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Article

Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry

1
School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
2
Geological Survey of Anhui Province, Hefei 230001, China
3
School of Environment and Tourism, West Anhui University, Lu’an 237012, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(4), 562; https://doi.org/10.3390/min13040562
Submission received: 28 February 2023 / Revised: 31 March 2023 / Accepted: 15 April 2023 / Published: 17 April 2023

Abstract

:
The Yangtze Block records Neoproterozoic magmatism and sedimentation related to the breakup of Rodinia and is an important piece in the reconstruction of the supercontinent. However, the tectonic setting and position of this block in Rodinia remain a subject of debate. In the present study, we report the zircon U-Pb ages and Hf isotopic composition of zircon and geochemical and Nd-Pb isotopic compositions for meta-volcanic rocks exposed in the Zhangbaling uplift of the NE Yangtze Block. The volcanic rocks, dominated by rhyolite and dacite, belong to the calc-alkaline series and show geochemical characteristics of arc rocks. Zircon U-Pb isotopic ages show that volcanic rocks in the Xileng Formation formed at ca. 790 Ma and ca. 760–700 Ma peaking at ~740 Ma. The late-stage volcanism was widely exposed in the uplift, characterized by a temporal-spatial trend becoming younger southwards. The old volcanic rocks have low initial εNd (−11.0) and εHf (−19.7 to −8.2) values and low Pb isotopic ratios, likely indicating an origin from ancient basement rocks underneath the Yangtze Block. The younger ones, being similar to continental arc andesite in trace element compositions, have relatively high initial εNd (mostly −4.6 to 0.5) and εHf (−0.4 to 8.8) values and high Pb isotopic ratios. These isotopic features point to an origin from the partial melting of juvenile crustal rocks. Sedimentary rocks of the Xileng Formation and the overlying strata also contain numerous zircon grains of ~700 Ma to ~630 Ma. The volcanic rocks in the Zhangbaling uplift might demonstrate long-lasting subduction along the northeastern margin of the Yangtze Block, probably active until ca. 700 Ma.

1. Introduction

The Yangtze Block, a part of South China, preserves well Neoproterozoic magmatism and sedimentation related to the breakup of Rodinia and, hence, is important for the supercontinent reconstruction. Over the past decades, the genesis and tectonic evolution of the extensive Neoproterozoic igneous rocks within this block have received considerable attention. However, there are still debates, especially on the coeval tectonic setting, including three competing models of plume-rift, slab-arc, and plate-rift [1,2,3,4,5]. The position of the South China Block in Rodinia reconstruction also remains unclear [2,6,7,8,9]. Scholars have proposed that the South China Block was originally placed in the interior of Rodinia [1,2,6] or adjacent to India and East Antarctica during the breakup of Rodinia to the Gondwana assembly in the Neoproterozoic [9,10,11].
Low-grade metamorphosed sedimentary and volcanic rocks of Neoproterozoic ages are widely exposed in the Zhangbaling uplift of the northeastern Yangtze Block. They are termed the Zhangbaling Group and distributed along the Tan-Lu fault zone within Anhui Province. These rocks were correlated with low-T/high-P metamorphic rocks in the Dabie-Sulu orogenic belt, such as the Hongan Group and the Yuntai Formation of the Haizhou Group [12,13]. The Zhangbaling Group is also an important object for understanding the tectonic evolution of the Yangtze Block during the Neoproterozoic. The age of Zhangbaling volcano-sedimentary rocks was previously interpreted as Paleo- to Mesoproterozoic [14], early Neoproterozoic (975–925 Ma) [15,16], or middle Neoproterozoic (770–720 Ma) [17,18,19]. Therefore, the ages of these meta-volcanic and meta-sedimentary rocks still need to be precisely constrained for clarifying the tectonic evolution of the Yangtze Block. In the present study, we report the U-Pb ages and Hf isotopic composition of zircon and bulk rock geochemical and Nd-Pb isotopic compositions of meta-volcanic rocks from the Zhangbaling uplift in the northeastern Yangtze Block, and discuss the nature of the magmas, magmatic and crustal evolution, and their tectonic relationship with Rodinia.

