Nb–Zr–REE Re-Mobilization and Implications for Transitional Agpaitic Rock Formation: Insights from the Sushina Hill Complex, India

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Journal of Petrology





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The formation of transitional agpaitic rocks is not a well understood process as there are few studies of miaskitic to agpaitic transitions. The Mesoproterozoic Sushina Hill complex (India) provides a suitable site to investigate these ‘transitions’ as this complex hosts diverse miaskitic and agpaitic nepheline syenites, together with syenites containing exotic mineral assemblages. In this study, we have used mineralogical and geochemical data to describe the evolution of the transitional agpaitic rocks occurring at Sushina Hill. In common with other occurrences, high field strength elements (HFSE) in miaskitic nepheline syenites are mainly sequestered by primary zircon and magnetite. In contrast, the major HFSE carriers in agpaitic nepheline syenites (agpaitic unit-I) are late-magmatic eudialyte and rinkite-(Ce)– nacareniobsite-(Ce), formed at T between 825°–784°C and aSiO2aSiO2 in the range of 0·41–0·44. With decreasing temperature (∼575°C) and aSiO2 aSiO2(0·30), coupled with an increase in aH2OaH2O⁠, this assemblage has undergone extensive subsolidus alteration leading to the decomposition of late-magmatic eudialyte to wöhlerite—marianoite, alkali-zirconosilicates (catapleiite/gaidonnyaite, hilairite) and pectolite—serandite. Decomposition of late-magmatic eudialyte resulted in a more alkaline fluid by increasing the a(Na+)/a(Cl-) ratio, facilitating crystallization of hydrothermal eudialyte replacing late-magmatic eudialyte. Crystallization of hydrothermal eudialyte leads to evolving fluids which are less alkaline, resulting in the crystallization of a transitional agpaitic assemblage of pyrochlore + zircon + niobokupletskite + wadeite in agpaitic unit-II in the temperature range 547º–455°C with aSiO2aSiO2 in the range 0·27–0·25. Regional scale deformation contemporaneous with the subsolidus alteration stage leads to separation of the evolving fluid from the system, resulting in extensive albitization, with superposition of a new miaskitic-like assemblage in syenite I in the form of late-stage zircon–magnetite–xenotime–monazite-(Ce) upon the early assemblage of primary zircon and magnetite. During deformation, syenite unit-II composed of eudialyte–albite– aegirine was also formed and considered as a later stage pegmatitic offshoot of agpaitic unit I. The mineralogical changes are also complemented by variations in the bulk-rock composition in which the total REE, Nb, U and Th concentrations increase in order from: miaskitic unit → agpaitic unit I → syenite unit II, -I → agpaitic unit II at constant Zr concentration. This suggests that the REE–Nb are mainly mobilized in agpaitic unit-II during the agpaitic to transitional agpaitic assemblage transformation in a relatively less alkaline environment.


Agpaitic-transitional agpaitic-miaskitic assemblage; Rinkite-(Ce)-nacareniobsite-(Ce); Wohlerite-marianoite; Niobokupletskite; Rare earth element mobility





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