Mineral Chemistry of Chillas Mafic-Ultramafic Complex, Kohistan Island Arc. N. Pakistan- Oxide Phases
Abstract
The Chilas Complex contains a great variety of Fe3+, Al, Cr, and Ti oxide phases. The principal gabbronorite association (PGA), which forms much of the complex, and the amphibolite dykes which represent a younger magmatic event in the Chilas Complex, contain magnetite-ilmenite pair exsolved from an initially single phase; no Cr-A1 spinel occurs in these rocks. The ultramafic-mafic-anorthosite association (UMAA) of rocks, which appears to be emplaced in the PGA, contains oxides covering the entire range from pleonaste through Al-Cr-F3+ spinel (with a maximum of 39 wt. % Cr2O3 to magnetite. These display complete ranges for Cr/ (Cr+Al) and Fe3+/R3+ ratios which decrease with increase in Mg/(Mg+Fe2+) ratio (0.7 to 0.0). The composition of the UMAA spinel varies from one to another sample and in some cases from grain to grain within a single specimen. The variations are attributed to subsolidus re-equilibration with adjacent silicate grains and to exsolution. The exsolved pairs define a complete solvus similar in shape to those reported elsewhere. The UMAA also contains pleonaste formed in reaction coronas between calcic plagioclase and olivine, and in few cases, ilmenite formed during exsolution of Cr-Al-Fe3+ spinel. Exsolved oxide phases in both the associations of rocks have re-equilibrated under low oxygen fugacity during cooling of the complex at temperatures ranging from >800 to <500˚C. The detailed microprobe analytical data for the oxide phases discussed in this paper support our earlier view that the Chilas Complex is related to the Kohistan arc and probably formed during intra-arc rifting in initial stages of development of a back-arc basin.
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