2. Geological Setting and Samples

The Yangtze Block is one of the most important tectonic units in China. This block collided with the North China Block along the Qinling-Dabie-Sulu orogenic belt in the Early Mesozoic [20]. The Dabie and Sulu orogenic belts were later offset by the Tan-Lu fault zone for ca. 350 km [21,22]. The Zhangbaling uplift, named after the wide exposure of greenschist-facies metamorphosed volcano-sedimentary rocks of the Zhangbaling Group, is located between the Dabie and Sulu belts along the Tan-Lu fault zone in eastern China and strikes NNE-ward with a length of ca. 150 km (Figure 1).
The Zhangbaling Group is exposed mainly in the northern segment of the uplift, locally on the eastern side of the southern Zhangbaling uplift, and sporadically along the southeastern edge of the Dabie orogenic belt (Figure 1). The major outcrops are located mainly in the Chuzhou, Chaohu, Lujiang, and Susong areas from north to south. The Zhangbaling Group was subdivided into the Beijiangjun and Xileng formations bottom to top (Figure 2). The Xileng Formation unconformably overlies the Beijiangjun Formation and previously was considered to be the upper part of this group. The Beijiangjun Formation consists mainly of low greenschist-facies metamorphosed clastic rocks, with major rock types of phyllite and meta-sandstone. The Xileng Formation is dominated by medium to high greenschist-facies metamorphosed intermediate volcanic rocks with major rock types of keratophyre, quartz keratophyre, and quartz schist. From bottom to top, it can be further subdivided into a white mica schist unit, a quartz-feldspar schist unit, and a blue amphibole schist unit [23]. The Xileng Formation is unconformably overlain by the Zhougang Formation, which is composed of low greenschist-facies rocks dominated by phyllite, pebble-bearing phyllite, and meta-sandstone.
We have systematically investigated low-grade metamorphosed volcanic rocks of the Xileng Formation. Seventeen samples were collected from different areas of the Zhangbaling uplift, including the Chuzhou, Chaohu, Lujiang, and Susong areas. These samples are mainly quartz keratophyre with minor keratophyre and quartz schist. The quartz keratophyres are mainly light gray in color with porphyritic textures and massive structures. They consist mainly of quartz (~40 vol.%–50 vol.%) and albite (~40 vol.%–50 vol.%) and their matrix is composed of microcrystalline albite (~40 vol.%–50 vol.%), quartz (~30 vol.%–40 vol.%), and sericite (~5 vol.%–10 vol.%). The quartz schists are mainly grayish-white in color with crystalloblastic texture and medium-bedded structure. They are composed mainly of quartz (~50 vol.%–60 vol.%), albite (~30 vol.%–35 vol.%), and sericite (~10 vol.%).

3. Analytical Methods

Rock samples of about 5 kg in weight were crushed for the zircon separation and fresh crushed pieces of ~20 g were milled for whole-rock powder in an alloy mortar. The contents of major elements were analyzed in fused lithium borate glass beads by X-ray florescence spectrometry (XRF) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences CAS. Calibration lines used in the quantification were produced by a bivariate regression of data from 36 reference materials encompassing a wide range of silicate compositions, and analytical uncertainties were between 1% and 5%. The contents of trace elements were analyzed on an Agilent 7500a ICP-MS in the Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China (USTC). Powdered samples were dried at 105 °C for 4 h. To achieve complete dissolution, ~50 mg of material was dissolved in a mixture of HF + HNO3 solution in Teflon bombs at 190 °C for 72 h. Dissolved samples were then diluted to 80 mL by the addition of 2% HNO3 solution prior to the analysis. The precision and accuracy of the analyses were better than 5% for trace elements, as estimated by analysis of the USGS rock standards BHVO−2 and AGV-2. Analyses of whole-rock Sm-Nd and Pb isotopic composition were performed at the Laboratory for Radiogenic Isotope Geochemistry, USTC. Elements were isolated from each other by standard chromatographic separation techniques. Measured Nd isotopic ratios were normalized to 146Nd/144Nd of 0.7219. Standard solutions of NBS987 for Sr and La Jolla for Nd were measured during the analysis. The precision of the 147Sm/144Nd ratios was better than 0.5%. The precision of the measured Pb isotopic ratios was better than 0.01%. Further details on the analytical procedures are described elsewhere [25].
Zircon grains were extracted from rock samples by standard crushing, sieving, heavy liquids, and magnetic separation techniques. Transparent zircon grains without cracks were subsequently handpicked under a binocular microscope, mounted in epoxy resin, and then polished down to expose grain centers. Prior to the U-Pb isotopic dating, zircon crystals were imaged by the cathodoluminescence (CL) technique. CL image and zircon U-Pb dating were performed at the USTC. The detailed analytical procedures are reported elsewhere [26]. Zircon 91500 was used as an external standard to correct isotopic fractionation during analysis. NIST610 was used as an external standard to normalize the U, Th, and Pb contents of the unknowns. Raw data were processed using the GLITTER program [27]. A common Pb correction was applied using the Andersen method [28]; data were then calculated and plotted using the ISOPLOT program [29]. The uncertainties of individual analyses were reported at the 1σ level and weighted average ages were calculated at the 2σ level. Zircon Hf isotopic analysis was performed using a double-focusing multi-collector Neptune MC-ICP mass spectrometer at USTC. Analytical procedures are given elsewhere [30]. A 175Lu/176Lu value of 0.02655 was used for the isotopic fractionation correction. Isobaric interference of 176Yb on 176Hf was corrected using the mean fractionation index of Iizuka [31]. A 176Hf/177Hf value of 0.282007 ± 7 (2σ, n = 36) was obtained for the standard zircon GJ1.

