Quantitative Petrology of a Part of Singhbhum Granite around Kuyali, Singhbhum District, Bihar

The composition of biotite granodiorite in an area of 11 square miles around Kuyali has been studied on the basis of modal analyses of 41 systematically collected samples. The average mineralogical composition of the sampled area of the granodiorite has been determined by superposing a square grid pattern (size of grid 1 sq. mile) over the area and considering the average composition of each grid. Isopleths for quartz percentage, K feldspar percentage and mafics percentage have been drawn on the basis of the deviation of the grid averages from the average for the entire sampled area. By using a procedure of analysis of variance, it is shown that the local variations in composition are considerable and that they tend to mask the regional variation. Poor and generally non-significant correlations amongst the major minerals of the granodiorite have been found.

The compositional data support the field evidences of considerable assimilation of basic metamorphic rocks and argillites having been an important factor in the evolution of these rocks. K feldspar-rich residual liquid is shown to have migrated towards the low-pressure southern border region of the granodiorite body.