Interpretation of Gravity Field Over the Perinthatta Anorthosite, Northern Kerala

Gravity surveys have been carried out over the Perinthatta anorthosite and the adjoining areas and the results are discussed. The Bouguer anomaly map reveals the absence of any conspicuous gravity anomaly corresponding to the pluton and thus precludes the existence of larger mafic portions of the pluton in the immediate subsurface region. The residual gravity map obtained by regional-residual separation method shows a minor positive gravity field of irregular shape with localised highs of 6 - 12 mgal within and around the anorthosite body. TWo-dimensional gravity interpretation of profiles across the pluton suggests subjacent nature and lensoid shape for the anorthosite which is underlain by small isolated cylindrical bodies (≃ 2.5 km diameter) of higher density (2.9 gm/cc) extending 3 - 7 km downwards. These cylindrical bodies are inferred as either the mafic/oxide rich portions of the pluton sagged down during crystallization or the individual feeders of the plagioclase-rich mushes, which coalesced in the upper crust to give rise to the massif.