Characterization of zircon megacrysts from an atypical occurrence of carbonatite at Kawisigamuwa, Sri Lanka
- 1Institute for Earth Sciences, NAWI, University of Graz, Graz, Austria (daniela.gallhofer@uni-graz.at)
- 2Department of Geology, The University of Kansas, Lawrence, USA
- 3School of Earth Sciences, The University of Western Australia, Perth, Australia
- 4Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia
- 5Department of Physics, The Open University of Sri Lanka, Nugegoda, Sri Lanka
- 6State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xian, China
Zircon megacrysts are unusually large crystals (> 5 mm) that are commonly associated with mantle-derived kimberlites, carbonatites, alkali basalts and syenitic pegmatites (e.g., Hoskin and Schaltegger 2003). Such zircons form during relatively short timespans and therefore, are often used as reference material for U-Pb geochronology. Here, we determine the geochemical and isotopic (U-Pb, Hf, O) characteristics of a little-known occurrence of zircon megacrysts at Kawisigamuwa, Sri Lanka.
The dark brown megacrysts are euhedral, commonly elongate crystals with double pyramidal terminations and have faintly corroded crystal surfaces. The zircons consist of oscillatory zoned and nearly featureless cathodoluminescence-bright patches, some of which appear to follow sealed cracks. All zircon domains show a low to moderate FWHM of the ν3 (SiO4) Raman band (2.5 to 7.3 cm-1), have a low to moderate radiation damage (total α-dose mainly <0.5 x 1018 events/g) and therefore are intermediate to well crystalline. Contents of most trace element (U, Th, REE, P) are elevated in the oscillatory zoned domains, while Hf content is elevated in the CL-bright domains and seems to be grain-dependant. The oscillatory zoned domains yielded a TIMS weighted mean 206Pb/238U age of 532.39 ± 0.66 Ma (2sd). The206Pb/238U dates within the CL-bright domains are partially reset by a single event of recrystallisation at ~518 Ma. The mean Hafnium isotopic compositions of the tested grains show a narrow range of 176Hf/177Hf from 0.281969 to 0.282003. Oxygen isotopes determined on two oscillatory zoned zircon megacrysts are homogeneous (mean δ18O of 12.1 and 12.2).
While some of the trace and major element characteristics (Th/U, Zr/Hf, Hf content) of the Kawisigamuwa megacrysts resemble those of carbonatite zircons, their hafnium and oxygen isotope ratios are clearly different from mantle values. The isotopic values indicate that a significant amount of a crustal component must be involved in the formation of the zircons. Recently, several studies have found evidence for melting of carbonate rocks under high grade metamorphic conditions in Sri Lanka (e.g., Wang et al. 2021). It might be feasible that zircons grow from interaction of crustal derived carbonate melts and silicate melts or wall rocks under high grade metamorphic conditions.
Hoskin P.W.O. and Schaltegger U. (2003). The Composition of Zircon and Igneous and Metamorphic Petrogenesis. Reviews in Mineralogy and Geochemistry, 53 (1), 27–62.
Wang J., Su B.-X., Chen C., Ferrero S., Malaviarachchi S.P.K., Sakyi P.A., Yang Y.-H. and Dharmapriya P.L. (2021). Crustal derivation of the ca. 475-Ma Eppawala carbonatites in Sri Lanka. Journal of Petrology, 62 (11), 1-18.
How to cite: Gallhofer, D., Skrzypek, E., Hauzenberger, C., Möller, A., Andrew, J., Parra-Avila, L. A., Martin, L., Kemp, A., Fernando, R., and Dengfeng, H.: Characterization of zircon megacrysts from an atypical occurrence of carbonatite at Kawisigamuwa, Sri Lanka, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12969, https://doi.org/10.5194/egusphere-egu23-12969, 2023.
