DEFORMATION CHARACTERISTICS OF MALANI VOLCANIC SUITE AROUND POKHRAN, RAJASTHAN

Abstract

Neoproterozoic Malani Volcanic Suite forms the largest anorogenic felsic volcanic unit in India. it took place afier Erinpura granitizalion and ceased before Marwar sedimentation. Volcanism occurred in three distinct phases. Main voluminous Rhyolitic flows are exposed through intracratonic extensional nil system in and around Jodhpur. Siwana. .Jhalorc. Pokhran. Kundal area of Rajasthan. Malani Rhyolites in and around Pokhran area are pervasively foliated and lineated. Dcfirniation on this foliation and lineation generates structural flatures which have been observed. There was no tectonic force acting at the period of Malani volcanism. l)cR)rmation took place in hot state ductile nature. known as Rheomorphism. Pyroclastic materials were deposited and welded to form the primary flow laminations. lextual and structural heterogeneity develops because of viscosity contrast in dillerent layers. I lere bubble content in dilirent layers influences the viscosity greatly. lextural layers of Malani rocks in Pokhran are recognised on the basis of relative vcsicle and microlite content. These are coarsely vesicular pumice. finely vesicular pumice and obsidian. l)imension and relative proportion of these layers are varying in vertical section from mm to i cm scales. Ductile flow of thick deposits of volcanic material give rise to Rheomorphic latures in Malani rocks. Features like intrafolial recumbent folds. sheath IbIds, flattened vesicles. and refolded flow folds are very common. Buckle folding occurs in less vesicular obsidian layers. This observation leads to the fact that bubble content influences viscosity mostly and viscosity difference in alternating layers generates rheological differences. It also influences fold geometry. For a single obsidian layer embedded in pumiceous matrix the fold geometry will be heterogeneous compared to 1111.111i layered structure. Vertical variation in texture and structure in Malani Rhyolite in Pokhran leads to the emplacement history. Also observation of structural parameters gives a general flow direction at that area. Pumice content decreases upwards also mechanism of folding changes from buckling to shearing. Ductile motion along with agglutination phenomena gives rise to an upward migrating shear zone which is more prominent in the upper part. Studies done here infer an explosive volcanism with high gas content followed by a more quiescent emplacement as volatile content decreases.