Journal of Earth Science, Vol. 29, No. 1, p , February 2018 ISSN X Printed in China

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1 Journal of Earth Science, Vol. 29, No. 1, p , February 2018 ISSN X Printed in China Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton: Constrains from the Mineral Composition, U-Pb Age and Hf Isotopes of Zircons Xingfu Jiang * 1, 2, 3, Songbai Peng 2, 3, Timothy M. Kusky 3, 4, 5, Lu Wang 3, 4, 5, Hao Deng 2 1. School of Earth Sciences, East China University of Technology, Nanchang , China 2. School of Earth Sciences, China University of Geosciences, Wuhan , China 3. Center for Global Tectonics, China University of Geosciences, Wuhan , China 4. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan , China 5. Three Gorges Center for Geohazard, Ministry of Education, China University of Geosciences, Wuhan , China Xingfu Jiang: ABSTRACT: The olivine-gabbroic rocks located at the Huangling anticline within the Yangtze Craton are dated at circa Ma by LA-ICP-MS method. The rocks belong to the sub-alkaline series and consist of pyroxene (35% 40%), plagioclase (40% 45%), olivine (8% 10%) and spinel (3% 5%). Olivine has Fo values of that is classified as chrysolite. Pyroxene has relative low contents of FeO (6.60 wt.% 8.23 wt.%) but high CaO (20.23 wt.% wt.%) contents, however, plagioclase has high Al 2 O 3 (31.78 wt.% wt.%), CaO (16.08 wt.% wt.%) and An (79 80) values, but low Na 2 O contents (1.95 wt.% 2.11 wt.%). Spinel are magnesioferrite with characteristics of high contents of MgO (13.65 wt.% wt.%), FeO (23.27 wt.% wt.%) and Al 2 O 3 (62.43 wt.% wt.%). Chemical compositions of these minerals are similar to those of gabbro rocks that were formed in the post-orogeny environment. The olivine-gabbro samples have negative zircon ε Hf values (-16.57±0.47) that resemble the mafic rocks in the same region, indicating that they are derived from the extremely enriched mantle source. On the compilation of documented Neoproterozoic mafic rocks in the Yangtze Craton, it is proposed that the mantle in the northern Yangtze Craton has experienced different degrees enrichment during the Neoproterozoic. KEY WORDS: Neoproterozoic olivine-gabbro, Huangling anticline, post-orogeny environment, Yangtze Craton. 0 INTRODUCTION The South China Block, composed of the Yangtze Craton and Cathaysia Block, was formed by Neoproterozoic collision event along the Jiangnan orogen (Fig. 1a), therefore, it is a perfect field for studying the coeval igneous rocks related to the aggregation and break-up processes of the Rodinia supercontinent (Zhang and Zheng, 2013; Li et al., 2008; Zhou et al., 2002 and reference therein). The Yangtze Craton has experienced complicated geological processes containing multiple tectono-thermal events, high-amphibolite and granulite facies metamorphism in the Precambrian Period (Kong et al., 2017; Jiang et al., 2016; Guo et al., 2014; Li et al., 2014; Chen et al., 2013; Peng et al., 2012; Wei et al., 2012; Gao et al., 2011; Wu et al., 2009; Zhang et al., 2009, 2006; Ma et al., 2002). Although it is full of controversy about the *Corresponding author: jiangxingfu229@163.com China University of Geosciences and Springer-Verlag GmbH Germany, Part of Springer Nature 2018 Manuscript received April 20, Manuscript accepted August 30, properties of the Precambrian geological events, recent studies suggest that the Yangtze Craton underwent a subduction process at ca Ma and a collisional event at ca Ma during the Late-Mesoproterozoic to Early-Neoproterozoic (Jiang et al., 2016; Peng et al., 2012; Qiu et al., 2011). There are large quantities of Neoproterozoic granitic intrusions and volcanic rocks, as well as minor mafic-ultramafic rocks in the Yangtze Craton (Zhang and Zheng, 2013; Zhao and Zhou, 2013; Zheng et al., 2013; Wang et al., 2008; Zhao et al., 2007). However, it is mainly focused on the granites and volcanic rocks for more than a decade in comparison with the mafic-ultramafic rocks. In this contribution, we present new electron probe micro-analyzer (EPMA) mineral compositions, U-Pb ages and Hf isotopic data of the olivine-gabbro rock in the Huangling anticline within the Yangtze Craton, our aims are: (1) to obtain the intrusion time of the rock; (2) to discuss the petrogenesis and tectonic setting of the gabbro; (3) to propose a geodynamic implication of the Yangtze Craton. 1 GEOLOGICAL BACKGROUND The Huangling anticline, located in the north-central part of Jiang, X. F., Peng, S. B., Kusky, T. M., et al., Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine- Gabbro within the Yangtze Craton: Constrains from the Mineral Composition, U-Pb Age and Hf Isotopes of Zircons. Journal of Earth Science, 29(1):

