Spilitic degradation of a tholeiitic basalt

Abstract

Transition from black tholeiite to green albite-chlorite spilite within a single Deccan flow unit at Bombay, India, provides direct evidence of the ancestry of a spilite. Clinopyroxenes occur throughout the sequence and are chemically similar in both basalt and spilite. All are low-Ca augites that separated from a tholeiitic melt undergoing fractionation until final quenching supervened. The spilites are secondary products formed from solidified tholeiite by local hydrothermal metamorphism. Two alkali feldspars are generated in the spilites and both become progressively ordered with advancing reconstitution of the host rocks. The Na-feldspar finally becomes borderline low-albite, K-feldspar is ordered to adularia. The occurrence affords a means of evaluating relict pyroxenes as indicators of pristine character in degraded mafic volcanic rocks. © 1974 Oxford University Press.