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Structures of the porphyritic granite and associated metamorphic rocks of East Manbhum, Bihar, India
Published in Geological Society of America
Volume: 67
Issue: 5
Pages: 647 - 670
Porphyritic granite, associated with metamorphic rocks and migmatites in East Manbhum (23°27'45″-23°35' N. Lat.; 86°30'-86°49' E. Long.), India, crops out as a lenticular body showing well-developed planar banding of alternating feldspar phenocrysts and a finer-grained assemblage of quartz, feldspar, and accessories. Arrangement of phenocrysts within the banding may be random, or they may show perceptible lineation. Lineation, foliation, and joints are primary. Regional distribution of flow layers suggests an inclined lens, with two secondary domes, one at Raghunathpur and the other at Bero. Lineation, generally within a few degrees from the horizontal, parallels b, except in the two domes where it parallels a. The porphyritic granite is concordant for the greater length except toward the eastern margin of the exposure. North-northeast of Raghunathpur, it is discordant with the structural trends of the country rocks. There are a few small-scale discordances where the granite boundary takes sharp turns, leaving the schistose alignments of the country rocks abutting against the boundary. Spatial distribution of foliation, lineation, and joints of the porphyritic granite is independent of that of the metamorphic and migmatitic country rocks. Wall rocks have been mylonitized, and three joint systems developed. Distribution and concentration of joints prove that the effect of tension was comparatively greater close to the contact. Compression diminishes more slowly and is effective over a greater distance. Lineation in the country rocks may be parallel to both a and b. It is chiefly of two types produced by (1) microfolds of different dimensions, or by (2) alignment of elongated minerals such as quartz or sillimanite. Biotite fabric of the metamorphic rocks is not indicative of their tectonic trend as the mineral is a result of neocrystallization. Quartz-axes diagrams show peripheral ac girdles with maxima I, II, and V, and occasionally an ac ̂ bc diagonal girdle. The orientation in the metamorphic rocks, especially as they belong to a high grade of regional metamorphism, presumably originated in solid flow by translation, chiefly along megascopic s planes. Fabric of migmatites, ultramigmatitic granite gneiss, and two types of quartz in leptynites (granulites) are very similar to that of the metasediments, and their orientation is ascribed to mimetic retention of tectonitic orientation during metasomatic recrystallization. Country rocks show simultaneous fabric reconstitution near the porphyritic granite. Along the granite contact the fabric is symmetrical about the bisectrices of the shear joints caused by intrusion of porphyritic granite. The intrusion also produced minor secondary folds or intensification of folding. Quartz fabric of the porphyritic granite is independent, with regard to its pattern and relation to geographic coordinates, of the country rock fabric. © 1956, The Geological Society of America, Inc.
About the journal
JournalBulletin of the Geological Society of America
PublisherGeological Society of America