Early Paleozoic arc-back-arc system evolution in the junction of the Qinling and Qilian Orogens: Geochemical constraints from ca. 445–430 Ma magmatic rocks in the Tianshui area

ABSTRACT The Tianshui area is located at the junction of the Qinling and Qilian Orogens, the evolution of which is crucial in understanding the Early Palaeozoic tectonic evolution of the Central China Orogenic System. We present here geochronological, geochemical, and Sr-Nd isotopic data for four representative Silurian magmatic rocks, termed the Dangchuan, Huamiao, Yanjiadian, and Hongtupu, along a cross section across the main rock units of the Tianhui area. The 432 Ma Dangchuan syenogranites are peraluminous with negative εNd(t) values, high initial 87Sr/86Sr ratios, and low Rb/Sr and high CaO/Na2O ratios. We interpret the syenogranites to be generated by partial melting of metagreywackes during a Silurian orogenic event. The 436 Ma Huamiao diorites are high-Mg diorites with high MgO contents and Mg# (63–65) and enriched Sr-Nd isotopic compositions. The 445 Ma Yanjiadian diorites are high-Mg adakitic rocks with high Sr and Mg# and low Y and Yb contents. They were both generated by the interaction between sediment/slab-derived fluids/melts and mantle peridotite in the subduction-related setting. The 445 Ma Hongtupu basalts include E-MORB and IAB with depleted Sr-Nd isotopic compositions. They were derived from enriched mantle and sediment-modified mantle, respectively, and formed in a back-arc basin setting. Synthesizing our new and published data, we propose a tectonic model involving generation of Baihua-Huluhe arc-back-arc system, subduction of the Huluhe back-arc oceanic basin, and collision between the North Qinling arc terrane and North China Block, which is closely associated with the closure of the Proto-Tethys Ocean.


Introduction
The Central China Orogenic System (CCOS) is a > 5000 km, ~E-W trending orogenic belt that separates the northern (e.g. North China and Tarim Blocks) and southern blocks (e.g. South China and Qiangtang Blocks) of China. It consists mainly of the Qinling-Tongbai-Dabie, Qilian, East Kunlun, and West Kunlun Orogens from east to west ( Figure 1) Dong et al. 2019). The CCOS experienced a long-lived (>600 Ma), complicated orogenic history during the Neoproterozoic to Indosinian periods (Dong et al. 2021). In the Palaeozoic, the tectonic evolution of North Qinling, Qilian, and Kunlun involved the generation of multiple arc-backarc systems and arc/continent-continent collision (Dong et al. 2011bSong et al. 2013;Zhang et al. 2015;Dong and Santosh 2016), which were closely associated with the evolution of the Proto-Tethys Ocean and assembly of the supercontinent Gondwana Li et al. 2018a). Among them, the North Qinling and Qilian Orogens, extending for ~2000 km, were spatially connected and genetically concomitant (e.g. Song et al. 2017). Their spatiotemporal relationship and evolutionary processes are crucial in unravelling the amalgamation of Chinese mainland and Gondwana.
The Tianshui area is located at the junction of the North Qinling and Qilian Orogens, an ideal region to address their spatiotemporal relationship and evolutionary history. In this area, Early Palaeozoic magmatic rocks are widespread (Figure 1). Previous studies reported preliminary geochemical and geochronological data for these magmatic rocks, and most of them were interpreted as subduction/collision-related rocks (e.g. Pei et al. 2006Pei et al. , 2007aZhang et al. 2006;Li et al. 2007Li et al. , 2018bYan et al. 2007;He et al. 2007a;Dong et al. 2011a;Gao et al. 2012;Ren et al. 2018;Yang et al. 2018a). On the basis of their geochemical features and petrogenesis, Xu et al. (2008) and Pei et al. (2009) had constructed the tectonic framework of the Tianshui area. They identified two oceanic basins (i.e. the Wushan-Guanzizhen -hereafter referred to as Wuguan -and Huluhe oceans) separated by the North Qinling Terrane (NQT), associated arc-back-arc systems, and a Silurian orogenic event. Despite this, at least three issues about the evolution of this area remain unaddressed: (1) What was the subduction polarity of the Huluhe ocean, double-sided  or southward subduction )? (2) Which ocean closure caused the Silurian orogenic event, Wuguan, Huluhe, or both? (3) How was the orogenic event associated with the evolution of Gondwana?
To address these questions, four representative Silurian volcanic and intrusive rocks -termed the Huamiao, Dangchuan, Hongtupu, and Yanjiadian from south to north -were collected along a cross section across the main rock units of the Tianshui area ( Figure 1). We present zircon U-Pb ages, whole-rock elements, and Sr-Nd isotopes for these magmatic rocks, constraining their crystallization ages, petrogenesis, and tectonic implications. Combined with published sedimentary, structural, geochemical, and geochronological data, we reconstruct the tectonic framework and evolutionary history of the Tianshui area. Finally, we discuss its tectonic implications in the context of the Early Palaeozoic tectonic evolution of eastern Gondwana.

Geological background
In this study, we divided the Tianshui area into southern magmatic complex belt (SMCB) and northern magmatic complex belt (NMCB) that were separated by the Xinyang-Yuanlong ductile shear zone   Wang et al. (2008) ( Figure 1). These two belts contain Proterozoic strata, Palaeozoic stratigraphic sequences, and voluminous Palaeozoic plutons.

Precambrian strata
The Precambrian strata of the SMCB is the Qinling Complex (map symbol Pt q ; Figure 2), which is mainly composed of garnet-sillimanite gneisses, amphibolebearing two-pyroxene granulites, marbles, amphibolites, and some gneissic granites (e.g. Mao et al. 2017). Detrital zircon ages of metasedimentary rocks suggested that the complex was formed during the Meso-to Neoproterozoic, representing the basement of the NQT (Diwu et al. 2014;Shi et al. 2018). Paragneisses of the Qinling Complex underwent amphibolite to granulite facies metamorphism at ~433−424 Ma (LA-ICPMS and SHRIMP U-Pb ages on metamorphic zircons and monazites; Mao et al. 2017) with peak metamorphic P-T conditions of ~793−803°C and ~8.8−9.5 kbar (phase equilibria modelling; Mao et al. 2018).

Palaeozoic stratigraphic sequences
The Palaeozoic strata include the Ordovician volcanosedimentary sequences and Silurian-Devonian terrestrial clastic sedimentary sequences.
Volcano-sedimentary sequences include the Liziyuan and Caotangou Groups ( Figure 2). The Liziyuan Group (O lz ) comprises greenschist-facies metamorphosed clastic sedimentary successions, carbonates, and minor intermediate to mafic volcanic rocks and boninites. They were deposited at ca. 451 Ma in the forearc setting (Ding et al. 2004;Yang et al. 2018a). Within the Liziyuan Group, Yang et al. (2018b) identified ca. 384 Ma HP granulites with peak P-T conditions of 757−792°C and 1.3−1.5 GPa (traditional geothermobarometer). The Caotangou Group (O c ) consists of sub-greenschist-facies metamorphosed clastic and volcanic sequences. They were interpreted as arc rock assemblages with zircon U-Pb ages of 472−456 Ma (Sun and Dong 1995;Wang et al. 2007;Yan et al. 2007).

Mafic to ultramafic magmatic suites
Several ophiolite suites discontinuously crop out along the southern margin of the SMCB, termed the Wushan (Yuanyangzhen), Guanzizhen and Yanwan (Yinggezui) ophiolites ( Figure 1). They consist mainly of serpentinized dunites, peridotites, (pillow) basalts, (cumulate) gabbros, (oceanic) plagiogranites, and cherts. Two groups (N-MORB-and E-MORB-type) can be classified on the basis of their distinct geochemical features. The Guanzizhen ophiolite suite is a typical N-MORB-type ophiolite (Pei et al. 2007a;Dong et al. 2011a). SHRIMP and LA-ICP-MS zircon U-Pb dating of gabbros and plagiogranites gave ages of ~534-471 Ma for the suite (Yang et al. 2006(Yang et al. , 2018aLi et al. 2007;Pei et al. 2007a). The Wushan and Yanwan ophiolites exhibit E-MORB-like features with zircon U-Pb ages of ~524-474 Ma (Chen et al. 2008c;Dong et al. 2011a). The two above-stated groups of ophiolites represented remnants of the oceanic lithosphere, which might be the westward extension of the Shangdan ocean of the East Qinling Orogen (Pei et al. 2007a;Dong et al. 2011a).
In addition to ophiolite suites, the Muqitan amphibolites, consisting mainly of metamorphosed basalts and minor gabbros and cherts, crop out in the Muqitan areas (Figures 1 and 2). The basalts exhibit E-MORB-like geochemical features with crystallization ages of ~763 Ma (Zhang et al. 2011).

Palaeozoic igneous complexes and granitoids
Two Early Palaeozoic igneous complexes (Liushuigou and Baihua) crop out in the SMCB. The Liushuigou complex is mainly composed of metamorphosed gabbros, gabbroic diorites, and diorites. LA-ICP-MS and TIMS zircon U-Pb dating gave ages of ~549-508 Ma (Pei et al. 2005;Gao et al. 2012). Together with the Guanzizhen ophiolites, they constituted an ophiolite complex (Yang et al. 2018a). The Baihua complex consists mainly of metamorphosed pyroxenites, gabbros, diorites, and quartz diorites. U-Pb dating of zircons from gabbros and quartz diorites yielded ages of ~450-435 Ma (Pei et al. 2005(Pei et al. , 2007c. The complex represented arc magmatism in response to the subduction of the oceanic lithosphere (Pei et al. 2005(Pei et al. , 2007c. Palaeozoic granitic rocks range in ages from 455 Ma to 411 Ma and vary in compositions from adakitic, I-to A-type granites. Adakitic granites can be subdivided into Na-and K-rich rocks. The Na-rich adakitic rocks, dated at ~455 Ma, were interpreted to originate from partial melting of the subducting oceanic slab in an arcrelated setting (Chen et al. 2008a(Chen et al. , 2008b. The K-rich adakitic rocks, with ages ranging from 438 Ma to 434 Ma, were suggested to be derived from partial melting of mafic lower crust Wang et al. 2008). I-type granites, formed at 451-449 Ma, were interpreted as arc-related magmas in response to the subducting oceanic lithosphere (Wang et al. 2006a;Yao et al. 2017;Ren et al. 2018). A-type granites, yielding zircon U-Pb ages of 414-411 Ma, were considered as extensionrelated magmas, which were formed due to the tectonic collapse of thickened crust (Wang 2013;Xu et al. 2017;Ren et al. 2021).

Precambrian strata
The Precambrian strata of the NMCB are the Longshan Group/Complex (Pt l ; Figure 3). The Longshan Group/Complex is an assemblage of metamorphosed volcanic and intrusive complexes and metasedimentary rocks, comprising granitic gneisses, amphibolites, aluminium gneisses, and marbles. LA-ICP-MS zircon U-Pb dating of tonalite gneisses indicated that they were predominantly formed at ~2.5 Ga and underwent Palaeoproterozoic (~1.9 Ga) metamorphism, consistent with the temporal evolution of the basement of the NCB (He et al. 2005). The Longshan Group/ Complex represented a basement exposing along the southwestern margin of the NCB.

Early Palaeozoic stratigraphic sequences
Palaeozoic stratigraphic sequences include the Chenjiahe Group (O-S ch ), Hongtupu Formation (Є-S ht ) and Huluhe Group (S hl ) from north to south (Figure 3). The Chenjiahe Group, dated at 462-443 Ma, is a volcanosedimentary unit comprising sub-greenschist-facies metamorphosed andesites, dacites, rhyolites, tuffs, sandstones, and siltstones. They exhibited calc-alkaline affinity and were interpreted to be formed in a continental arc setting (He et al. 2007a;Li et al. 2018b). The Hongtupu Formation mainly comprises sub-greenschist-  Li (2008) facies metamorphosed (pillow) basalts, basaltic andesites, and minor dolerite dikes and siliceous rocks. The basalts and dolerite dikes were suggested to be formed in a back-arc basin during ~500-438 Ma (He et al. 2007a, b;Li et al. 2018b;Fu et al. 2019). The Huluhe Group is a metamorphosed flysch sequence, which is composed of biotite/two-mica quartz schists, phyllites, and quartzites. The youngest age group of detrital zircons from biotite quartz schists is 443-434 Ma, indicating a Silurian depositional age . These lithological units were separated from each other by either brittle faults or ductile shear zones.

Early Palaeozoic magmatic rocks
One ophiolite suite, termed the Huluhe ophiolite, is exposed in the central NMCB ( Figure 1). It is mainly composed of peridotites, olivine pyroxenites, gabbros, and basalts interbedded by minor cherts and siliceous limestones with the latest Neoproterozoic to Cambrian fossils. The ophiolite suite was interpreted to be formed in a back-arc basin (Zhang et al. 2004). To the north of the Huluhe ophiolite suite, several Early Palaeozoic (457-440 Ma) intermediate-felsic plutons crop out ( Figure 3) Chen et al. 2007;Pei et al. 2007d;Wei et al. 2012). All of them share geochemical characteristics of I-type granites, interpreted as arc magmatism due to the subduction of the oceanic plate Chen et al. 2007;Wei et al. 2012). Mafic rocks, dated at 453-440 Ma, intruded into the Longshan Group/ Complex ( Figure 3) and exhibited geochemical characteristics of enriched mantle, representing extensionrelated magmas generated in a rifting setting He et al. 2006;Meng 2017).
The Hongtupu volcanic samples were collected from the ~NW-trending Hongtupu Formation (Figures 2 and  3). They are massive basalts exhibiting porphyritic texture: the phenocrysts are dominated by plagioclase and clinopyroxene; the matrix consists mainly of plagioclase, clinopyroxene, and secondary chlorite and epidote (Figure 4g-h).

Analytical methods
Fresh rock samples were crushed in a specially designed steel crusher and then powdered in an agate mill to a grain size of 200 mesh. Geochemical analysis was carried out at the Wuhan Institute of Geology and Resources, Wuhan, China. Major elements -including loss on ignition (LOI) -were measured by X-ray fluorescence, with analytical uncertainties <0.5%. Trace elements, including rare earth elements (REEs), were measured using ICP-MS with analytical uncertainties <5% for elements with an abundance >10 ppm, and 5-10% for those <10 ppm.
Zircons for U-Pb dating were separated using conventional heavy liquid and magnetic techniques. They were mounted in epoxy and polished to about half thickness. Cathodoluminescence (CL) images were obtained using a JEOL scanning electron microscope to reveal the internal structures. In situ zircon U-Pb dating was performed using laser ablation (LA)-MC-ICP-MS, attached to a New Wave UP193FX ArF Excimer laser ablation system, at the Tianjin Institute of Geology and Mineral Resources. Laser sampling was conducted using a spot size of 35 μm, at a repetition rate of 8-10 Hz and an energy density of 7-8 J/cm 2 . Two standard zircons (GJ-1 and 91,500) were used to calibrate the U-Th-Pb ratios and absolute U abundance. The detailed analytical techniques and operating conditions have been described by . All age calculations and Concordia-diagram plots were made using ISOPLOT (ver 3.0) (Ludwig 2003). Uncertainties on single analyses are reported at the 1σ level. Uncertainties on the weighted mean ages are reported at the 2σ level.
For the analysis of Rb-Sr and Sm-Nd isotopes, 50-200 mg sample powder (200 mesh) was placed in an oven at 80°C for drying of 12 hours. Ion exchange separation method was applied to isolate and purificate Rb-Sr and Sm-Nd using Dowex50W resin in the purified laboratory. Sr isotopic analyses were performed on a thermal ionization mass spectrometer (MAT261) at the Wuhan Institute of Geology and Resources. Mass discrimination correction was carried out via internal normalization to an 88 Sr/ 86 Sr ratio of 8.375209. NBS987, NBS607, and GBW04411 standard materials were measured to evaluate the reproducibility and accuracy of the instrument and analytical processes. They yielded 87 Sr/ 86 Sr ratios of 0.71021 ± 0.00003 (2σ), 1.20025 ± 0.00004 (2σ) and 0.76007 ± 0.00005 (2σ), respectively, which are consistent with their recommended values (0.71024 ± 0.00026, 1.20039 ± 0.00020 and 0.75999 ± 0.00020) within error. The total blanks for Rb and Sr are 0.2 × 10 -9 and 1.3 × 10 -9 g, respectively. Nd isotopic analyses were conducted on a thermal ionization mass spectrometer (Triton) at the Wuhan Institute of Geology and Resources. Mass discrimination correction was carried out via internal normalization to a 146 Nd/ 144 Nd ratio of 0.7219. GBW04419 and JNdi-1 standard materials were measured to monitor the accuracy of the instrument and analytical processes. The 143 Nd/ 144 Nd ratios of them were 0.512721 ± 0.00005 (2σ) and 0.512117 ± 0.00005 (2σ), respectively, consistent with recommended values within error. Total blanks for Sm and Nd are less than 3 × 10 -10 and 1 × 10 -10 g, respectively.

