Title: Fayalite, pyroxene, amphibole, annite and their decay products in mafic clots within Younger Granites of Nigeria: Petrography, mineral chemistry and genetic implications
Abstract: The Younger Granites of Nigeria, emplaced in the basement complex 213–141 Ma ago, occur in the form of ring complexes. About 50 complexes are known with the town of Jos approximately in the center of the area. Apart from biotite granite (main rock type) granites and porphyries occur that contain olivine, pyroxene, amphibole, and mica. Granites and porphyries of five ring complexes were investigated in this study. These are the Jos-Bukuru, Shere, Ganawuri, Rop, and Ririwei complexes. Among the granitic minerals (feldspar and quartz) mafic clots were observed being mainly rounded to oval in shape (300 μm to 3 mm) often showing reaction rims and corroded outlines. These clots contain extremely iron-rich minerals: Olivine, exclusively found in the granites, is represented by nearly pure fayalite; Ca–Fe pyroxene, restricted to rocks (granites and porphyries) of the Shere complex, consist of hedenbergite and iron-rich augite; calcic amphiboles (occurring in all complexes) have Mg/(Fe2+ + Mg)-ratios mainly below 0.2 and vary from hastingsite via ferro-edenite to ferro-hornblende and ferro-actinolite; and mica (not observed in the Rop complex) consist nearly of the annite endmember. With decreasing temperature (lying in the range 850–600 °C) the following mineral succession was found: fayalite, Ca–Fe pyroxene, calcic amphibole, and annite. These minerals were directly formed from the melt. Due to cooling and resulting disequilibrium conditions, the primary minerals became unstable and were replaced by a variety of minerals of sub-solidus origin and a temperature lower than 600 °C. Fayalite decomposes into grunerite + magnetite or alters into ferro-saponite; pyroxene is rarely replaced by calcic amphibole and often by an unidentified water-rich mineral; calcic amphibole may be replaced by titanite and ilvaite or decomposes into grunerite, chlorite and stilpnomelane; and annite into chlorite (iron-rich chamosite or delessite). Different in composition are pyroxene and amphibole of the albite-rich granite of the Ririwei complex where fayalite and annite do not occur. Pyroxene is dominated by the aegirine endmember and amphibole is arfvedsonite. The formation of these minerals is attributed to sodic metasomatism and higher oxygen fugacity conditions than observed in the other rocks.
Publication Year: 2003
Publication Date: 2003-01-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 24
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