Geochronology and Geochemistry of the 890 Ma I-Type Granites in the Southwestern Yangtze Block: Petrogenesis and Crustal Evolution

The Yangtze Block in South China Craton is considered to have been part of the Mesoproterozoic to Neoproterozoic Rodinia supercontinent (Zhu et al., 2020a, b, 2019a, b; Zhao et al., 2019, 2018, 2017, 2011; Zhao and Zhou, 2007a, b; Zheng et al., 2007; Li et al., 2003, 2002; Zhou et al., 2002a, b). The Mesoproterozoic to Neoproterozoic magmatism along the Yangtze Block is critical for the crustal evolution of the Yangtze Block and its paleogeographic location in the reconstruction of the Rodinia. In the past few decades, large quantities of magmatic rocks throughout the Yangtze Block have carried out. However, the petrogenesis and geodynamic of the Neoproterozoic igneous rocks remain controversial, and three contrasting geodynamic viewpoints were proposed. The first model (mantle plume model) suggests that the Early Neoproterozoic (> 900 Ma) magmatic events were related to the Grenvillian orogenic events coinciding with the assembly of the Rodinia supercontinent (Zhu et al., 2016; Li X H et al., 2008; Greentree et al., 2006; Li Z-X et al., 2002). The Middle Neoproterozoic (860–740 Ma) magmatic rocks were generated in an extensional basin setting triggered by mantle plume related to the breakup of Rodinia (Yang et al., 2016; Li et al., 2003, 2002). The second model (subduction model) proposes that the Early Neoproterozoic (> 960 Ma) magmatic rocks were generated in a continental rift basin setting, and the 860–740 Ma magmatic rocks formed in an active continental margin setting (Zhu et al., 2020b; Chen et al., 2018, 2014; Zhao et al., 2018, 2007a, b; Zhou et al., 2006a, b, 2002a, b). The third model (plate rifting model) prompts that the Early Neoproterozoic magmatic activity was generated in an island arc setting, and the Middle Neoproterozoic magmatic events generated in an intra-plate rift environment (Wang et al., 2007; Zheng et al., 2007).

In recent years, an increasing number of geochronological research recognized voluminous Neoproterozoic igneous assemblages throughout the southwestern Yangtze Block, providing valuable information about the tectonic evolution process of the Yangtze Block (Li and Zhao, 2018a, b; Zhao et al., 2018; Du et al., 2014; Qiu et al., 2011; Huang et al., 2009; Sun and Zhou, 2008; Li et al., 2002). The Neoproterozoic magmatic rocks latter than 860 Ma were widely distributed (Zhao et al., 2019; Zhu et al., 2019a; Lai et al., 2015; Zhao and Zhou, 2007a, b; Zhou et al., 2006a, b), However, earlier studies proposed that limited magmatic records are available in the time interval of 1 000 to 860 Ma along the southwestern Yangtze Block. It, therefore, is meaningful to uncover the petrogenesis of igneous rocks with ages ranging from 1 000 to 860 Ma in the southwestern Yangtze Block.

In this contribution, we conducted new zircon U-Pb ages, whole-rock geochemical and Nd isotopes for the Early Neoproterozoic granodiorites in the southwestern Yangtze Block. On the one hand, the new dataset in this paper allows us to understand the petrogenesis and tectonic setting of the Early Neoproterozoic magmatism, and to illuminate the tectonic evolution process of the southwestern Yangtze Block at this period. On the other hand, this study will provide significant implications for the role of the Yangtze Block in the reconstruction of Rodinia.

