| Citation: |
Anas A. Karimov, Marina A. Gornova, Vasiliy A. Belyaev, Aleksander Ya. Medvedev, Nikolay V. Bryanskiy, 2020. Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia), China Geology, 3, 299-313. doi: 10.31035/cg2020035
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Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia)
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Abstract
Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain (Central Asian Orogenic Belt, Northern Mongolia). The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals, calculations of closure temperatures and composition of equilibrium melt. The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho- and clinopyroxene due to melt-rock interaction. Relics of peridotite olivines are found in pyroxenites, Cr# of spinel increases from peridotites to pyroxenites, and compositions of ortho- and clinopyroxene change from peridotite to pyroxenite. The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain. Therefore, formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites. Orthopyroxenite veins formed at first, followed by websterite veins. Thus, the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.
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References
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Anas A. Karimov, Marina A. Gornova, Vasiliy A. Belyaev, Aleksander Ya. Medvedev, Nikolay V. Bryanskiy, 2020. Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia), China Geology, 3, 299-313. doi: 10.31035/cg2020035
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Figure 1.
Geological schemes of the Bayangol accretionary complex (a), Dzhida terrain (b), Egiingol massif (c) and fields photo of pyroxenite veins (d–f). a–Bayangol accretionary complex (after Gordienko IV et al., 2007): 1–Narmandal serpentinite mélange with pyroxenites, gabbro, metabasalts and boninites; 2–tectonic melange with a matrix of apobasite tectonites and with large xenoblocks of carbonate rock; 3–metabasalts; 4–differentiated volcanic rocks; 5–terrigenous rocks; 6–flysch; 7–island arc diorite-granodiorite complex; 8–Late Paleozoic granitoids; 9–faults and shear zones. b–Dzhida terrain scheme (after Almukhamedov AI et al., 1996). c–Egiingol massif (after Pinus GV, 1984): 1–conglomerates, sandstones, siltstones, mudstones, interbeds of coal (Middle Jurassic); 2–sedimentary-volcanic deposits (Vendian-Lower Cambrian); 3–sub-alkaline leucocratic granites; 4–syenites; 5–serpentinites and serpentinized harzburgites; 6–quartz-carbonate and talc-carbonate rocks; 7–chrysotile-asbestos mineralization; 8–magnesite mineralization; 9–diluvial outcrops of chromitites; 10–pyroxenite vein area; 11–faults. d–large orthopyroxenite vein. e–websterite veins. f–web of small orthopyroxenite veins. Hz–wall-rock harzburgite, Opxt–orthopyroxenite vein, Web–websterite vein.
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Figure 2.
Petrographic features of orthopyroxenite veins and pyroxenite-harzburgite contacts. a–contact of Hz and thin Opxt vein (about 1–2 cm), Ol assemblages of Hz are permeate to vein (MP13-01/4-2); b–contact of Hz and thin Opxt vein (about 10 cm), Ol assemblages of Hz are also permeate to vein, and the presence of small Ol “inclusion” in Opx grain (M11-71); c–Cpx exsolution lamellae and small Cpx inclusion in large Opx grain, in orthopyroxenite (MP13-30); d–Opx porphyroclast with Cpx exsolution lamellae in harzburgite (MP13-10/5); e–idiomorphic grains of Opx and interstitial Cpx in medium-grained orthopyroxenite (M11-71) f–large irregular Cr-spl grain with Mag rim, harzburgite (MP13-08); g–boundary layer between wall-rock Hz and Opxt vein, the presence of both type of Spl (Spl1-irregular shape and Spl2-idiomorphic Spl grain) (MP13-29B); h–Opx porphyroclast with Cpx exsolution lamellae in contact harzburgite and interstitial Cpx (MP13-21-2). Hz–harzburgite, Opxt–orthopyroxenite, Ol–olivine, Opx–orthopyroxene, Cpx–clinopyroxene, Spl–spinel, Amph–amphibole, Spt–serpentine, Mag–magnetite.
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Figure 3.
