slabs for arc magma generation occurs at depths of ca. 100 ± 20 km (Grove et al. 2009 and references in it), mainly through conversion of amphibolite facies into eclogite facies (Stern 2002).U, as a fluid-mobile element, is released into arc magmas, as confirmed by its high depletion in eclogite (Cannaò and Malaspina 2018).Therefore, U should be enriched in arc magmas as an indicator of subduction fluid input.The lower continental crust is generally intermediate (Rudnick and Gao 2014), with a high Th/U ratio of approximately 6 (1.2 ppm Th and 0.2 ppm U).Following Yakymchuk et al. (2018), melts of crustal metapelite yield a high Th/U range from 1 to 4. Thus, Th/U ratios enable to reveal a variety of crustal involvements in parental magmas in association with tectonic environment.Zircon largely incorporates U and Th from melts owing to its high affinities for them (Bea 1996;Grimes et al. 2007); therefore, U and Th are widely used to reveal related magmatism and tectonism.
As Yb is incompatible in arc melts, it can reduce deviations of elements in various settings by normalizing elements to Yb (Grimes et al. 2015).To further investigate variation in subduction fluid input, the log 10 (U/Yb) with age for detrital igneous zircons reduced significantly by stages in the SW ECS (Figure 9a).To indicate crustal involvement in magmas, the Th/ U changes of detrital igneous zircons are investigated (Figure 9b).Low Th/U values occur to magmatic episodes at 200-170 Ma (0.14 to 1.28) and 170-150 Ma (0.19 to 1.68), but they become variously high at 150-124 Ma (0.10 to 2.95), 124-102 Ma (0.20 to 4.05), and 102-86 Ma (0.35 to 2.55), corresponding to an increased addition of crustal components in magmas from the Jurassic to Cretaceous.McKay et al. (2018) suggested elevated Th/U ratios of zircons (>1.0) with a wide range in association with extensional magmatism, so that the Cretaceous magmatism (124-102 Ma) is related to the control of extensional tectonism, possibly by arc-root removal.Zircon crystallization temperature (T Ti-in-zircon ) can be calculated from Ti content in zircons, which has been widely used as a single mineral trace element thermometer (Watson et al. 2006).Figure 9d shows T Ti-in-zircon calculations of detrital igneous zircons.T Ti-in-zircon is relatively low in magmatic episodes at 200-170 Ma (median 678°C) and 170-150 Ma (median 668°C), followed by elevated temperatures at 150-124 Ma (median 704°C), 124-102 Ma (median 693°C), and 102-86 Ma (median 718°C).Miller et al. (2003) defined types of hot (>800°C) and cold (<800°C) granites: hot granites mostly form in (transitional) extensional tectonism, while cold granites reflect compression and crustal thickening settings.T Ti-in-zircon variation coupled with Th/U ratios in the West ECS reveals a change from Jurassic compressional arc to Cretaceous extensional magmatism with an increased addition of crustal fluid-fluxed melts.
In summary, detrital igneous zircons from the SW ECS are formed in continental magmatic arc, registered by elements and ratios of U, Sc, Yb, Th, Nb, and Hf.The swells of Yandang and Zhemin, as main provenances of the LJS, constitute parts of the Late Mesozoic magmatic arc.It accords with part of continental magmatic arc, as revealed by detrital zircons from the Xihu and Fuzhou sags (Li et al. 2020b;Si et al. 2021).In region, the swells of Hupijiao, Haijiao, Yushan, and Zhemin in the West ECS outline a Late Mesozoic magmatic arc zone, controlled by the Izanagi subduction beneath South China.The oceanic slab subducted in the Jurassic, then steepened and rolled back in the Cretaceous.As a consequence, a Jurassic magmatic arc by slab subduction in the West ECS contrasts Cretaceous extensional magmatism by arc-root removal.