South China consists of the Yangtze Block to the north and the Cathaysia Block to the south, amalgamated along the Sibao/Jiangnan orogenic belt (Zhao, 2015; Li et al., 2008; Zhou et al., 2006a). The Yangtze Block is separated from the Songpan-Ganzi terrane by the Longmenshan fault, from the North China Craton by the Qinling-Dabie Orogen, and from the Cathaysia Block by the Jiangnan Orogen. The oldest crystalline basement rocks of the Yangtze Block are represented by the high-grade metamorphosed complexes to the north. For instance, the Kongling Complex dominantly is composed of metasedimentary rocks and felsic gneisses with formation ages of 3 200–2 900 Ma that were intruded by the 1 850 Ma Quanyishang granite and the 850 Ma Huangling granite complex (Han et al., 2019; Zhang et al., 2011; Peng et al., 2009). The 2 700 Ma Yudongzi Complex consists of tonalite, trondhjemite and granodiorite gneisses and metasedimentary rocks (Hui et al., 2017; Zhang et al., 2001). More recent studies reported the occurrence of numerous 2.9–2.6 Ga magmatic activities, namely Zhongxiang, Douling and Houhe complexes in the northern Yangtze Block (Lu et al., 2019; Zhao et al., 2018, 2017). The Neoproterozoic strata and igneous rocks are widely distributed along the margins of the Yangtze Block (Wang et al., 2018; Zhao et al., 2018; Sun and Zhou, 2008; Zhou et al., 2006a, 2002b).

Paleoproterozoic to Mesoproterozoic sequences are mainly exposed in the southwestern Yangtze Block (Zhao et al., 2018; Chen et al., 2013; Zhou et al., 2012; Greentree and Li, 2008). Late Paleoproterozoic to Early Mesoproterozoic strata, from south to north, including the Dongchuan, the Dahongshan and the Hekou groups experienced lower grade amphibolite facies metamorphism (Geng et al., 2017; Wang et al., 2014; Zhao et al., 2010; Greentree and Li, 2008). All these sequences are intruded by early Mesoproterozoic mafic plutons (Guan et al., 2011; Zhao et al., 2010; Greentree and Li, 2008). The Dahongshan and the Hekou groups consist of volcano-sedimentary rocks; however the Dongchuan Group is characterized by sedimentary rocks, covering sandstone, shale and carbonatite (Wang et al., 2014; Zhao et al., 2010). The Late Mesoproterozoic to Early Neoproterozoic sequences include the Huili, Kunyang, Julin and Yanbian groups (Fig. 1b). The Julin Group underwent lower amphibolite facies metamorphism (Deng, 2000), while the other groups metamorphosed to lower greenschist facies (Zhang et al., 2007; Li et al., 1988; SBG, 1972). The Huili Group to the north and Kunyang Group to the south predominantly comprise a sequence of sandstone, siltstone, carbonaceous slate and carbonate with minor interlayered volcanic rocks. The Julin Group consists of quartzite, phyllite and schists with minor marbles and volcanic rocks. The Late Mesoproterozoic strata show a faulted contact with the Paleoproterozoic to Early Mesoproterozoic sequences. Sparse Early–Middle Neoproterozoic volcano-sedimentary units are also distributed in the study area, as represented by Yanbian Group, which comprises a sequence of basaltic lava in its lowermost part, and flysch deposit sequence in its upper part, and were intruded by the 857 Ma Guandaoshan diorite pluton (Du et al., 2014; Sun and Zhou, 2008). These Late Paleoproterozoic to Middle Neoproterozoic sequences are unconformity overlain by unmetamorphosed Sinian to Cenozoic cover sequences that are widespread across the southwestern Yangtze Block (Yan et al., 2003).

Figure 1.  Sketch geological map of the western margin of Yangtze Block (modified after Zhao et al., 2018).

This paper studied granodiorite samples that were recognized in the Yanbian area in the southwestern Yangtze Block (Fig. 2). The collected granodiorites are medium-grained and composed of plagioclase (35%–40%), quartz (30%–40%), hornblende (10%–15%) and biotite (5%–10%). Some plagioclases are partly altered to sericite on the surface, and other plagioclases show the typical polysynthetic twinning textures. The hornblendes are subhedral to euhedral and weakly altered. The subhedral biotite filled into the fissure between plagioclase and quartz (Fig. 3).

Figure 2.  Geological map showing disposition of studied samples in the Yanbian area from the southwestern Yangtze Block, South China (modified after SBG, 1972).

Figure 3.  Field petrography (a) and microscope (b) photographs of the granodiorite in the southwestern Yangtze Block. Pl. Plagioclase; Qz. quartz; Bt. biotite; Hb. hornblende.