4. Results

4.1. Zircon U-Pb Isotopic Ages and Hf Isotopic Composition

Eight volcanic rock samples of the Zhangbaling Group, collected from different localities, were selected for the zircon U-Pb isotopic dating, including three from Chuzhou and two from Feidong in the north, one from Lujiang and two from Susong in the south. The analytical results are given in the Table S1 in Supplementary Materials and are summarized in Table 1. Most zircon grains from meta-volcanic rocks are colorless and transparent, and euhedral to subhedral, with sizes ranging from 100 μm to 200 μm in length, with length-to-width ratios between 1:1 and 3:1. In the CL images, most of the grains exhibit weak to clear oscillatory zoning (Figure S1 in Supplementary Materials). In general, the analyzed grains had variable U (14–410 ppm) and Th contents (6–3580 ppm), but all of them had high Th/U ratios of 0.33–6.11, indicating magmatic origin [32,33]. In this study, 206Pb/238U ages are used for zircon grains younger than 1.0 Ga and 207Pb/206Pb ages for the older ones [34].
Chuzhou area: Sixty grains from sample XL02 were analyzed, but some of them were discarded for being highly discordant (Figure 3a). Fifty-two grains gave U-Pb isotopic ages ranging from 3390 ± 29 Ma to 129 ± 3 Ma. Broadly, these concordant analyses fell into three age populations according to their 206Pb/238U or 207Pb/206Pb ages. Thirty grains yielded Paleo-Archean to Paleo-Proterozoic ages of 3390 ± 29 Ma to 2026 ± 61 Ma, with a minor peak at ~2.5 Ga. They can be interpreted as captured zircon grains, indicating the existence of ancient basement rocks underneath the Yangtze Block. Seventeen grains gave Neoproterozoic ages of 922 ± 15 Ma to 670 ± 13 Ma, and fifteen of them yielded a weighted mean 206Pb/238U age of 790 ± 11 Ma (MSWD = 2.5), interpreted as the formation time of this volcanic rock. Five grains had very young ages from 211 ± 6 Ma to 129 ± 3 Ma, perhaps resulting from Pb-loss in late thermal event(s). Twenty-six out of thirty-two grains from sample XL06 had concordant 206Pb/238U ages ranging from ~830 Ma to 720 Ma (Figure 3b). Sixteen of them yielded a weighted mean 206Pb/238U age of 791 ± 10 Ma (MSWD = 2.9), likely indicating captured grains of an early magmatic event. The remaining ten grains gave younger ages between 757 ± 17 Ma and 723 ± 12 Ma, yielding a weighted mean age of 741 ± 7 Ma (MSWD = 0.7), interpreted as the formation time of quartz keratophyre. Twenty-three out of thirty-two grains from sample XL13 displayed concordant 206Pb/238U ages of ~820 Ma to 720 Ma (Figure 3c). They clustered around two age peaks with weighted mean ages of 795 ± 7 Ma (n = 13, MSWD = 1.1) and 746 ±6 Ma (n = 10, MSWD = 0.7). The latter is interpreted as the intrusion age and the former as an early magmatic event recorded in the captured zircon grains.
