Petrogenesis of Neoproterozoic mafic dykes in western Yangtze Block, South China: implications for the assembly and break-up of Rodinia

ABSTRACT Neoproterozoic mafic dyke swarms are an important probe for reconstructing the evolutionary history of the Rodinia supercontinent. Here, we present detailed geochronological, geochemical, and Sr–Nd–Hf isotopic data to constrain the petrogenesis and geodynamic setting of Neoproterozoic mafic dyke swarms in the western Yangtze Block, South China. Petrological and zircon U–Pb dating studies reveal that the studied dykes mainly comprise subophitic-textured dolerite and intergranular dolerite with crystallization ages of ca. 823–817 Ma and ca. 795–794 Ma, respectively. The two groups of dykes exhibit varied geochemical and Sr–Nd–Hf isotopic compositions. Among them, the older group has moderate rare earth elements (∑REE = 71.0–94.9 ppm) with flat REE patterns ((La/Yb)N = 1.38–1.81) and exhibits enriched large ion lithophile elements and Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.704865–0.707641, εNd(t) = −6.15 to 1.26, εHf(t) = −2.59 to 2.11), resembling typical island arc basalts. Whereas, the younger group has high Nb contents (4–10 ppm) with high Nb/U values (6.93–29.3), high REE contents (∑REE = 123.3–180.8 ppm) with moderate (La/Yb)N values (1.84–4.51), and slightly depleted but variable Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.702280–0.706761, εNd(t) = −2.67 to 4.07, εHf(t) = −0.74 to 5.91), akin to Nb-enriched basalts in the subduction zone. Petrogenesis studies indicate that the ca. 823–817 Ma and ca. 795–794 Ma dykes may be derived from different degrees of melting of the enriched mantle wedge that was metasomatized by subducted slab-related fluids and melts, respectively. Varying degrees of the interaction of the slab-derived fluid/melts with mantle peridotite account for their elemental–isotopic heterogeneity. The geochemical database compilation of Neoproterozoic igneous rocks suggests that the Yangtze Block may be located on the periphery of the Rodinia supercontinent and slab tearing or break-off, rather than mantle plume, may account for the Rodinia break-up and associated transition of the mantle metasomatic mechanism at ca. 830–820 Ma.


Introduction
Supercontinents play an important role in global tectonic evolution, crust-mantle dynamics, mineral systems, and surface geological processes; thus, their assembly and break-up histories and associated geodynamic mechanisms are always the focus of international geosciences (Cawood et al. 2016;Gamal EL Dien et al. 2019;Jordan et al. 2020;Shu et al. 2021). The Rodinia supercontinent, which formed during the late Mesoproterozoic to early Neoproterozoic, is one of the most important Precambrian supercontinents (Cawood et al. 2016(Cawood et al. , 2013Li et al. 1995;Peng et al. 2019;Zhou et al. 2007). The Yangtze Block in the South China Craton is considered an integral part of Rodinia (Zhao and Cawood 2012) and the widely distributed Neoproterozoic plutonic or volcanic rocks along its periphery are magmatic responses to the assembly and break-up of Rodinia, which can be used to reveal the tectonic evolution and palaeogeography of the Yangtze Block in the Rodinia supercontinent (Ao et al. 2019;Cawood et al. 2018;Chen et al. 2019;Gan et al. 2017;Li et al. 2002Li et al. , 1995Luo et al. 2018;Zhao et al. 2019;Zhu et al. 2021). However, the petrogenesis and tectonic setting of these rocks have been debated with many diverse geodynamic models, including two representative controversial models, that is mantle plume (Li et al. 1999(Li et al. , 2003 and slab subduction (Ao et al. 2019;Luo et al. 2018;Zhao et al. 2019;Zhou et al. 2002), which hinders further understanding of whether the palaeogeographic location of the Yangtze Block lies within or on the periphery of the Rodinia supercontinent.
Mafic dyke swarms, one special igneous rock representing conspicuous extensional tectonics, are of great significance as they record critical information regarding the timing and processes of continental break-up, which can be used to reveal the mantle source and tectonic evolutionary history of paleocontinents (Ernst et al. 1995;Li et al. 2015Li et al. , 2020. Mafic dyke swarms can be formed in various tectonic settings displaying distinct geochemical characteristics, such as island arcs, mid-ocean ridges, oceanic islands and continental rifts; thus, they are important petrological probes for studying regional tectonic evolution (Zhu et al. 2008;Cui et al. 2015). Neoproterozoic mafic dykes are widespread in the Yangtze Block, South China, providing an important means of reconstructing the Rodinia supercontinent and understanding its break-up mechanisms (Lin et al. 2007;Zhu et al. 2008;Cui et al. 2015;Yang et al. 2017;Li et al. 2019). Previous studies primarily focused on the mid to late Neoproterozoic mafic dykes in the Yangtze Block and revealed three episodes of mafic dyke magmatism during the break-up of Rodinia: 800-790 Ma, 790-780 Ma and 780-730 Ma (Lin et al. 2007;Zhu et al. 2008;Li et al. 2019). However, the geochronological and geochemical characteristics of the early Neoproterozoic (before 800 Ma) mafic dyke swarms have not been reported in detail and their petrogenesis is not well constrained. The variation in mantle sources of these episodic mafic dyke magmatism and its relationship with the geodynamic setting during the break-up of Rodinia is not well understood. In this paper, we present new, detailed geochronological, whole-rock geochemical, and zircon Lu-Hf isotopic data of two groups of mafic dyke swarms from the Mianning area in the western Yangtze Block, South China. Combined with published data on Neoproterozoic mafic igneous rocks in the western Yangtze Block, this study attempts to (1) constrain the petrogenesis and geodynamic setting of these mafic dykes and (2) gain insights into the assembly and break-up of the Rodinia supercontinent.

