Occurrence and Stability of Allanite and Monazite in the Greater Himalayan Sequence, Dhauliganga Valley, Garhwal Himalay
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Occurrence and Stability of Allanite and Monazite in the Greater Himalayan Sequence, Dhauliganga Valley, Garhwal Himalaya C. Chaurasia1, K. Madhavan2, S.S. Thakur1,*, S.C. Patel2, A.K. Samal3, S. Nema1, P.K. Dixit1 1
Wadia Institute of Himalayan Geology, 33 GMS Road, Dehradun – 248 001, India Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076, India 3 Department of Geology, Banaras Hindu University, Varanasi – 221 005, India E-mail: [email protected]; [email protected]; [email protected]*; [email protected]; [email protected]; [email protected]; [email protected]
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ABSTRACT The occurrence and relative stability of metamorphic allanite and monazite in metapelites of the Greater Himalayan Sequence (GHS) in the Dhauliganga valley, Garhwal Himalaya have been studied. The GHS is marked by kyanite grade metamorphism in the lower and middle structural levels and sillimanite grade metamorphism in the upper structural level. The occurrence of allanite is restricted to the lower structural level, while monazite occurs in the middle and upper structural levels. The allanite-out reaction occurred at ~660 oC and 9.5 kbar. Monazite shows widely varying ThO2 content which can be explained mostly by brabantite substitution. The rocks of the GHS of the Dhauliganga valley display inverted metamorphic sequence and the allanite-out reaction is the result of prograde metamorphism. INTRODUCTION Allanite and monazite are the main rare earth element (REE)bearing accessory minerals in metapelites which are known for their use as petrochronometers. Among these, monazite is a more frequently used petrochronometer, however, allanite has rather limited use because of quick change in its crystalline state. The relative stabilities of allanite and monazite depend on pressure (P)–temperature (T) conditions, bulk rock composition and fluids (Wing et al., 2003; Rasmussen et al., 2006; Spear, 2010; Budzyn et al., 2017; Engi, 2017). Phase relations between the two minerals can be used to relate precise ages to P–T conditions of metamorphism, provided these are identified and understood. In the past three decades, considerable progress has been made to understand the occurrence and origin of monazite and allanite, especially the monazite geochemistry (DeWolf, 1993; Zhu et al., 1999a, 1999b; Ayers et al., 1999; Catlos et al., 2002; Wing et al., 2003; Kohn and Malloy, 2004; William et al., 2007; Janots et al., 2008; Spear, 2010; Pandey et al., 2013; Finger et al., 2016; Thakur et al., 2018). Despite various studies, however, the conditions at which allanite and monazite are produced and consumed during prograde metamorphism of pelites still need to be better understood. Petrological investigations in metapelites show that metamorphic allanite generally forms at the expense of detrital monazite or allanite (± xenotime) close to the biotite-in reaction, while allanite breaks down to monazite at the staurolite isograd (Wing et al., 2003; Yang and Pattison, 2006; Janots et a
