Anisotropy of magnetic susceptibility reveals the magma flow direction and emplacement history of Malani rhyolites, northwest India

Abstract

Magma flow directions and emplacement may offer crucial details on the geodynamic and tectonic settings of magmatism. Malani Igneous Suite (MIS) in northwest India is a large silicic igneous province of the Neoproterozoic age. The eruption of MIS has been divided into three phases, with felsic (rhyolitic) lava flows and granitic plutons dominating the first phase and mafic lavas and felsic and mafic dykes acting as secondary components. The magma flow direction and emplacement history of Malani rhyolites are poorly understood. We study the magnetic mineralogy, microscopic observations, and anisotropy of magnetic susceptibility (AMS) of the Malani rhyolites to determine the magma flow directions and the emplacement mechanism. Ti-magnetite and Ti-hematite pseudo-single domains are the primary carrier minerals in the rhyolites. Prolate, oblate, and triaxial magnetic ellipsoid are observed and describe normal, intermediate, and inverse fabrics related to magma flow during emplacement. The principal susceptibility axes are well-clustered and independent of their shape factor (T) with a triaxial distribution, indicating well-developed magnetic lineation and foliation. In all three magnetic susceptibility axes, the magnetic lineation (K₁) aligns finest with the microscopic flow indicators. AMS investigation shows that the magnetic fabric is consistent with an ∼ E-W trending flow. The azimuthal orientation of magnetic lineation is consistent in all the sites representing an undisputed ∼ E-W trending flow sense; the plunge of the lineation is mutually opposite, hindering an unambiguous flow direction. AMS fabrics also show downward and upward flow that could be related to flank and summit eruptions.