Geological background
The Yangtze Block is an important part of the South China Craton, which is bounded by the North China Craton to the north, the Cathaysia Block to the east, and the Songpan-Ganze Tethyan terrane to the west (Li et al. 1999;Zhao and Cawood 2012). The Yangtze Block consists of an Archaean crystalline basement, including the Kongling, Yudongzi, and Douling complexes, which are covered by Mesoproterozoic metamorphic sedimentary rocks and Neoproterozoic-Cenozoic volcanosedimentary strata Cawood et al. 2020). Neoproterozoic magmatic rocks and sedimentary strata are primarily distributed in the northern and western parts (Figure 1), including the Kangding, Pengguan, and Baoxing plutonic complexes (Meng et al. 2015;Yan et al. 2008;. Voluminous Neoproterozoic granitoids are widely preserved with subordinate maficultramafic rocks in the western Yangtze Block (Figure 2a), which show predominant zircon crystallization ages ranging from ca. 870-750 Ma (Hu et al. 2020;Meng et al. 2015;Yan et al. 2008;Zhu et al. 2021).
Field mapping identified two groups of mafic dyke swarms distributed in the Mianning area in the western Yangtze Block, which have different occurrence and petrological characteristics (Figure 2b). Among them, one group intruded into the Mesoproterozoic strata comprises dolerites with subophitic texture (group I) and displays a prevalent NNW-SSE strike, while the other group comprises intergranular dolerites (group II) which were emplaced into the Lugu Neoproterozoic granite along a preferred NE-SW direction ( Figure 2b). Previous zircon U-Pb dating demonstrates that the Lugu granite intruded by intergranular dolerites was emplaced at ca. 815-806 Ma ), which is the upper limit of the formation age for group II mafic dykes. All these dykes are either vertical or sub-vertical with a width of 0.5-5 m (Figure 3a-b). The group I dykes show subophitic texture with some lath-shaped plagioclases embedded in several large anhedral clinopyroxene crystals. Some of the plagioclases are wholly embedded, while others penetrate beyond the anhedral clinopyroxenes (Figure 3c-d). The ophitic clinopyroxene crystals are 1-2 mm in size and less than 10% in volume (Figure 3c-d). Group II dykes show a fine-grained intergranular texture with anhedral equant crystals of clinopyroxene occupying the spaces between the lathshaped euhedral plagioclase crystals (Figure 3e-f). Moreover, all the studied dykes contain many euhedral titanium-bearing minerals, such as ilmenite (Figure 3c-f).

LA-ICP-MS zircon U-Pb dating
Zircons were separated using conventional heavy liquid and magnetic techniques at the Langfang Regional Geological Survey, Hebei Province, China. Zircon grains were photographed with an optical microscope, and their internal structure was checked by cathodoluminescence (CL). The U-Pb dating was done by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm) and a MicroLas optical system. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. The spot size, frequency and energy of the laser were set to 32 μm, 6 Hz and ~60 mJ, respectively. Zircon 91,500 was used as the external standard for U-Pb dating and was analysed twice every 5 analyses. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3, and data were processed by ICPMSDataCal (Liu et al. 2010). Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as described by Liu et al. (2010).

Whole-rock geochemistry
Whole-rock samples were crushed in a corundum jaw crusher (to 60 mesh). About 60 g was powdered in an agate ring mill to <200 mesh for whole-rock geochemistry analysis. The major element analysis was conducted by standard X-ray fluorescence methods, which was carried out on a Shimadzu Sequential 1800 spectrometer at the GPMR laboratory, Wuhan. Precision is better than 4% and accuracy is better than 3% for major elements. The detailed techniques for analysis of major elements are described by Ma et al. (2012). Trace elements were analysed with an Agilent 7500a ICP-MS at GPMR laboratory, Wuhan. The samples were digested by HF + HNO3 in Teflon bombs. Analyses of USGS standards (AGV-2, BHVO-2, BCR-2 and RGM-2) indicate accuracy better than 5-10% for most trace elements. The detailed sample-digesting procedure of ICP-MS analyses for trace elements is the same as described by Liu et al. (2008).
Bulk-rock Sr and Nd isotopic ratios were determined on a Finnigan Triton thermal ionization mass spectrometer at the GPMR laboratory, Wuhan. Isotopic measurements were performed on a Finnigan MAT-261 thermal ionization mass spectrometer. Total procedural blanks were <50 pg for Sm and Nd, as well as <1 ng for Rb and Sr. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219, respectively. Analyses of standards during the period of analysis are as follows: NBS987 gave 87 Sr/ 86 Sr = 0.710236 ± 16 (2σ); La Jolla gave 143 Nd/ 144 Nd = 0.511862 ± 5 (2σ). The analytical precision and accuracy for Sr-Nd isotopic compositions are as described by Gao et al. (2004).

In-situ zircon Lu-Hf isotope
In-situ zircon Lu-Hf isotope analyses were conducted on the dated zircon grains using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 laser ablation system (Lambda Physik, Göttingen, Germany) that was hosted at the GPMR laboratory, Wuhan. All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm. Each measurement consisted of 20s of acquisition of the background signal followed by 50s of ablation signal acquisition. Off-line selection and integration of analytical signals were performed using ICPMSDataCal (Liu et al. 2010). Zircon standards 91,500 and GJ-1 were used to check instrument reliability and stability. Detailed operating conditions for the laser ablation system, the MC-ICP-MS instrument and analytical method are the same as described by Hu et al. (2012).

Zircon U-Pb dating
Two representative subophitic-textured dolerites (samples D0020 and D0021) and two intergranular dolerites (sample PM-402-39 and PM-402-95) were analysed to determine their crystallization ages. The CL images of representative zircon grains are shown in Supplementary Figure S1. The analysis result of LA-ICP-MS zircon U-Pb dating is presented in Supplementary  are euhedral to subhedral and exhibit oscillatory, banded zoning, or sector zoning. A few grains show weak zoning with dark colour in the CL images ( Figure  S1). Although all studied zircons have high Th and U contents with high Th/U values (0.11-0.97; Table S1), most oscillatory, banded, or sector-zoned zircons do not have inherited cores and have similar apparent age, indicating their magmatic origin. However, those with inherited cores all have much older ages (such as 868 Ma for spot 1 in sample D0200), indicating their inherited origin. Certain grains are dark in colour, show weak zoning and much younger 206 Pb/ 238 U apparent ages (such as 743 Ma for spot 5 in sample D0200), possibly indicating a younger thermal event (Fig. S1a). Thus, we believe that the U-Pb dating results are reliable and that the U-Pb ages of these magmatic zircons represent the crystallization ages of the studied mafic dykes. Nineteen zircon grains from sample D0020 (subophitic-textured dolerite) were analysed, among them, one zircon grain is dark in CL without magmatic zoning structure, and its 206 Pb/ 238 U age of 743 Table S1), which are interpreted to be zircon xenoliths captured during the mafic magma emplacement. Thus, the age of 823-817 Ma is interpreted as the crystallization age of the Mianning subophitic-textured dolerite (group I) in the western Yangtze Block.    Twenty-four zircon U-Pb isotopic analyses were acquired from sample PM402-39 (intergranular dolerite). The analysed zircon grains have apparent 206 Pb/ 238 U ages of 808 ± 10 to 783 ± 9 Ma, yielding a weighted mean age of 794 ± 4 Ma (MSWD = 0.48, Figure 4c-d), which is explained as the crystallization age of the sample PM402-39. Twenty-one grains from sample PM402-95 (intergranular dolerite) were analysed. Two xenocrystal zircons showing dark cores mantled by newly grown rim yield apparent 206 Pb/ 238 U ages from 881 ± 10 to 822 ± 9 Ma ( Figure   S1d, Figure 4g;
Zr, and Hf) (Table S2; Figure 6a-b). In the chondritenormalized REE patterns, the group I dykes show relatively low REE contents of (71.0-94.9 ppm) and weak to unobvious REE fractionation without significant Eu anomalies (δEu = 0.84-1.05), exhibiting trace element compositions identical to E-MORB ( Figure 6a). In contrast, the group II dykes show much higher total REE contents of 125-181 ppm. They display slightly stronger REEs fractionation with (La/Yb) N values of 1.84-4.51 that are similar to those of oceanic island basalt (OIB) in the REE patterns (Sun and McDonough 1989; Figure 6c). Primitive mantle-normalized trace element spider diagrams illustrate that the group I dykes are relatively enriched in large ionic lithophile elements (LILEs, such as Rb, Ba, Th, U, and K), and depleted in Nb and Ta (Figures 6b and 6d). Group II dykes are more enriched in LILEs and less depleted or even more enriched in HFSE (such as Zr, Hf, and Ti) and Y (Figures 6b and 6d).

Whole-rock Sr-Nd and zircon Lu-Hf isotope
The whole-rock Sr-Nd isotopic compositions are presented in Supplementary Table S3. The initial Sr and Nd isotope ratios and epsilon Nd values (εNd) of the group I and group II dykes were calculated using their average crystallization ages of 820 and 795 Ma, respectively (  Figure 6. Chondrite-normalized REE and primitive mantle-normalized trace element patterns for samples from the studied mafic dykes. Normalizing values are after Taylor and McLennan (1985) and Sun and McDonough (1989) (Table S3). Similarly, the analyses in the sample PM402-95 have positive εHf(t) values of 0.76 to 5.91 and exhibit T DM1 ages of 1.14-1.33 Ga (Table S4).

Timing of neoproterozoic mafic magmatism in western Yangtze block
In recent decades, an increasing number of Neoproterozoic mafic igneous rocks in the western Yangtze Block have been reported and studied, including the ca. western Yangtze Block is not consistent with the mantle plume model but is similar to the typical arc-related subduction model, such as the Mesozoic to early Cenozoic magmatic arcs in the western North American Cordillera (Ducea et al. 2015). Previous studies on the Neoproterozoic mafic igneous rocks in the western Yangtze Block have provided good data for understanding the variations in mantle sources and regional tectonic evolution, although the petrogenesis and tectonic implications of these igneous rocks are still controversial (Yan et al. 2004;Sun et al. 2007;Xiao et al. 2007;Du et al. 2014;Meng et al. 2015;Zhao et al. 2017). For example, existing studies on mafic dyke swarms show that there are at least three episodic emplacement events of mafic dyke swarms in the western Yangtze Block, South China: 800-790 Ma, 790-780 Ma, and 780-730 Ma (Lin et al. 2007;Zhu et al. 2008;Yang et al. 2017;Li et al. 2019). The group II mafic dykes in this study have weighted mean 206 Pb/ 238 U ages of ca. 795 Ma, which is almost the same as the previously reported first episode of mafic dyke magmatism in the western Yangtze Block. The ca. 795 Ma episodic emplacement of mafic dyke swarms is synchronous with the flare-up event of mafic magmatism in the western Yangtze Block, which may represent the peaking of regional lithospheric extension (Figure 8). The group I mafic dyke swarms in this study have weighted mean 206 Pb/ 238 U ages of 823-817 Ma, indicating the presence of emplacement of early Neoproterozoic mafic dyke swarms in the western Yangtze Block.

Cumulation, crustal contamination and fractional crystallization?
The studied mafic dyke swarms show subophitic or intergranular texture and comprise a large amount of fine-grained plagioclase and clinopyroxene crystals with less than 10% anhedral clinopyroxene phenocrysts (Figure 3c-d), precluding their cumulate origins. Their high contents of total REEs (71.0-94.9 ppm and 125-181 ppm, respectively) without significant positive Eu anomalies ( Figure 6) also suggest that the studied dykes are not derived by the cumulation of pyroxene and plagioclase crystals (Escuder-Viruete et al. 2014;Otamendi et al. 2016;Chin et al. 2018).
As discussed above, the studied dykes have high concentrations of compatible elements, such as V (183-256 ppm and 182-313 ppm, respectively) and Cr (140-380 ppm and 110-500 ppm, respectively). They have slightly enriched or weakly depleted Sr-Nd-Hf isotopes, which are significantly different from the crustal basement of the Yangtze Block (Table S3; Figure 7). These geochemical features, as well as their low SiO 2 (45.9-49.1 wt.% and 43.9-50.1 wt.%, respectively) and relatively high contents of MgO (7.62-8.33 wt.% and 6.30-10.45 wt.%, respectively) and FeO T (8.44-12.08 wt. % and 8.17-14.84 wt.%, respectively), suggest their mantle origins (Shervais 1982;Sun and McDonough 1989). The concentrations of high field strength elements (such as Th, Zr, Hf, Nb, Ta, and Y), REE, and transition metals (such as Cr and V), the values of Mg#, and the ratios of Sr-Nd-Pb isotopes may help to reveal the magmatic processes such as crustal contamination and fractional crystallization. Minor crustal contamination might result in depletion of Nb and Ta but enrichment of Zr and Hf due to their enrichment in crustal materials (Halliday et al. 1995;Gao et al. 2004;Castillo et al. 2007). The studied dykes show significant depletion of Nb and Ta with minor enrichment of Zr and Hf in the spider diagrams (Figures 6b and 6d), suggesting the possibility of crustal contamination. However, their low contents of SiO 2 , high contents of Cr and V, varied and moderate Mg# (52-64 and 44-64, respectively), nearly flat REEs patterns ((La/Yb) N = 1.38-1.81, and 1.84-4.51, respectively), and relatively homogenous and depleted zircon εHf(t) values (−2.59 to 2.11 and −0.74 to 5.91, respectively; Figure 9), argue against the intensive occurrence of crustal contamination. In addition, there is no positive correlation between the whole-rock initial ɛNd(t) values and Mg# of the studied dykes (Figure 10a), which excludes significant crustal contamination. Thus, we propose that the studied mafic dykes do not show significant crustal contamination and that their trace element ratios and Sr-Nd-Hf isotopes could be used to constrain their mantle sources.
The studied mafic dykes have variable Mg# values (52-64 and 44-64, respectively), which are much lower than those of primitive mantle melts (Mg# = 73-76; Kinzler and Grove, 1992), suggesting that these samples represent evolved rather than primary melts. There is a wide range in the concentrations of SiO 2 (45.9-49.1 wt.% and 43.9-50.1 wt.%, respectively), V (183-256 ppm and 182-313 ppm, respectively), and Cr (140-380 ppm and 110-500 ppm, respectively), which together with the moderate Mg#, indicates that the studied mafic dykes have experienced a certain degree of fractional crystallization. The positive correlations between Mg# and MgO, Al 2 O 3 , and Cr highlight the significant fractionation of olivine and clinopyroxene (Figure 10b-d). The negative correlations between Mg# and FeO T and TiO 2 may have resulted from the accumulation of Fe-Ti oxides (Figure 10e-f). The relatively flat REE patterns (Figure 6), as well as the constant Dy/Yb ratios with decreasing Mg# (Figure 10g), suggest that amphibole and garnet did not significantly fractionate from the primary magma. The absence of any pronounced Eu anomalies in the REE patterns ( Figure 10h) and their high Al 2 O 3 content (16.20-16.75 wt.% and 14.25-16.30 wt.%, respectively), argues against significant fractional crystallization of plagioclase. Thus, we propose that the studied mafic dykes may have experienced fractional crystallization of olivine and clinopyroxene with a certain degree of accumulation of the Fe-Ti oxides.

Nature of the mantle sources and their heterogeneities
Mantle peridotite usually contains various types and amounts of aluminium-rich phases at different depths, that is, plagioclase at low pressures, spinel at medium pressures, and garnet at higher pressures (Sun and McDonough 1989;White 2010). All REEs are strongly incompatible in spinel, whereas heavy REEs (HREE) are preferentially partitioned into garnet (Nicholls and Harris 1980). Thus, the abundances of REEs in mantle-derived mafic rocks are particularly useful for determining mantle sources and evaluating their melting degrees.
Although the curves in Figure 11 can vary with different melting models, distribution coefficients, and mantle source composition, the variable (Yb/Sm) P values with constant (Tb/Yb) P values make the conclusion that melting occurred in spinel-facies rather than the garnetfacies mantle, which was difficult to avoid. The low (Tb/Yb) P ratios (< 1.5) further suggest that their primary melts were derived from spinel-bearing peridotites ( Figure 11; Zhang et al. 2006). The variable (Yb/Sm) P values also imply that the group I and group II dykes were derived from varying degrees of melting of similar mantle sources, i.e. 10 − 15% and 5 − 10%, respectively ( Figure 11). The variable degree of melting could explain their varied REE contents and the variations in the incompatible element ratios, such as Th/Y, Nd/U, and Sm/Th (Figure 12a-c). However, it cannot explain why they have a wide range of Sr−Nd−Hf isotopic composition (Figures 7 and 9 Age (Ma) ɛHf(t)

D e p l e t i o n t r e n d
Group Ⅰ dykes (sample D0020) Group Ⅱ dykes (sample PM402-39) Group Ⅰ dykes (sample D0021) Group Ⅱ dykes (sample PM402-95) Neoproterozoic mafic igneous rocks in western Yangtze Block Figure 9. Plots of zircon εHf(t) values versus zircon U-Pb ages for the Neoproterozoic mafic rocks in the western Yangtze Block, South China. The data sources are the same as . Figure 8 εHf(t) = −0.74 to 5.91). Since crustal contamination has been excluded, as discussed above, this variable Sr−Nd −Hf isotopic composition may indicate that their chemical compositions are also controlled by other factors besides the various degrees of partial melting. Primitive mantle-normalized trace element spider diagrams show that the group I dykes exhibit Nb, Ta, P, and Ti depletion, resembling arc-related mafic magmas (Castillo et al. 2007;Pearce 2008). However, in contrast to the slight enrichment of Sr in the group I dykes, the group II dykes show depletion of Sr, which is different from most arc-related igneous rocks ( Figure 6; Otamendi et al. 2016;Reagan and Gill 1989), implying their different origins. The incompatible element ratios of mafic rocks, such as Th/Y, Nd/U, Nb/Yb, Th/Yb, and Ta/Yb, are critical for revealing the nature of mantle sources, because the ratios of two incompatible elements with similar geochemical behaviour can exclude the effects of partial melting and fractional crystallization (Wood et al. 1979;Castillo et al. 2007; White 2010). Both groups of dykes have high ratios of Th/Y and (La/Yb) N but low ratios of Sm/Th, Nd/U, and Nb/Yb, which are akin to E-MORB or OIB (Figure 12a−c). The significant variation in incompatible element ratios and their wide range of ( 87 Sr/ 86 Sr)i, εNd(t), and εHf(t) values, suggests enriched mantle affinity. Mantle enrichment was probably controlled by slab-related fluids or melts in the subduction zone (Castillo et a. 2007;Pearce 2008;Zhao et al. 2019). The partitioning of LILEs, REE, and HFSEs between aqueous fluids and melts is different; therefore, LILEs (such as Rb, Ba, Sr, and U) can be transported effectively by fluid phases, whereas REEs and HSFEs are mainly mobilized in the melt rather than fluid phases (Hawkesworth et al. 1993;Class et al. 2000). Therefore, mafic magmas with high Rb/Y and Th/Yb but low Nb/Y, Nb/Yb, and Ta/Yb are commonly interpreted to originate from a mantle source enriched by aqueous fluid, while mafic magmas that have high Nb/Y, Nb/Yb and Ta/Yb are thought to be derived from a mantle source modified by slab melts (Class et al. 2000;Castillo et al. 2007;Pearce 2008). The group I dykes all have high ratios of Rb/Y (3.14-5.27) and Th/Yb (0.14-0.59), but relatively low ratios of Nb/Y (0.04-0.06), Nb/Yb (0.35-0.58) and Ta/Yb (0.04-0.06), suggesting the involvement of significant slab-related fluid in their origin (Figure 12d-f). In contrast, the group II dykes have low ratios of Rb/Y (0.66-3.37, except for one sample with 7.22) but relatively high ratios of Nb/Y (0.09-0.25), Nb/Yb (0.86-2.98) and Ta/Yb (0.08-0.20), suggesting the addition of significant slab-derived melt in their petrogenesis (Figure 12d−f). Varied degrees of the interaction of the slab-derived fluid/ melts with the mantle peridotite can lead to the generation of an elementally-isotopically heterogeneous mantle source, which may be the reason for the large variations in Sr−Nd−Hf isotopes and immobile element ratios of the group I and group II dykes.
Melt-related metasomatism also explains the high content of HFSEs in the group II dykes. As shown in Figure 6, the group II dykes share some geochemical characteristics similar to those of the island arc basalts (such as enrichment in LILEs), but they have higher Nb, Ta, and other HFSEs than most arc basalts (Figure 6d). Group II dykes contain relatively high Nb contents (4-10 ppm; Table S2) with high Nb/U values (6.93-29.3), resembling typical Nb-enriched basalts (NEBs; Castillo et al. 2007; Figure 5d). Previous studies have suggested that (1) magma mixing between OIB-like melt and depleted MORB-type melt and (2) metasomatism of the mantle wedge by slab-related melts are the most common mechanisms for the generation of NEBs (Reagan and Gill 1989;Castillo et al. 2007;Hastie et al. 2011). Their tholeiitic characteristics (Figure 5b−c), E-MORB-like REEs distribution pattern (Figures 6c), enriched to slightly depleted Sr−Nd−Hf isotopes (Figures 7 and 9), arcrelated trace element spidergram and incompatible element ratios (Figures 6d and 12) are akin to those generated in a subduction zone rather than intraplate lavas, . (Tb/Yb) P versus (Yb/Sm) P for the studied groups of mafic dykes in western Yangtze Block. All the ratios are normalized by primitive mantle. The grid indicates the range of model melt compositions produced by 1%, 5%, 10%, and 15% of aggregated fractional melting of peridotite in which the amount of melting that occurs in the presence of garnet varies from 0 to 100%. The mantle source, mineral assemblage, partition coefficients are as described by . Zhang et al. (2006) arguing for the model of melt-related mantle wedge metasomatism accounting for the petrogenesis of the group II mafic dykes.

