Crustal evolution of island-arc ultramafic magma: Galmoenan pyroxenite-dunite plutonic complex, Koryak highland (Far East Russia)

Alaskan-type platinum-bearing plutons and potassium-enriched mafic to ultramafic volcanic rocks are temporally and spatially associated within the Late Cretaceous-Paleocene Achalvayam-Valaginskii intra-oceanic palaeo-arc system, allochthonousy present in the Kogak Highland and Kamchatka Peninsula (Far East Russia). The compositions of the parental magmas to the Alaskan-type complexes are estimated using the Galmoenan platonic complex as an example. This complex, composed of dunites, pyroxenites and minor gabbros, is the largest (~ 20 km3) in the system and the best studied owing to associated platinum placer deposits. The compositions of the principal mineral phases in the Galmoenan intrusive rocks [olivine (Fo(79-92)), clinopyroxene (1-3.5 wt % Al2O3, 0.1-0.5 wt % TiO2), and Cr-spinel (5-15 wt % Al2O3 and 0.3-0.7 wt % TiO2)] are typical of liquidus assemblages in primitive island-arc magmas in intra-oceanic settings, and closely resemble the mineral compositions in the Achaivayam-Valaginskii ultramafic volcanic rocks. The temporal and spatial association of intrusive and extrusive units, and the similarity of their mineral compositions, suggest that both suites were formed from similar parental magmas. ne composition of the parental magma for the Galmoenan platonic rocks is estimated using previously reported data for the Achaivayam-Valaginskii ultramafic volcanic rocks and phenocryst-hosted melt inclusions. Quantitative simulation of crystallization of the parental magma in the Galmoenan magma chamber shows that the compositions of the cumulate units are best modelled by fractional cryallization with periodic magma replenishment. The model calculations reproduce well the observed mineral assemblages and the trace element abundances in clinopyroxene. Based upon the estimated composition of the parental magmas and their mantle source, we consider that fluxing of a highly refractory mantle wedge (similar to the source of boninites) by chlorine-rich aqueous fluids is primarily responsible for both high degrees of partial melting and the geochemical characteristics of the magmas, including their enrichment in platinum-group elements.