Feidong area: Twenty-four out of thirty-three grains from sample XL15 had concordant 206Pb/238U ages ranging from 830 ± 10 Ma to 688 ± 9 Ma (Figure 3d) and ten of them yielded a weighted mean 206Pb/238U age of 789 ± 11 Ma (MSWD = 0.16), indicating an early magmatic event. The remaining fourteen grains gave younger ages between 763 ±11 Ma and 718 ± 7 Ma, yielding a weighted mean age of 732 ± 6 Ma (MSWD = 1.4), interpreted as the formation time of this quartz keratophyre. Sixteen out of twenty-eight grains from sample XL17 had concordant 206Pb/238U ages of 815 ± 14 Ma to 254 ± 4 Ma (Figure 3e). They clustered around two age peaks with weighted mean ages of 790 ± 11 Ma (n = 9, MSWD = 1.1) and 747 ± 16 Ma (n = 5, MSWD = 0.5), interpreted as the ages of an early magmatic event and the extrusion of this volcanic rock. Two grains showed extremely young ages of 255 ± 4 Ma and 254 ± 5 Ma, likely representing Pb-loss during a later thermal event.
Lujiang area: Twenty-five out of thirty-two grains from sample XL18 had concordant 206Pb/238U ages ranging from 827 ± 19 Ma to 707 ± 10 Ma (Figure 3f). Fifteen grains gave a weighted mean age of 789 ± 8 Ma (MSWD = 1.3), indicating an early magmatic event. The remaining ten grains had younger ages of 754 ± 13 Ma to 721 ± 15 Ma, yielding a weighted mean age of 738 ± 7 Ma (MSWD = 0.7), interpreted as the crystallization time of quartz keratophyre.
Susong area: Twenty-two out of thirty-two grains from sample XL23 had concordant 206Pb/238U ages ranging from 814 ± 11 Ma to 683 ± 15 Ma (Figure 3g). Eight grains yielded an old weighted mean 206Pb/238U age of 791 ± 15 Ma (MSWD = 1.7), while the other fourteen grains had ages of 748 ± 13 Ma to 720 ± 13 Ma, yielding a weighted mean age of 736 ± 7 Ma (MSWD = 0.3), interpreted as the formation time of this rock. Twenty-three out of thirty-two grains from sample XL24 displayed concordant 206Pb/238U ages of 847 ±18 Ma to 697 ± 10 Ma (Figure 3h). They clustered around two age peaks with weighted mean ages of 797 ± 9 Ma (n = 13, MSWD = 1.2) and 739 ± 8 Ma (n = 10, MSWD = 0.7), interpreted as the ages of an early magmatic event and the extrusion of this volcanic rock.
Zircon grains from three meta-volcanic rock samples of the Xileng Formation were analyzed for Hf isotopic composition. The analytical results are given in the Table S2 in Supplementary Materials and the data are shown in Figure 4a. The initial εHf values were calculated back to the average zircon crystallization ages of each sample. The analyzed grains had variable Hf isotopic compositions ranging from positive to strongly negative initial εHf values. Fifteen zircon grains from sample XL02 gave initial 176Hf/177Hf ratios of 0.280744 to 0.282002. Their initial εHf values varied from −19.7 to −8.2 and TDM values from 2.31 to 4.07 Ga. Eighteen zircon grains were analyzed for each sample XL06 and XL23, respectively. These grains yielded initial 176Hf/177Hf ratios of 0.282310 to 0.282542. Most of them had positive initial εHf values between 0.7 and 8.8 and TDM values of 1.01 Ga to 1.32 Ga. Only one grain gave a negative initial εHf value of −0.4 with a TDM value of 1.66 Ga.
Based on the results of the zircon ages and Hf isotopic compositions mentioned above, we subdivide the volcanic rocks of the Xileng Formation into two stages: the early-stage of ~790 Ma and the late-stage of 760–700 Ma peaking at ca. 740 Ma.