Petrographic features of websterite and orthopyroxenite veins. a–coarse-grained texture of websterite vein, presence of secondary amphibole between Opx and Cpx (MP13-08/3); b–interstitial grain of Cpx between large orthopyroxenes, orthopyroxenite (MP13-29); c–large pyroxenes with irregular boundaries, and view of coarse-grained texture of websterite vein (MP13-22); d–large tabular Opx with Cpx exsolution lamellae and large tabular Cpx with Opx inclusion, websterite (MP13-23/2); e–large irregular shape Opx and Cpx grains, Opx inclusion in Cpx, websterite (MP13-22); f–irregular shape of Cpx grain without exsolution lamellae, located between large Opx grains, orthopyroxenite (MP13-08/6); g–small idiomorphic spinel in websterite vein (MP13-22); h–Opx grains with irregular boundaries and idiomorphic Cr-spinel in thick orthopyroxenite vein, presence of amphibole (MP13-10/4). Abbreviations of rocks and minerals are the same as in Fig. 2.
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Figure 4.
Cr#[Cr# = Cr/(Cr+Al)] vs Mg# [Mg# = Mg/(Mg+Fe2+)] (a–c) and TiO2 content (d–f) in Cr-spinels. a, d–composition of spinel from harzburgites; b, e–composition of spinel from orthopyroxenites; c, f–composition of spinel from websterites and Bayangol boninites. Large circles–harzburgites near the contact with pyroxenite vein; small circles–orthopyroxenite veins; srtars–websterite veins; crosses–Bayangol boninites. Each color matches to contact of harzburgite – orthopyroxenite or harzburgite–websterite vein or both, in one contact there can be several samples. The fields of the supra-subduction zone peridotites (SSZ) are given according to the source Parkinson IJ and Pearce JA (1998), abyssal peridotites by the source Dick HJB and Bullen T (1984), high-Ca boninites by Sobolev AV and Danyushevsky LV (1994) are shown for comparison.
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Figure 5.
Olivine composition. a–Egiingol peridotites; b–Egiingol orthopyroxenite veins. Symbols are same as in Fig. 3. Fields of olivine mantle array (Takahashi E et al., 1987), SSZ peridotites (Ishimaru S et al., 2006), high-Ca boninites (Cameron WE 1985; Sobolev AV and Danyushevsky LV, 1994) are shown for comparison.
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Figure 6.
Al2O3 and Cr2O3 contents versus Mg# in orthopyroxenes of contact harzburgites (a, b) and pyroxenite veins (c–e). Composition of orthopyroxenes from contact harzburgites shown by grey field (c, d). Symbols are same as in Fig. 3. Field of secondary Opx (Khedr MZ and Arai S, 2010), and fields of SSZ peridotites and high-Ca boninites are the same as in Fig. 5 are shown for comparison.
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Figure 7.
Al2O3 and Cr2O3 contents vs Mg# in clinopyroxenes of contact harzburgites (a, b) and pyroxenite veins (c–e). Composition of clinopyroxenes from contact harzburgites shown by grey field (c, d). Cpx phenocrysts of BAWC boninites are shown by white crosses. Field of secondary Cpx (Murata K et al., 2009; Nozaka T, 2005; Li XP et al., 2004; Peacock SM, 1987), and fields of SSZ peridotites and high-Ca boninites are the same as in Fig. 5 are shown for comparison.
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Figure 8.
Trace element composition of clinopyroxenes (a, b) and equilibrium melt for pyroxenite veins composition (c, d). Fields of Cpx from abyssal peridotites and pyroxenites of South-West Indian Ridge (SWIR; Dantas C et al., 2007), Cpx composition and equilibrium melts for Oman orthopyroxenite veins (Tamura and Arai S, 2006) and melt inclusions of in clinopyroxene from Bayangol boninites (Simonov VA et al., 2004) are shown for comparison. Field of Bayangol boninites on Fig. 8d is according to Almukhamedov AI et al. (1996) and authors' unpublished data.
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Figure 9.
Equilibrium temperature estimates of REE (Liang Y et al., 2013) and major-element (Brey G and Köhler T, 1990) two-pyroxene thermometers. a–orthopyroxenite vein (sample eg01-5/27), b–websterite vein (sample M11-71).