LA-ICP-MS in Wuhan Sample Solution Analysis Technology Co., Ltd., simultaneously analyzed the zircon U-Pb isotope dating and trace element content. Zong et al. (2017) explain the detailed instrument parameters and analysis procedures. The GeolasPro laser denudation system consists of a COMPexPro 102 ArF 193 nm excimer laser and a MicroLas optical system. The ICP-MS model is Agilent 7700e. In the laser denudation process, helium gas is used as the carrier gas and argon gas as the compensation gas to adjust the sensitivity. Before entering the ICP, the two are mixed through a T-joint. The laser denudation system is equipped with a signal smoothing device (Hu et al., 2017). The laser beam spots and frequencies that were analyzed were 30 m and 6 Hz, respectively. Zircon standard 91500 and glass standard NIST610 were used to calibrate the U-Pb isotope dating and trace element content. Each time-resolution analysis data includes about 20–30 s blank signal and 50 s sample signal. Software ICP-MS-Data Cal completed. Isoplot/Ex_ver3 (Ludwig, 2003) was used to plot U-Pb age harmonic plots and calculate the age-weighted average of zircon samples.

Major element analyses of whole-rock were conducted on XRF (Primus Ⅱ, Rigaku, Japan) at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The sample processing process for XRF analysis is as follows: (1) place the 200 mesh sample in a 105 ℃ oven to dry for 12 h; (2) weigh ~1.0 g drying samples and place them in a constant weight ceramic crucible, burn them in a muffle furnace at 1 000 ℃ for 2 h, take them out, cool them to room temperature, weigh them, and calculate the loss of firing; (3) place 6.0 g flux (Li₂B₄O₇ : LiBO₂ : LiF=9 : 2 : 1), 0.6 g sample, and 0.3 g oxidant (NH₄NO₃) in a platinum crucible and melt it in a sample melting furnace at 1 150 ℃ for 14 min.

Trace element analysis of whole-rock was conducted on Agilent 7700e ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The sample treatment for ICP-MS analysis is as follows: (1) place 200 mesh samples in a 105 ℃ oven to dry for 12 h; (2) accurately weigh the powder sample of 50 mg and place it in a Teflon sample cartridge; (3) slowly add 1 mL of high-purity HNO₃ and 1 mL of high-purity HF; (4) place the Teflon sample cartridge into the steel sleeve, tighten it, and heat it in the oven at 190 ℃ for more than 24 h; (5) to dissolve and cool the sample, open the cover and place it on a 140 ℃ electric heating plate to dry, then add 1 mL HNO₃ and dry it again; (6) add 1 mL of high-purity HNO₃, 1 mL of MQ water, and 1 mL of internal standard (concentration of 1 ppm), then place the Teflon dissolved sample cartridge into the steel sleeve again, tighten it, and heat it in the oven at 190 ℃ for more than 12 h; (7) transfer the solution into a polyethylene bottle and dilute it to 100 g with 2% HNO₃ for the ICP-MS test.

The study adopted the static method to determine neodymium (Nd) isotopes. The measurement of ¹⁴⁰Ce is to monitor the interference of ¹⁴²Ce to ¹⁴²Nd, and the measurement of ¹⁴⁷Sm is to monitor the interference of ¹⁴⁴Sm and ¹⁴⁸Sm to ¹⁴⁴Nd and ¹⁴⁸Nd. Before testing the sample, Neptune Plus was optimized using ThermoFisher tuning fluid (Nd content 200 g/L), including plasma portions (parameters such as torque tube position and carrier gas flow rate) and ion lens parameters for maximum sensitivity. The samples after chemical separation were introduced into mass spectrometry with 2% HNO₃ so that the signal strength of ¹⁴⁶Nd was about 8 V (the concentration of Nd in the solution was about 200 g/L). After the sample test is completed, the injection system is washed with a 2% HNO₃ solution, and the next sample is measured. The ratio of ¹⁴³Nd/¹⁴⁴Nd was adjusted using ¹⁴⁶Nd/¹⁴⁴Nd=0.721 9 for exponential normalization.

