Petrographic evolution of metagabbro from south Delhi fold belt

Abstract

In north-western Indian shield South Delhi Fold Belt (SDFB) is a northeast southwest trending, multiply folded and poly-metamorphosed rock of Proterozoic age. The western margin of the SDFB is demarcated by Phulad Shear Zone (PSZ) of mid-Proterozoic age (~810 Ma). The PSZ is characterized by north-easterly striking ductile shear zone with a well-developed mylonitic foliation and a down-dip stretching lineation. The deformation in the PSZ has developed in a transpressional regime with top-to-the-west reverse sense of movement. The present study deals with petrological evolution of a meta-gabbro that occur parallel to this shear zone. The meta-gabbros are coarsed grained and are variably deformed. The gabbro shows strong development of dominant foliation with attitude 38/87˚E and prominent down dip lineation. Systematic study of mesoscopic structures of the meta-gabbro and the shear zone rocks demonstrate a similarity in geometry and style. This indicates that the deformation in the meta-gabbro and the shear zone is broadly synchronous. The meta-gabbro contains clinopyroxene, plagioclase, ± orthopyroxene, amphibole with ilmenite, sphene in minor amounts. In the least altered meta-gabbro, the rock preserves prominent evidence of igneous texture. In the altered variety, the clinopyroxene grains are rimmed by amphibole grains with relict of clinopyroxene at the center. In more deformed variety the amphibole grains are completely replaced the clinopyroxene grains. Electron microprobe analysis data show the amphibole composition varies from actinolite to hornblende and edenite. Textural relation suggests actinolite formed earlier and hornblende and edenite formed later. Mylonitic gabbro comprised of amphibole minerals which exhibit well defined foliation appearance along with dynamically recrystallised plagioclase grains. Textural relations show formation of coronal sphene over ilmenite grains. Integrating the mesoscopic and textural relation it is suggested that all the metamorphic reactions are fluid driven reactions that further facilitate the exhumation of the present suite of gabbro.