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The Origin of Low-Mgo Eclogite Xenoliths from Obnazhennaya

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The Origin of Low-Mgo Eclogite Xenoliths from Obnazhennaya Contributions to Mineralogy and Petrology (2020) 175:25 https://doi.org/10.1007/s00410-020-1655-6 ORIGINAL PAPER The origin of low‑MgO eclogite xenoliths from Obnazhennaya kimberlite, Siberian craton Jing Sun1,2 · Roberta L. Rudnick3 · Sergey Kostrovitsky4,5 · Tatiana Kalashnikova4 · Kouki Kitajima6 · Ranpeng Li3 · Qiao Shu7,8 Received: 11 August 2019 / Accepted: 11 January 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11– 16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fne-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure–temperature conditions 2.3–3.7 GPa and 855–1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability feld, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as refected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like δ18O of the gar- nets (5.07–5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO 2 contents and depleted light REE ((Nd/Yb)N < 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy REE patterns ((Dy/Yb)N < 1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability feld, based on melting model calculations. We, therefore, suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melt- ing processes at high pressures, where garnet was the main residual phase. Keywords Obnazhennaya · Low-MgO eclogite xenolith · Low-pressure origin · Garnet exsolution · Cumulates Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0041 0-020-1655-6) contains supplementary material, which is available to authorized users. * Jing Sun 5 Institute of Earth Crust, Siberian Branch of the Russian [email protected] Academy of Sciences, 664033 Irkutsk, Russia * Qiao Shu 6 Department of Geoscience, University [email protected] of Wisconsin-Madison, Madison, WI 53706, USA 7 State Key Laboratory of Ore Deposit Geochemistry, 1 State Key Laboratory of Petroleum Resources Institute of Geochemistry, Chinese Academy of Sciences, and Engineering, Beijing 102249, China Guiyang 550081, China 2 College of Geosciences, China University of Petroleum 8 CAS Center for Excellence in Comparative Planetology, (Beijing), Beijing 102249, China Hefei, China 3 Department of Earth Science and Earth Research Institute, University of California, Santa Barbara, CA 93106, USA 4 Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 664033 Irkutsk, Russia Vol.:(0123456789)1 3 25 Page 2 of 22 Contributions to Mineralogy and Petrology (2020) 175:25 Introduction A large number of high-MgO eclogites and pyroxenites from Obnazhennaya have previously been studied (Qi et al. Eclogite is a volumetrically small (< 1%), but important 1994; Taylor et al. 2003; Alifrova et al. 2012, 2015). How- and integral part of the continental lithosphere (Schulze ever, far fewer studies have been carried out on the Obnaz- 1989). Xenolithic eclogites exhibit a broad compositional hennaya low-MgO eclogites. spectrum (Aulbach and Jacob 2016), and their origins The eclogite xenoliths invested here contain both granular could vary accordingly. Several possible origins have been garnet and garnet exsolutions in host clinopyroxenes, which proposed for xenolithic eclogites: (1) cumulates of man- allows us to place constraints not only on their origin and tle melt formed at high pressure (e.g., O’Hara 1969); (2) evolution but also the thermal state of the lithospheric man- residues of melted subducted oceanic crust (e.g., Ireland tle beneath the margin of the Siberian craton. In this paper, et al. 1994; Jacob and Foley 1999; Barth et al. 2001); (3) we present new major, trace element, and oxygen-isotope subducted low-pressure cumulates (e.g., Barth et al. 2002; results for eight fresh low-MgO Obnazhennaya eclogites. Shu et al. 2016); and (4) products of melt-rock reaction We integrate our new data with the previously published (e.g., Smart et al. 2009; Wang et al. 2015). Due to their data for three Group B eclogites from this pipe (Taylor et al. possible origin via ancient subduction, eclogites may hold 2003) to decipher their origins and garnet exsolution history. clues to the past thermal, redox, and chemical state of the ambient convecting mantle (e.g., Aulbach and Stagno 2016; Aulbach and Arndt 2019). Hence, it is important to characterize eclogite xenoliths geochemically to place Geological setting and prior work better constraints on their origins, and the implications for the operation of major earth processes in the past. The Siberian platform extends more than 2500 km from Eclogites with low bulk rock MgO contents are gener- Lake Baikal in the south to the Arctic Ocean in the north, ally interpreted to originate as subducted oceanic crust. and from the Yenisey River in the west to the Aldan shield Examples include low-MgO eclogite xenoliths from South in the east (Fig. 1). Moho depths beneath the platform range Africa (e.g., Gréau et al. 2011; Huang et al. 2012; Huang from 35 to 60 km (Rosen et al. 1994). Phanerozoic kimber- et al. 2014; Shu et al. 2013), Finland (Peltonen et al. lites with ages from 420 to 150 Ma (Sun et al. 2014) erupted 2002), West Africa (e.g., Barth et al. 2001; Aulbach et al. within the region marked “Yakutian kimberlite Province” 2019), Slave craton, Canada (e.g., Aulbach et al. 2007; in Fig. 1. The Obnazhennaya kimberlite pipe lies within the Smart et al. 2012) and from Udachnaya pipe in the middle Kuoika feld, which occupies the extreme northeastern por- of the Siberian craton (Agashev et al. 2018; and refer- tion of the platform (Fig. 1). The age of the Obnazhennaya ences therein). A further important locality for eclogite kimberlite is not well defned. Zircon and perovskite U–Pb and pyroxenite xenoliths, the non-diamondiferous Obnaz- dates from other pipes in the Kuoika feld range from 156 to hennaya kimberlite, is located on the northeastern edge of 167 Ma (Davis et al. 1980; Kinny et al. 1997; Blanco et al. the Siberian craton. These xenoliths can provide informa- 2013; Sun et al. 2014). Paleomagnetic studies of the Obnaz- tion on the shallower lithosphere evolution history of this hennaya pipe suggest an age of 168 Ma (Blanco et al. 2013). region, and provide information about processes occurring Therefore, ~ 160 Ma is assumed to be the eruption age of this at the edge of the platform. pipe (Smelov and Zaitsev 2013). Eclogite or pyroxenite xenoliths from the Obnazhen- We collected a suite of fresh samples and chose those with- naya kimberlite pipe can be divided into two broad groups out orthopyroxene and with low MgO contents in the recon- based on their petrological characteristics. One group of structed whole rock for this study. The size of most xenoliths samples is bimineralic, composed of garnet and clinopy- was 4 cm × 6 cm × 4 cm (Fig. 2a). Eclogite and pyroxenite roxene containing lamellae-shaped garnet exsolutions; xenoliths from Obnazhennaya have previously been investi- they were named Group B/C eclogites in previous studies gated for major, trace elements, oxygen isotope, and Sr–Nd (Taylor et al. 2003) according to their garnet composition. isotopes by Qi et al. (1994); Taylor et al. (2003); Alifrova The MgO content of reconstructed bulk rocks of this group et al. (2012, 2015) and Spengler and Alifrova (2019). In these is always lower than 16%, and we call this group “low- studies, nearly all of samples were of group A (or high-MgO MgO eclogites”. The other group of samples, previously eclogites and websterites). Only three samples from Taylor named Group A eclogites (Qi et al. 1994; Taylor et al. et al. (2003) are Group B eclogites with low MgO contents 2003; Alifrova et al. 2012, 2015) or websterites (Spen- and without orthopyroxene and are, thus, similar to the sam- gler and Alifrova 2019), contain minor orthopyroxene ples in this study. No detailed petrographic descriptions for and rutile in addition to garnet and clinopyroxene. This these three samples were reported, but their mineral major and group of samples has higher MgO content in the recon- trace-element compositions and garnet oxygen isotopes exist; structed bulk rock, and is referred to as “high-MgO” here. these data are included and discussed in this study. 1 3 Contributions to Mineralogy and Petrology (2020) 175:25 Page 3 of 22 25 90° Fig. 1 Simplifed geological 72° 78° 84° 96° 102° 108° 114° 120° 126° 132° 138° 144° 150° 156° map of the Siberian platform, which is modifed. The Obnaz- Kara sea hennaya kimberlite pipe, host Laptev sea of the eclogite xenoliths in this study, is located at the north- eastern margin of the platform. The Udachnaya kimberlite pipe 68° 68° from the Daldyn fled and Mir kimberlite from Mirninsky feld Anabar Kuoika field are located in the central part shield Obnazhennaya of the platform [Modifed from Enisey river Grifn et al.
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