4.2. Whole-Rock Geochemical and Nd-Pb Isotopic Compositions

Whole-rock major and trace elemental contents for seventeen samples of meta-volcanic rocks are given in the Table S3 in Supplementary Materials. They include two samples chosen from the early-stage volcanic rocks and fifteen from the late-stage volcanic rocks.
The early-stage volcanic rocks had high SiO2 (66.78 wt.%–68.95 wt.%) and Al2O3 (15.37 wt.%–15.9 wt.%) and low MgO contents (0.69 wt.%–0.9 wt.%). Two samples (XL13 and XL18) had low Na2O and very high K2O-contents, and correspondingly high K2O/Na2O ratios. Most of the late-stage volcanic rocks showed similar geochemical characteristics in major elements to those of the early-stage volcanic rocks, with high SiO2 (67.4 wt.%–78.5 wt.%) and Al2O3 (12.6 wt.%–19.0 wt.%) and low MgO (0.03 wt.%–0.93 wt.%) contents. In the Nb/Y versus Zr/TiO2 diagram, the early-stage volcanic rocks are dominated by rhyolite and dacite (Figure 5a), while the late-stage volcanic rocks are dominated by trachyandesite. In the FeO versus FeO/MgO diagram, most volcanic rocks belong to the calc-alkaline series (Figure 5b).
In the chondrite-normalized REE patterns (Figure 6a), the early-stage volcanic rocks exhibited low REE contents, highly fractionated REE patterns (LREEs/HREEs of 8.1 to 16.5, (La/Yb)N of 24.6 to 36.7), and variable Eu/Eu* values of 0.81 to 1.32. The late-stage volcanic rocks had relatively high REE contents, moderately fractionated REE patterns (LREEs/HREEs of 3.5 to 9.1, (La/Yb)N of 2.4 to 9.7), and more negative Eu-anomalies (Eu/Eu* values of 0.54 to 1.06). In the primitive mantle–normalized trace element diagram (Figure 6b), all the volcanic rocks were significantly enriched in large ion lithophile elements (LILEs) and Pb but depleted in P and Ti; they also showed slightly negative Nb-Ta anomalies, but positive Zr-Hf anomalies. The early-stage volcanic rocks had significantly higher Sr contents and Sr/Y ratios (Sr/Y of 16 to 66) and fell in the adakitic area when plotted in the (La/Yb)N versus YbN diagram (not shown here). Combined with low MgO contents, they were of low-Mg adakitic affinity.
Whole-rock Nd and Pb isotopic data of thirteen volcanic rock samples are given in the Table S4 in Supplementary Materials. Sample XL02 of the early-stage volcanic rock had a low initial εNd value of −11.0 (TDM2 age of 2.35 Ga) and low initial Pb isotopic ratios (206Pb/204Pb of 15.91, 207Pb/204Pb of 15.32, and 208Pb/204Pb of 36.38). Samples of the late-stage volcanic rocks had relatively high initial εNd values of −4.6 to +0.5, except for sample XL20 (Figure 4b), when calculated back to 740 Ma. They displayed similar initial Pb isotopic compositions, i.e., 206Pb/204Pb of 16.15–17.08, 207Pb/204Pb of 15.43–15.51, and 208Pb/204Pb of 36.72–37.33 (Figure 7).

