Petrogenesis of the Neoproterozoic low-δ18O granitoids at the western margin of the Yangtze Block in South China

doi:10.1016/j.precamres.2020.105953

Precambrian Research

Volume 351, December 2020, 105953

https://doi.org/10.1016/j.precamres.2020.105953 Get rights and content

Highlights

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The ca. 800 Ma Moutuo pluton consists of calc-alkaline I-type granitoids.
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Low-δ18O signatures were inherited from the mantle source.
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The heterogeneous mantle resulted from variable addition of AOC and sediment materials.

Abstract

The western margin of the Yangtze Block in South China preserves voluminous arc-like intrusive and extrusive igneous rocks which were formed in an active continental margin during the Neoproterozoic time. Rocks from the ca. 800 Ma Moutuo pluton are calc-alkaline I-type granitoids which have high SiO₂ (67.13–77.96 wt%), K₂O (2.08–4.32 wt%) and low MgO (0.16–1.81 wt%). They show concave chondrite-normalized rare earth element patterns, and are rich in large ion lithophile elements (e.g. Rb, Ba, Th, U and Pb), but depleted in high field strength elements (e.g. Nb, Ta and Ti). The magmatic zircons have low δ¹⁸O (3.29–4.80‰) and high εHf(t) values (+6.10 to +13.16) with two-stage Hf model ages (T_DM2) of 0.87 to 1.33 Ga. These lines of evidence suggest that the juvenile mafic crust, protolith of the low-δ¹⁸O granitoids, was derived from the sub-arc mantle that was modified by slab-derived materials (~90% altered oceanic crust and ~10% oceanic sediments). Partial melting of the newly formed mafic crust followed by fractional crystallization played an important role in the petrogenesis of the Moutuo granitoids. Based on the available oxygen and hafnium isotopic data for the Neoproterozoic magmatic zircons in this region, the low-δ¹⁸O character of the I-type granitoids in active continental margins was probably inherited from the sub-arc mantle that was mixed with the altered oceanic crust and oceanic sedimentary materials.

Introduction

Active continental margins are the places where continental crust is generated and recycled into the mantle by subduction (Hawkesworth et al., 2019). Calc-alkaline I-type granitoids, constituting the dominant fractionated intrusions in active continental margins (Brown, 2013), generally show typical arc-affinity chemical compositions which may have resulted from crustal contamination or inherited from subducting slab-derived materials, including marine sediments and hydrothermally altered oceanic crust (AOC) (Castro et al., 2010, Depaolo, 1981, Kemp et al., 2009). It remains unclear how the crust- and mantle-derived materials play variable roles in the petrogenesis of the I-type granitoids in active continental margins (Hammerli et al., 2018). As the precrusor of I-type granitoids, basaltic rocks in arc settings were derived from a mantle wedge that had been enriched by slab-derived fluids and melts (Hawkesworth et al., 1993, Kelemen et al., 1998, Kimura and Yoshida, 2006, Yogodzinski et al., 1994). Alternatively, they are suggested to have been sourced from mélanges which were physical mixtures of mantle peridotite, AOC and oceanic sediments (Castro, 2020, Castro, 2014, Codillo et al., 2018, Cruz-Uribe et al., 2018, Nielsen and Marschall, 2017).

Oxygen and hafnium isotopes have been widely used to constrain petrogenesis of igneous rocks. Zircon is the robust accessory mineral in granitoids, and its U-Pb ages and Hf-O isotopes systematically record magmatic process from which they crystallized (Bindeman, 2008, Kemp et al., 2007, Spencer et al., 2019, Wang et al., 2013a). Granitoids generally have high or mantle-like δ¹⁸O values (5.3 ± 0.6‰, 2σ, VSMOW; Valley et al., 1998), whereas those formed in calderas settings and rift zones show low even negative δ¹⁸O values (Lackey et al., 2008, Valley et al., 2005). High and low δ¹⁸O values reflect the protolith suffered water–rock interactions under different temperatures (Grimes et al., 2011, Hoefs, 2018, Page et al., 2007). Therefore, low-δ¹⁸O felsic igneous rocks are suggested to be produced by partial melting or assimilation of ¹⁸O-depleted continental crust rocks (e.g. Bindeman, 2008, Blum et al., 2016, Harris and Ashwal, 2002, Zheng et al., 2004, Zhu et al., 2017).

