Modal Composition Variations in the Dalima Biotite Granodiorite Member of the Singhbhum Granitic Complex

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Abstract
Analysis of variance and polynomial trend surface techniques are used to determine the levels of variability and geographic variability, of five modal attributes and specific gravity of the Dalima Biotite Granodiorite. It is found that modal analyses from one measurement area in one hand specimen collected from one exposure in the pluton gives a reasonable estimate of that exposure, but that more than one exposure per one-square-mile grid is needed to estimate the variability within the grid. There are significant differences between the samples from each one-square-mile grid. The degree 6 polynomial trend surface maps of the modal attributes and specific gravity, based on149samples, represent good appraisals of the regional variability; there is good correspondence between the trends of primary foliation and the compositional contours of the trend surfaces. The structural evidence of the existence of two distinct intrusive units within the pluton is supported by (a) comparison of the cumulative frequency distributions of the modal attributes, and (b) the distinctive patterns of the individual trend surfaces for the two units. Compositional and structural evidences suggest that the western unit is the older of the two.

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