5. Discussion

The formation ages of the low-grade metamorphosed volcano-sedimentary rocks in the Zhangbaling uplift are crucial for understanding the tectonic evolution of the northeastern Yangtze Block during the Neoproterozoic. Traditionally, the protolith’s age for the Xileng Formation of the Zhangbaling Group was proposed to be early Neoproterozoic (975–925 Ma) [15,16], or middle Neoproterozoic (762–723 Ma) [17,18,19,39,40,41]. The sedimentary sequences of the Beijiangjun Formation were assumed to have formed in the Paleo- to Meso-Proterozoic [14]. Recent studies have shown that the Beijiangjun Formation was deposited in the late Neoproterozoic after the Xileng Formation [19].
Most of the volcanic rocks analyzed in this study contained concordant 206Pb/238U ages, clustering at two age peaks of ca. 790 and ca. 740 Ma. One volcanic rock (sample XL02) recorded an earlier formation at ca. 790 Ma and contained very old Paleo-Archean to Paleo-Proterozoic (3390 Ma to 2026 Ma) zircon grains. This volcanic rock was geochemically different from other rocks in elemental and Nd-Hf-Pb isotopic compositions. Based on these features, as summarized in Table 1, we can propose two magmatic stages in ~790 Ma and ~760–700 Ma (peaking at ~740 Ma) for the volcanic rocks in the Zhangbaling uplift. In addition, the sedimentary rocks of the Beijiangjun Formation and the overlying strata contained many young 700 Ma to ~630 Ma detrital zircon grains (unpublished data). Therefore, we tentatively conclude that a long-lived, and possibly multiple-stage, magmatism might have taken place along the northeastern margin of the Yangtze block.
Two major stages of magmatism of ~830–800 Ma and ~780–740 Ma are widely distributed along the northern and western margins of the Yangtze Block and were interpreted as related to the evolution of Rodinia [1]. In the present study, Neoproterozoic volcanic rocks of ~790 Ma and ~760–700 Ma (peaking at ~740 Ma) could be discriminated from the Xileng Formation of the Zhangbaling Group. These magmatic activities roughly coincide with the major magmatic episodes in the Yangtze Block. Therefore, the origin of these volcanic rocks can provide valuable information for the tectonic evolution of the northeastern margin of the Yangtze Block during the Neoproterozoic.
The volcanic rocks of the Xileng Formation showed similar geochemical characteristics of arc rocks and most of them belong to the calc-alkaline series. They had highly fractionated REE patterns, negative to weakly positive Eu-anomalies, and were significantly enriched in large ion lithophile elements. However, many differences in geochemical characteristics of trace elements and Nd-Pb-Hf isotopes could be observed between the early-stage (~790 Ma) and late-stage (~760–700 Ma) volcanic rocks.
The trace elemental characteristics of the early-stage volcanic rocks were similar to those of low-Mg adakitic rocks, which are generally interpreted as the products of partial melting of a thickened, garnet-bearing lower crust [42]. They had low initial εNd values and low initial Pb isotopic ratios, close to those of ancient basement rocks in the lower crust. Volcanic rock sample XL02 contained many Archean-Paleoproterozoic (∼3.4–2.0 Ga) zircon grains, having very low initial εHf values (−19.7 to −8.2). These features point to an origin from melting of ancient basement rocks of the lower crust for the early-stage magmatism. The representative Archean igneous rocks in the eastern Yangtze Block are TTG gneisses in the Kongling area [43,44,45,46,47]. Magmatic zircon grains from the TTG gneisses and other meta-volcanic rocks commonly have negative εHf values with Hf model ages of ca. 4.0–3.0 Ga [48,49,50,51,52]. Archean rocks are also exposed in the northern Yangtze Block, documented by ca. 2.8–2.6 Ga TTG rocks in the Yudongzi complex and ~2.5 Ga orthogneisses in the Douling complex [53,54,55]. Moreover, significant amounts of ~2.5 Ga detrital zircon were reported in Neoproterozoic sedimentary rocks exposed in the Yangtze Block [56]. In combination with the previously reported results, it seems reasonable to conclude that the early-stage volcanic rocks originated from partial melting of ancient rocks underneath the Yangtze Block.
Late-stage volcanic rocks (~760–700 Ma) of the Xileng Formation had arc-like geochemical features in trace elements, which were characterized by enrichment of LILEs and LREEs and depletion of HFSEs in the primitive mantle-normalized spider diagrams [57]. These rocks had relatively high initial εNd values and high Pb isotopic ratios compared with those of the early-stage volcanic rocks. Almost all the zircon grains from samples XL06 and XL23 had positive initial εHf values, obviously different from those of the early-stage volcanic rock (sample XL02). The late-stage volcanic rocks contained two zircon groups with crystallization ages clustering at ~790 Ma and ~740 Ma (Table 1). These old 790 Ma zircon grains had significantly different Hf isotopic compositions from the zircon grains of the early-stage volcanic rocks. This implies that the magma sources of the late-stage volcanic rocks might have some contribution from juvenile materials, probably of ~790 Ma mafic rocks. According to previously reported data, numerous Neoproterozoic mafic rocks exist in the Yangtze Block (Figure 8).
Three distinctly different models have been proposed to explain the geodynamics of the Neoproterozoic magmatism in the Yangtze Block [60,61]. Neoproterozoic volcanic rocks of the Xileng Formation can be classified in two magmatic stages: ~790 Ma low-Mg adakitic rocks originated from partial melting of ancient basement rocks and ~740 Ma continental arc rocks derived from partial melting of juvenile crustal rocks. Previous studies have identified many subduction–accretion–arc formations in the periphery of the Yangtze Block [62]. Within south China, the coeval (820–720 Ma) rift-related igneous rocks and sedimentary sequences are suggested to have been formed due to long-term subduction along the western margin of the Yangtze Block [63]. Integrating the results of the previous studies, we propose a long-lived magmatism (~820 to ~700 Ma or later) along the northeastern margin of the Yangtze Block. Temporally, it is consistent with the Neoproterozoic magmatism along the northern and western margins of the Yangtze Block [64,65,66].

