Preliminary magnetic susceptibility results on the Neoproterozoic Mt. Abu granite, NW Indian craton: correlation between magnetic fabric and deformation
de Wall, Helga; Pandit, Manoj K.
Neoproterozoic acid magmatism in NW India has been described to represent a transition from late-tectonic Erinpura granitoids to the non-orogenic Malani Igneous Suite (MIS). The Erinpura granitoids, occurring in the Mt. Abu - Sirohi region, are variably deformed; the relatively undeformed 'pink granite' phase being regarded as correlative with the MIS peraluminous granite on the basis of apparent textural and mineralogical similarities. The peraluminous (pink) Jalore granite (MIS) and Mt. Abu pink granite (Erinpura granite) show marked differences in their low field magnetic susceptibility characteristics. The Mt. Abu granitoids fall in the range of paramagnetic susceptibilities with biotite as the principal paramagnetic mineral, whereas in the here investigated Malani granites a contribution of paramagnetic (biotite) and ferrimagnetic (magnetite) constituents has been inferred. The magnitude of anisotropy of magnetic susceptibility (AMS) for both the granitoid types is in the range of hypersolidus AMS fabrics with anisotropy values below 10% but with lower values in the Malani granites. A typical magmatic fabric, without any indication of solid state deformation, is identified in the Malani pink granites while the Mt. Abu samples show a tectonic fabric in the transition from submagmatic to high-temperature solid state deformation. The geometry of this fabric (steep NE-SW trending foliations and subvertical lineations) is clearly documented by the AMS data. Geometrical similarities with shear zones in the Sirohi belt favour an interpretation of coeval fabric evolution for both, the Sirohi belt and the Mt. Abu pink granite and relate the granite intrusion to a syn-tectonic stage.