Abstract
Several dykes of 'porphyry' rock and of Newer dolerite occur as intrusives into the Singhbhum granite around Rairangpur in Mayurbhanj District. The 'porphyry' rock contains large xenoliths of vein quartz and granitic rocks in a fine to medium grained aggregate of granophyric intergrowth. plagioclase and quartz together with some amphibole, pyroxene, epidote, chlorite and iron-ore; in places, the xenoliths show preferred orientation indicating direction of movement of the magma. The 'porphyry' dykes and the Newer dolerite dykes have similar trends and they intersect mutually. A four-mile long dyke of porphyry rock shows transition to dolerite in places along its length; this is considered to be the result of injection of 'porphyry' magma along the continuation of a Newer dolerite dyke .
It is suggested that the Newer dolerite magma locally differentiated at depth forming two fractions-one doleritic and the other syeno-dioritic, which were emplaced simultaneously or alternately. The abundance of xenoliths of granite and vein quartz in the 'porphyry' dykes is considered to be due to shattering of the granitic walls during the uprise of the syenodioritic magma which produced these dykes.
