Geochemistry of olivine-hosted melt inclusions in the Baekdusan (Changbaishan) basalts: Implications for recycling of oceanic crustal materials into the mantle source
Introduction
Silicate melt inclusions (hereafter ‘melt inclusions’) are small droplets entrapped in phenocrysts during their growth within magmatic systems (e.g., Lowenstern, 1995, Roedder, 1984). Melt inclusions entrapped by phenocrysts that formed early in the process of magma evolution may represent snapshots of primitive magmatic conditions (Kent, 2008, Schiano, 2003, Shimizu, 1998, Sobolev, 1996). Studies of such melt inclusions can provide important insights into the nature of mantle sources and the melting and melt transport processes that occur in primitive magma and are subsequently erased from the basaltic rock by fractional crystallization, magma mixing, or crustal contamination during its evolution. Meanwhile, it is well recognized that the original composition of a melt inclusion may be altered by post-entrapment processes during natural cooling, such as crystallization of host minerals on the walls of inclusions, crystallization of other daughter minerals inside the inclusions, formation of shrinkage bubbles, or diffusive re-equilibration between host minerals and the external magma (e.g., Chen et al., 2011, Cottrell et al., 2002, Danyushevsky et al., 2000, Gaetani and Watson, 2000, Qin et al., 1992, Roedder, 1984). Such modification, however, can be reversed by rehomogenization experiments and/or numerical reconstruction (e.g., Danyushevsky et al., 2002, Kent, 2008, Schiano, 2003).
Late Cenozoic intraplate volcanism is widespread in the eastern North China Craton (NCC; Fig. 1a, b). Numerous petrological and geochemical studies have shown that Cenozoic basaltic rocks have oceanic island basalt (OIB)-like patterns of trace element distribution, including enrichment of light rare earth elements (LREE) and a lack of depletion of high field strength elements (HFSE), and thus might be derived from a peridotitic source in the subcontinental lithospheric mantle or asthenosphere (Basu et al., 1991, Chen et al., 2003, Chu et al., 2013, Kuritani et al., 2009, Kuritani et al., 2013, Li et al., 2014, Sakuyama et al., 2013, Xu et al., 2005, Xu et al., 2012a, Yan and Zhao, 2008, Zhang et al., 1995, Zhang et al., 2015, Zou et al., 2008). However, there is a general consensus that the peridotitic source contains a significant proportion of mafic lithologies such as pyroxenite, eclogite, or hornblendite, which may play an important role in generating OIB-like intraplate basaltic magmatism (e.g., Eisele et al., 2002, Gao et al., 2004, Gao et al., 2008, Hauri, 1996, Hirschmann et al., 2003, Keshav et al., 2004, Kogiso and Hirschmann, 2006, Kogiso et al., 2003, Pertermann and Hirschmann, 2003a, Pertermann and Hirschmann, 2003b, Rehkämper and Hofmann, 1997, Sobolev et al., 2005, Sobolev et al., 2007, Yaxley and Green, 1998). Recycled oceanic crust or melt-peridotite reaction products may provide the mafic source rocks (e.g., Hauri, 1996, Herzberg, 2011, Sobolev et al., 2005, Sobolev et al., 2007, Straub et al., 2008), and some Cenozoic basaltic rocks from the eastern NCC (e.g., Abaga-Dalinuoer, Chifeng, Jilin, Hebei, Hannuoba, Liaoning, Bohai Bay, Shandong, Anhui, Jiangsu) may be derived from a source lithology containing pyroxenite in addition to peridotite (Hong et al., 2013, Li et al., 2016, Liu et al., 2008, Qian et al., 2015, Zhang and Guo, 2016, Zhang et al., 2009).