2 94 Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang and Hao Deng the Yangtze Craton, outcrops the oldest basement rocks in South China (Guo et al., 2014; Gao et al., 2011, 1999; Qiu et al., 2000; Ma et al., 1997). The old Precambrian metamorphic rocks are mainly distributed in the core area of the anticline, and intruded by the Neoproterozoic Huangling batholith that is overlain unconformably by Sinian red sandstone (Fig. 1b). The WNW-striking Wuduhe fault separates the Huangling anticline into two segments, which consist mainly of (1) Archean tonalitic, trondhjemitic and granodioritic (TTG) gneisses; (2) Paleoproterozoic metasedimentary rocks dominated by Khondalite series; and (3) Neoproterozoic Huangling batholith and minor Meso-Neoproterozoic mafic-ultramafic rocks (Peng et al., 2012; Gao et al., 2011, 1999; Ma et al., 2002; BGMRHP, 1991; Gao and Zhang, 1990). The Neoproterozoic Huangling batholith in the anticline mainly consists of five magmatic suites from bottom to top, namely the Duanfangxi gabbro/hornblende gabbro, the Maoping quartz diorite-tonalite, the Huanglingmiao trondhjemitegranodiorite, the Dalaoling monzodiorite-monzogranite, and the Xiaofeng mafic-felsic dike swarms (Fig. 1b), and formed ranging from 860 to 760 Ma through the SHRIMP and LA-ICP-MS zircon U-Pb methods (Wei et al., 2012; Zhang et al., 2009; Ma et al., 2002), however, the Duanfangxi mafic rocks have long been neglected for studying on account of some uncertainties, especially for the scientific issues of their precise intrusion time, properties of the mantle source and tectonic setting. The olivine-gabbro from the Duanfangxi magmatic suite in the Huangling anticline is medium- to fined-grained (Fig. 3a), and mainly composed of pyroxene (35% 40%), plagioclase (40% 45%), olivine (8% 10%) and spinel (3% 5%) (Fig. 3b). Accessory minerals contain zircon and opaque Fe-Ti oxides. Pyroxene is generally euhedral with irregular shapes (Figs. 3b, 3c). The grain size is generally 2 4 mm and surrounded by anhedral-subanhedral plagioclase mineral with typical polysynthetic twinning texture (Fig. 3d). Plagioclase grains have typical lath shapes, are more euhedral and range from 3 to 5 mm (Fig. 3b). Olivine is anhedral and granular, and has a typical grain size of 3 5 mm (Fig. 3b). Spinel is green, and occurs as subhedral grains (Fig. 3b). 2 ANALYTICAL METHODS 2.1 LA-ICP-MS Zircon U-Pb Dating Two olivine-gabbro rock samples were selected for LA-ICP-MS zircon U-Pb dating, and the geochronological results are presented in Table 1. Cathodoluminescence (CL) imaging of typical magmatic zircons from the two samples was carried out at the Chinese Academy of Sciences (CAS) in Beijing and the State Key Laboratory of Geological Processes and Figure 1. (a) A simplified tectonic map of China (Qiu et al., 2011). (b) Geological map of the Huangling anticline in the Yangtze Craton (modified after Wei et al., 2012; BGMRHP, 1991).

3 Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton 95 Figure 2. (a) Geological map of the southern Huangling anticline in the central Yangtze Craton (modified after Peng et al., 2012). Figure 3. Field and microscopic photos of the olivine-gabbro in the southern Huangling anticline. (a) medium- to fined-grained olivine-gabbro; (b) mineral assemblage of the olivine-gabbro (plane polarized light); (c) pyroxene was retrograde into hornblende (plane polarized light); (d) polysynthetic twinning of the plagioclase (cross polarized light). Ol. Olivine; Py. pyroxene; Hb. hornblende; Pl. plagioclase; Sp. spinel.