Results
Tables S1-S2 in the supporting information list details of the zircon U-Th-Pb isotopic and whole-rock elemental and Sr-Nd isotopic results. The zircon U-Th-Pb isotopic data are shown on concordia diagrams in Figure 5, and whole-rock elemental results are plotted in Figures 6-9 and S1-S2. For the Dangchuan pluton, results from previous work ) have also been shown in Figures 7a-b and 8a-b.

Zircon U-Pb ages
Sample HM08-7 is a diorite collected from the Huamiao dikes ( Figure 2). Zircons from this sample are euhedral, subhedral, and long prismatic. They range from 100 to 200 μm in length, and have length/width ratios of 1.5:1 −3:1. In CL images, all zircon grains exhibit clear oscillatory zoning, indicating an igneous origin ( Figure 5a). We conducted 30 analyses on 30 zircon grains. They have Th contents of 59−230 ppm, U contents of 94−402 ppm, and Th/U ratios of 0.34−1.36 (Table S1). All these analyses are concordant, and 29 of them define a weighted mean 206 Pb/ 238 U age of 436 ± 2 Ma (MSWD = 0.18) (Figure 5b), interpreted as the crystallization age. The remaining one analysis yields a 206 Pb/ 238 U age of 832 ± 10 Ma ( Figure 5b, Table S1), interpreted as the age of the xenocrystic zircon grain.
Sample DC027-1 is a syenogranite collected from the Dangchuan pluton ( Figure 2). Zircons from this sample are euhedral, subhedral, and long prismatic. They range from 100 to 150 μm in length with aspect ratios of ~1.5 −3.0. In CL images, most zircon grains are weakly luminescent and display oscillatory zoning, indicating an igneous origin (Figure 5c). Several zircon grains show core-rim structures, including highly luminescent cores representing inherited/xenocrystic grains and weakly luminescent rims with oscillatory zoning representing magmatic overgrowth (Figure 5c). Twenty-six analyses were conducted on 24 zircon grains. Among them, seven analyses exhibit extremely high Th (1580−5117 ppm) and U (1224−4151 ppm) contents (Table S1), indicating the contamination of Th-and U-rich inclusions during analysis. These analyses are unreliable and not used for age calculation. Thirteen zircons have normal Th and U contents of 68−845 ppm and 133−2174 ppm, respectively, with Th/U ratios of 0.06−0.64 (Table S1) Table S1).
Sample YJD77-1 is a diorite collected from the Yanjiadian pluton (Figure 3). Zircons from this sample are euhedral, subhedral, and long prismatic. They range from 100 to 300 μm in length and have length/width ratios of 1.5:1−4:1. In CL images, all zircon grains exhibit clear oscillatory zoning, indicating an igneous origin (Figure 5e). We conducted 32 analyses on 32 zircon grains. They have Th contents of 58−221 ppm, U contents of 57−402 ppm, and Th/U ratios of 0.47 −1.20 (Table S1). All these analyses are concordant and define a weighted mean 206 Pb/ 238 U age of 445 ± 2 Ma (MSWD = 0.6) (Figure 5f), interpreted as the crystallization age.
Sample HTP45-6 is a basalt collected from the Hongtupu volcanic rocks (Figure 2). Zircons from this sample are 50−100 μm in length and have length/width ratios of 1:1−3:1. In CL images, some zircon grains are subhedral and have faint or sector igneous zoning, indicative of an igneous origin; the other grains are round and show oscillatory zoning, representing xenocrystic zircons (Figure 5g). We conducted 16 analyses on 16 zircon grains. They have Th contents of 38−149 ppm, U contents of 35−654 ppm, and Th/U ratios of 0.07−1.51 (Table S1). Five analyses conducted on igneous zircons are concordant and yield a weighted mean 206 Pb/ 238 U age of 445 ± 4 Ma (MSWD = 0.9) (Figure 5h), herein interpreted as the crystallization age. The remaining analyses were conducted on xenocrystic zircons. Most of them are discordant, and four concordant analyses yield ages of 1800−461 Ma (Figure 5h; Table S1).  (Table S2). On the Harker diagrams ( Figure S1), SiO 2 correlates positively with P 2 O 5 and negatively with CaO, FeOt, MgO, MnO, and TiO 2 ; its correlation with Al 2 O 3 , K 2 O and Na 2 O is insignificant. All these samples have high total alkali contents (K 2 O + Na 2 O = 7.56−8.61 wt%; Table S2), and show high-K calcalkaline affinity ( Figure S1). The syenogranites are strongly peraluminous with A/CNK ratios of 1.08−1.19 ( Figure 6b, Table S2). All samples except one (DC027-1) have high total REE contents of 251−378 ppm (Table  S2). They show variable enrichments in LREE with La N /Yb N ratios = 8.33−93.57, and display negative Eu anomalies with Eu/Eu* = 0.29−0.63 (Figure 7a, Table  S2). On the N-MORB-normalized spider diagram (Figure 7b), all these samples show enrichments in Rb and Th, and significantly negative Nb, Ta, Sr, P and Ti anomalies, characterizing crust-derived melts. Major element compositions and Zr contents constrain their zircon saturation temperatures (T Zr ) to 730−816°C (Table  S2). On the basis of our new zircon U-Pb age (432 Ma),   Table S2).

