Carbon–Strontium Isotope Decoupling in Carbonatites from Caotan (Qinling, China): Implications for the Origin of Calcite Carbonatite in Orogenic Settings. (17th February 2020)

• Record Type: Journal Article
• Authors: Wei, Chun-Wan; Xu, Cheng; Chakhmouradian, Anton R; Brenna, Marco; Kynicky, Jindrich; Song, Wen-Lei
• Is Part Of: Journal of petrology. Volume 61:Number 2(2020)
• Publication Date: 2020-02-17
• Subjects: dolomite and calcite carbonatites -- isotope geochemistry -- C–Sr isotopic decoupling -- calcite carbonatite petrogenesis -- wall-rock metasomatism -- Qinling orogenic belt; Petrology -- Periodicals; 552
• DOI: 10.1093/petrology/egaa024
• Languages: English
• ISSNs: 0022-3530

Abstract

Abstract: Mantle-derived carbonatites emplaced in orogenic belts and some extensional settings are hypothesized to contain recycled crustal material. However, these carbonatites are typically composed of calcite showing a typical mantle range of C–O isotopic values devoid of recognizable sedimentary fingerprints. Here, we report the first known instance of C–Sr isotope decoupling between intimately associated dolomite carbonatites and magnetite–forsterite–calcite carbonatites from the northern Qinling orogen, central China. The calcite-dominant variety is developed at the contact between the dolomite carbonatite and metasomatized wall-rock gneiss. The two types of carbonatites have similar δ 18 OVSMOW (6·98‰ to 9·96‰), εNd(i) (-3·01 to -6·47) and Pb ( 206 Pb/ 204 Pb(i) = 17·369–17·584, 207 Pb/ 204 Pb(i) = 15·443–15·466) isotopic compositions, but significantly different C and Sr isotopic signatures (δ 13 CVPDB = -3·09 to -3·58‰ and -6·11 to -7·19‰; 87 Sr/ 86 Sr(i) = 0·70373 to 0·70565 vs 0·70565 to 0·70624 for the dolomite and calcite rocks, respectively). The relative enrichment of the early-crystallizing dolomite carbonatite in 13 C and its depletion in 87 Sr are primary isotopic characteristics inherited from its mantle source. The observed field relations, petrographic and geochemical characteristics of the Caotan dolomite and calcite carbonatites imply that the strong C–Sr isotopic decoupling between them could not result from mixing of different mantle reservoirs (e.g. HIMU and EM1), or from magma fractionation processes. We propose that the calcite carbonatites were a by-product of metasomatic reactions between primary dolomitic melts and felsic wall-rock. These reactions involved the loss of Mg and CO2 from the magma, leading to depletion of the evolved calcite-saturated liquid in 13 C and its enrichment in radiogenic Sr. We conclude that calcite carbonatites in plate-collision zones may not represent primary melts even if their isotopic signature is recognizably 'mantle-like'.