4 96 Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang and Hao Deng Table 1 LA-ICP-MS U-Pb data for zircons analyzed from olivine-gabbro in the Huangling anticline Spot Th (ppm) U (ppm) Th/U Corrected isotope ratio Corrected age (Ma) 206 Pb 1σ 235 U 1σ 206 Pb/ 238 U 1σ 206 Pb 1σ 235 U 1σ 206 Pb/ 238 U 1σ Sample No. 13YJS Sample No. 13YJS

5 Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton 97 Table 1 Continued Spot Th (ppm) U (ppm) Th/U Corrected isotope ratio Corrected age (Ma) 235 U 1σ 206 Pb/ 238 U 1σ 206 Pb 1σ 235 U 1σ 206 Pb/ 238 U 1σ 206 Pb 1σ Sample No. 13YJS Mineral Resources (GPMR), China University of Geosciences (CUG), Wuhan. The in-situ zircon U-Pb and trace element analytical methods were carried out by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the GPMR in CUG, Wuhan. Zircon is selected for external standard of U-Pb dating, and common Pb was calibrated by obtained 204 Pb contents. The spot size was 32 μm. GJ-1 zircon yielded weighted average 235 U and 206 Pb/ 238 U ages are 594±12 Ma (MSWD=1.05; n=8) and 600.1±5.2 Ma (MSWD=0.98; n=8), respectively, which are consistent with its references age of 599.8±1.7 Ma (Jackson et al., 2004). 2.2 EPMA Mineral Analysis Mineral compositions of the olivine-gabbro rock in the Huangling anticline were determined through the JXA-8100 electron microprobe micro-analyzer at CAS in Beijing. The quantitative analysis was carried out on the 15 kv accelerating voltage and A specimen current with 2 μm beam diameter. The analytical results of mineral compositions of the olivine-gabbro rock are shown in Table Zircon Hf Isotopic Analyses In-situ zircon Lu-Hf isotopic composition of the olivinegabbro rock in the southern Huangling anticline was obtained in the GPMR in CUG, Wuhan. The analytical work was measured on Neptune Plus MC-ICP MS (Thermo Fisher Scientific, Germany) and GeoLas The spot size was 44 μm. Weighted average 176 Hf/ 177 Hf ratio of GJ-1 is ± (MSWD=1.4, n=8), which is consistent with recommended value of ± (2 SD, n=25) (Elhlou et al., 2006). Detailed work conditions of the ICP-MS instrument, the data processing and calibrating were presented by Hu et al. (2012). Hf isotopic results of the olivine-gabbro rock are shown in Table 3. 3 ANALYTICAL RESULTS 3.1 LA-ICP-MS Zircon U-Pb Dating Results Zircons from the Sample 13YJS-1 from the olivine-gabbro have typical oscillatory zoned structure (Fig. 4a), Th/U ratios of the twenty analytical spots are from 0.70 to 1.4. All the geochronological results close to the concordia line, giving a weighted mean 206 Pb/ 238 U age of 857±11 Ma (MSWD=3.8), which is interpreted as the crystallization time of the gabbro. Zircons selected from the olivine-gabbro Sample 13YJS-2 have euhedral to subhedral and prismatic morphologies, and range from 60 to 180 μm in length (Fig. 4b), Th/U ratios are from Twelve analytical spots yield young ages ranging from 808±9 and 512±6 Ma, and deviate from the concordia line. The remaining nine analytical spots are concordant, giving a weighted mean 206 Pb/ 238 U age of 854±14 Ma (MSWD=3.7) (Fig. 4b), which is accounting for the emplacement age of the olivine-gabbro. 3.2 Mineral Compositions Olivine grains selected from the olivine-gabbros have restricted Fo values ranging from 73 to 83, which is classified as chrysolite. There is negative correlation between Fo and SiO 2. Pyroxene grains in the thin-sections have relative low Mg # (=(100 Mg/(Mg+Fe 2+ )) values from 76 to 81 with characteristics

6 98 Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang and Hao Deng Table 2 Mineral composition (wt.%) of the olivine-gabbro in the southern Huangling anticline Olivine Pyroxene Plagioclase Spinel Na 2 O MgO Al 2 O SiO CaO P 2 O K 2 O TiO NiO FeO MnO Cr 2 O Total Figure 4. Representative cathodulominescence (CL) images and condordia plots for zircons from the olivine-gabbro. Circles on the zircon grains show the locations of U-Pb age measurement. of low contents of Al 2 O 3 (less than 4 wt.%) and FeO (6.60 wt.% 8.23 wt.%), but high SiO 2 (more than 50 wt.%) and CaO (20.23 wt.% wt.%) values. Plagioclase from the mafic rocks belongs to the basic-plagioclase sub-group and could be classified as bytownite on the basis of An values of 79 80, and has high contents of Al 2 O 3 (31.78 wt.