Petrography
The Sonakhan Group of rocks as a whole has suffered greenschist facies of metamorphism but most of the basaltic rocks of the Baghmara Formation have well-preserved original igneous texture. These rocks are fine to medium grained, and in some cases, they exhibit porphyritic texture. In many basaltic samples primary pyroxenes have not reverted to chlorite and actinolite and the rock remains quite black. Since the rocks have overall preserved original igneous texture and mineralogy in most of the cases, we have retained the basalt term. The basaltic rocks mainly contain pyroxene, plagioclase and opaque minerals. The samples, which show effects of greenschist metamorphism, consist of chlorite, actinolite, plagioclase, epidote, sphene and opaque minerals. In some samples, biotite and hornblende have also been observed. The basaltic rocks exhibit limited alteration to chlorite, actinolite and epidote. Restricted alterations of plagioclase to sericite, and biotite and hornblende to chlorite, have been observed in many cases which may indicate in situ grain boundary controlled changes representing isochemical changes (Pollard et al., 1983; Taylor and Pollard, 1988). Replacement of original pyroxene crystal by chlorite, and in some cases by amphiboles, without any change in original igneous texture, may also suggest isochemical changes (Redman and Kay, 1985).