Layered Gabbro of the Igneous Complex of Phenai Mata, Gujarat State

Phenai Mata hill (Lat. 22°8'N., Long. 73°55'E.), believed to be a composite plug, is successively filled by basalt, gabbro, granophyre and nepheline syenite. The paper focusses attention on the layered character of the gabbro. Rhythmic layering is due to the repetition of thick anorthositic and gabbroic layers. Cryptic layering is marked by extreme variation in anorthite content of plagioclase. The differentiation trend is established by the chemical and petrographic studies of samples collected at regular intervals. Up to 600 feet the enrichment is towards both iron and alkali, after which the gabbro (tholeiitic) differentiates to a granitic residuum.