Zircon U-Pb data of the studied samples from the Yanbian area in southwestern Yangtze Block are shown in Table 1. Zircon grains from granodiorites are mainly transparent and subhedral to euhedral prismatic crystals and range from 100 to 200 μm in length (Fig. 4). All of the analytical data samples have Th (46 ppm–766 ppm) and U (107 ppm–569 ppm) contents with Th/U ratios of 0.3–1.3, indicating a magmatic origin. Twenty-five analyses spots yield a weighted mean age of 894.6±7.4 Ma (MSWD=5.1) (Fig. 4a). This age is interpreted as the best estimate of the crystallization age for Yanbian granodiorite. In addition, zircon trace element analyses were conducted simultaneously with U-Pb analyses. They display depletion in LREE and enrichment in HREE as well as negative Eu anomalies and positive Ce anomalies (Fig. 4b), suggesting a magmatic origin.

Figure 4.  LA-ICP-MS U-Pb isotopic concordia plots of zircon grains from Yanbian granodiorite in the southwestern Yangtze Block. Also shown the CL images of several zircon grains (the red circles are dating spots).

Whole-rock major and trace geochemical data for the studied granodiorites are shown in Table 2. The Yanbian granodiorites have moderate SiO₂ (60.2 wt.%–65.8 wt.%) and relatively high and variable Fe₂O₃^T (3.9 wt.%–9.4 wt.%) and Al₂O₃ (14.5 wt.%–18.4 wt.%) contents. In addition, these samples have low CaO=0.2 wt.%–7.1 wt.%, (Na₂O+K₂O)=4.0 wt.%–5.5 wt.%, P₂O₅=0.1 wt.%–0.4 wt.% and MgO=0.5 wt.%–3.9 wt.%, with Mg^# between 17.3 and 56.3. On the TAS diagram, the studied granodiorites plot in the field of granodiorite and diorite (Fig. 5a). Four granodiorite samples have A/CNK values of 0.8–1.1 and plot in the metaluminous and peraluminous field in the A/CNK-A/NK diagram (Fig. 5b). In addition, one sample has high A/CNK ratios of 3.1, which may be the result of alteration.

Figure 5.  (a) TAS diagram for classification of the rocks (after Middlemost, 1994) and (b) A/NK vs. A/CNK (after Frost et al., 2001) diagram for the Yanbian granodiorites from the study area.

The Yanbian granodiorites are characterized by variable REE contents (ƩREE=72 ppm–194 ppm). In the chondrite-normalized REE diagram (Fig. 6a), these rocks exhibit enrichment of LREE and depletion of HREE patterns ((La/Yb)_N=1.4–12.9), with moderate to insignificant negative Eu anomalies (δEu=0.6–0.9). In the primitive mantle-normalized diagram (Fig. 6b), the Yanbian granodiorites display similar trace element patterns, with enrichment in Th, La and Ce and depletion in Nb, Ta, Sr, P, and Ti.

Figure 6.  Chondrite-normalized REE and primitive mantle-normalized trace element patterns for the studied felsic intrusions (after Sun and McDonough, 1989).

Whole-rock Nd isotopic data is presented in Table 3. The granodiorites have measured ¹⁴⁷Sm/¹⁴⁴Nd ratios ranging of 0.101 4 –0.128 8 and ¹⁴³Nd/¹⁴⁴Nd varying from 0.511 797 to 0.511 841. They have positive ε_Nd(t) from +5.8 to +6.8 at their ages of magma emplacement. The T_DM1 and T_DM2 Nd model ages of the granodiorites are 974–1 095 and 989–1 071 Ma, respectively (Fig. 7).

Figure 7.  Whole-rock Nd isotopic compositions of the Yanbian granodiorites. CHUR. Evolution of the Chondritic Uniform Reservoir.

All the studied samples in this paper were the freshest based on their outcrop appearances, whereas they were altered to various degrees inferred by the variable LOI values of 0.7–7.1. Thus, we must assess the effects of alteration of these rocks before further discussion. For granite, some elements, such as K, P, Ca, Na and LILE, are easily transported during the chemical weathering and low degree alteration (Bédard, 1999). Zr is commonly considered the most immobile element during low-grade metamorphism or alteration (Gibson et al., 1982). Consequently, a bivariate diagram of Zr against these elements can be used to evaluate the mobility of such elements (Polat et al., 2002). Except for Ti, the other elements overall correlate tightly with Zr, indicating that these elements were essentially immobile (not shown). As a consequence, the immobile elements can be used for petrogenetic discussion.

