Applicability of AMS technique as a flow fabric indicator in dykes: insight from Nandurbar–Dhule Deccan dyke swarm

Abstract

Determination of the nature of magma flow (vertical or lateral) from rock fabrics in a dyke swarm holds significant importance in deciphering its emplacement mechanism, depth of magma chamber and its association with mantle plumes. Nandurbar–Dhule dyke swarm from Western India consists of ~ 210 mappable, tholeiitic, large, linear dykes intruding the country rocks of Deccan basalt. We discuss here the development of flow-controlled rock fabrics in these dykes as a function of their thicknesses. Magnetic fabric obtained from anisotropy of magnetic susceptibility (AMS) analysis of the samples collected from these dykes was used as a proxy for rock (flow) fabric. Samples from the central parts of the thicker dykes (strike perpendicular thickness > 10 m) show random and inconsistent development of magnetic fabrics than their marginal counterparts. Petrography suggests the development of relatively clear shape-preferred orientations of the elongated mineral grains only at the margin of a thick dyke. Thinner dykes (strike perpendicular thickness < 10 m) show both consistent magnetic fabrics and clear shape-preferred orientations. We propose that the center of a thick dyke fails to provide (due to high flow velocity, slower cooling rate, lack of friction-induced shearing, and draining back of magma) a favorable condition where (a) flow fabric can efficiently develop and (b) magnetic fabric can effectively mimic the flow fabric. Thinner dykes and the margins of the thicker dykes are, hence, better candidates for observing flow fabrics represented by magnetic fabrics. This work is also motivated by the reservations expressed by earlier researchers in using the AMS technique for magma flow characterization, especially for giant dyke swarms. We have critically evaluated their reservations in light of our results and commented on the applicability of the AMS technique on such studies.