Re–Os Isotopic Compositions of Picrites from the Emeishan Flood

Re–Os Isotopic Compositions of Picrites from the Emeishan Flood

Earth and Planetary Science Letters 276 (2008) 30–39 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Re–Os isotopic compositions of picrites from the Emeishan flood basalt province, China Zhaochong Zhang a,⁎, Xiachen Zhi b, Lei Chen b, Andrew D. Saunders c, Marc K. Reichow c a State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China b CAS Key Laboratory of Crust–Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China c Department of Geology, University of Leicester, Leicester LE1 7RH, UK article info abstract Article history: Picrites from the Lijiang area, in the western part of the Emeishan flood basalt province, are considered to be Received 23 January 2008 one of the most important indicators of the Emeishan mantle plume, and may represent initial mantle plume Received in revised form 29 August 2008 activity. This paper presents the first Re–Os isotope data for picrites from the province. Eighteen Accepted 2 September 2008 geochemically well characterised samples, including 12 picrites and 6 basalts from two sections near Available online 18 October 2008 Lijiang, have been analyzed for Os isotopes. The picrites from the two sections have overlapping Re and Os Editor: R.W. Carlson concentrations and initial Os isotopic compositions. They have higher Os concentrations (1.5–3 parts per billion), and slightly lower Re concentrations (b0.07 ppb) than those from many other flood basalt provinces Keywords: (FBPs) and ocean island picrites. The picrites have distinctly unradiogenic Os isotopes with γOs(t) Re–Os isotope values (t=259 Ma) of −0.9 to −2.4, but the associated basalts have suprachondritic Os isotope compositions picrite with γOs(t) of 1.8 to 73.3, which overlap with published data for basalts from other localities in the province. plume The high γ (t) values of the basalts could be the result of crustal contamination, which is enabled by the low fl Os Emeishan ood basalt province Os concentrations in the parental basalts. The low γ (t) values of the picrites are unusual in terms of plume- upper–lower mantle boundary Os related magmatism. There is no clear evidence for recycled crustal material in the mantle source of the Emeishan picrites, or for a significant degree of interaction of the picrites with ancient subcontinental lithospheric mantle of the Yangtze craton. The low γOs(t) values relative to many other plume-related picrites (from ocean islands or flood basalt provinces) indicate that there was no clear Os isotope evidence of involvement of the Earth's core or recycled crustal components in the formation of Emeishan flood basalt province. © 2008 Elsevier B.V. All rights reserved. 1. Introduction information on the structure of the deep mantle, timescales of mantle mixing and possibly interaction between the mantle and the outer Continental flood basalt provinces (CFBs), characterized by core. However, incompatible elements, and Sr, Nd and Pb isotope voluminous volcanic sequences erupted in a short period of time ratios, are of limited efficiency as indicators of plume sources, because and transient surface uplift, are thought to originate from the rapid they are easily overwhelmed by continental lithospheric mantle and ascent of mantle plumes (Larsen and Yuen, 1997; Larsen et al., 1999; crust. In contrast, the chalcophile and siderophile nature of Re and Os, Saunders et al., 2007). The plumes are probably derived from combined with the compatible behavior of Os during mantle melting, boundary layers, such as the interface between the upper and lower set this isotope system apart from conventional lithophile isotope mantle (i.e. 670 km discontinuity), or the D$ layer at the core–mantle systems and provide a unique insight into mantle plume processes. boundary (CMB, Allègre and Turcotte, 1985; Hofmann, 1997), although Thus, if chemical exchange occurs across the CMB, the hypothesized there has been little direct evidence for either location. Recent seismic long-term elevation in Re/Os and Pt/Os in the outer core would impart evidence suggests that the roots of the Iceland and Hawaiian plumes a coupled enrichment of 187Os and 186Os to mantle plumes derived may reside at the CMB, supporting the view that some plumes may from the CMB. This isotopic fingerprinting might be observed in a originate at this interface (e.g., Bijwaard and Spakman, 1999; plume, because the outer core likely has approximately 300 times Helmberger et al., 1998; Russell et al., 1988; Montelli et al., 2003, higher abundances of Os than the mantle. Such coupled enrichments 2006; Nolet et al., 2006, 2007). have been confirmed via high precision isotope analyses of picrites Constraints on the composition and location of the source regions from Hawaii and the Permo-Triassic Siberian Traps (Walker et al., that ultimately give rise to mantle plumes would provide important 1997; Brandon et al., 1998). Unfortunately, many CFB lavas are crustally contaminated and/or fi ⁎ Corresponding author. they contain signi cant contributions from the subcontinental litho- E-mail address: [email protected] (Z. Zhang). spheric mantle (SCLM, Ellam et al., 1992; Gallagher, and Hawkesworth, 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.09.005 Z. Zhang et al. / Earth and Planetary Science Letters 276 (2008) 30–39 31 1992; Arndt et al., 1993), making inferences about deeper (i.e., sub- dikes and sills, syenite, and other alkaline intrusions, form part of the lithospheric) mantle compositions difficult. This study presents the Emeishan FBP. first Re–Os isotope data for a suite of picrites and associated basalts The lava sequence is underlain by late Carboniferous–Permian from the Lijiang area, in the western part of the Emeishan flood basalt sedimentary beds atop a Mesoproterozoic to late Paleoproterozoic province, which was previously characterized geochemically by Zhang metamorphic basement. A recent biostratigraphic and stratigraphic et al. (2006a). These lavas represent early magmatism associated with investigation has shown that starting in the early Permian, thick the initiation of the Emeishan plume. High Os concentrations in these (N1 km) carbonate sequences were deposited as a response to picritic lavas mean they are relatively insensitive to crustal contam- extensive transgression and basin subsidence in the southwest. ination and therefore represent the best samples of sublithospheric Later, basement uplift took place, suggesting rapid kilometer-scale (possibly plume-related) material so far analysed from this area. doming, accompanied by crustal thinning, before emplacement of the Emeishan lavas (He et al., 2003; Xu et al., 2004). 2. Geological setting and timing of magmatism In contrast to the Siberian Traps, which formed at relatively high northern latitude, emplacement of the Emeishan flood basalts The Late Permian Emeishan flood basalt province (Emeishan FBP) occurred near the equator (Enkin et al., 1992). Overall, the province is exposed over a large area in southwest China (and possibly in appears to be slightly older than the much better-dated, ~251 Ma northern Vietnam), extending from the eastern margin of the Tibetan Siberian Traps. Recent U–Pb dating of zircons in the Xinjie layered Plateau to the western margin of the Yangtze craton (Fig. 1). The flood mafic–ultramafic intrusion in the central part of the province basalts cover an area of more than 2.5×105 km2 with a diameter of yielded an age of 259±3 Ma (Zhou et al., 2002), whereas 40Ar–39Ar ~500 km, and the thickness of the volcanic succession ranges from ages of 254±5 Ma (Boven et al., 2002), 255 Ma, and 251–253 Ma (Lo ~5 km in the west to ~100 m in the east (Chung and Jahn 1995; Song et al., 2002) have been reported for lava flows and two late-stage et al. 2001; Xu et al. 2001). The Emeishan FBP consists predominantly intrusions. More recent SHRIMP U–Pb dating for mafic intrusions, of a succession of tholeiites, with minor picritic and rhyolitic/trachytic dykes and volcanic rocks have yielded an age range from 257 to flows. In addition to lava flows, mafic–ultramafic layered complexes, 263 Ma (Guo et al., 2004; Zhong and Zhu, 2006; He et al., 2007; Zhou Fig. 1. Map showing principal outcrops of the Emeishan flood basalts (shown in black) (modified from Chung and Jahn, 1995). Square symbol located near Lijiang marks the sample locality. 32 Z. Zhang et al. / Earth and Planetary Science Letters 276 (2008) 30–39 et al., 2008), which unequivocally indicates a temporal link between grains reach 4 mm. Olivine is generally replaced by serpentine, but some the Emeishan large igneous province and the end-Guadalupian mass grains retain cores of unaltered olivine (up to 91.6% Fo), many of which extinction (Middle Permian). contain scattered melt inclusions. Strained, kink-banded crystals are absent. Some olivine crystals host equant, euhedral to rounded Cr-spinel 3. Mineralogy and petrography of the Lijiang picrites and basalts a few tens of microns across. Cr-spinel is also present as solitary grains in the groundmass. The groundmass retains an unaltered appearance and The recently discovered picritic flows in the Lijiang area are located consists principally of very fine grained, probably originally glassy, in an 800 m thick lava section near Shiman and a 5500 m thick section mesostasis plus lesser amounts of olivine, anhedral diopside, and small at Daju, one of the thickest in the entire Emeishan province (see Fig. 2). plagioclase crystals. The groundmass olivine tends to be less altered than These two occurrences are about 26 km apart. In both sections, the the olivine phenocrysts; some are nearly equant, but elongated skeletal picritic flows are intercalated with augite–phyric basalt flows (Fig.

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