Phase petrology with special reference to pyroxenes of the acid igneous complex of Barda hills,Western Saurashtra (Gujarat)

The Barda igncous complex of western Saurashtra (Gujarat) consists of a group of volcanic and subvolcanic igneous rocks, viz., pitchstone, rhyolite, felsite, granophyre and dioritic rocks. They are generally grouped under the Deccan Trap series and have been emplaced through the Deccan Trap basalt flows. Using different varieties of clinopyroxenes as characteristic phases several units of felsites and granophyres have been distinguished in the southeastern part of the Barda hills. The augites of the adjacent Deccan Trap basalt flows range between Ca₃₃Mg₆₁Fe₆ and Ca₄₁Mg₄₄Fe₁₅. (All compositions of minerals are derived from optical data). The clinopyroxene of a monzodiorite is Ca₄₁Mg_42.5Fe_16.5. In the felsites and granophyres the clinopyroxenes range from salite (Ca₄₆Mg₃₁Fe₂₃) through ferrosalite (Ca₄₆Mg₂₀Fe₃₄) and ferroaugite (Ca₄₃Mg₁₇Fe₄₆) to hedenbergite (Ca₄₆Mg₉Fe₄₅). Beyond that a peralkaline trend is indicated by common occurrence of aegirine-augite granophyre in which the clinopyroxene is alkalic with 20 percent of NaFe³ Si₂O₆ (acmite) molecule. Several bodies of acmite-riebeckitebearing granophyres have been found; they also indicate a high degree of oxidation. A fayalitic olivine-bearing granophyre is of interest as it shows the strongest iron enrichment with 13 weight percent FeO + Fe₂O₃; its mineralogy indicates a very low oxygen fugacity of 10¹³ atmospheres during crystallization.