Proterozoic Mafic Dykes from the Southern Margin of Cuddapah Basin, India: Part 2 – Palaeomagnetism and Ar/Ar Geochronology

Abstract

Diopsidites and rodingites are two specific kind of dyke cropping out in the mantle section of the Oman ophiolite. Diopsidites show a diopside monomineralic modal composition, associated in some exceptional cases with anorthite. Typical rodingites mineralogical assemblages include grossular, chlorite, tremolite, zoisite/clinozoisite, epidote, prehnite, diopside, magnetite and other accessory minerals in various proportions. Both are result from the alteration in Ca-rich environment of the mantle or pre-existing gabbroic dykes. In spite of this apparent common origin, significant divergences are observed between these two lithologies and their host, allowing us to think that they are two different kind of rock which have been generated by two different processes: rodingites are the result of the interaction between gabbroic dykes and serpentinisation-issued fluids, while diopsidites formed by genuine Ca-metasomatism of the mantle with an external source of calcium. A precise petrographical study of the diopsidites and rodingites, their characteristics and their interaction with their host show that these two lithologies are related to two different type of alteration in the mantle. 1. The first is a classical phenomenon of serpentinisation, which led to the transformation of a primary harzburgite to an assemblage of serpentine, brucite, and magnetite. Rodingite dykes are present in these serpentinised peridotites. They resulted from the interaction of the fluid responsible of the alteration with any mafic lithology included in the serpentinised zone. 2. The second type of alteration involves a Ca-rich fluid, it leads to abundant crystallisation of tremolite, replacing orthopyroxene and serpentine, and results in a nephrite with preserved porphyroclastic texture.