Implications for the neoproterozoic tectonic evolution of western Yangtze block
Determining the tectonic settings of the early Neoproterozoic magmatic rocks in the western Yangtze Block is the key to the palaeogeographic reconstruction of the Rodinia supercontinent, but it remains controversial. Some authors have suggested an active continental arc setting (Cawood et al. 2013(Cawood et al. , 2020Hu et al. 2020;Wang et al. 2021;Yan et al. 2008;Zhao et al. 2017;Zh2019;Zhou et al. 2002), whereas others have argued for a mantle plume setting (Li et al. , 2003. This controversy stems primarily from insufficient research on the temporal evolution of magma sources. Group I and group II dykes in this study have moderate to high contents of TiO 2 (0.81-1.52 wt.% and 1.62-2.68 wt.%, respectively), resembling the MORB or OIB in typical tectonic discrimination diagrams (Figure 13a). However, the two groups of mafic dykes exhibit increasing tendency of FeO T and TiO 2 with decreasing Mg# values Thus, we propose that the V-Ti discrimination diagram does not apply to the studied dykes. Herein, we used high-field strength elements with good stability for tectonic setting discrimination. The results demonstrate that all the studied dykes were formed in a subduction arc environment (Figure 13b-d).
Meanwhile, the Sr-Nd-Hf isotopic compilation of Neoproterozoic mafic igneous rocks in the western Yangtze Block reveals that the mantle sources were enriched as early as 880-870 Ma and a depletion trend appeared after 830-820 Ma (Figures 7 and 9). Considering that the mafic rocks before 830-820 Ma have arc-related geochemical affinities (including enrichment in LILEs and depletion in HFSEs), such as the studied group I dykes, the ca. 877 Ma Liujiaping gabbro (Xiao et al. 2007) and ca. 856 Ma Guandaoshan gabbro (Du et al. 2014), we propose that the early mantle metasomatism is controlled by progressive slab subduction and associated LILE-enriched but HFSE-depleted slab fluid. However, most of the reported 820-780 Ma mafic rocks in the western Yangtze Block exhibit high Th/La and Nb/Zr ratios (Figure 14), suggesting that late mantle metasomatism was mainly dominated by LILE-depleted but HFSE-enriched slab melts (Halliday et al. 1995;Pearce et al. 1999). The variation in the metasomatic mechanism was also recorded in the zircon oxygen isotopes of Neoproterozoic gabbros in the western Yangtze Block, that is, the 870-830 Ma gabbros have overall normal or relatively high δ 18 O values (4.79-7.42‰) with an increasing trend, while the 820-780 Ma gabbros have relatively low δ 18 O values (4.22-5.49‰) with a decreasing trend . The low δ 18 O values, and their overall high εHf(t) and εNd(t) values compared with the 870-830 Ma high δ 18 O mafic rocks (Figures 7b and 9), require an additional  Shervais (1982)); (b-c) Hf/ 3-Th-Ta and Hf/3-Th-Nb/16 diagrams (after Wood et al. (1979)); (d) 2*Nb-Zr/4-Y diagram (after Meschede (1986)).
metasomatic component for the generation of the 820-780 Ma mafic rocks. As discussed above, subducted basaltic slab-derived melts may be the most reasonable candidates for the mantle metasomatism. Several studies have reported that oceanic crust melts typically have δ 18 O values of approximately 2-5‰ (Bindeman et al. 2005), which may have contributed to the low δ 18 O mantle sources of the 820-780 Ma mafic rocks in the western Yangtze Block. The significant involvement of oceanic crust-derived melts in the mantle sources of the 820-780 Ma Neoproterozoic mafic rocks may have been a response to the geodynamic evolution of the subducted slab. We further propose that Neoproterozoic slab break-off or slab tearing, rather than mantle plume, accounts for the transition of the mantle metasomatic mechanism (see discussed below). Compared with the extensive middle Neoproterozoic continental rifting-related anorogenic magmatism developed in other Rodinia blocks or cratons, such as Tarim (Xu et al. 2005;Zhu et al. 2008;Li et al. 2020), eastern Australia (Li et al. 1995) and western Siberia (Likhanov and Santosh 2017), those in the western Yangtze Block mostly exhibit arc-or orogeny-related geochemical affinities. For example, the ca. 780 Ma Mianning granites and the ca. 800 Ma Daxiangling granites are slightly peraluminous, high-K calc-alkaline series, and show petrological and geochemical characteristics of A2-type granite rather than within-plate rifting-related A1-type (Eby 1992), indicating an orogeny-related extension setting (Huang et al. 2008;Zhao et al. 2008). Widespread younger TTG-type and S-type granitoids, such as the ca. 750 Ma Xuelongbao adakitic plutons ) and the ca. 750 Ma Kuchahe S-type granites , indicate that oceanic subduction in the western Yangtze Block lasted at least until the mid-Neoproterozoic (ca. 750 Ma). This long-lived subduction has also been recorded in metamorphic rocks, for example, the amphibolite-granulite facies metamorphic event caused by subduction compression lasted for a long time with monazite metamorphic ages of 885-778 Ma Li et al. 2021).
In summary, the low δ 18 O values and high εHf(t) and εNd(t) values of the 820-780 Ma mafic rocks in the western Yangtze Block imply significant addition of the oceanic crust-derived melts into the mantle source. Since the oceanic slab subduction, associated slab dehydration, and fluid-dominated metasomatism started no later than 870 Ma in the western Yangtze Block, the further melting of the old and cold oceanic crust during 820-780 Ma required abnormally high temperatures to elevate the thermal gradient (Hawkesworth et al. 1993;Hastie et al. 2011;Ducea et al. 2015;Zhao et al. 2019). Combined with the long-lived oceanic subduction in the western Yangtze Block , the presence of Neoproterozoic slab-derived adakitic rocks, such as the Xuelongbao pluton (ca. 750 Ma; εNd(t) = 0.36-2.88; , and the Datian pluton (ca. 760 Ma; εNd(t) = −0.92-0.66; Zhao and Zhou 2007), we propose that abnormally high temperatures are most likely a result of slab tearing or break-off because these processes usually result in large-scale slab melting (Ducea et al. 2015;Wang et al. 2018;Zhao et al. 2019). The ca. 815-780 Ma, OIB-like mafic dykes and basalts in the western Yangtze Block were probably the products of the upwelling asthenospheric mantle through the slab windows Lin et al. 2007;Cui et al. 2015), whereas the TTG-type or adakitic granitoids were the responses to the following oceanic slab melting Zhao and Zhou 2007;Luo et al. 2018). The Neoproterozoic mafic magmatism in the northern Yangtze Block also exhibits a transition trend from arc affinity to MORB-and OIB-like characteristics at around 820 Ma, such as the ca. 820 Ma Wangjiagnshan  Figure 14. Plots of trace element ratio versus crystallization age for the Neoproterozoic mafic rocks in the western Yangtze Block. The data sources are the same as . Figure 8. The value of N-MORB is from Sun and McDonough (1989) mafic-ultramafic intrusion, ca. 815 Ma Dahongshan mafic pluton, and ca. 780 Ma Bijigou gabbro (Dong et al. 2011;Liu and Zhao 2019;Zhang et al. 2020), which may also have resulted from slab tearing or break-off. Therefore, combined with the published data of magmatic and metamorphic rocks, we propose that the tectonic transition from syn-subduction compression to continental arc extension occurred at ca. 830-820 Ma, although the data presented in this study are not sufficient to identify whether the extension was triggered by slab tearing or break-off. In our proposed tectonic model, the ongoing extension would lead to the upwelling of the asthenospheric mantle followed by subsequent melting to form the ca. 820-780 Ma OIB-like magma. The upwelling asthenosphere might heat the subducted oceanic slab and promote its partial melting to generate adakitic melts, followed by melt-mantle interaction and melting to generate NEBs, such as the studied group II dykes (Figure 15a). Recent studies have demonstrated that the ca. 780 Ma MORB-and OIB-like large igneous province magmatism was widely developed in NW India . The ca. 800-700 Ma rocks in NW India exhibit a decreasing trend in zircon δ 18 O values but an increasing trend in zircon εHf(t) values , which is similar to the case in the western Yangtze Block , implying that a common Neoproterozoic slab tearing or slab break-off may have occurred in the western Yangtze and NW India. Interestingly, the 820-780 Ma oceanic slab tearing or break-off in the western Yangtze and NW India is simultaneous with the Rodinia break-up (Cawood et al. 2013(Cawood et al. , 2018Li et al. 1995Li et al. , 2003Zhao et al. 2019), suggesting that the extension triggered by slab tearing or break-off may be a key mechanism for the break-up of Rodinia. The data presented in this study, therefore, indicate that the western Yangtze Block is located on the periphery of the Rodinia supercontinent (Figure 15b), just like Greater India and Madagascar, which have similar arc-related Neoproterozoic magmatism (Cawood et al. 2013(Cawood et al. , 2018Wang et al. 2018Wang et al. , 2020, suggesting a large-scale Andean-type subduction system along western Rodinia.

Conclusion
(1) Petrology and LA-ICP-MS zircon U-Pb ages indicate that the two groups of mafic dyke swarms in the western Yangtze Block, that is, the subophitictextured dolerite and intergranular dolerite, were emplaced at ca. 823-817 Ma and ca. 795-794 Ma, respectively. (2) Geochemical and Sr-Nd-Hf isotopic studies suggest that the ca. 823-817 Ma and ca. 795-794 Ma mafic dykes were derived from different degrees of melting of the enriched mantle wedge that was metasomatized by subducted slab-related fluids and melts, respectively. Varying degrees of mantle metasomatism accounted for the elementalisotopic heterogeneity of the studied mafic dykes. (3) The geochemical database compilation of Neoproterozoic igneous rocks in the western Yangtze Block suggests that slab tearing or breakoff, rather than mantle plume, may account for the transition of the mantle metasomatic mechanism at approximately 830-820 Ma. (4) The Yangtze Block may be located on the periphery of the Rodinia supercontinent, and its convergence and break-up may be related to slab subduction and subsequent slab tearing or break-off.

Plumes
Yangtze Block

Acknowledgments
We sincerely thank the Editor-in-Chief Prof. Robert Stern, as well as the reviewer Prof. Peter. A Cawood and two anonymous reviewers for their constructive comments and suggestions, which substantially improved our manuscript.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
Xiong acknowledges support from Chengdu University of Technology (2022ZF11412) and the National Science and Technology Major Project (No. 2017ZX05008-00W-009). We thank Wen Zhang and Keqing Zong for their help in the laboratory.