% wt.%) and CaO (16.08 wt.% wt.%) but low Na 2 O contents (1.95 wt.% 2.11 wt.%). The analytical data of spinel minerals reveals that they are magnesioferrite characterized by high contents of MgO (13.65 wt.% wt.%), FeO (23.27 wt.% wt.%) and Al 2 O 3 (62.43 wt.% wt.%). 3.3 Hf Isotope Compositions Ten spots in different zircons were analyzed for Lu-Hf isotopic compositions of olivine-gabbro Sample 13YJS-1. The 176 Hf/ 177 Hf ratios have a narrow span ranging from to (Table 3). From the Hf isotope analytical results, it is evidence that all zircon grains with the 206 Pb/ 238 U age of ca. 860 Ma have negative ε Hf values from to (Fig. 5), with an average value of ±0.47 (MSWD=0.55), and Hf depleted mantle model ages are Ma. Number Mean= ±0.47 MSWD=0.55; n= ε Hf () t Figure 5. Zircon ε Hf (t) values of the olivine-gabbro. 4 DISCUSSION 4.1 Time of the Mafic Magmatism of the Huangling Batholith Neoproterozoic magmatism was widely distributed in the Yangtze Craton, and large quantities of igneous zircons from the Huangling batholith, Zhaigun, Bendong, Dongma, Sanfang and Relative probability

7 Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton 99 ~818 Ma Table 3 Lu-Hf isotopic data of zircons from the olivine-gabbro (13YJS-1) 176 Spot No. U-Pb age (Ma) Yb/ 177 Hf 176 1σ Lu/ 177 Hf 176 1σ Hf/ 177 Hf 1σ εhf(t) 1σ T DM1 (Ma) TDM2 (Ma) flu/hf Number ~755 Ma n= Age (Ma) Figure 6. Age spectra of Neoproterozoic zircon grains for the Yangtze Craton. Tha data are from Zhao et al. (2013); Wei et al. (2012); Zhang et al. (2009, 2008); Zhou et al. (2009); Wang et al. (2006). Tianpeng plutons, Yiyang basaltic rocks and gabbroic rocks in the Fanjingshan Group were reported to be concentrated on ages of ca and ca Ma, with the relict zircon ages of ca. 900 Ma (Fig. 6) (Zhao et al., 2013; Wei et al., 2012; Zhang et al., 2009, 2008; Zhou et al., 2009; Wang et al., 2006). However, recent studies show that crystallization ages of the Sandouping quartz diorite-tonalite in the Huangling batholith are ca Ma (Wei et al., 2012), older than the emplacement time documented before. In this contribution, the zircons from the olivine-gabbro samples 13YJS-1 and 13YJS-2 in the Huangling anticline show oscillatory zoning structure, and high Th/U ratios ( and ), which are classified as the typical magmatic zircon (Wu and Zheng, 2004). The geochronological results yield weighted mean 206 Pb/ 238 U ages of 857±11 (MSWD=3.8) and 854±14 Ma (MSWD=3.7), respectively, which are interpreted as the formation ages of the olivine-gabbro in the Huangling anticline. To our knowledge, it is the first time for reporting the ca Ma olivine-gabbro in the Huangling batholith within the Yangtze Craton, moreover, the crystallization age of the mafic rocks is older than the majority of the Neoproterozoic Huangling batholiths. Combination with the previously reported zircon data of the ~860 Ma granitc pluton and ~850 Ma tonalites in the Huangling anticline, it is proposed that formation time of the mafic magmatic event of the Huangling batholith in the Yangtze Craton could be ca Ma, which is coeval with the initial time of the granitic magmatism. 4.2 Petrogenesis and Tectonic Background Gabbro rocks could be formed in various tectonic settings (Zhao et al., 2008; Chen et al., 2006; Zhou et al., 2005; Zhang et al., 1995; Sun, 1994), and show unique petro-geochemical compositions for different minerals. Olivine grains of the gabbro samples in the southern Huangling anticline have relative low Fo values (from 73 to 83), which are less than the typical mantle peridotite (Fo>90). Curvilinear relationship between the Fo values in different composition and mass fraction of MgO in olivine of equilibrated magma suggests that MgO contents of the olivine-gabbro rocks in the Huangling anticline within the Yangtze Craton should be no more than 8% (Révillon et al.,