Petrogenesis
The high loss on ignition (LOI) of the Huamiao and Hongtupu Group II samples (2.51−3.96 wt% and 3.43 −4.85 wt%, respectively; Table S2) suggests that they have undergone some alteration. To evaluate the effects of alteration on their elemental compositions, representative mobile elements, including CaO, K 2 O, Rb, Sr, Th, and U, were plotted against LOI ( Figure S2).
For the Hongtupu Group II samples, the correlations of these elements with LOI are insignificant (Figure S2), indicating that they were unchanged during post-magmatic alteration. For the Huamiao samples, CaO, Rb, and Sr define a broadly positive correlation with LOI ( Figure S2), indicating their mobility during post-magmatic carbonatization. The other elements show no correlations with LOI ( Figure S2), indicating that their contents were controlled by protolith composition rather than alteration. In addition, most HFSEs and REEs of the Huamiao and Hongtupu (Group II) samples are also immobile during alteration, as expressed by their subparallel chondrite-normalized REE and primitive  Wang et al. (2008). Data for metabasaltic and eclogite experimental melts (1-4 GPa) are from Rapp et al. (1999); data for MORB and GLOSS-II are from Workman and Hart (2005) and Plank (2014), respectively. Symbols in Figure 8b-h are the same as those in Figure 8a mantle-normalized incompatible-element patterns (Figure 7c-f). Therefore, in the following discussion, we will prioritize the immobile elements to trace the source of the Huamiao diorites.

The Dangchuan syenogranites
The Dangchuan syenogranites show high A/CNK ratios, low MgO contents, negative ε Nd (t) values, relatively high initial 87 Sr/ 86 Sr ratios, and crust-like REE and trace element patterns (Figures 6b and 7a-b; Table S2). These geochemical features suggest that they were generated by remelting of crustal rocks. Furthermore, their high CaO/Na 2 O and low Rb/Sr ratios indicate that the source rocks are clay-poor, plagioclase-rich greywackes (Figure 8a, b).
In addition, they show strong fractionation between LREE and HREE, steep HREE patterns, and low Y (10.1 −36.4 ppm) and Yb (0.78−3.74 ppm) contents (Figure 7a; Table S2), indicating residual garnet in the source at pressures ≥0.5 GPa (Rapp et al. 2003;Patiño Douce 2004). Wang et al. (2008) interpreted the early-stage (~438 Ma) monzogranites as C-type adakitic rocks that resulted from partial melting of a thickened crust, with a possible eclogitic residue containing garnet without plagioclase. By comparison, the late-stage syenogranites (~432 Ma) exhibit higher HREE and Y contents and significant Eu negative anomalies (Figure 7a, b), indicating garnet and plagioclase residua in the source. Thus, we deduce that the late-stage syenogranites were generated at medium to high pressures corresponding to   Table 1 amphibolite to granulite facies metamorphism. All above characteristics, together with the calculated T Zr (730−816°C; Table S2), suggest that the Dangchuan syenogranites were derived from a clay-poor, plagioclase-rich greywacke source, which might have undergone amphibolite to granulite facies metamorphism. Partial melting experiments from a quartz-rich metagreywacke suggested that melts formed at >0.5 GPa and 800-850°C are silicic and peraluminous with K 2 O+Na 2 O = 7.33−8.16 wt% and Al 2 O 3 = 13.54−14.20 wt % (Montel and Vielzeuf 1997), which matches well with our granitoid samples, further confirming this interpretation.

The Huamiao diorites
The Huamiao diorites show typical features of high-Mg diorites expressed by moderate SiO 2 and extremely high MgO contents and Mg# (Figure 8c; Table S2), suggesting a major mantle source (Kuroda et al. 1978;Tatsumi 2001). In addition, they display LREE-enriched REE patterns, enrichments in LILEs, and depletions in HFSEs (Figure 7c,  d), indicating that the mantle source had been metasomatized by subduction-related fluids (Figure 8d, e) (e.g. Kelemen et al. 2003;Spandler and Pirard 2013). It is suggested that basaltic oceanic crust and seafloor sediments can liberate subduction zone fluids with variable compositions, which might finally result in distinctive geochemical features of subduction-related magmas (e.g. Zheng 2019). The following lines of evidence suggest that fluids from both seafloor sediments and basaltic oceanic crust had participated in the generation of the Huamiao diorites. First, the diorites show enriched Sr-Nd isotopic compositions (Table S2), enrichments in Th, Zr, and Hf (Figure 7d), and high Th/Nb and Th/Nd ratios (Figure 8f, g), indicating that their mantle source was probably metasomatized by sedimentderived fluids Class et al. 2000). Second, the diorites show enrichments in Ba, K, and U and high Ba/La ratios (Figures 7d and 8 g), suggesting the participation of slab-derived fluids in their source (e.g. Woodhead et al. 1998;Hanyu et al. 2006).
In a subduction zone, the oceanic crust is blanketed by sediments that are dominated by terrigenous components resembling upper continental crust (Plank and Langmuir 1998;Shimoda et al. 1998). These sediments dehydrate at shallow levels due to shear-heating and advection of hot mantle material, generating sedimentderived fluids . Fluids move into the overlying mantle and react with it, creating a sediment-modified mantle wedge (e.g. Elliott et al. 1997). The sediment-modified mantle is dragged downdip by the subducting slab due to solid advection and relatively high mantle shear viscosity at the base of the mantle wedge (e.g. Cagnioncle et al. 2007;Cerpa et al., 2017). At deeper levels, the altered oceanic crust dehydrates and releases aqueous fluids at or near the slab surface (McGary et al. 2014). The fluids induce fluxed melting of sediment-enriched mantle, producing magmas that finally resided in the crust. Owing to the separate addition of slab-and sediment-derived fluids, their contributions are both documented in the arc magmatism (e.g. Ma et al. 2021), the best explanation of geochemical observations of the Huamiao diorites. This petrogenetic model is also consistent with high-Mg andesites/diorites in the Setouchi volcanic belt, southwest Japan (e.g. Shimoda et al. 1998;Tatsumi 2001).