Neoproterozoic (870–750 Ma) granitoids are widely distributed in the western margin of the Yangtze Block (Fig. 1; Li et al., 2003, Zhao et al., 2018, Zhou et al., 2002, Zhou et al., 2006b). These igneous rocks are generally considered to have been formed in a long-term subduction system (Dong et al., 2011, Dong et al., 2012), and thus provide a good example for discussing petrogenesis of the calc-alkaline granitic magmatism in an active continental margin. In this study, we present new zircon U–Pb ages, Hf- and O isotopes, as well as whole rock geochemical data for the Moutuo pluton in the region, and propose that their low-δ¹⁸O signatures were inherited from the sub-arc mantle which was probably mechanically mixed with the AOC and sedimentary materials. The study also enlightens the sub-arc melting dynamics in the active continental margin.

Section snippets

Geological background

South China comprises the Yangtze Block in the northwest and the Cathaysia block in the southeast (Shu et al., 2011, Zhao et al., 2011, Zheng et al., 2013). The Yangtze Block is separated from the North China Craton to the north by the Triassic Qinling–Dabie orogenic belt (Bader et al., 2019, Stern et al., 2018, Zheng et al., 2018), and from the Tibetan Plateau by the Songpan–Ganze Terrane to the west (Fig. 1; Zhao, 2015). The Yangtze Block consists of Archean crystalline basement and

Zircon U–Pb dating

Zircons were separated using conventional magnetic and density techniques, mounted in epoxy and polished for analysis. Both reflected and transmitted photomicrographs, as well as cathodoluminescence (CL) images were used to reveal internal textures and locate analysis spots. Zircon U–Pb data were collected using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences

Zircon U–Pb ages

Two samples, NX12 and NX13, from the Moutuo intrusion were used for zircon U–Pb dating (Table 1). The zircon grains are subhedral to euhedral and range in size from 60 to 150 μm with length/width ratios of 2:1 to 1:1. Most grains exhibit oscillatory zonings in CL images, typical of magmatic origin. A few grains preserve core and mantle structures (Fig. 4a). Twenty-five analyses were carried out on 25 zircon grains for each sample. They have moderate U (409 to 2221 ppm) and Th (281 to 1619 ppm)

Effect of supracrustal contamination

Rocks from the Moutuo pluton contain biotite and hornblende and lack Al-rich minerals (Fig. 3). They are calc-alkalic to alkali-calcic in composition (Fig. 5a), rich in LILEs (e.g. K and LILE) and depleted in HFSEs (Fig. 6b), and display negative correlations between SiO₂ and P₂O₅, similar to I-type granites (Fig. 5e; Broska et al., 2004, Chappell et al., 2001, Wolf and London, 1994).

Magmatic zircons from the Moutuo granitoid have lower δ¹⁸O values (3.29 to 4.80‰) than those of the mantle

Conclusions

The ca. 800 Ma Moutuo pluton in the western margin of the Yangtze Block consists of calc-alkaline I-type granitoids which were formed by partial melting of the juvenile mafic crust followed by fractional crystallization. Their low-δ¹⁸O and high-εHf isotopic characters were inherited from the sub-arc mantle that was mechanically mixed with the slab-derived materials, including ~90% AOC and ~10% sedimentary materials. The O-isotope compositions and variations among the Neoproterozoic intrusions

CRediT authorship contribution statement

Han Qi: Formal analysis, Investigation, Writing - original draft, Visualization. Jun-Hong Zhao: Methodology, Resources, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was substantially support by the National Nature Science Foundation of China (41573020, 41773027 and 41973031) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan). This manuscript benefitted from discussion with Dr. T.E. Johnson.

