Emplacement and growth of alkaline dikes: Insights from the shonkinite dikes (Elchuru alkaline complex, SE India)

Abstract

The Mesoproterozoic Elchuru alkaline complex in the Prakasam Alkaline Province (PAP), Eastern Ghats Belt, SE India is intruded by coeval shonkinite dikes. Magmas parental to both the host nepheline syenite and shonkinite dikes formed during ca. 1350 Ma continental-rift related magmatism. The shonkinite dikes show fine-to medium-grained equigranular to foliated textures with clinopyroxene, biotite, amphibole, K-feldspar perthite and nepheline as major mineral phases. The dikes grew by inflation and coalescence of dike segments along intrusive steps, anastomosis and subsequent coalescence, and thermal/mechanical erosion of the host rocks. A few dikes resulted by filling the fractures created by shearing. Inflation of intrusive steps between magma segments by Mode I opening is the major mechanism of shonkinite dike emplacement and growth. However, the shonkinite dikes also demonstrate the application of remote shear stress on the dike walls resulting in anastomosis, entrapment and segmentation of tabular host rock xenoliths, and in extreme cases brecciation of the host rocks under volatile-rich and low-viscous melt conditions. Fractal analysis of xenolith-bearing dike further illustrates that thermal erosion was facilitated and accentuated by mechanical breakdown of host rock xenoliths due to interactions of tensile and shear forces. Cross-cutting relationships between the shonkinite dikes suggest local deviation in the least compressive stress direction during dike emplacement. The shonkinite dikes of Elchuru demonstrate that spatially- and temporally-restricted high-volatile low-viscous alkaline magmas may have distinctly different styles of emplacement, as controlled by interaction of near field (magmatic) tensile stress and far field (tectonic) shear stress, in an evolving continental-rift setting.