The Yanjiadian diorites
The Yanjiadian diorites show high Al 2 O 3 (>15 wt%), Sr (>700 ppm) and Ba (>700 ppm), and low Y (<16 ppm) and Yb (<1.9 ppm) contents, corresponding to high Sr/Y and La N /Yb N ratios (Table S2), akin to the geochemical features of the adakitic rocks (Figure 8h) (e.g. Defant and Drummond 1990;Castillo 2006Castillo , 2012. Several petrogenetic models have been proposed for the generation of the adakitic rocks, including the partial melting of (1) a young and hot slab (Defant and Drummond 1990) or (2) the eclogitic lower crust (Zhang et al. 2001;Rapp et al. 2002;Wang et al. 2006b) and (3) fractional crystallization from parental basaltic magmas (Castillo et al. 1999;Macpherson et al. 2006;Rodriguez et al. 2007;Rooney et al. 2011). The Yanjiadian diorites show constant major element compositions (Figure S1), precluding their generation by fractional crystallization. They exhibit calc-alkaline affinity and high Na 2 O/K 2 O ratios (1.76−1.95; Table S2), suggesting a MORB-like source rather than lower-crust source that would show high-K calc-alkaline affinity and low Na 2 O/K 2 O ratios (Zhang et al. 2001;Rapp et al. 2002). We suggest that the Yanjiadian diorites were derived from the basaltic oceanic crust in the light of the following evidence: (1) all Yanjiadian samples fall into the field of oceanic crust-derived melts on the Mg# vs. SiO 2 diagram ( Figure 8c); (2) they are enriched in Ba and Sr but depleted in Th ( Figure 7d); (3) they have high Ba/Th, Ba/La and low Th/Yb, Th/Nd ratios (Figure 8f, g) (Woodhead et al. 1998;Hanyu et al. 2006). In addition, their high Mg# (>50 ; Table S2) suggests that the slabderived melts might have interacted with mantle peridotite (Kay 1978;Yogodzinski et al. 1995;Stern and Kilian 1996). Thus, we can infer that the Yanjiadian diorites were generated by the interaction between slab-derived melts and mantle peridotites.

The Hongtupu basalts 5.1.4.1 Fractional crystallization and crustal contamination. Before evaluating the mantle sources of the basalts, it is necessary to eliminate the effects of magma differentiation and crustal contamination.
Significant crustal contamination can be precluded by the homogeneous Sr-Nd isotopic compositions ( Table S2), lack of Zr-Hf positive anomalies (Figure 7f) and no correlations of Nb/La with Mg# (Figure 9a). In addition, the variable Mg# suggests that fractional crystallization might have been involved in the generation of both two groups of basalts (Table S2; Figure 9a). For Group I basalts, the decreasing Mg# but constant FeOt contents and CaO/Al 2 O 3 ratios indicate the olivine fractionation (Figure 9b, c). For Group II basalts, the decreasing Mg# and constant FeOt contents, and slightly negative correlations between Mg# and CaO/Al 2 O 3 ratios, testify to the fractionation of olivine and clinopyroxene (Figure 9b, c). They exhibit negative Eu and Sr anomalies on the REE and trace element diagrams (Figure 7e, f), suggesting the plagioclase fractionation. (Figure 6d), respectively, and exhibit negative correlations between Nb/La and Mg# ratios and distinct trace element patterns (Figures 7f and 9a), indicating that they were derived from different mantle sources.

Magma source. Group I and II basalts are tholeiitic and calc-alkaline
Group I basalts show low initial 87 Sr/ 86 Sr ratios and positive ε Nd (t) values (Table S2), slight enrichments in LREE (Figure 7e), flat HFSE patterns with insignificant Nb, Ta, and Ti anomalies (Figure 7f), high Nb/Th ratios (Figure 8d), and low Ba/Nb, La/Nb, Ba/Th and Th/Nd ratios (Figure 8d-g). All these features are consistent with those of the E-MORB, suggesting that they were derived from an enriched mantle source.
For Group II basalts, they exhibit enrichments in LILEs (e.g. Ba and Th) and LREE and significant depletions in HFSEs (e.g. Nb, Ta, Zr, and Ti) (Figure 7e, f) that result in high La/Nb and Ba/Nb and low Nb/Th ratios (Figure 8d, e), consistent with geochemical characteristics of the island-arc basalts (IAB). These features suggest that the Group II basalts originated from a metasomatized mantle source. Moreover, their low abundances of K and Rb, but high contents of Th and LREE (Figure 7f), low Ba/La, and Ba/Th ratios and high Th/Nb and Th/Nd ratios (Figure 8f, g) indicate that the mantle source was metasomatized by sediment-derived fluids (e.g. Woodhead et al. 1998;Hanyu et al. 2006).
In summary, we can conclude that the Group I basalts were derived from an enriched mantle source and Group II basalts originated from a sediment-modified mantle source. Their primary magmas experienced the fractionation of olivine and, to a lesser extent, clinopyroxene and plagioclase.