The Yanbian granodiorites have medium SiO₂ contents and are calcalkaline, metaluminous to weakly peraluminous series (A/CNK=0.8–1.1). Compared to typical A-type granitoids, the rocks in this paper have low Fe₂O₃^T/MgO and 10 000×Ga/Al ratios and Zr contents. Their samples also plot in the ''I- and S-type granitoids" field in the Zr-10 000 Ga/Al and Nb-10 000 Ga/Al discrimination plots (Figs. 8a, 8b). Furthermore, in the diagrams of 10 000Ga/Al–Zr+Nb+Y+Ce and FeO^T/MgO, all rocks also exclusively plot into the field of ''I- and S-type granitoids" (Figs. 8c, 8d). In addition, typical S-type granites possess higher A/CNK values (> 1.1) values than those of I-type granites (A/CNK < 1.1) because of Na and Ca elements leaching out from sedimentary rocks by the alteration and weathering process (Clemens, 2003). The Yanbian granodiorites are metaluminous to weakly peraluminous series with low A/CNK values (0.8–1.1), which is different from the S-type granites. Furthermore, the negative correlation between P₂O₅ and SiO₂ also display the characteristics of I-type granites (not shown). In summary, we argue that the studied Yanbian granodiorites can be classified as I-type granites.

Figure 8.  Geochemical discrimination diagrams (Whalen et al., 1987) for the Yanbian granodiorites in the southwestern Yangtze Block. FG. Fractionated granites; OGT. unfractionated I-, S-, and M-type granites.

Published researches demonstrated that the genetic models for the origin of I-type rocks are complicated. It essentially falls into three mechanisms (1) the partial melting of mafic igneous rocks (Clemens et al., 2009); (2) magma mixture of mantle-derived and crust-derived magma members (Kemp et al., 2007); and (3) extremely differentiation of mantle-derived mafic magmas (Soesoo, 2000).

Grimes et al. (2007) proposed that the average Y contents and U/Yb ratios for zircons from different tectonic settings are disparate, and the continental granitoid has relatively high Y contents (> 100) and low U/Yb ratios (1.07). The Yanbian I-type granites cannot be derived from the mantle-derived mafic magmas by fractional crystallization or a assimilation fractional crystallization process because the granodiorites in this paper have high Y contents and low U/Yb ratios, which is the characteristics of the typical continental granitoids (Mahoney et al., 2008). Moreover, the high SiO₂ and low MgO and Cr contents also illustrate that the mantle-derived source is illogical (Smithies, 2000). Experimental results suggest that magma mixing process could produce massive mafic enclaves (Kemp et al., 2007). The possibility of magma mixing model is very low because no mafic or ultramafic enclaves were observed in the Yanbian granodiorites. The studied Yanbian granodiorites have high Al₂O₃ and low MgO contents, indicating their original magma is primarily composed of the mafic continental crust (Rapp and Watson, 1995). Considering that the samples have positive ε_Nd(t) values, it might be reasonable to interpret the Yanbian granodiorites as products of the partial melting of the juvenile mafic lower crust. The P₂O₅ content decreased with increasing SiO₂ (Fig. 9), implying that they could have experienced fractionation of apatite. Thus, subsequent fractional crystallization could also determine the geochemical composition of the Yanbian granodiorites. As discussed above, we suppose that the parental magma of the Yanbian granodiorites was most likely derived from the juvenile mafic rocks underplated in the lower crust that underwent apatite fractionation.

Figure 9.  Harker diagrams of the Yanbian granodiorites in the southwestern Yangtze Block.

