Platinum‐group mineral and silicate inclusions in the low‐Al chromitites of the Manipur ophiolite, northeast India: Implications on Cr‐PGE mineralization in nascent subduction zone settings

摘要

Low‐Al chromitites from the Manipur ophiolite contain a wide range of mineral inclusions, including hydrous and anhydrous silicates, sulphides, and platinum group minerals (PGM). The common inclusions are primary silicates, such as clinopyroxene, orthopyroxene and secondary silicate inclusions, such as chlorite, serpentine, and garnet. These inclusions are small (<30 μm in PGM, <100 μm in silicates) and randomly distributed throughout their host chromite grains. The shape and texture indicate some inclusions (clinopyroxene, orthopyroxene, PGMs) trapped during the magmatic stage and most inclusions (chlorite, serpentine, and uvarovite) trapped during recrystallization of chromite. Clinopyroxene was discrete inside the chromite, with Cr2O3 content ranging from 0.62 to 1.77 wt., and Al2O3 varying between 0.96 to 1.63 wt.. Orthopyroxene is enstatite (En93Wo2Fs4), Cr2O3 varies from 0.58 to 1.8 wt. and the minor amount of CaO (0.41–0.81 wt.), and Al2O3 (0.58–0.83 wt.). Uvarovite is present in two forms: discrete crystal inclusions (anhydrous) in chromites and fractures (hydrous) as fillings. The chlorite occurs as lath shaped, and patchy phase is Cr‐rich variety ‐ kammererite (clinochlore) and penninite with variable Cr2O3 (0.22–7.25 wt.) and FeO (2.5–3.9 wt.). The PGM in the studied chromite are laurite (Ru, Os, Ir)S2. The parental melts of chromitites had low TiO2 (0.16–0.48 wt.) and Al2O3 (10.18–14.58 wt.) contents, and primary mineral inclusions, suggesting an arc‐like geochemical affinity, and the estimated melt composition is comparable with that of boninitic magma.