In situ U–Th–Pbtotal dating of polychronous monazite in the Koraput anorthosite pluton, Eastern Ghats Granulite Belt (India), and implications

Abstract

AbstractU–Th–Pbtotal age determinations in monazite in a noritic anorthosite at the margin of the Koraput anorthosite pluton constrain the time of emplacement and sub-solidus chemical modifications in the Grenvillian-age anorthosite pluton in the Eastern Ghats Province (EGP), India. The monazites hosted within dynamically recrystallized orthopyroxene and plagioclase grains are large (50–500 μm in diameter) and complexly zoned. Based on the textural–chemical heterogeneities, these monazites are classified into three groups. Group-I monazites exhibit a low-ThO2 core mantled by a high-ThO2 rim. By contrast, the group-II monazites exhibit high-ThO2 cores laced by ThO2-poor mantles, and outermost rims with still lower ThO2 contents. Skeletal group-III monazites at polygonized grain/phase boundaries exhibit patchy and concentric zones with decreasing ThO2 towards the margin. The U–Th–Pbtotal chemical ages obtained using electron probe microanalyses exhibit four age clusters. The oldest age population (mean 939 ± 4.5 Ma) obtained in cores in group-I, II and III monazites with patchy zones corresponds with the emplacement of the Koraput anorthosite, and this age population is consistent with 980–930 Ma emplacement ages reported in other EGP anorthosite massifs. Younger monazites mantling the cores in group-II monazites and in group-III monazites with mean ages of 877 ± 5 Ma and 749 ± 18 Ma possibly reflect episodic monazite growth by fluid-aided dissolution–precipitation culminating with the disintegration of Rodinia at ~750 Ma. The youngest age population (mean 574 ± 19 Ma) in the outermost monazite rims and monazite veins represents renewed monazite growth during the Pan African assembly of the Grenvillian-age EGP domain with the proto-Indian cratons.