Geochemistry of Feldspar intergrowth microtextures from paleoproterozoic granitoids in Central India: Implications to exsolution processes in granitic system

Abstract

Various forms of feldspar intergrowth microtextures occurred in granitic rocks. Characteristically, two distinct types of microtextures formed due to intergrowth between K-feldspar and plagioclase i.e. perthites and myrmekites. Malanjkhand and Dongargarh granitoids are two contemporaneous units of Paleoproterozoic granitoids in central India. Three varieties of perthites have been identified in these granitoids i.e. flame, braid, and vein types. Flame perthite bearing K-feldspar is common. Braid and vein perthites occur occasionally. Paleopiezometry estimation indicates that flame perthite formed under the influence of low to moderate differential stress on granitoids. Most of the perthites occurs in these granitoids are compositionally albite to oligoclase in nature. The composition ranges XAb: 0.98–0.49 for the Malanjkhand and XAb: 0.96–0.49 for Dongargarh granitoids. Binary feldspar thermometry calculation indicates that perthites have formed at moderate to high temperature conditions in the Malanjkhand granitoid whereas low to moderate temperature conditions in Dongargarh granitoids during sub-solidus phase. Myrmekites is another feldspar intergrowth microtexture found in these granitoids. Myrmekites are identified in these granitoids based on their characteristic morphologies such as ghost, bulbous, rim, etc. These myrmekites were formed under moderate temperature conditions during magmatic-hydrothermal transition phase. Formation of flame perthite can be explained by replacement reaction mechanism (Na-K exchange) between K-feldspar and plagioclase under low to moderate differential stress condition during rapid cooling. Further, extensive deuteric alteration results in the formation of braid and vein perthites. Formation of myrmekites can be assumed by couple of replacement mechanisms (Na-K and Ca-K exchange) between K-feldspar and plagioclase. Paleopiezometry and feldspar thermometry results infer that orthomagmatic-hydrothermal transition in the Malanjkhand and Dongargarh granitoids were favored at upper or shallow levels in the continental crust.