Behaviors of monazite and zircon during incipient dehydration melting of charnockite and mafic granulite at the Northern Eastern Ghats Belt, India: insights on the mobility of LREE and Zr at amphibolite-granulite facies transition

Abstract

The present study focuses on the textural and chemical evolutions of monazite and zircon during biotite and hornblende dehydration melting in charnockite and mafic granulite at amphibolite to granulite facies transition in the northern Eastern Ghats Belt. In charnockite, primary monazite with oscillatory-zoned interiors remained intact in the quartzofeldspathic layers but are partially or completely transformed to fluorapatite + secondary monazite + allanite corona in the biotite-garnet-bearing layers. In partially transformed monazite, huttonite-rich domains are conspicuous in the monazite boundary. Monazite transformation occurred in the presence of incipient melt of granitic composition generated during biotite dehydration melting, with the formation of neoblastic garnet overgrowing biotite during the aforesaid transition. Biotite released fluorine during this melting reaction which was subsequently incorporated into the melt that accelerated the monazite transformation. While silica in the huttonite-rich patches seems to have been introduced from such melt, light rare earth elements (LREEs) were concomitantly liberated from primary monazite and partitioned into the allanite corona. The Th-U-total Pb age of ca. 928 Ma from huttonite-rich patches indicates the age of the aforesaid melting at the amphibolite to granulite facies transition in charnockite. During such a transition, hornblende-bearing assemblage in mafic granulite was transformed to orthopyroxene + clinopyroxene + ilmenite with incipient melt of trondhjemitic composition, which crystallized as albitic plagioclase + quartz + K-feldspar intergrowth on cooling. Unlike monazite, zircon growth was limited during the transition, and it formed a narrow outer rim surrounding an earlier formed overgrowth and recrystallized core following crystallization of the rock from a mafic protolith. The present study, therefore, demonstrates that while LREEs show micro-scale mobility in the presence of fluorine-rich melt, Zr remained relatively immobile, particularly where fluorine is lacking. The present study also points towards the importance of felsic and mafic protoliths having magmatic ancestries in preserving the petrological and geochronological evidence of multiple thermal events in granulite terranes, which sometimes remains undetected.