Slab subduction models
Many concerns have focused on the Late Mesozoic models of the Izanagi subduction beneath East Asia in the past decades.The representative models include: 1) Zhou and Li (2000) used a dip-increased slab subduction to explain Jurassic to Cretaceous eastward magmatic migration in South China; 2) Li and Li (2007) proposed a flat-subduction model since the Permian, and the slab foundered and rolled back in the Jurassic causing vast magmatism in South China; 3) Wang et al. (2013) suggested a post-Indosinian orogenic collapse in response to Early-to-Middle Jurassic back-arc extension in SE South China, and a latest Jurassic to earliest Cretaceous transpression due to paleo-Pacific oblique subduction, followed by a slab retreat and Cretaceous coastal magmatism; 4) Suo et al. (2019) supported a model of Late Mesozoic Andean-type continental margin in South China, replaced by the Late Cretaceous western Pacifictype continental margin; 5) Li et al. (2018aLi et al. ( , 2020a) also considered a paleo-Pacific flat subduction in the Jurassic, and slab sank into the mantle in the Early Cretaceous with break-off and rollback, followed by crustal contraction and the Late Cretaceous magmatic lull; 6) Niu et al. (2015) used a non-Andean-type continental margin and an exotic terrane to explain the Late Mesozoic geology in SE South China; the vast granitoid generation of 190-90 Ma inland was caused by dehydration of the stagnant paleo-Pacific slab in the mantle transition zone.These models are mostly concerned about onshore geology, while the records of offshore ECS, as a key connection between South China and the Pacific domains, are less attended.
The Pacific plate growth continued outwards from the Izanagi-Farallon-Phoenix triple junction since the Early Jurassic (ca. 190 Ma;Nakanishi and Winterer 1998;Boschman and van Hinsbergen 2016).This process drove subduction of the Izanagi slab beneath East Asia, and the Izanagi met its demise in the Early Paleocene (ca.60 Ma; Müller et al. 2016).The swells of Yushan and Zhemin together with Haijiao and Hupijiao outline a Late Mesozoic magmatic arc through the West ECS (Li et al. 2020b), as registered by detrital zircon and bulk-rock data.Jurassic to Cretaceous fluvial, lacustrine, and delta sequences in the East ECS constitute the forearc depositions, covering the Fuzhou, Xiamen, Yushan, and Minjiang Fms (Zheng et al. 2005;Yang et al. 2012a).Remnants of subduction-related accretionary complexes from SW Japan and eastern Taiwan Island to central Indonesia spatially outline a Late Mesozoic trench (Maruyama et al. 1997;Wakita and Metcalfe 2005;Isozaki et al. 2010;Yui et al. 2012).Late Mesozoic vast felsic magmatism that dominated SE South China became younger coastward from the Jurassic to Cretaceous (Zhou and Li 2000), which are associated with the lithospheric extensional environments (Li et al. 2007;Wang et al. 2013).Therefore, units of SW Japan to eastern Taiwan Island subduction complexes, East ECS forearc, West ECS magmatic arc, and intraplate lithospheric extension in SE South China, form a Late Mesozoic 'trench-forearc-arc-intraplate' architecture responding to Izanagi subduction beneath East Asia (Figure 10; Xu et al. 2023).Observations and data indicate interactions of arc, intraplate, and subduction as follows, as well as tectonomagmatic episodes.(2) During the Middle to Late Jurassic (170-150 Ma), the NW subduction of Izanagi slab beneath East Asia was enhanced (ca.11 cm/yr; Müller et al. 2016); arc-related magmatism occurred in the SW ECS, but voluminous granites dominated the Cathaysia interior (Zhou and Li 2000).Prior to the Paleocene grabens, sediment provenances such as the Yushan and Zhemin swells allied with the Pingtan-Dongshan metamorphic zone (Zhao et al. 2007b) are arc-related, but not activated as the main arc locus when compared to the Haijiao swell in the east (Li et al. 2020b).The Pingtan-Dongshan metamorphic zone formed as a largescale ductile shear zone between the West ECS magmatic arc and SE South China intraplate (Wang and Lu 2000;Liu et al. 2012).When the slab subducted deeply and entered the mantle transition zone, the increase in viscosity from the upper to the lower mantle sufficed to stagnate the subduction slab (Huang and Zhao 2006;Goes et al. 2017;Zhao 2017).The slab dehydrated in the mantle transition zone leading to a large-scale upwelling of hot melt plumes in the asthenosphere (Zhao et al. 2007a).Mantle convection below the Cathaysia block would intermittently remove the lithospheric root, and cause mafic underplate and abundant magmatism in the Cathaysia interior.(3) In the Early Cretaceous (150-102 Ma), arc-related magmatism started to develop in the SW ECS, and abundant igneous rocks occurred along the Cathaysia coast (Zhou and Li 2000).We consider that slow subduction could force stagnated slab rollback (150-124 Ma), and the slab continually dehydrated in the mantle transition zone.This dynamic tended to decouple the subducted slab from the overlying plate and promoted upwelling of melt plumes, generating arc-related magmatism in the SW ECS.

