Structural state transformation in alkali feldspar: evidence for post-crystallization deformation from Proterozoic granite, Kumaun Himalaya (India)

The Paleoproterozoic Amritpur Granite Series (AGS) constitutes a cogenetic magmatic suite of S-type granites. The granitic complex shows variation with respect to chemical and mineralogical properties. However, it shows uniformity in the structural state of alkali feldspars. Homogenization experiments on alkali feldspars of the AGS indicate >700 °C magmatic melt temperatures. The alkali feldspars are intermediate to maximum microcline (Δ=0.604-0.833) with Al concentrations ranging from 0.738-1.00 in the T1O tetrahedral site. This indicates a higher degree of ordering following an ideal one-step ordering path. The two-feldspar thermometry for granite suggests a temperature of 400±50 °C, which is the inversion temperature of orthoclase into microcline due to deformation. This suggests that the structural state transformation of alkali feldspar in the Amritpur Granite took place during a post crystallization thermal event. Subsequent to the crystallization of AGS in the Himalayas, an Early Palaeozoic and a Tertiary Himalayan event associated with a low temperature event are recognized. The P-T conditions of the Himalayan event are more conducive for the phase transformation of the alkali feldspar.