The Yanbian granodiorites have relatively low Sr/Y and (La/Yb)_N values and characterized by enrichment in LILE and LREE and depletion in Nb, Ta and Ti. These features are similar to typical modern subduction-related igneous rocks (Zhou et al., 2006a). All the studied samples fall into the volcanic arc granitoids field in the Nb-Y diagram (Fig. 10). Considering the above features, we propose that the Yanbian granodiorites show arc affinity compositions generated in a subduction-related setting.

Figure 10.  Y-Nb tectnic discriminant diagram for Yanbian granodiorites in this study (Pearce et al., 1984). VAG. Volcanic arc granites; syn-COLG. syn-collisional granites; WPG. within-plate granites; ORG. ocean ridge granites.

In recent years, increasing Neoproterozoic magmatic rocks have been discovered in the southwestern Yangtze Block, with ages ranging from 860 to 740 Ma. These include the 860 Ma Guandaoshan pluton (Du et al., 2014), the 750 Ma Shaba pluton (Zhao et al., 2008), the 800 Ma Suxiong alkaline basalts (Li et al., 2002), the 780 Ma Dalu I-type granite (Zhu et al., 2019c), and the 782 Ma Mopanshan adakitic complex (Huang et al., 2009). Meanwhile, numerous Late Mesoproterozoic magmatic rocks have also been recognized along the southwestern Yangtze Block (Table 1), best represented by the 1 020 Ma Tianbaoshan mafic dykes and felsic volcanic rocks (Zhu et al., 2016), the 1 050 Ma Julin metabasalts (Chen et al., 2014), and the 1 028 to 1 019 Ma Huili A₁- and A₂-type felsic rocks (Wang et al., 2019).

Notably, it appears that the magmatic events along the southwestern margin of the Yangtze Block were interrupted for approximately 140 Ma from 1 000 to 860 Ma. Although extensive detrital zircon age dates show that there is no obvious age gap between 1 000 and 860 Ma in South China (Sun et al., 2007), the corresponding magmatic rocks remain undiscovered. So far, the 860 Ma Guandaoshan pluton represents the oldest magmatic activity in the region during the Neoproterozoic. The 894 Ma Yanbian granodiorite in this paper can fill the magmatic gap on the southwestern Yangtze Block. Furthermore, the 894 Ma igneous event in this study is consistent with the 960–890 Ma Shuangxiwu felsic rocks in the southeastern periphery of the Yangtze Block and 900 Ma A-type rhyolite in the northern margin of the Yangtze Block (Zhou et al., 2019; Li et al., 2008). This indicates that Early Neoproterozoic igneous rocks (1 000 to 860 Ma) could be more widely distributed along the west, east and north margins of the Yangtze Block than previous proposed. The zircon U-Pb age obtained in this study together and published data show that there is no obvious age gap between the Late Mesoproterozoic and Middle Neoproterozoic igneous rocks in the Yangtze Block.

The tectonic evolution process of the southwestern Yangtze Block during the Neoproterozoic time is still controversial. One widely accepted model interprets the Middle Neoproterozoic (860–740 Ma) subduction-related magmatism that formed in a convergence setting (Chen et al., 2018; Sun and Zhou, 2008; Zhou et al., 2006a, 2002a). On the contrary, the mantle plume/plate rift model argues that the 860–740 Ma anorogenic magmatism formed in an intra-continental setting (Wang et al., 2008; Li et al., 2002, 1999). Although the 890 Ma arc-related magmatism in this study has no direct contact with the tectonic evolution of the Neoproterozoic (860–740 Ma) magmatic events, it could provide important information for the geodynamics of the southwestern Yangtze Block at Early Neoproterozoic. For the conception of the first tectonic model, the 894 Ma subduction-related magmatism might represent the early stage of the oceanic crust subduction in the Neoproterozoic. On the contrary, within the context of the second model, the 894 Ma subduction-related magmatic activity is likely the late stage of the Grenvillian continental collision (Jiangnan/Sibao Orogen) lasting from the Late Mesoproterozoic to Early Neoproterozoic. The continental rifting system was subsequently initiated at 894–860 Ma.