Summary
(1) Sandstones from the LJS in delta facies are typical proximal deposits.They, coupled with the main provenances of Zhemin and Yandang swells, constitute the source-to-sink system.Sediment provenances at the active continental margin supply Late Mesozoic felsic materials with minor metabasite for the LJS.

Figure 1 .
Figure 1.(a) Tectonic units of the ECS, along with sampling borehole locations.(b) A seismic section showing representative graben structures filled by tertiary sequences across the southern ECS.(c) Simplified stratigraphic chart (modified from CNOOC Limited-Shanghai) and sampling positions.Sketches of sedimentary facies for (d) Upper Lingfeng Fm and (e) Upper Mingyuefeng Fm in the Lishui sag (modified from Chen 2005).

Figure 3 .
Figure 3. Quartz-Feldspar-rock fragment characteristics of clastic rocks from the Lishui sag (base map after Pettijohn 1975, colour samples from this study, grey samples from CNOOC Limited-Shanghai.).

Figure 5 .
Figure 5. Probabilistic histogram of U-Pb zircon ages revealing evolved basements.U-Pb (<1000 Ma) and Pb-Pb (≥1000 Ma) age data (with concordance ≥90%) of detrital and inherited zircons from the West ECS are comparable to U-Pb zircon age spectra of the Cathaysia block (Xu et al. 2023).
zircon dating and trace element analyses are performed on 41 sandstones sampled from 11 boreholes in the SW ECS.Within analyses of 2,731 detrital zircons, those with oscillatory zones represent the products of magmatism.There are 2,103 detrital igneous zircons (Th/ U>0.1), with 8.8% Jurassic (200-145 Ma) and 91.2% Cretaceous (145-85 Ma; Figure 6 a and b; Table

Figure 6 .
Figure 6.(a) Concordia plots of LA-ICP-MS U-Pb data of detrital igneous zircons from the West ECS (with concordance ≥90%), coupled with representative cathodoluminescence images, showing the Late Mesozoic crystallization ages of magmatism.(b) Probabilistic histogram of detrital zircon ages.(c) Th/U ratios of detrital igneous zircons (base map after Hoskin and Schaltegger 2003).(d) Trace element contents of Jurassic to Cretaceous detrital igneous zircons (data >1.5 times IQR were removed).
It has high log 10 (U/Yb) values in magmatic episodes of 200-170 and 170-150 Ma, whereas the relatively low values are shown in the episodes of 150-124, 124-102, and 102-86 Ma.Moreover, from 200-150 to 150-86 Ma, the decrease of average U content from 303.8 to 230.5 ppm reveals that addition of slab-derived fluids in the parental magmas would prominently reduce.Various U/Yb in 124-102 Ma may reveal changes in subduction situation.Sc is considered to be related to subduction fluid input.It has a high distribution coefficient in amphibole from the subduction slab, which is involved in calc-alkaline magmas formed by slab dehydration; therefore, crystallized zircons from arc magmas have high Sc concentrations (Grimes et al. 2015).As shown in Figure 9c, high log 10 (Sc/Yb) values in zircon are recorded in magmatic episodes of 200-170 and 170-150 Ma, but they decreased in 150-124, 124-102, and 102-86 Ma.Sc/Yb variations point to prominent changes in subduction regimes at 124-86 Ma.In summary, reduction in log 10 (U/Yb) and log 10 (Sc/Yb) of zircons over time likely indicates a decline of subduction fluid inputs in melts from the Jurassic to Cretaceous, revealing the Cretaceous slab steepening and rollback in the West ECS.

