Compositional zoning, crystal chemistry and metamorphic growth of monazite in the Greater Himalayan Sequence, Dhauliganga Valley, Garhwal Himalaya

Abstract

X-ray elemental mapping and quantitative analysis of monazite grains contained in metapelites from the Greater Himalayan Sequence (crystalline core of the Himalaya) of the Dhauliganga Valley, Garhwal Himalaya were performed by electron probe micro-analyser to evaluate the zoning patterns. The rocks, from which monazites have been studied, include those with peak metamorphism (650–715 °C and 8.0–9.9 kbar) under subsolidus conditions and also those showing evidences of partial melting (<800 °C and 10.5 kbar). All the monazite analyses taken together show that Light Rare Earth Elements (LREEs) constitute 80–89 % of the total cation proportion (exclusive of P) and Heavy Rare Earth Elements (HREEs) 1–4 %, while other elements make 9–18 %. Zoning patterns vary from core–rim to patchy and irregular types. The rim is invariably Y-rich compared to the core. Compositional variability of monazites can be explained mostly by the brabantite substitution: 2REE³⁺ = (Th,U)⁴⁺ + Ca²⁺. Y shows negative correlation with LREE and positive correlation with HREE. The U-Th-Pb in situ analyses of one of the samples (HH52) suggests that the monazite grew at 26.28 ± 0.19 Ma. Likely metamorphic reactions for the growth of Y-poor monazite core and its resorption followed by overgrowth of Y-rich rim during progressive metamorphism have been explored. Phase equilibria considerations indicate that resorption and regrowth of monazite can occur in both subsolidus and suprasolidus regimes. The P–T path for the development of resorption–regrowth texture of monazite is inferred to be isothermal decompression in the case of subsolidus regime, and isobaric cooling to retrograde decompression in the case of suprasolidus conditions.