Geochemical internal variations of Salem Dolerite dykes from Southern Granulite Terrane: Implications of petrogenetic processes in the dyke conduits

Abstract

Salem dolerites were collected across the dyke from various parts of the studied area for identifying the differentiation process of magma in the conduits from the compositional profile. The thick dolerites show NNW-SSE, NE-SW, and NW-SE trends. The studied dyke shows systematic composition increasing and decreasing in the chilled margin and centre of the dyke as the texture and concentration of plagioclase and pyroxene increase. Chilled margins show microcrystalline to intersertal, and the centre of the dyke show sub ophitic textures. The compatible oxide MgO, element Ni and the Mg number (100Mg/(Mg+FeT)) increased and the incompatible oxides TiO2, P2O5, and elements Zr decreased from the chilled margin to the centre of the dyke in the compositional profile show reverse fractionation trends (opposite to fractional crystallization) in the studied dykes except for Seeliyampatti dyke. The Seeliyampatti dolerites show opposite compositional variations indicating a normal fractionation trend from the other dykes of the study area. The reverse fractional trends in dykes resulting in the progressive increase of cumulate minerals growth against the dyke wall called the «cumulate process», however, the normal fractional trend in Seeliyampatti resulting in the progressive increase of more evolved magma successively removes the early chilled margin and fills the thick dyke with less compatible and more incompatible components toward the centre of the dyke. Although the normal trend in thick dyke considered the exceptional liquid process of magma differentiation that was formed more like fractional crystallization. Factor analysis also supports the differentiation process of magma. The first factor accounts total variance of 57.68% showing the positively loaded incompatible element and negatively loaded compatible elements.