Geochemistry, mineral chemistry and Re-Os isotopes of refractory peridotites of the North Balkhash ophiolite zone in the West Central Asian Orogenic Belt (Central Kazakhstan): multi-stage melt evolution of a late Precambrian forearc mantle

In this study we report on the petrography, major and trace element and mineral chemistry, platinum-group elements and Re-Os isotope systematics of depleted ultramafic rock suites from the Itmurundy Block in the North Balkhash ophiolite zone in Kazakhstan. Represented mainly by variably serpentinized harzburgites and dunites, our samples are characterized by low whole-rock Al2O3 (0.33-0.86 wt%), CaO (0.51-0.86 wt%) and Na2O (0.07-0.25 wt%) concentrations, and high-Mg olivine (Fo = 91-92) and orthopyroxene (Mg# = 92-93) contents, together with moderately high spinel Cr-numbers (Cr# = 63-68). They are depleted in incompatible elements (sigma REE, Nb, Sc) and enriched in compatible elements, such as Cr (up to 2817 ppm) and Ni (up to 2327 ppm), representing highly refractory mantle residues derived from a forearc mantle wedge. They underwent 19-23% hydrous partial melting to produce boninitic melts.Os- 187/Os-188 values vary from 0.1202 to 0.12599, and Re-187/Os-188 ratios from 0.230 to 0.316. The Re-Os model ages (T-MA) and maximum Re depletion model age (T-RD) were calculated based on the obtained data. Re-Os isotope systematics suggests that the analysed peridotites formed in two stages: a first stage around 1.5 Ga and a later stage around 668-589 Ma.