Linking monazite geochronology with fluid infiltration and metamorphic histories: Nature and experiment

Abstract

Migmatised metapelites from the Kodaikanal region, central Madurai Block, southern India have undergone ultrahigh-temperature metamorphism (950–1000°C; 7–8kbar). In-situ electron microprobe Th–U–Pb isochron (CHIME) dating of monazites in a leucosome and surrounding silica-saturated and silica-poor restites from the same outcrop indicates three principal ages that can be linked to the evolutionary history of these rocks. Monazite grains from the silica-saturated restite have well-defined, inherited cores with thick rims that yield an age of ca. 1684Ma. This either dates the metamorphism of the original metapelite or is a detrital age of inherited monazite. Monazite grains from the silica-poor restite, thick rims from the silica-saturated restite, and monazite cores from the leucosome have ages ranging from 520 to 540Ma suggesting a mean age of 530Ma within the error bars. In the leucosome the altered rim of the monazite gives an age of ca. 502Ma. Alteration takes the form of Th-depleted lobes of monazite with sharp curvilinear boundaries extending from the monazite grain rim into the core. We have replicated experimentally these altered rims in a monazite-leucosome experiment at 800°C and 2kbar. This experiment, coupled with earlier published monazite-fluid experiments involving high pH alkali-bearing fluids at high P–T, helps to confirm the idea that alkali-bearing fluids, in the melt and along grain boundaries during crystallization, were responsible for the formation of the altered monazite grain rims via the process of coupled dissolution–reprecipitation.