Formation conditions of Paleozoic ultrabasic complexes of the basement of the West Siberian sedimentary basin

Recent studies have shown the wide distribution of ultrabasic rocks in the basement of the West Siberian sedimentary basin [Kuzovatov et al., 1988, 1996; Bochkarev et al., 2003; Ivanov et al., 2003, 2007, 2009; Simonov et al., 2006-2010, 2012, 2018-2020; Stupakov et al., 2008]. The overwhelming majority of ultrabasic rocks of the West Siberia are duniteharzburgite assemblages of ophiolite associations of Paleozoic age [Ivanov et al., 2007, 2009; Erokhin et al., 2008]. Another more rare type of ultrabasic rocks in the basement of the West Siberian sedimentary basin is represented by picrites [Kuzovatov et al., 1988, 1996; Simonov et al., 2018, 2019] of the Early Paleozoic age [Simonov et al., 2020]. The article is a continuation of studies of ultrabasic complexes of the West Siberia. It is based on the results of processing the original (including partially previously published) data, obtained mainly during study of clinopyroxene containing peridotites of the Hultursky and Festivalny massifs and also picrites of the Chkalovsky area. The carried out studies of clinopyroxenes from peridotites of the Hultursky and Festivalny massifs indicate that its crystallization was connected with the “oceanic” magmatic tiend, while clinopyioxenes fiom piciites of the Chkalovsky aiea aie associated with the “island-arc-plume” magmatism. At the same time, clinopyioxenes fiom the peiidotites aie close to the data on mineials from ultrabasic rocks of standard ophiolites (Polar Urals). Analysis of the composition of Cr-spinels showed that the evolution of ultrabasic complexes of the basement of the West Siberian sedimentary basin took place during the geodynamic situation changed from oceanic to island arc. In this case, clinopyroxene containing parageneses were formed both at the beginning (ocean) and at the end (island arc) of the history of evolution of ultramafic rocks. Results of studying of the features of the distribution of rare-earth elements (as well as data on minerals) clearly indicate the participation of magmatic systems of mid-oceanic ridges in the formation of clinopyroxene containing peridotites of the Hultursky massif, in contrast to picrite porphyrites of the Chkalovsky area, which are characterized by island-arc and plume melts. As a whole, study of the Paleozoic clinopyroxene containing ultrabasic complexes of the basement of the West Siberian sedimentary basin (including computational modeling with the help of modern software WinPLtb [Yavuz, Yildirim, 2018], COMAGMAT [Ariskin, Barmina, 2004], PETROLOG [Danyushevsky, Plechov, 2011]) testify about participation of mid-oceanic magmatic systems during successive (9.7-2.5 kbar) crystallization of olivines (1560-1470 °C) and clinopyroxenes (1355-1100 °C) of peridotites of Hultursky and Festivalny massifs. Formation of picrite porphyrites of the Chkalovsky area is connected with a drop in PT - parameters during crystallization of olivine (1540-1490 °C, 12.8-7 kbar), clinopyroxene (13151215°C, 8-4.5 kbar) and amphibole (1105-1060 °C, 6.1-4.5 kbar) from the island arc and plume melts.

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