PLATINUM GROUP ELEMENT CONCENTRATIONS OF CHILAS COMPLEX GABBROS, KOHISTAN ARC LOWER CRUST, PAKISTAN
PGE concentrations in layered gabbros are generally low and highly fractionated (Ir < 0.04 ppb, Pt = 0.23–0.67 ppb, Pd = 0.07–7.67 ppb, Pt/Os = 11–224), whereas higher PGE concentrations with similarly fractionated patterns are locally found in km-scale dunite bodies (Ir = 0.69–1.05 ppb, Pt = 25.3–29.6 ppb, Pd = 7.7–22.4 ppb, Pt/Os =14–104; one sample contained macroscopically visible interstitial sulfides with ppm levels of PGE. Initial Os isotopic compositions (187Os/188Os)i of the gabbros range from 0.129 to 0.295, with two samples having significantly higher values up to 1.47, reflecting variable degrees of contamination by mature crustal rocks. Dunite (187Os/188Os)i ratios vary from 0.128 to 0.165. From both absolute and relative PGE concentrations and Os isotopic compositions, the gabbros are interpreted to have been derived by partial melting of supra-subduction zone metasomatized mantle wedge under mildly sulfide-saturated conditions, such that the evolved melts are depleted and fractionated in PGE, but mostly retain mantle-like Os isotopic compositions. The dunite appears to be a more primitive melt, derived from a source with similar Os isotope ratios to the gabbro source, but which has undergone lower degrees of crustal contamination during emplacement in the upper plate lithosphere.