Magmatic Recharge Buffers the Isotopic Compositions Against Crustal Contamination in Formation of Continental Flood Basalts
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Lithos 284–285 (2017) 1–10 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Magmatic recharge buffers the isotopic compositions against crustal contamination in formation of continental flood basalts Xun Yu, Li-Hui Chen ⁎,GangZeng State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China article info abstract Article history: Isotopic compositions of continental flood basalts are essential to understand their genesis and to constrain the Received 2 August 2016 character of their mantle sources. Because of potential crustal contamination, it needs to be evaluated if and to Accepted 27 March 2017 which degree these basalts record original isotopic signals of their mantle sources and/or crustal signatures. Available online 08 April 2017 This study examines the Sr, Nd, Hf, and Pb isotopic compositions of the late Cenozoic Xinchang–Shengzhou (XS) flood basalts, a small-scale continental flood basalt field in eastern China. The basalts show positive Keywords: 87 86 143 144 143 144 Continental flood basalts correlations between Sr/ Sr and Nd/ Nd, and negative correlations between Nd/ Nd and 176 177 Mantle heterogeneity Hf/ Hf, which deviate from compositional arrays of crustal contamination and instead highlight variations Magmatic recharge in magmatic recharge intensity and mantle source compositions. The lava samples formed by high-volume mag- Crustal contamination matic recharge recorded signals of recycled sediments in the mantle source, which are characterized by moderate Ba/Th (91.9–106.5), excess 208Pb/204Pb relative to 206Pb/204Pb, and excess 176Hf/177Hf relative to 143Nd/144Nd. Thus, we propose that magmatic recharge buffers the original isotopic compositions of magmas against crustal contamination. Identifying and utilizing the isotope systematics of continental flood basalts generated by high volumes of magmatic recharge are thus crucial to trace their mantle sources. © 2017 Elsevier B.V. All rights reserved. 1. Introduction and/or temporary storage in the crust (Arndt et al., 1993; Brandon et al., 1993; Meshesha and Shinjo, 2007; Yu et al., 2015b). Continental flood basalts are thought to be the volcanic manifesta- To recognize effects of crustal contamination during magma evolu- tions of large volumes of magma derived from Earth's mantle including tion, previous studies have examined correlations between SiO2 and mantle compositions with variable elemental and isotopic compositions incompatible element ratios (e.g., Ba/Th and Ce/Pb) and isotopic ratios (e.g., Hooper et al., 2007; Sobolev et al., 2009, 2011; Zhang et al., 2006). (e.g., 87Sr/86Sr and 143Nd/144Nd) (e.g., Arndt and Christensen, 1992; Understanding the compositional record of continental flood basalts Sharma et al., 1991; Xiao et al., 2004). This approach is based on the is thus crucial for a better understanding of mantle compositional principle assumption that continental flood basalt compositions are variations and ultimately the dynamics that lead to these variations. essentially mixtures of two end-members: mantle-derived melts and Radiogenic isotope systematics of melts derived from a mantle continental crust. Thus, ideally, mixing between these two end- source can help provide clues on the formation mechanisms members yields a mixing line indicating crustal contamination, other- (e.g., asthenosphere versus continental lithosphere, identifying the wise insignificant crustal contamination is suggested (e.g., Li et al., properties of the recycled crustal materials in mantle sources) of 2015; Xu et al., 2005). However, mixing or contamination by crustal continental flood basalts (e.g., Chung and Jahn, 1995; Hooper et al., materials in mantle sources can also form geochemical correlations as 2007; Sharma et al., 1992; Xiao et al., 2004). However, crustal contami- mentioned above. Besides of source mixing or crustal assimilation and nation commonly obscures the primary compositions and needs to be fractional crystallization (AFC), another kind of magmatic processes constrained (Carlson et al., 1981; Cox, 1980; Devey and Cox, 1987; for continental flood basalts is magmatic recharge in magma chambers Ramos et al., 2005; Silver et al., 2006). It remains uncertain and debated (e.g., Camp and Roobol, 1989; Devey and Cox, 1987; Kumar et al., 2010). whether the radiogenic isotopic compositions of continental flood Magmatic recharge means continuous or episodic magma replenish- basalts record elemental and isotopic heterogeneities of the mantle ment in the long-lived magma chambers, which are situated above a re- source or if this information is lost during their passage through gion undergoing relatively continuous magmatic fluxing (e.g., Bohrson and Spera, 2003; Davidson and Tepley, 1997; Lee et al., 2014; O'Hara and Mathews, 1981; Spera and Bohrson, 2002; Yu et al., 2015b). ⁎ Corresponding author. Although magmatic recharge commonly occurs in magma chambers E-mail address: [email protected] (L.