Structural state transformation in alkali feldspar: evidence for post-crystallization deformation from Proterozoic granite, Kumaun Himalaya (India) Prabha Pandey * , R.S. Rawat, T.N. Jowhar Wadia Institute of Himalayan Geology, Dehra Dun-248001, India Received 7 February 2003; revised 20 May 2004; accepted 16 June 2004 Abstract 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 O700 8C magmatic melt temperatures. The alkali feldspars are intermediate to maximum microcline (DZ0.604–0.833) with Al concentrations ranging from 0.738–1.00 in the T 1 O 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 400G50 8C, 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. q 2004 Elsevier Ltd. All rights reserved. Keywords: Feldspar; Proterozoic Granite; Himalayan Orogeny 1. Introduction The Himalaya represents a continent–continent colli- sional belt that formed due to collision and continued convergence of the Indian plate with the accreted Tibetan landmass. Continued convergence since the Eocene col- lision has resulted in the formation of intracrustal thrusts, e.g. Main Central Thrust (MCT) zone, Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) from north to south, respectively (Fig. 1 inset; Valdiya, 1980a). These thrusts tectonically separate the Higher Himalaya, Lesser Himalaya, Outer Himalaya and the alluvial plains. In the Himalayan fold-thrust belt, three major phases of granitoids have been mapped. The youngest Eocene to Miocene leuco-granites occurs in the upper part of the Higher Himalayan Crystalline belt. Early-Paleozoic (w500 Ma) granites are observed in the Higher and Lesser Himalayan crystalline sequences (Fig. 1 inset). The oldest Paleoproterozoic granitic suites, namely the Wangtu Gneissic Complex (WGC) and Bandal Gneissic Complex (BGC) in the Kulu-Rampur window zone and Amritpur Granite Series (AGS) in Kumaun, are exposed in the Lesser Himalayan zone of NW Himalaya (Fig. 1 inset; Varadarajan, 1978; Bhargava, 1980; Trivedi and Pande, 1993; Miller et al., 2000). The AGS is the southern most granitic body, which lies in juxtaposition with the Siwalik Group of the Outer Himalaya along the MBT. Homogenization experiments on the alkali feldspars of the AGS indicate O700 8C magmatic melt temperatures (Rawat and Kumari, 1998). Alkali feldspar constitutes the dominant feldspar phase in the AGS. In addition to textural characteristics, the feldspars can be classified on the basis of ordering and structural state. The ordering and structural state of alkali feldspar is a function of temperature and annealing time (Yund, 1983; Parsons and Brown, 1991), and, therefore, is an indicator of 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.06.007 Journal of Asian Earth Sciences 25 (2005) 611–620 www.elsevier.com/locate/jaes * Corresponding author. Present address: Department of Geology, Banaras Hindu University, Varanasi 221005, India. E-mail address: pandey_prabha@rediffmail.com (P. Pandey).