Abstract

The Chilas complex contains accessory and segregated spinels that display a complete range from Cr-poor pleonaste through Al-Fe3+ -rich chromite to Cr-poor magnetite. Microprobe data reveal complete ranges for the Crl(Cr + Al) and Fe3+/R3+ ratios, whereas the Mg/(Mg + Fe2+) ratio ranges from 0.7 to 0.0; the former two ratios increase as the latter decreases. Grain to grain compositional variation within a thin section is common. It is particularly drastic in tiny spinel grains and is mainly because of subsolidus reequilibration with adjacent grains of olivine, pyroxenes, and amphibole. Many samples also contain aluminum chromium spinel with blebs and lamellae of exsolved Fe3+-rich spinel. The exsolved grains define a complete solvus, the shape of which is similar to those of other documented examples of exsolved spinels in the system (Mg-Fe2+)Fe23+O4-(MB-Fe2+)Cr2O4-(Mg-Fe2+)Al2O4. Exsolution probably took place during cooling at <600 °C.

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