-H. Chen). that produce flood basalts (Camp and Roobol, 1989; Devey and Cox, http://dx.doi.org/10.1016/j.lithos.2017.03.027 0024-4937/© 2017 Elsevier B.V. All rights reserved. 2 X. Yu et al. / Lithos 284–285 (2017) 1–10 1987; Kumar et al., 2010; Song et al., 2006; Yu et al., 2015a, 2015b)and contamination, thereby isotopic compositions of flood basalts with sig- layered intrusions (e.g., Bohrson and Spera, 2003; Howarth and Prevec, nificant volume recharge can provide constraints on the compositional 2013; Maydagan et al., 2014; Pang et al., 2009; Wark et al., 2007), the variations of the mantle. role of magmatic recharge has not been addressed adequately in previ- ous studies. Magmatic recharge represents a plausible mechanism of di- 2. Background review luting the signal of crustal contamination and resulting in more primitive magma and the composition of the mantle-derived recharge Eastern China is composed, from north to south, of the Xingmeng magma may vary with time (Yu et al., 2015b). Therefore, it is important Block, the North China Craton, the Yangtze Craton, and the Cathysia to understand possible temporal variations in mantle-derived magma Block. Cenozoic basalts that have erupted from monogenetic volcanoes supply in addition to processes and compositional evolution in the crust. or as small-scale flood basalts are widely distributed in eastern China To quantify magmatic recharge and crustal contamination when an (e.g., Liu et al., 1992). According to the geophysical observations, isotopic study is conducted, radiogenic isotopic compositions of Sr, Nd, subducted paleo-Pacific oceanic slab is stagnant at the mantle transition Hf, and Pb of the Xinchang–Shengzhou (XS) flood basalts, a small- zone covering the distribution area of Cenozoic basalts of eastern China scale continental flood basalt field in Zhejiang Province, southeastern (Huang and Zhao, 2006)(Fig. 1a). Geochemical studies further China (Fig. 1a), were analyzed and evaluated. The XS flood basalts are suggested that the formation of these Cenozoic basalts has close known to have been influenced by magma chamber processes relationship with the stagnant slab which contributed recycled crust (i.e., crustal contamination and magmatic recharge) in the lower conti- and sediment materials (e.g., Li et al., 2015; Sakuyama et al., 2013; Xu nental crust (Yu et al., 2015a). Consequently, the XS flood basalts are et al., 2012; Xu, 2014). suitable for investigating the impact of magmatic recharge on the In the Cathysia Block, small-scale flood basalts occur in the coastal isotope systematics of continental flood basalts. Combined with major areas of Zhejiang, Fujian, Guangdong, and Hainan provinces. Among and trace elemental compositions (Yu et al., 2015a), the isotopic them, XS flood basalts crop out in Zhejiang Province of the northeastern compositions of the XS flood basalts give clues to the interplay of crustal Cathysia Block (Fig. 1a). The main body of the XS flood basalts is ‘contamination’, magmatic ‘recharge’, and mantle ‘heterogeneity’ (here- composed of high-SiO2 alkaline/tholeiitic basalts (here we defined after termed the CRH model) and possible short-term variations. It is basalts with SiO2 N 47.5 wt.% and positive correlation between SiO2 and proposed that continuous and significant recharge of the basaltic Na2O+K2O as high-SiO2 basalts while basalts with SiO2 b 47.5 wt.% magma chamber can suppress the isotopic variations by crustal and negative correlation between SiO2 and Na2O+K2O as low-SiO2 Fig. 1. Distribution of Cenozoic basalts in eastern China (a) and in the Zhejiang Province (b). Three major wrench faults in the Zhejiang Province are the Jiangshan–Shaoxing fault (A), the Lishui–Yuyao fault (B), and the Zhenhai–Wenzhou fault (C). The studied basalts crop out north and south of the surface trace of the Lishui–Yuyao fault (B). The sampling locations are those of Yu et al. (2015a). The indicated K–Ar and Ar–Ar ages of the basalts are from Ho et al. (2003) and Yu et al. (2015a). White symbols stand for low-SiO2 alkaline basalts, and blue and green symbols stand for high-SiO2 basalts sampled from south and north of the Lishui–Yuyao fault, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) The whole map is modified from Yu et al. (2015a). X. Yu et al. / Lithos 284–285 (2017) 1–10 3 basalts (Fig. 2)). Negative correlation of Ar–Ar ages versus latitudes of high-SiO2 basalts (Fig. 2BinYu et al., 2015a) suggests that the flood basalts were first emplaced in the south (Xinchang), and then moved northward continuously to the north (Shengzhou), across the Lishui– Yuyao trans-lithospheric fault from 9.4 Ma to 3.0 Ma (Fig. 1b). It indicates that the magmatism lasted more than six million years and the volcanic center was migrating northwards (Ho et al., 2003; Yu et al., 2015a). Besides of low SiO2 contents and negative correlation of SiO2 versus Na2O+K2O(Fig. 2a), low-SiO2 alkaline basalts (including nephelinite, basanite and minor alkaline olivine basalt) are characterized with high MgO contents (N8.5 wt.%), which are distinct from high-SiO2 alkaline/ tholeiitic basalts (Fig.