Carbon-Strontium Isotope Decoupling in Carbonatites from Caotan (Qinling, China): Implications for the Origin of Calcite Carbonatite in Orogenic Settings

journal article in Web of Science

English

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 delta O-18(vsmow) (6.98 parts per thousand to 9.96 parts per thousand), epsilon Nd-(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 (delta C-13(VPDB) = -3.09 to -3.58 parts per thousand and -6.11 to -7.19 parts per thousand; Sr-87/Sr-86((ii)) = 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 C-13 and its depletion in Sr-87 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 EMI), 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 C-13 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'.

dolomite and calcite carbonatites; isotope geochemistry; C-Sr isotopic decoupling; calcite carbonatite petrogenesis; wall-rock metasomatism; Qinling orogenic belt

Wei, CW.; Xu, C.; Chakhmouradian, AR.; Brenna, M.; Kynicky, J.; Song, WL.

1. 2. 2020

OXFORD UNIV PRESS

OXFORD

1460-2415

Journal of Petrology

61

2

United Kingdom of Great Britain and Northern Ireland

egaa024-1

egaa024-26

26

https://academic.oup.com/petrology/article/61/2/egaa024/5739450