Boninites and ophiolites: Problems of their relations and petrogenesis of boninites

Abstract

. Dobretsov et al. [1985] first described the rock complexes in Eastern Sayan as ophiolites. Ophiolites formed in Dunzhugur island arc and were obducted onto Gargan block, a Neoarchean crystalline basement of the Tuva-Mongolian Massif (TMM), as a single nappe [Khain et al., 2002;Kuzmichev, 2004]. Zircons from plagiogranite were dated at 1021±5 Ma by multigrain TIMS and 1020±1 Ma by Pb-Pb single-grains evaporation method [Khain et al., 2002]. Later [Kuzmichev, Larionov, 2013] analysed 12 grains of detrital zircons from gravelstone of the Dunzhugur formation and obtained 206 Pb/ 238 U ages from 844±8 to 1048±12 Ma. Careful examination of these data shows that 206 Pb/ 238 U ages for concordant zircons only vary from 962±11 to 1048±12 Ma. Two groups of data give Concordia ages of 974±11 and 1028±10 Ma. Rocks of the Dunzhugur complex are characterized by slightly negative to slightly positive εNd(t) values from -1.0 to +1.5 and Late Palaeoproterozoic depleted mantle Nd model ages of 1.8-1.6 Ga [Sklyarov et al., 2016]. The Dunzhugur complex was intruded by tonalite plutons of the Sumsunur complex dated at 785±11 Ma [Kuzmichev et al., 2001] and 811±7 Ma [Kovach et al., 2012]. These tonalites have strongly negative ɛNd(t) values from -13.2 to -12.3 and Nd model ages of 2.5-2.4 Ga, suggesting formation of these melts from a mixture of N.L.

Geochemistry and Origin of the Eastern Sayan Ophiolites, Tuva-Mongolian Microcontinent (Southern Siberia)

“…Nd isotope analysis revealed variations in εNd(1020) from -2.8 to +2.7 in Dunzhugur boninites and andesites, -0.2 to +0.4 in Ilchir boninites and andesites, and +2.3 to +4.5 in tholeiites from Ilchir and Dunzhugur massifs. The range of εNd(1020) from -1.0 to +1.5 was previously reported for the Dunzhugur andesite and boninite lavas, dikes, and gabbro [Sklyarov et al, 2016]. Overall, samples with lower εNd tend to be more enriched in terms of trace element ratios such as Zr/Ti, Th/Yb, La/Yb (Fig.…”

“…The Eastern Sayan ophiolites form two belts surrounding Gargan block, with Dunzhugur massif in northern belt, and Ilchir massif in southern belt, which are the subject of this work. Previous studies reported boninites and boninite-series andesites, peridotitepyroxenite-gabbroic rocks and mantle harzburgites and argued for the supra-subduction zone setting and formation of the ophiolites in an incipient Duznhugur island arc [Dobretsov et al, 1985;Khain et al, 2002;Kuzmichev, 2004;Sklyarov et al, 2016]. In this study, elemental and Nd isotope geochemistry of lavas and dikes in the Dunzhugur and Ilchir massifs are investigated to constrain the origin of the Eastern Sayan ophiolites.…”

“…F -Ti -V diagram, [Shervais, 1982] with field subdivision of [Pearce, 2014]. Previously reported data for the Dunzhugur lavas and dikes [Dobretsov et al, 1985;Khain et al, 2002;Kuzmichev, 2004;Sklyarov et al, 2016] are shown in A, B, E. [Pearce, 2008[Pearce, , 2014, tholeiites plot within oceanic arc, whereas boninites and andesites locate within oceanic to continental arc fields. On Ti-V plot [Shervais, 1982], tholeiites are classified as island arc tholeiites, and boninites to andesites plot within the boninite field.…”

“…The studied Eastern Sayan boninite-series rocks are similar in major-element composition to low-Ca and intermediate-Ca boninites and related rocks of Izu-Bonin-Mariana forearc [Pearce et al, 1992], but in terms of trace elements vary between Izu-Bonin and Papua New Guinea low-Ca boninites [König et al, 2010]. Low-Ca type of boninites is more typical of subduction initiation settings, while high-Ca boninites occur in environments varying from subduction initiation to back-arc and subduction zone influenced by a mantle plume [Reagan et al, 2017;Shchipansky, 2016;Sklyarov et al, 2016]. Boniniteseries rocks in association with tholeiites are common for intra-oceanic subduction initiation settings recorded in modern forearcs [Reagan et al, 2010[Reagan et al, , 2017 and ophiolites [König et al, 2010;Pearce, Robinson, 2010].…”

Depleted SSZ Type Mantle Peridotites in Proterozoic Eastern Sayan Ophiolites in Siberia

“…Two groups of data give Concordia ages of 974±11 and 1028±10 Ma. Rocks of the Dunzhugur complex are characterized by slightly negative to slightly positive εNd(t) values from -1.0 to +1.5 and Late Palaeoproterozoic depleted mantle Nd model ages of 1.8-1.6 Ga [Sklyarov et al, 2016]. The Dunzhugur complex was intruded by tonalite plutons of the Sumsunur complex dated at 785±11 Ma [Kuzmichev et al, 2001] and 811±7 Ma [Kovach et al, 2012].…”