Hydrothermal alteration studies of gabbros from Northern Central Indian Ridge and their geodynamic implications

Abstract

Mylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection, adjacent to Vityaz transform fault of the slow spreading (32–35 mm/yr, full spreading) Northern Central Indian Ridge. Both the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents high temperature metamorphism (∼700–800°C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered gabbro from corner high generally includes low temperature greenschist grade (∼300°C) mineralogical assemblages: chlorite, albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of gabbro.