Deformation pattern in the Precambrian basement around Masuda, central Rajasthan

Detailed mapping of the Precambrian rocks around Masuda in central Rajastha n reveals that the basement is made up of a granulite-granitoid terrane and a sheared gneiss-migmatite terrane. The gneiss-migmatite terrane mainly co mprises a multitude of the monzogranite gneisses, amphibolites and migmatis ed biotite schists. These gneissicmigmatitic rocks are repeatedly mylonitis ed and folded at scales ranging from thin section to map. The granulitegran itoid terrane consists of massive bodies of granulite and granitoid hosting numerous enclaves of gneiss, monzogranite porphyry and mafic-ultramafic ro ck. Two successive groups of folds (F-1 and F-2), each containing two or more s ets of co-axial folds are common in the gneiss-migmatite terrane. Whereas t he interference between different fold sets belonging to the individual gro ups has resulted in type 3 interference patterns, the superposition of F-2 group folds on F-1 group folds has resulted in type 2 interference patterns . Several lines of evidence suggest that F-1 and F-2 groups of folds have e volved as sheath folds during two phases of ductile shearing, respectively. It is primarily due to the isoclinal nature of the late folds that both th e early and late folds display similar style and coplanar relationship in m ost of the area. A critical comparison reveals that the gneiss-migmatite terranes in the sou thern and central Rajasthan are different from each other with respect to s tyle of deformation. It is proposed that the basement-cover contact in cent ral Rajasthan may have acted as a decollement surface during a major part o f the early deformation in the cover rocks.