Behavior of monazite during incipient dehydration melting of charnockite at the northern Eastern Ghats Belt, India: Insights on the mobility of REE at amphibolite-granulite facies transition

Monazite has the potential to place temporal constraints on the crustal melting in high-grade metamorphic rocks like granulites and migmatites. Melt loss in granulite-grade metamorphic rocks plays a key role in progressively depleting LREE in the residue and enhancing the dissolution of monazite during heating to the metamorphic peak. Newly formed monazite are therefore more abundant in leucosomes than the residue. Higher degree of partial melting and subsequent melt loss, therefore, poses a major hindrance to constraining the mobility of these elements in the micro-domain scale, particularly during the early stage of melting at amphibolite to granulite facies transition. To overcome such an issue and to understand the behavior of this mineral during the onset of granulite facies metamorphism, metamorphic rocks that have reached the P-T conditions culminating at the aforesaid transition should be targeted. Considering this, the present study has been carried out on charnockite from the northern Eastern Ghats Belt, India which underwent such transition (M₂) following crystallization during an earlier granulite facies metamorphic event (M₁). The rock is composed of plagioclase (Pl), K-feldspar (Kfs), quartz, orthopyroxene, biotite, and garnet with apatite, allanite, and monazite as accessory phases. The rock has well-developed gneissic foliation, demarcated by alternate biotite +garnet-rich and quartzofeldspathic layers. While both the feldspars show grain boundary migration recrystallization, quartz grains are deformed by sub-grain rotation recrystallization. Garnet is porphyroblastic and post-kinematic as it overgrows the matrix biotite. The former phase is closely associated with cuspate Kfs and quartz grains which developed as a result of incipient dehydration melting of moderately fluorine rich biotite during the aforesaid transition. Monazite grains are coarse (up to 200 µm across), mostly elliptical and either partially or completely replaced by the reaction rim of apatite+ thorite with an external corona of allanite in the biotite+garnet-rich layers. In case of partial replacement, the oscillatory-zoned relict monazite core is preserved. Th-rich patches are present in such cores. Interestingly, the coronitic assemblage overgrows the matrix biotite is always associated with porphyroblastic garnet. On the contrary, corona-free monazite grains are abundant in quartzofeldspathic layers. Spot dates from the oscillatory-zoned relict monazite core yield a weighted mean age of 960±6 Ma. Th-rich patches, showing prominent huttonite substitution, yield a weighted mean age of 938±7 Ma. Integrating monazite textural and age data, we interpret that the ca. 960 Ma represents the crystallization age of the charnockite magma which coincides with the M₁ metamorphic event of the Eastern Ghats Province (EGP). The ca. 938 Ma, additionally, corresponds to the age of the M₂ event when biotite dehydration melting occurred and porphyroblastic garnet was formed. Based on the textural evidence and mineral phase chemical data, we propose that the replacement of primary monazite occurred via coupled dissolution precipitation process in the presence of incipient melt originated during biotite dehydration melting. Such melt was fluorine rich and helped to mobilize REEs by forming REE-fluoride complexes and was incorporated in allanite corona. Monazite grains in quartzofeldspathic layers must have escaped the melting reaction and the melt-induced element mobility.