( 1 )
Succeeding a strike-slip boundary (200-180 Ma), the Izanagi slab transferred to westerly subduction beneath East Asia (180-170 Ma; Müller et al. 2016).The subduction dynamics triggered an oblique strike-slipping along the South China margin, registered by the Changle-Nanao and Guangning-Bobai dextral strike-slip fault zones (e.g.Xu et al. 2017).It resulted in some arc-related magmatism (mostly <700°C) in the SW ECS, southerly covering the SE Taiwan Island (Yui et al. 2009, 2017) and the NE South China Sea (Xu et al. 2017), regionally defining an Early Jurassic low-T magmatic arc zone.Continuous subduction in the West ECS and dehydration of the slab resulted in enriched fluid mobile elements in arc magmatism.Most of the Early Jurassic igneous rocks in the Cathaysia block form a WNW-ESE-trending rift-related Ningyuan-Fankeng igneous zone between the Wuyishan and Nanling-Yunkai terranes as a result of slab strikeslip reactivation (Xu et al. 2017).

Figure 10 .
Figure 10.A cartoon illustrating an evolved Late Mesozoic trench-arc-intraplate architecture in South China continental margin, controlled by the Izanagi subduction.
(2) Detrital zircon data from the LJS reveal a Late Mesozoic low-T evolved magmatic arc, well developed in 150-102 Ma.Zircon element variations in the Jurassic to Cretaceous arc magmatism reveal a decline in slab-dehydrated fluid input due to slab rollback and an increase in fluid-fluxed crustal materials due to arc-root removal.(3)A residual Late Mesozoic magmatic arc is outlined by the swells of Yushan, Zhemin, Haijiao, and Hupijiao in the West ECS, which together with the SE South China intraplate and subduction complexes in SW Japan to Taiwan Island, spatially formed a trench-arc-intraplate architecture due to the Izanagi subduction beneath East Asia.It evolved from slab strike-slip subduction (200-170 Ma), slab stagnation and intraplate foundering (170-150 Ma), slab rollback and arc-root removal (150-102 Ma), to arc migration trenchward (102-86 Ma).(4) Zircon data support formation of the West ECS basement at ca. 1.85 Ga (Wendong Group) in accordance with the Cathaysian basement.Reworkings of the unified Cathaysia block include the break-up of South China (805-750 Ma), Wuyi-Yunkai orogeny (480-400 Ma), and Indosinian intracontinental orogeny (250-200 Ma).
However, slab rollback decreased addition of dehydrated fluids in arc magmas.With the subduction of Izanagi slab accelerated in 124-102 Ma (ca.13.0-14.8cm/yr; Müller et al. 2016), slab rollback and mantle convection intensified, the thickened lithospheric arcroot was detached.It resulted in the SW ECS extension and vast generation of arc-related magmatism, largely with lower crust components.Synchronously, the lithospheric-root removal caused coastal abundant magmatism in the Cathaysia due to mafic underplate.(4) In the Late Cretaceous (102-86 Ma), the subduction of Izanagi beneath East Asia further accelerated (ca.19.5 cm/yr; Müller et al. 2016) and slab rollback continued.The related magmatic arc migrated trenchward (Chen et al. 2016b).Meanwhile, coastal SE South China occurred as a backarc domain (Zhu and Xu 2019).