Establishment of the Baihua-Huluhe arc-backarc system associated with the subduction of the Wuguan ocean
At the southern margin of the NQT, several faultbounded, lenticular ophiolite slices outcrop in the Wushan (Yuanyangzhen), Guanzizhen, and Yanwan areas, from west to east (❶ in Figure 10). In the Guanzizhen and Wushan areas, deep-marine carbonates were also identified (Dong et al. 2011a). The basalts within them exhibit features of either N-MORB or E-MORB, and yield zircon U-Pb ages of 534-471 Ma (Table 1). These ophiolite slices represent the remnants of the oceanic lithosphere, suggesting the existence of an ocean, i.e. the Wuguan ocean, during the Cambrian to Ordovician periods (Pei et al. 2007a;Dong et al. 2011a).
In this study, the 436 Ma Huamiao high-Mg diorites, intruding into the Qinling Complex, the basement of the NQT, are interpreted to be generated by the interaction between sediment/slab-derived fluids and mantle peridotite. They were formed in the subduction-related setting, indicating that the Wuguan ocean had been subducting beneath the NQT at ca. 436 Ma. Similar forearc-and arc-related magmatic rocks are widespread in the NQT, including the 472-451 Ma Liziyuan and Caotangou volcano-sedimentary sequences (Pei et al. 2006;Yan et al. 2007;Yang et al. 2018a), 450-435 Ma Baihua igneous complexes (Pei et al. 2005(Pei et al. , 2007cGao et al. 2012), 455 Ma Tangzang adakitic granitoids (Chen et al. 2008a), and 439-438 Ma Honghuapu and Yangjiazhuang high Ba-Sr diorites (Ren et al. 2018) (❶-❷ in Figure 10). All these magmatic rocks constitute typical arc-related magmatic assemblages, yielding crystallization ages of 472-435 Ma (Table 1). This suggests that a magmatic arc, termed here as the Baihua magmatic arc, was built on the NQT due to the northward subduction of the Wuguan ocean during ~472-435 Ma.
To the north of the NQT, the Hongtupu basalts crop out (❸ in Figure 10). Our new geochemical data suggest that the Hongtupu basalts include E-MORB (Group I) and IAB (Group II) with depleted Sr-Nd isotopic compositions. In addition, He et al. (2007a) and Li et al. (2018b) identified basalts with transitional geochemical features between IAB and MORB in the Yangjiasi and Huluhe areas. All these basalts share similar crystallization ages of 445-438 Ma (He et al. 2007b;Li et al. 2018b; this study (Table 1). Coeval basalts showing diverse geochemical features matches well with basalts in the Lau Basin, the back-arc basin of the Tonga arc (e.g. Gill 1976;Pearce et al. 1994), indicating that the Hongtupu basalts were formed in a back-arc basin setting. The back-arc basin, named here as the Huluhe back-arc basin, was formed during the Cambrian to Silurian periods, as evidenced by zircon U-Pb ages of 500-438 Ma of basalts and dolerite dikes (Table 1) and the latest Neoproterozoic-Cambrian fossils discovered from the siliceous rocks (Zhang et al. 2004).
Spatially, the ~NW-SE striking Huluhe back-arc basin is parallel to the Baihua magmatic arc ( Figure 10); temporally, the duration of the Huluhe back-arc basin matches well with the Baihua magmatic arc. Given their spatial and temporal relationship, we infer that the Baihua and Huluhe tectonic units constitute a complete arc-back-arc system, which was closely associated with the subduction of the Wuguan ocean ( Figure 11a).

Generation of the Chenjiahe arc in response to the subduction of the Huluhe Ocean
The Hongtupu Group I basalts in this study exhibit features of the E-MORB, indicating that the Huluhe back-arc basin had evolved into a mature one (e.g. Klein and  1987). Together with the ophiolite suite identified in the Huluhe area (Zhang et al. 2004), we can deduce that the Huluhe back-arc basin had evolved into a small oceanic basin, termed here as the Huluhe ocean.
The 445 Ma Yanjiadian diorites of this study, located to the northeast of the Huluhe back-arc basin (Figures 1 and  Figure 10), were defined as high-Mg adakitic rocks that were formed by the interaction between slab-derived melts and mantle peridotite. Thus, we interpret the Yanjiadian diorites to be formed in a subduction-related setting. Coeval subduction-related magmatic rocks widely crop out to the northeast of the Huluhe back-arc basin (Table 1; ❹ in Figure 10). Examples include 462-443 Ma Chenjiahe volcano-sedimentary sequences ) and 441 Ma Huangmenchuan and 455 Ma Wangjiacha I-type granites Wei et al. 2012). All these subduction-related magmatic rocks constitute a ~NW-SE striking continental arc along the southwestern NCB (❹ in Figure 10). As the continental arc is coeval with and parallel to the Huluhe back-arc oceanic basin (Figure 10), we infer that the Chenjiahe arc was generated by the northeastward subduction of the Huluhe ocean (Figure 11b).
In addition to subduction-related sequences, several EM-derived, extension-related mafic dikes (453 −440 Ma) intruded into the Longshan Group/ Complex (❹ in Figure 10, Table 1). These ~NW-SE trending dikes indicate ~NE-SW extension, which is parallel to the coeval subduction direction of the Huluhe ocean. Therefore, we infer that these mafic dikes were formed in response to the back-arc extension induced by the roll-back and retreat of the Huluhe oceanic plate.
In summary, the Chenjiahe arc was built along the southwestern NCB during 462-440 Ma in response to the subduction of the Huluhe ocean ( Figure 11b).

