Structural and Metamorphic Evolution of the Bengal Anorthosite and Associated Rocks

The anorthosite is emplaced along the core of a doubly plunging (east-west) antiformal structure developed on the lithologic contacts of the associated metasediments and metanorites.

In the first phase of deformation (prior to the emplacement of anorthosite) the country rock together with numerous noritic sills was metamorphosed to pyroxene granulite facies. During the second phase of deformation a hydrous gabbroic anorthosite magma was synkinematically emplaced and slowly consolidated expelling considerable amount of water-rich volatiles which permeated the associated rocks. Metanorites (pyroxene granulites) and anorthosites were then metamorphosed to hornblende granulite facies followed by local diaphthoresis to greenschist-amphibolite transition facies. At the waning of deformation, palingenesis of the leptynites at places resulted in younger microcline-granites which brought about metasomatic changes in the anorthosite.