Chromite chemistry as petrogenetic indicator of altered ultramafic rocks from Higher Himalayan Crystalline, Kishtwar, India

The altered ultramafic rocks from Higher Himalayan Crystalline (HHC) are characterised by the mineral assemblage of olivine, pyroxene, phlogopite, chromite, and opaques. Olivine and pyroxenes show alteration to serpentine, talc, anthophyllite, and tremolite. In the present research work, the petrogenesis and paleo-tectonic setting of the ultramafic rocks from the Paddar sapphire mining area are understood using chromite chemistry. The investigated chromite is characterised by a higher abundance of Cr 2 O 3 , FeO, Al 2 O 3 , and low MgO and is classified as Al-chromite. They show higher Cr# (71.10–87.80) and Fe# (71.66–86.04) and lower Mg# (13.95–28.16), suggesting their affinity towards the ophiolitic peridotite. Chromite geochemistry further suggests that the parent magma originated from the mantle and intruded under high pressure and low oxygen fugacity. Chromite chemistry suggests arc to supra-subduction tectonic setting for the crystallisation of chromite from ultramafic magma. Based on the correlation matrix and binary diagram, it is interpreted that Ti + Cr ↔ Al; Cr ↔ Fe + substitution prevails at the octahedral site, while Fe ↔ Mg; Al ↔ Fe and Al ↔ Mg substitution mechanism operated at the tetrahedral site during the crystallisation of chromite from magma. Further, compositional re-equilibration occurs during hydrothermal alteration. Research highlights The ultramafic rocks from Paddar area are mainly composed of olivine and pyroxenes with a variable amount of opaque minerals and are highly altered to serpentine and talc. Chromite occurs as isolated subhedral to anhedral grains and scattered fine dust associated with altered major mineral phases. The chromites shows very high Cr# (100*Cr/Cr+Al) and wider range of the Mg# (100*Mg/Mg+Fe 2+ ). The chromite chemistry suggests arc to supra-subduction tectonic setting for the generation of magma in the study area. Graphical abstract The present research paper is focused on chromite chemistry from altered ultramafcs from HIgher Himalyan Crystalline Complex. The host rocks of chromites are intruded in the garnetiferous-biotite gneiss. The investigated chromite is characterised by higher abundance of Cr 2 O 3 , FeO, Al 2 O 3, Cr# (71.10 - 87.80) and Fe# (71.66-86.04) while low content of MgO and Mg# (13.95-28.16) is recorded. They are classified as Al-chromiteexhibiting affinity towards the ophiolitic peridotite. They have intrected with hydrothermal fluids resulting post magmatic compositional re-equilibration.