Differentiated Gabbro-Granophyre Composite Sill from Ramagiri Greenstone Belt, Andhra Pradesh

The differentiated gabbro-granophyre composite sill, emplaced in the basic tuffs of Ramagiri greenstone belt, represents the first record of such an intrusion in the Eastern Dharwar craton. The sill consists of four differentiated gabbroic units, namely, the carbonated, mafic, normal and knotted gabbro forming a locally inverted stratigraphy along a steep fold limb in the belt. A faintly layered quartz diorite band, visually resembling anorthosite but compositionally different in having euhedral zoned feldspars and quartz-plagioclase "granophyric ' intergrowths, occurs in association with the differentiated gabbroic units but is separated from it by a later metadoleritic amphibolite dyke. The granophyric quartz diorite and the associated quartz gabbro are not directly related to the other four differentiated gabbros of the composite Ramagiri sill. The quartz diorite is chemically similar but tectonically dissimilar to continental trondhjemites forming tbe ischolar_main of island arcs and is different from oceanic plagiogranites and continental granophyres in terms of their K₂O, Si0₂, Rb and Sr contents, although such rocks are not uncommon in the ocean floor. Such quartz diorites are believed to have been formed by fractional crystallisation of oceanic tholeiite magma which interacted with sea water to produce low potassic trends, as distinct from the more potassic granophyre differentiates of continental layered sills. These non-continental granophyres in basic sills, together with the associated pillow lavas and pelagic sediments indicate a nascent oceanic to transitional environment generated by rifting of the basement gneisses to produce the Ramagiri greenstone belt.