Spilitic Degradation of a Tholeiitic Basalt

Transition from black tholeiite to green albite-chlorite spilitewithin a single Deccan flow unit at Bombay, India, providesdirect evidence of the ancestry of a spilite. Clinopyroxenesoccur throughout the sequence and are chemically similar inboth basalt and spilite. All are low-Ca augites that separatedfrom a tholeiitic melt undergoing fractionation until finalquenching supervened. The spilites are secondary products formedfrom solidified tholeiite by local hydrothermal metamorphism.Two alkali feldspars are generated in the spilites and bothbecome progressively ordered with advancing reconstitution ofthe host rocks. The Na-feldspar finally becomes borderline low-albite,K-feldspar is ordered to adularia. The occurrence affords ameans of evaluating relict pyroxenes as indicators of pristinecharacter in degraded mafic volcanic rocks.     

Mafic Rock Alteration and Isochemical Development of Some Cordierite-Anthophyllite Rocks

Mafic lavas can undergo extreme alteration and chemical reconstitutionwhen exposed to the low-temperature, bydrous conditions of late-magmaticand diagenetic environments. Of the various alteration productsfound some have the compositions (apart from extra H₂O) of cordierite-anthophylliterocks. Such altered mafic lavas occur at Yalwal, N.S.W., andhave been converted locally to cordierite-anthophyllite hornfelsesby essentially isochemical thermal metamorphism.