Origin of clouded red feldspars; petrologic contrasts in a granitic porphyry intrusion

Abstract

A granitic porphyry emplaced at a high crustal level in the Gaspe Peninsula exposes broad areas of altered porphyry containing the sub-assemblage red albite + chloritized biotite grading into restricted areas of gray porphyry with the critical subassemblage clear oligoclase-andesine + less altered biotite. These are end membersof a gradational change. Hematiferous albites are relict phenocrysts of plagioclase initially more calcic and potassic. They are related to potassic oligoclases and andesines in less altered porphyries by a paragenetic model involving reaction between ternary feldspar, a vapor phase, and Fe + or - Mg released during decomposition and oxidation of the primary mafic phase, on a fixed K-Al basis. Formation of low-Al chlorite as a minor additional inclusion phase may involve loss of Al (super 3+) from the site of the host feldspar. Contrasts in feldspar zoning further illustrate the sequence.