Petrogenesis, platinum-group element geochemistry and geodynamic evolution of the Cretaceous Chilas gabbros, Kohistan island arc, NE Pakistan

Abstract

Kohistan, an intra-oceanic arc, formed due to the India-Asia collision-related tectonics. The arc is comprised of mafic-ultramafic and crustal lithologies. Chilas Complex, one of the major mafic-ultramafic body that lies at the base of the Kohistan arc, is dominantly composed of massive gabbronorites with local ultramafic-mafic intrusive bodies. The ultramafic rocks were considered as the products of fractional crystallization whereas gabbronorites were reported to have formed at the root zone of the magma chamber either from melts or due to fractional crystallization, based on major and trace element data. To understand the petrogenesis and origin of the Chilas gabbros, we conducted platinum group elements (PGEs) geochemistry combined with whole-rock major and trace element compositions. Geochemical data of the Chilas gabbros show tholeiite series, likely derived from a depleted lithospheric mantle source via peridotite melts. Presence of hornblende and relatively higher Al/Ti ratios of clinopyroxene in the Chilas gabbros suggest hydrous magma, ascribed to mantle wedge metasomatism by slab-derived fluids. Poor correlations between MgO vs. Ni, Cr, and V, Ni vs. Cr, Dy/Yb vs. SiO2, Sr/Y vs. Eu/Eu* and relatively low TiO2 indicate a minimal role of fractional crystallization. Low contents of LOI (0–3.24 wt%), Ce/Ce* (0.92–1.01), and positive relation of La, Ta, and Hf with Zr strongly support the insignificant impact of hydrothermal alteration. Enrichment of PGEs, such as Pt (0.63–13.75 ppb), Pd (0.43–13.87 ppb), and lower Pd/Ir ratios (0.76–40.04) suggest low degree partial melting of the parent magma. Higher Pd, Pd/Ir ratios compared to Pt and Pt/Pd, combined with enrichment of PPGE over IPGE, suggest the participation of slab-dehydrated fluids and mantle metasomatism however Cu/Ir vs. Ni/Pd, V-Pd/Ir and La-Pd/Ir ratios point towards olivine fractionation in the magma. Lower Cu/Pd ratios, relationships of Ru and Pt against Cr, and Os diminution corroborate S-undersaturation in parental magma. The PGEs contents and Pd/Pt ratios attest depleted mantle source for their production. In view of Cu/Zr and Cu/Pd ratios, it is substantiated that the sulphide segregation cannot be a promising reason for PGEs depletion. Therefore, we infer PGEs depletions in parental magma are due to PGEs-depleted mantle source, though sulfide retention in mantle source cannot be eliminated. The geochemical fingerprints in tectonic diagrams and negative anomalies of Nb and Zr in the studied samples provide credible facts that favour arc magmatism. Based on major and trace element data and PGEs values, we consider that the Chilas gabbros were formed from a lithospheric mantle that was metasomatized by fluids generated at the mantle wedge and derived from mafic melts in an arc setting. Geochemical data and their plotting on various tectonic discrimination diagrams indicate Chilas Complex as a remnant of the Neo-Tethyan oceanic arc system.