Abstract
Pyroxenite, almost entirely composed of iron-rich orthopyroxene with very little fayalite and quartz, forms the extremity of a lens of pyroxenites and basic charnockites. This lens is one of a group of numerous bodies of basic and ultrabasic rocks belonging to the charnockitic suite of the Gour Oumelalen (Ahaggar, Algeria), which are surrounded by garnet-sillimanite gneisses and marbles. This orthopyroxenite gives place in few decimeters to a clinopyroxenite with orthoferrosilite and fayalite. Basic charnockite is a norite with brown amphibole and biotite. The interaction between a vein of hypersthene pegmatite and the clinopyroxenite has produced secondary minerals, amphibole (hastingsite) and garnet (almandine). Physical properties of the iron-rich OPX from the "orthoferrosilitite" are given. Microprobe analyses of ferromagnesian silicates in these rocks permit some estimates, based upon experimental data, of temperature and pressure. The composition of coexisting clinopyroxene-orthopyroxene suggests that temperatures ranged from 800 °C to 860 °C, The composition of orthoferrosilite (OFS 87-92) coexisting with fayalite (Fa 93 -95) applied to various experimental data indicates pressures of 8. 1-8.7 kbar at 860 °C (8-9 kbar at 800 °C). Temperatures estimated from garnet-biotite geothermometry in gamet-sillimanite gneisses are between 760 °C and 850 °C. The metamorphic pressure, measured from the garnet-plagioclase-sillimanite-quartz equilibrium, is close to 8.5 kbar at 790 °C. These diverse P-T estimates define the final metamorphic crystallisation conditions as 760 °C-860 °C and 8-9 kbar, which agree well with those recorded from other Proterozoic granulite facies series.
Published Version
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