The Baekdusan volcanic field (also called Changbaishan in China; N41°20′–42°40′, E127°00′–129°00′), located on the border between China and North Korea, represents the largest exposure of intraplate volcanic rocks on the NCC (Fig. 1b, c). The volcanoes in this area have erupted repeatedly in historical times. The most powerful eruption, called “the Millennium Eruption”, occurred with a Volcanic Explosivity Index (VEI) of 6 or 7 in ca. 938–946 CE (Horn and Schmincke, 2000, Iacovino et al., 2016, Wei et al., 2013, Xu et al., 2013, Yin et al., 2012, Zou et al., 2010). Voluminous Plinian fallout, ignimbrite, lahar, and other erupted materials covered 33,000 km2 of northeastern China and Korea (Stone, 2011, Sun et al., 2014), and extended as far as northern Japan, ~ 1200 km from the volcano (Machida and Arai, 1983). After the Millennium Eruption, minor volcanic activity continued, with eruptions in 1413, 1597, 1668, 1702, 1898, and 1903 CE, exhibiting a roughly 100-year periodicity (Cui et al., 1995, Stone, 2011, Xu et al., 2012b, Xu et al., 2013). Although the volcano has not erupted in the past 100 years, all available geological data including seismicity, ground deformation, and geochemical monitoring of springs indicate that Baekdusan is an active volcano with the potential for eruption in the near future (Ramos et al., 2016, Wei et al., 2003, Wei et al., 2013, Xu et al., 2012b, Yun and Lee, 2012). Thus, it is of the utmost importance to understand the nature of the magmatism to anticipate its behavior at this site. Previous studies have primarily focused on the history of Baekdusan volcanic activity (Wei et al., 2003, Wei et al., 2007, Wei et al., 2013), magma evolution and the eruptive mechanism (Liu et al., 2015a, Wang et al., 2003, Zhang et al., 2015, Zou et al., 2008, Zou et al., 2010, Zou et al., 2014), and geochemical lines of evidence of enriched mantle sources (Basu et al., 1991, Chen et al., 2007, Hsu et al., 2000, Kuritani et al., 2009, Liu et al., 2015a). Here, we describe for the first time the major and trace element compositions of re-heated melt inclusions entrapped in olivine phenocrysts from the Baekdusan basaltic rocks. Combining these data with whole-rock major element, trace element, and host olivine compositions, we shed light on (1) the particular mineral phases and lithologies of the mantle source, (2) the nature of source heterogeneities, and (3) the geodynamic processes that gave rise to volcanism at Baekdusan.
Section snippets
Geological setting and sampling
The NCC is one of the oldest cratons in the world (~ 3.8 Ga; Liu et al., 1992, Zheng et al., 2004), and the Baekdusan volcanic field is located on the northeastern margin of the NCC (Fig. 1a, b). The basement of the eastern NCC consists primarily of Archean tonalite-trondhjemite-granodiorite (TTG) gneisses, granitoids, and supracrustal rocks, which are locally overlain by Proterozoic to Paleozoic strata (e.g., Wang et al., 2003, Zhang et al., 2015, Zhang et al., 2017). The eastern NCC experienced
Analytical procedures
All samples used in this study were freshly collected. Samples for the whole-rock analysis were crushed into small pieces (< 5 mm in diameter) in a tungsten carbide mortar, and cleaned in an ultrasonic bath containing Milli-Q water. Fresh fragments were pulverized in an agate ball mill prior to geochemical analysis. Whole-rock major element contents were analyzed by X-ray fluorescence spectrometry (XRF) at Pukyong National University in Pusan, South Korea. The data were reduced using a weighted
Host basaltic rocks
Major and trace element concentrations in the host rocks are given in Table S1. On a total alkali versus silica (TAS) diagram (Le Maitre et al., 1989) (Fig. 2), they are compositionally basalt to basaltic andesite, belonging to the sub-alkaline suite. The host rocks have a limited range of SiO2 (51.1–52.3 wt%) with Mg#s [= 100 Mg/(Mg + Fe2 +)] varying from 46.0 to 59.0 (Table S1; Fig. 3). The Ni, Co, and Cr contents of the samples were 50–110, 34–39, and 70–150 ppm, respectively (Table S1). The Mg#
Source mineralogy and lithology
Melt inclusions exhibit a wider diversity of compositions than their host basalts (Fig. 3). For example, TiO2, K2O, and P2O5 contents of melt inclusions varied from 2.4 to 5.3, 0.2 to 3.0, and 0.2 to 5.3 wt%, respectively, but those species in the host basalt are lower, from 2.1 to 3.0, 1.3 to 1.8, and 0.3 to 0.7 wt%, respectively. Significant compositional diversity has often been observed in melt inclusions from a single sample (e.g., Choi et al., 2013, Gurenko and Chaussidon, 1995, Kamenetsky
Conclusions
Olivine-hosted melt inclusions in Baekdusan basalts can be divided into two major groups in terms of their major and trace element compositions: a low-Si group and a high-Si group. The low-Si group is characterized by a distinct positive spike for P in the spidergram, as well as a positive correlation between CaO and P2O5. The high-Si group exhibits pronounced positive anomalies in Eu, Ba, Rb, K, Pb, and P. They are characterized by fractionated (Zr/Hf)N ratios higher than the OIB. Geochemical
Acknowledgements
This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMIPA2015-7090. We would like to thank to Nadja Omara Cintron Franqui for her help during the experimental work. Insightful reviews by Takeshi Kuritani and an anonymous reviewer greatly improved the manuscript. We thank Sun-Lin Chung for the editorial handling.
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