Sediment-derived origin of the putative Munnar carbonatite, South India

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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.

Introduction

The granulite terranes of southern India host a number of metacarbonate assemblages whose mineralogical and geochemical characteristics have stimulated discussions regarding mantle-derived or sedimentary protoliths (Subbarao et al., 1995, Schleicher et al., 1998, Kumar et al., 1992, Kumar et al., 2001, Le Bas et al., 2002, Le Bas et al., 2004, Srivastava et al., 2005, Ackerman et al., 2017). The reasons for diverging interpretations of the nature of their protoliths lie in the circumstance that primary characteristics such as structural relationships, textures, and geochemical signatures were often obscured by rock deformation, fluid-rock interaction, and granulite-facies metamorphism. Limited data sets have additionally contributed to ambiguous interpretations. An understanding of the origin of these metacarbonate assemblages is desirable for targeting mineralized carbonatites that may be economically important Nb-LREE deposits. Identification of carbonatites is also of immense interest for unravelling the geochemical and isotopic evolution of the Indian subcontinental mantle. This paper examines new mineralogical, geochemical and isotope data for the putative Munnar carbonatite and presents new interpretations regarding its source, age, and origin. The metacarbonates were previously described as sövite and alvikite carbonatite varieties (Nair et al., 1984, Santosh et al., 1987). The authors noted similarities in the Th-REE systematics with mantle-derived (primary) carbonatites as well as low Sr and Ba concentrations, unlike those in carbonatites. Accordingly, they suggested an origin of the rocks by CO2 degassing of the mantle, fluid-induced melting of mantle and lower crust, and immiscibility of an alkali silicate melt. The rocks were subsequently treated in the literature as mantle-derived carbonatite (e.g., Veevers, 2007, Woolley and Kjarsgaard, 2008, Catlos et al., 2008, Krishnamurthy, 2019, Randive and Meshram, 2020, Paul et al., 2020). We use metacarbonate as a non-genetic description for granulite-facies carbonate-rich rocks (i.e., limestone, dolomite, impure limestone, calcio-carbonatite) and marble as genetic description for a carbonate-rich rock of sedimentary origin. The definition of carbonatite follows IUGS (Le Maitre et al., 1989) and includes the currently used trace-element, isotope, and process-oriented criteria, such as an ultimate mantle origin and the exclusion of crustal melts (e.g., Bell, 1989, Bell et al., 1998, Mitchell, 2005, Jones et al., 2013; and references in these publications). This more detailed definition is helpful to avoid misidentification of pegmatitic and sedimentary carbonate material (e.g., marble) as carbonatite.

Section snippets

Geological setting of the study area

The study area is located in the Western Madurai Domain (WMD; also known as northern Madurai Block, NMB) of the Southern Granulite Terrain (SGT) in southern India (Fig. 1). For reviews of the tectonic assemblage of the SGT and its temporal evolution we refer to e.g., Drury and Holt, 1980, Drury et al., 1984, Harris et al., 1994, Chetty, 1996, Ramakrishnan, 2003, Rao et al., 2006, Santosh et al., 2012, Chetty and Santosh, 2013, Collins et al., 2014, Clark et al., 2015, Brandt et al., 2011,

Samples and field observations

We sampled metacarbonate material from dikes, veins, and pods at roadcuts near the village of Kadugumudi (samples M12a, M12b, and included minerals) and from massive layered metacarbonate and calc-silicate assemblages at Yellapatti (samples M1 to M7), respectively (Fig. 2). Metacarbonate at these localities was previously interpreted as carbonatite (Nair et al., 1984, Santosh et al., 1987). Details of the field relationships at these roadcuts are presented further below in this section.

In the

Analytical methods

The analytical methods are described in detail in Appendix B and reported here in brief. The major-element concentrations of whole-rock samples were determined at Tübingen University, using a Bruker AXS S4 Pioneer XRF spectrometer. The trace-element analyses were carried out at Tübingen and Göttingen Universities, using ThermoFisher Scientific iCAP-Q mass spectrometers. The precision and accuracy of the trace element data are estimated at <5% and <8%, respectively. The major- and trace-element

Petrography of Kadugumudi and Yellapatti metacarbonate and calc-silicate samples

Thin section photomicrographs of metacarbonate and calc-silicate samples of dikes and veins at Kadugumudi are shown in Fig. 5. The thin sections of two metacarbonate and one calc-silicate sample from Yellapatti reveal a similar mineral assemblage and, for brevity, are shown and described in Appendix C. All thin sections show mineral assemblages representative of granulite-facies carbonate and calc-silicate rocks.

The dikes and veins in gneiss at Kadugumudi show a mineralogical zonation with

Conclusions

Magmatic zircons from the enigmatic metacarbonate dikes and veins near Munnar township yielded an intrusion age of 1020 ± 70 Ma compatible with the late Grenvillian orogeny and development of Rodinia. A cross-cutting quartz syenite, previously thought to be cogenetic yielded a magmatic age of 620 ± 35 Ma.

Field relations, petrographic and geochemical-isotope assessment of the marble and metacarbonate dike samples indicate old crustal characteristics and absence of carbonatite. Little

CRediT authorship contribution statement

Ernst Hegner: Conceptualization, Writing - original draft, Writing - review & editing, Visualization, Methodology, Investigation. Selvaraj Rajesh: Writing - review & editing, Visualization, Investigation. Matthias Willbold: Writing - review & editing, Methodology, Investigation. Dirk Müller: Writing - review & editing, Investigation. Michael Joachimski: Writing - review & editing, Investigation. Mandy Hofmann: Writing - review & editing, Visualization, Methodology, Investigation. Ulf Linnemann:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the staff at the Universities of Tübingen, Göttingen, Erlangen-Nürnberg and Senckenberg Museum, Dresden, for analytical support and D. Weidendorfer for discussions. We are indebted to Debjyothi Paul at the Advanced Centre for Material Science of the Indian Institute of Technology at Kanpur for C-O isotope analyses for a pilot study. Tomson J. Kallukalam at the National Centre for Earth Science Studies, Trivandrum, provided access to mineral separation facilities. Thanks go to Jean

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