Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Amaljyoti Sengupta ¹

Affiliations
1 Indian School of Mines, Dhanbad, IN

Abstract

The Barrovian zones of regional metamorphism developed in the pelitic rocks of the Sakoli Series have an average NE-SW trend. Intensity of metamorphism decreases on either side of a "Metamorphic High" of nearly the same trend, represented now by a strip of kyanite-schist running through Gangajhiri. West of the High, the zones are interrupted by a stretch of feldspathic gneisse. High grade metamorphism is not associated in space with granites or migmatites. The eastern boundary of the Sakoli Series is delimited by granites and granite gneisses which are located in areas of low grade metamorphism. There is, however, an abundant development of porphyroblasts of plagioclase in the "High" strip and its surroundings.

Crystallisation of the metamorphic minerals was not an instantaneous process, involving development of all minerals under certain combination of conditions. The existing mineral and rock-associations are the end result of a process of evolution in which one mineral followed the other in a time-sequence. Very rarely two index minerals crystallised side by side. The metamorphic minerals have been dated with reference to the different phases of rock movement. Major (first) folding on ss (bedding) was followed by development of axial plane foliation (s1) and crystallisation of quartz, muscovite and biotite probably during a static phase. To start with, the entire metamorphic belt was occupied by low grade phyllites. The first mineral to crystallise was kyanite, with some sporadic garnet and staurolite outside the kyanite zone. During the succeeding phase of relative movement along foliation (s1), almost the entire content of garnet crystallised out. Staurolite was the next to follow, in a subsequent static phase, over an extensive tract. Part of the staurolite crystallised at the initial stage of second folding. Fluctuations of metamorphic conditions were common. Crystallisation of plagioclase took place during all phases of tectonic evolution. Staurolite and kyanite, which are believed to be typical stress minerals, crystallised during static phases.

Examples of alteration of one mineral into the other are few, and are products of processes which followed the main phase of metamorphism.

Full Text

     

Amaljyoti Sengupta ¹

Affiliations
1 Department of Applied Geology, Indian School of Mines, Dhanbad, IN

Abstract

Porphyroblasts from Sakoli Series, Bhandara Dist. have been studied with a view to get a better insight into Sakoli metamorphism. Importance of the study of porphyroblasts for proper understanding of the metamorphic and tectonic end results has been discussed. Some hypothetical end results with probable conclusions are described and discussed in outline. Porphyrohlasts developed at different stages in the Sakoli metamorphites. The first phase of folding was associated with crystallisation of micas and quartz controlled by Wegsamkeit. The development of kyanite probably predated the second period of folding after which garnet and staurolite appeared, and finally biotite at the closing stage of metamorphism. Coarsening of fabric started only after the second phase of folding. Crystallisation at this stage took place under stress-free, static conditions.

Full Text