6. Conclusions

The low-grade metamorphosed volcano-sedimentary sequences in the Zhangbaling uplift record multiple-stage Neoproterozoic magmatism. Volcanic rocks in the Xileng Formation formed in an early stage of ca. 790 Ma and a late stage of ~760 Ma to ~700 Ma peaking at ca. 740 Ma. Detrital zircon grains in meta-sedimentary rocks of the Xileng Formation and the overlying strata likely record magmatic episodes between ~700 Ma and ~630 Ma. The multiple-stage magmatism recorded in the Zhangbaling uplift might demonstrate a long-lasting subduction along the northeastern margin of the Yangtze Block that lasted until ca. 700 Ma in response to the formation of Rodinia.
The early-stage volcanic rocks contained many Archean to Early Paleoproterozoic zircon grains and had low initial εNd and εHf values (−11.0; −19.7 to −8.2) and low Pb isotopic ratios. Geochemical features indicated low-Mg adakitic affinities, probably resulting from partial melting of ancient basement rocks underneath the Yangtze Block.
The late-stage volcanic rocks were similar to continental arc rocks in trace element compositions. They had relatively high initial εNd and εHf values (−9.5 to +0.5; −0.4 to +8.8) and Pb isotopic ratios and most of them contained zircon grains of ~800–790 Ma. The parental magmas of these volcanic rocks might be derived from partial melting mainly of the juvenile mafic crust.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13040562/s1, Table S1: Zircon U-Pb isotopic data obtained by the LA-ICPMS for meta-volcanic rocks from the Xileng Formation; Table S2: Lu-Hf isotopic compositions of zircon grains from volcanic rocks of the Xileng Formation; Table S3: Contents of major (wt.%) and trace (ppm) elements for meta-volcanic rocks from the Xileng Formation; Table S4: Whole rock Nd-Pb isotopic compositions of meta-volcanic rocks from the Xileng Formation; Figure S1: Cathodoluminescence images of representative zircon grains from volcanic rocks of the Xileng Formation.

Author Contributions

J.W. and J.H. designed the study; J.W., Y.Y. and F.C. co-wrote the manuscript; J.W., J.H., J.Z., Y.Y. and F.C. carried out the field work and the analyses. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (NSFC) (Grant Nos. 42202069 and 41872049).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its Supplementary Materials.

Acknowledgments

Sincere thanks are due to A. Wittmann and two anonymous reviewers for their constructive suggestions, to P. Xiao, J.-F. He and Z.-H. Hou for assistance in analysis, and to H. Zhang and H. Nie for help in fieldwork.

Conflicts of Interest

The authors declare no competing interests.