Numerous Neoproterozoic tectonothemlal events are widespread in the southwestern periphery of the Yangtze Block, including the 857 Ma Guandaoshan pluton (Sun and Zhou, 2008; Li et al., 2002), the 810 Ma Gaojiacun and Lengshuiqing mafic pluton (Zhou et al., 2006a), the 806–812 Ma Panzhihua gabbro-diorites and 749 Ma Panzhihua K-feldspar granites (Zhu et al., 2019a) and the 790 Ma Shimian I-type granites (Zhao et al., 2008). The Neoproterozoic magmatism generated a long-term active arc system along the southwestern Yangtze Block (Zhao et al., 2018; Zhou et al., 2006a, 2002b). On the other hand, Zhao et al. (2018) proposed that the Neoproterozoic mafic and ultramafic rocks in the southwestern Yangtze Block were characterized by arc-related trace elemental compositions and are illustration mantle sources modified through slab melts or fluids. Furthermore, these 860–740 Ma calc-alkaline I-type granitoids and 780–740 Ma adakitic granitoids in the study area were formed by melting of the juvenile underplated mafic rocks and partial melts of a subducted oceanic slab, respectively. Recently, Zhu et al. (2020b) recognized 850–835 Ma Shuilu high Mg^# diorites in the western Yangtze Block and proposed they were generated by partial melting of a metasomatized mantle source enriched by subduction fluids and sediment melts. All the magmatic rocks possess arc-affinity geochemical features. They recorded persistent oceanic subduction along the southwestern Yangtze Block during the Neoproterozoic time. However, magmatism records available from Xichang to Panzhihua between 1 000–860 Ma are very limited. Therefore, we consider the 894 Ma Yanbian granodiorites in this paper to represent one of the initial Neoproterozoic subduction-related igneous events along the southwestern Yangtze Block.

The most important evidence to support the subduction model is that the Grenvillian Orogen does not exist in the Yangtze Block. Therefore, to understand whether a Grenvillian orogeny occurred will shed light on the tectonic regime of the southwestern Yangtze Block in the Early Neoproterozoic. The Late Mesoproterozoic magmatism in the region was previously considered as the product of Grenvillian orogeny coinciding with the assembly of Rodinia. However, recent geochronology and geochemistry research precluded the presence of a Grenvillian time orogeny in the southwestern Yangtze Block. This inference corresponds well with the existing felsic rocks (1 050 Ma, Chen et al., 2018; 1 023 Ma, Zhu et al., 2016) in the Tianbaoshan Formation of the Huili Group and the 1 050 Ma metabasalts (Chen et al., 2014) in the Pudeng Formation of the Julin Group, which were attributed intercontinental rift setting. The extensional rifting setting for the Late Mesoproterozoic sedimentary sequences in southwestern Yangtze Block is further supported by the sedimentary facies. For instance, Deng (2000) presented that Late Mesoproterozoic sedimentary sequences in the study area were deposited in a shallow marine environment in a passive continental margin. Thus, we exclude the existence of a Mesoproterozoic subduction event.

Based on the above discussions, the results obtained in this paper demonstrate that the 894 Ma subduction-related granodiorites could represent the early stage of the Neoproterozoic orogeny in the southwest Yangtze Block. The 894 Ma Yanbian granodiorites, together with abundant subduction-related igneous rocks in study region, record the existence of a long-term Neoproterozoic active arc system along the southwestern Yangtze Block, which lasted more than 160 ma from 894 to 740 Ma. (Fig. 11).

Figure 11.  Sketch diagrams exhibiting tectonic transformation from a passive continental margin to an active continental margin in the southwestern Yangtze Block (modified after Chen et al., 2018).

(1) The 894.6±7.4 Ma Yanbian granodiorites in the southwestern Yangtze Block have I-type affinity and formed in a subduction-related setting.

(2) The Yanbian granodiorites are calc-alkaline and metaluminous to weakly peraluminous, which were derived from the partial melting of juvenile mafic lower crust.

(3) The Early Neoproterozoic arc-related igneous rocks in this study represent the early stage of subduction at the southwestern margin of the Yangtze Block.

This research was financially supported by the China Geological Survey (Nos. DD20190370, 121201111120117). We thank the assistances of Zemin Bao, Shiwen Xie, and Linlin Li during SHRIMP dating. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1339-1.