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Citation Excerpt :
In addition, extensive Late Mesoproterozoic A-type magmatic rocks and S-type granites have been documented at 1040–1020 Ma (Zhu et al., 2016; Zhu et al., 2020b). Voluminous Neoproterozoic granitic and mafic–ultramafic intrusive complexes, such as the Pengguan, Kangding, Shimian, Miyi, Tongde, Datian, and Yuanmou complexes as well as Moutuo, Jiaoziding, Yonglang, Dalu, Shuilu granitoid plutons, were well preserved and show predominant zircon U-Pb ages of ca. 870–750 Ma (Hu et al., 2020; Li et al., 2018; Qi and Zhao, 2020; Zhao et al., 2018 and references therein, 2019; Zhu et al., 2019a, 2019b, 2019c, 2020a, 2021a, 2021b). In this study, the Mopanshan pluton, covering an area of ∼ 280 km2, crops out at the middle segment of the western Yangtze Block (Fig. 1b), and is cut by the NS-trending Mopanshan Fault to the west and the Anninghe Fault to the east (Huang et al., 2009) (Fig. 1c).
Crust- and/or mantle-derived igneous rocks in the Neoproterozoic continental magmatic arc along the western Yangtze Block (South China) have been extensively studied, but the crust-mantle interaction that might have been involved in the formation of these rocks was poorly understood. In this paper, we present an integrated study on petrology, whole-rock compositions, in-situ plagioclase Sr and zircon U-Pb-Hf isotopes for granodiorites and monzogranites from the Neoproterozoic Mopanshan pluton in the western Yangtze Block. Our objective is to provide vital constraint on the magma mixing process in the formation of these rocks. The new LA-ICP-MS zircon U-Pb dating results reveal that both granodiorites and monzogranites of the Mopanshan pluton were coeval and formed at ca. 820 Ma. They are calc-alkaline to high-K calc-alkaline (SiO₂ = 66.06–70.30 wt%, K₂O = 2.24–3.34 wt%) and metaluminous to slightly peraluminous (A/CNK = 0.99–1.10) rocks, and characterized by middle-upper crustal trace element patterns (e.g., enrichment in Rb, K, Th, U, and Pb, and depletion in Nb, Ta, Sr, and Ti). The magma mixing process is supported by following ample evidence, including (1) the disequilibrium mineral textures and an abruptly increased An values (up to 60) as well as significant decrease of (⁸⁷Sr/⁸⁶Sr)_i ratios from core to rim in plagioclase crystals, (2) higher Mg^# values (45–50 > 40) than those of experimental melts from basalts at the same silica contents, (3) positive and negative whole-rock εNd(t) (–0.72 to + 1.01) values as well as variable whole-rock (⁸⁷Sr/⁸⁶Sr)_i (0.702778–0.705404) and zircon εHf(t) (+1.76 to + 7.72) values. The special geochemical characteristics and element modeling support that the Mopanshan pluton was generated by magma mixing of ancient crust-derived melts and relatively mafic melts from metasomatized mantle source. The adakitic signatures observed in the Mopanshan pluton were attributed to subsequent hornblende-dominant intra-crustal fractional crystallization, rather than derived from a thickened lower crust source. In combination with regional geology and our compilation for Nd-Hf isotopes of Neoproterozoic igneous rocks, the Mopanshan pluton is magmatic response of intensive crust-mantle interaction induced by underplating of voluminous mantle-derived magma in a back-arc extension setting. This research highlights that the crust-mantle interaction is significant for the petrological and geochemical diversity of Neoproterozoic granitoids from the western Yangtze Block.
A Neoproterozoic low-δ<sup>18</sup>O magmatic ring around South China: Implications for configuration and breakup of Rodinia supercontinent
2021, Earth and Planetary Science Letters
We report Neoproterozoic (ca. 785-780 Ma) granites from the western margin of the Yangtze Block that are characterised by magmatic zircons with $δ^{18}$ O values as low as 2.98‰. The lack of low- $δ^{18}$ O magmatic zircons in the ca. 820-805 Ma rhyolite samples from the Neoproterozoic Suxiong Formation indicates that there is no recycling of pre-existing hydrothermally altered crust in the study area prior to the emplacement of the ca. 785-780 Ma granites. Thus the ca. 785-780 Ma granites with low- $δ^{18}$ O values from the western margin of the Yangtze Block can be linked to assimilation of syn-magmatically altered rocks (rather than the assimilation of a pre-existing hydrothermally altered crust). The granites have a source constrained by their depleted Hf isotopes and low- $δ^{18}$ O values resulting from high temperature hydrothermal alteration. The $δ^{18}$ O values of the zircons from core to rim exhibit a decrease indicative of the remelting of material during the interaction between magma and water at high temperatures. In combination with the widespread low- $δ^{18}$ O signatures that occur in the northern and southern margins of the Yangtze Block and Cathaysia Block, the locations of low- $δ^{18}$ O magmatic zircons exhibit a Neoproterozoic low- $δ^{18}$ O magmatic ring around South China. This continent-scale Neoproterozoic extensional and magmatic event cannot be attributed to subduction processes in South China resulting in the emplacement of a magmatic ring of felsic igneous rocks with low- $δ^{18}$ O values. We therefore propose a model involving a Neoproterozoic super-mantle plume with a diameter of approximately 1500 km controlling the development of the rift systems around South China.

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