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Figure 1. (a) Sketch map of the Sorth China Plate; (b) Simplified geological map in the southern Tan-Lu fault zone after [17]. HP: high pressure; UHP: ultra-high pressure.
Figure 1. (a) Sketch map of the Sorth China Plate; (b) Simplified geological map in the southern Tan-Lu fault zone after [17]. HP: high pressure; UHP: ultra-high pressure.
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Figure 2. Simplified geological map of the northern Zhangbaling uplift after [24].
Figure 2. Simplified geological map of the northern Zhangbaling uplift after [24].
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Figure 3. Zircon U-Pb isotopic concordia diagrams for volcanic rocks of the Xileng Formation. (a) Early-stage volcanic rocks in Chuzhou; (b,c) Late-stage volcanic rocks in Chuzhou; (d,e) Late-stage volcanic rocks in Feidong; (f) Late-stage volcanic rocks in Lujiang; (g,h) Late-stage volcanic rocks in Susong.
Figure 3. Zircon U-Pb isotopic concordia diagrams for volcanic rocks of the Xileng Formation. (a) Early-stage volcanic rocks in Chuzhou; (b,c) Late-stage volcanic rocks in Chuzhou; (d,e) Late-stage volcanic rocks in Feidong; (f) Late-stage volcanic rocks in Lujiang; (g,h) Late-stage volcanic rocks in Susong.
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Figure 4. Plots of (a) zircon initial εHf value versus formation age; (b) whole-rock initial εNd value versus formation age for volcanic rocks of the Xileng Formation. DM: depleted mantle; CHUR: chondritic uniform reservoir.
Figure 4. Plots of (a) zircon initial εHf value versus formation age; (b) whole-rock initial εNd value versus formation age for volcanic rocks of the Xileng Formation. DM: depleted mantle; CHUR: chondritic uniform reservoir.
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Figure 5. (a) Zr/TiO2 versus Nb/Y diagram after [35] and (b) FeO versus FeO/MgO diagram after [36] for volcanic rocks of the Xileng Formation.
Figure 5. (a) Zr/TiO2 versus Nb/Y diagram after [35] and (b) FeO versus FeO/MgO diagram after [36] for volcanic rocks of the Xileng Formation.
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Figure 6. (a) Chondrite-normalized rare earth element diagram, and (b) primitive mantle-normalized element spider diagram for volcanic rocks of the Xileng Formation. Data of OIB (ocean island basalt), OAB (oceanic arc basalt), CAA (continental arc andesite), and MORB (mid-ocean ridge basalt) after [37]; normalization values from [38].
Figure 6. (a) Chondrite-normalized rare earth element diagram, and (b) primitive mantle-normalized element spider diagram for volcanic rocks of the Xileng Formation. Data of OIB (ocean island basalt), OAB (oceanic arc basalt), CAA (continental arc andesite), and MORB (mid-ocean ridge basalt) after [37]; normalization values from [38].
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Figure 7. Plots of initial Pb isotopic ratios for volcanic rocks of the Xileng Formation: (a) 207Pb/204Pb versus 206Pb/204Pb; (b) 208Pb/204Pb versus 206Pb/204Pb. LC: lower crust; EM: enriched mantle; N-MORB: normal mid-ocean ridge basalt; NHRL: northern hemisphere reference line.
Figure 7. Plots of initial Pb isotopic ratios for volcanic rocks of the Xileng Formation: (a) 207Pb/204Pb versus 206Pb/204Pb; (b) 208Pb/204Pb versus 206Pb/204Pb. LC: lower crust; EM: enriched mantle; N-MORB: normal mid-ocean ridge basalt; NHRL: northern hemisphere reference line.
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Figure 8. Distribution of Neoproterozoic rocks in south China modified after [58,59].
Figure 8. Distribution of Neoproterozoic rocks in south China modified after [58,59].
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Table 1. Summary of zircon ages and Nd-Hf isotopic compositions for volcanic rocks of the Xileng Formation in the Zhangbaling uplift.
Table 1. Summary of zircon ages and Nd-Hf isotopic compositions for volcanic rocks of the Xileng Formation in the Zhangbaling uplift.
Sample
No.
Sampling
Area
Total Number
of Grains
Major Age Group
(Ma)
Zircon
εHf(t) Value
Whole-Rock
εNd(t) Value
Early-stage
XL02Chuzhou60~3300–2000, 790 ± 11−19.7 to −8.2−11.03
Late-stage
XL06Chuzhou32791 ± 10, 741 ± 7−0.4 to +8.8−3.34
XL13Chuzhou32795 ± 7, 746 ± 6 −2.54
XL15Feidong33789 ± 11, 732 ± 6 −1.73
XL17Feidong28790 ± 11, 747 ± 16 −0.19
XL18Lujiang32789 ± 8, 738 ± 7 −1.86
XL23Susong32791 ± 15, 736 ± 7+1.3 to +6.1+0.49
XL24Susong32797 ± 9, 739 ± 8 −4.13
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Wang, J.; He, J.; Zhao, J.; Yang, Y.; Chen, F. Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry. Minerals 2023, 13, 562. https://doi.org/10.3390/min13040562

AMA Style

Wang J, He J, Zhao J, Yang Y, Chen F. Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry. Minerals. 2023; 13(4):562. https://doi.org/10.3390/min13040562

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Wang, Jing, Jun He, Jingxin Zhao, Yizeng Yang, and Fukun Chen. 2023. "Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry" Minerals 13, no. 4: 562. https://doi.org/10.3390/min13040562

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