8 2 100 Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang and Hao Deng 1999). However, MgO contents of the gabbro are from wt.% to 21 wt.% (Yunxu Wei, private communication), which are far above the theoretical value. The result implies that the olivine-gabbro was produced by accumulation of mafic magma rather than crystallization differentiation of ultramafic magma. SiO 2 and Al 2 O 3 contents of the pyroxene grains from the olivine-gabbro samples are wt.% wt.% and 3.43 wt.% 3.92 wt.%, respectively, which are plotted in the subalkaline field (Fig. 7). They have relative low w(feo) (%) values in comparison with the gabbros formed in the intra-plate extensional setting (Chen et al., 2006), but are close to gabbros reported in the subduction zone environment and post-orogeny setting (Zhao et al., 2008; Zhang et al., 1995), which are further supported by the high contents of CaO (20.23 wt.% wt.%). Plagioclase in the olivine-gabbro has high contents of Al 2 O 3 (31.78 wt.% wt.%) and CaO (16.08 wt.% wt.%) that are similar to those of the gabbro rocks in the subduction zone and post-orogeny settings (Zhao et al., 2008; Zhang et al., 1995), in addition, they are much higher than the mafic rocks occurred in the intra-plate extension environment (Chen et al., 2006), which are further proved by the low Na 2 O contents (1.95 wt.% 2.11 wt.%). The mineral chemical data of pyroxene from the olivinegabbro in the southern Huangling anticline show that the rocks could be classified as the sub-alkaline series, and formed in the subduction zone or post-orogeny settings. However, combined with the mineral compositions of the olivine and plagioclase, it is proposed that the olivine-gabbro in the southern Huangling anticline could be formed in the post-orogeny environment rather than the subduction zone. In addition, the large negative zircon ε Hf value of ±0.47 from the olivine-gabbro sample could be resulted from the extremely enriched mantle source or caused by the crustal contamination. However, the values of radiogenic Hf isotope ratios for the olivine-gabbro are very narrow, such as the 176 Hf/ 177 Hf ratios range from to , and ε Hf (t) values are between and Therefore, it is proposed that the olivine-gabbro in the Huangling anticline do not suffer a significant crustal contamination process. In comparison with reported Neoproterozoic mafic rocks in the Yangtze Craton, the 54 olivine-gabbro in the Huangling anticline has large negative zircon ε Hf values that are similar to those of the Huangling granitoids (Zhang et al., 2009). Therefore, as suggested by Zhang et al. (2009, 2008) and Zheng et al. (2008), the tectonic collapse environment is a plausible mechanism for the the Neoproterozoic Huangling batholith on the basis of TTG-affinity and adakitic rocks in the granotoids. 4.3 Geodynamic Implications Existence of the ca. 860 Ma olivine-gabbro in the southern Huangling anticline suggests that the Yangtze Craton were undergoing a tectonic collapse process during the Neoproterozoic era, meanwhile, large negative ε Hf values (-16.57±0.47) of the olivine-gabbro suggest that the rocks could be derived from the extremely enriched lithospheric mantle before the generation of calc-alkaline magmas (Zhao and Zhou, 2013). The lithospheric mantle could be generated from arc-continent collision process during the Paleoproterzoic time associated with aggregation of the supercontinent Columbia (Han et al., 2017; Wu et al., 2012), which formed the Paleoproterozoic mafic dyke in the Kongling anticline with characteristics of mantle property (Peng et al., 2008). Then, the lithospheric mantle formed the olivine-gabbro by accumulation, and the ancient crust was melted to generate the ca. 850 Ma tonalites and adakitic granites, as well as voluminous ca Ma Huangling granites in the Middle-Neoproterozoic (Zhao et al., 2013; Wei et al., 2012; Zhang et al., 2009), which marked crustal growth and ancient crust reworking in response to breakup process of the Rodinia supercontinent (Zhang et al., 2009; Zheng et al., 2007). 5 CONCLUSIONS (1) LA-ICP-MS zircon U-Pb data suggests that the olivinegabbros in the southern Huangling anticline in the Yangtze Craton were emplaced at ca Ma. (2) Mineral compositions of olivine, pyroxene and feldspar of the olivine-gabbro samples indicate formation environment of tectonic collapse, and the large negative zircon ε Hf values (-16.57±0.47) suggest that the gabbro rock was derived from the extremely enriched mantle source that could be generated from the Paleoproterozoic arc-continent collision process. 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