Explosive ultramafic volcanism in Phanerozoic accreted terranes: A case study of the Taragai peridotite complex (Russian Far East)

Abstract

In the Bureya accreted terrane of the Central Asian Orogenic Belt, the Taragai peridotite complex displays geologic characteristics of an explosive volcanic pipe. The crater-like, central topographic depression is composed of ultramafic pyroclastic rock that is surrounded by partially eroded tephra walls with imbedded peridotite blocks. Both peridotite blocks and ultramafic pyroclastics consist of olivine, orthopyroxene and spinel with minor phlogopite and display similar subduction-related geochemical characteristics. A certain degree of crustal contamination is suggested by highly radiogenic Sr isotope compositons of peridotites. LILE- and LREE-enriched basaltic dikes intrude ultramafic pyroclastic rocks. Peridotite blocks, ultramafic pyroclastic rocks and basaltic dikes contain three types of microspherules composed of 1) magnetite, 2) sulfide (mostly pyrrhotite) and 3) Cu-Ag-Au alloy. Magnetite microspherules reflect oxidation of sulfide in peridotite. Cu-Ag-Au microspherules were formed through re-melting of Cu-Ag-Au particles in peridotite blocks. Sulfide microspherules represent solidified droplets of sulfide melt generated during explosive emplacement of the Taragai complex. Assemblages of metals, alloys and ore minerals along with distribution of trace and ore elements in the Taragai peridotite complex are similar to the Ildeus ultramafic massif in the Stanovoy superterrane located to the northwest from the Bureya terrane. We propose that Taragai complex was formed as a result of a focused asthenospheric fluid flux from the flat-slab window beneath the Bureya terrane through the subduction-related ultramafic massif. Observed metallogenic features of the Taragai explosive volcanic complex thus may reflect contributions from mineralized ultramafic intrusive target, host accreted terrane rocks and asthenospheric fluid.