Silurian orogenic event associated with the closure of the Huluhe ocean
Our new geochronological and geochemical data suggest that the Dangchuan granitoids were derived by partial melting of crustal rocks at medium to high pressures at ca. 432 Ma. Coeval crust-derived granites are also documented across the Tianshui area, yielding zircon U-Pb ages of ~438-425 Ma ( Table 1). The generation of these crust-derived granites, together with the clockwise P-T path of the Qinling Complex at ca. 433−424 Ma (Mao et al. , 2018, indicated that the basement of the NQT had been deeply buried and experienced metamorphism and anatexis due to a Silurian orogenic event.
An orogenic event is generally caused by the closure of the oceanic basin. As discussed above, two oceanic basins existed in the Tianshui area during the Early Palaeozoic, so the remaining key issue is that the closure of which ocean induced the Silurian orogenic event. Our new geochemical data suggested that the 436 Ma Huamiao diorites were formed in the subduction-related setting. Other coeval subduction-related intrusions, such as 435 Ma Baihua gabbros and 438 Ma Honghuapu diorites (Table 1), were also reported in the Tianshui area. These subduction-related intrusions suggested that the Wuguan oceanic basin was still subducting during ~438−435 Ma. Furthermore, ~384 Ma HP granulites (Yang et al. 2018b) and ~420−370 Ma sinistral oblique shearing, in which the Dangchuan pluton was involved (Mao et al. 2021), were identified at the southern margin of the NQT, indicating that the closure of the Wuguan ocean might take place after ca. 420 Ma. All these evidence preclude the possibility that the Silurian orogenic event was caused by the Wuguan ocean closure. In addition, the following lines of evidence suggest that the Silurian orogenic event might have been caused by the closure of the Huluhe oceanic basin: (1) the latest magmatism caused by the subduction of the Huluhe oceanic basin was at ~440 Ma (Table 1); (2) the 447−434 Ma Huluhe Group metamorphosed flysch sequences marked that the Huluhe ocean had evolved into a shrank remnant sea during the Early Silurian ; (3) the 439 −435 Ma Renda appinite suite and granite porphyry in the NMCB, which were generated by partial melting of water-rich lithospheric mantle and juvenile crust, respectively, suggest a tectonic transition from oceanic subduction to continental collision (Li et al. 2021). The coexisting of subduction and collision processes in the NQT during 438−420 Ma ( Figure 11C) caused the complex origin of some collision-related plutons of the SMCB that involved mixing of melts derived from the slab and terrigenous sediments (e.g. Ren et al. 2018Ren et al. , 2021. In summary, we can conclude that the Silurian orogenic event documented in the NQT was caused by the closure of the Huluhe ocean. The closure of the Huluhe ocean resulted in the collision between the NQT and NCB and intense reworking of the basement of the NQT during ~438−425 Ma (Figure 11c).
In the global context, the Wuguan and Huluhe oceans were both segments of the Proto-Tethys Ocean (e.g. Song et al. 2017;Dong et al. 2021), which separated the NQT, the NCB and eastern Gondwana (Zhang et al. 2015;Li et al. 2018a, b;Yang et al. 2018a). The Silurian-Devonian closure of them caused the collision between the NQT and NCB and finally attached them to eastern Gondwana (Metcalfe 1996(Metcalfe , 2006Zhao et al. 2018;Li et al. 2018a). In addition to the NQT and NCB, other East Asian blocks, including South China, Tarim, Indochina, and Qiangtang, were also attached to the eastern Gondwana during the Early Palaeozoic as suggested by the depositional environment and faunal similarities (Metcalfe 2006(Metcalfe , 2013. The Silurian-Devonian orogenic event recorded in the Tianshui area was attributed to the closure of the Proto-Tethys Ocean, which assembled in eastern Gondwana.

Conclusions
(1) Zircon U-Pb dating suggests that the Dangchuan syenogranites were formed at ca. 432 Ma. The syenogranites are peraluminous with low MgO, FeOt contents and Rb/Sr ratios, high CaO/Na 2 O and initial 87 Sr/ 86 Sr ratios, and negative ε Nd (t) values. They were interpreted to be generated by partial melting of metagreywackes during an orogenic event.
(2) The Huamiao and Yanjiadian diorites were formed at ca. 436 Ma and 445 Ma, respectively. The Yanjiadian and Huamiao diorites are high-Mg adakitic rocks and high-Mg diorites, respectively. They were both generated by the interaction between sediment/slab-derived fluids/melts and mantle peridotite in the subductionrelated setting.
(3) The 445 Ma Hongtupu basalts are divided into two groups: Group I basalts share features of the E-MORB that were derived from an enriched mantle source; Group II basalts exhibit features of the IAB that originated from a sediment-modified mantle source. The basalts were formed in a back-arc basin setting.
(4) The Early Palaeozoic evolution of the Tianshui area involved the generation of the Baihua-Huluhe arc-backarc system, subduction of the Huluhe back-arc oceanic basin and collision between the North Qinling arc terrane and the NCB, which is closely associated with the closure of the Proto-Tethys Ocean.