Sediment-derived origin of the putative Munnar carbonatite, South India. (15th September 2020)

• Record Type: Journal Article
• Authors: Hegner, Ernst; Rajesh, Selvaraj; Willbold, Matthias; Müller, Dirk; Joachimski, Michael; Hofmann, Mandy; Linnemann, Ulf; Zieger, Johannes; Pradeepkumar, Anakkathil Purushothaman
• Is Part Of: Journal of Asian earth sciences. Volume 200(2020)
• Publication Date: 2020-09-15
• Subjects: Metacarbonate -- Carbonatite -- Marble -- Geochemistry -- Nd-Sr-C-O isotopes -- Zircon geochronology; Earth sciences -- Asia -- Periodicals; Sciences de la terre -- Asie -- Périodiques; Earth sciences; Asia; Periodicals; 555.05
• DOI: 10.1016/j.jseaes.2020.104432
• Languages: English
• ISSNs: 1367-9120

Abstract

Graphical abstract: Highlights: Reported carbonatite from Munnar, South India, is interpreted as a carbonate melt of sedimentary protolith. Crustal trace-element and Nd-Sr isotope characteristics preclude a classification as carbonatite. Mantle-like O and C isotope compositions reflect granulite-facies metamorphism. Zircon ages support formation of metacarbonate dikes in the framework of Rodinia. Abstract: Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.