A geochemical and isotopic perspective on tectonic setting and depositional environment of Precambrian meta-carbonate rocks in collisional orogenic belts
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In this contribution we review the possibility of establishing the depositional age and tectonic settings of metamorphosed carbonate rocks from continental collision zones in the East African-Antarctic Orogen. The geochemical characteristics of regionally distributed meta-carbonate rocks from the Highland Complex (HC) in Sri Lanka are considered in detail and compared with similar occurrences in East Antarctica, India, Madagascar and Africa. The variations seen in the Highland Complex of Sri Lanka imply that carbonate deposition was younging from west to east, spanning apparent ages from Mesoproterozoic to Neoproterozoic. In the case of East Antarctica, such variations are within the Neoproterozoic, whereas in southern India, Madagascar and Mozambique they have a broader age range possibly from the Paleoproterozoic to Mesoproterozoic. There is also clear evidence that some carbonates were deposited in an open ocean surrounding volcanic islands in the Mesoproterozoic. Shale-normalized REE patterns have typical signatures of open ocean deposition in a passive continental margin with variable continental input in platforms nearby to island arcs. In comparison to Phanerozoic equivalents, the absence of a Ce anomaly is most significant, whereas other parameters such as (Pr/Yb)SN, (Pr/Tb)SN, and (Tb/Yb)SN were used to evaluate relative enrichments of the LREE, MREE and HREE fractions that are characteristic of ambient seawater. Pronounced La, and Y anomalies with minor Eu and Gd anomalies and correlations of REE parameters and anomalies with carbon and oxygen isotopes, 87Sr/86Sr initial ratios and εNd values are evaluated for meta-carbonate rocks in the Proterozoic collision zone. The εNd values and Sr initial ratios suggest that basins in the western Mozambique Ocean that separated the East Gondwana from West Gondwana received contributions from Archean continental crust and ambient seawater, whereas the eastern Mozambique Ocean had REE contributions from specific cratonic continents in passive margins or from continental/volcanic island arcs in active margins.
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