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The layered gabbro-anorthosite suite in the eastern Indian shield is ideal for a texture based study of the crystallization history of a mafic magma in a static magma chamber. The magmatic suite comprises 40-150 m thick persistent layers of different lithounits, which have been divided into two cycles. The lower cycle (I) is represented from bottom to top by gabbro, gabbronorite, leucogabbronorite, anorthositic gabbronorite and anorthositic gabbro. The upper cycle (II) is similar to cycle I, except that the anorthositic gabbronorite and anorthositic gabbro units are not present there. The characteristic textures of different lithounits in each cycle repeats with repetition in lithology. Distinctive textures observed in the suite have been classified into three categories, Types I, II and III. Adcumulus growth was the principle crystallization mechanism for Types I and III textures, which are characteristically present in the lower layers of cycle I and cycle II. The upper layers of cycle I show Type II texture, typically with bimodal size distribution of plagioclase, and pyroxene oikocrysts. Thus, Type II texture indicates a possible interplay between adcumulus growth and either polybaric crystallization or textural coarsening processes. Electron microprobe analysis (EPMA) data indicate uniform composition of plagioclase (An 76-85 ), both from inclusions in pyroxene oikocrysts and from groundmass grains, hence ruling out the possibility of involvement of the polybaric crystallization in the layers with Type II texture. Crystal size distribution (CSD) analysis of plagioclase grains stands for textural coarsening process, and the graphical plots have been utilized to quantify the process. P-T calculation using EPMA data from selected pyroxene grains suggests that the textural coarsening took place at about 13.3 kb pressure and 990°C temperature. We propose that adcumulus growth operating in a static magma chamber for a long period of time might have created conditions conducive for textural coarsening process to take place.

The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE-SSW to NE-SW trending NDDs in two sectors in the northern and southwestern part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba