New insights into formation of the dolomite – magnesite – siderite series in the Newania carbonatite complex, India: Mineralogy, petrology and in situ SIMS analyses (REE, δ13C, δ18O, U-Th-Pb ages)

We investigated magnesio-carbonatites of the Precambrian plutonic carbonatite complex of Newania, NW India, to clarify the genesis of this unusual carbonatite type. Ferroan magnesite and ferroan dolomite are the main constituents in the studied samples indicating deep melting in the mantle and fast magma ascent in multiple magma batches. Several formation stages can be texturally distinguished: upon emplacement magnesite, dolomite, apatite and (magmatic) siderite co-crystallized, and subsequently siderite partly or fully decomposed to ferroan dolomite or magnesite during metamorphic reequilibration. Rare earth element (REE) concentrations in carbonates and apatite are depleted compared to other carbonatite occurrences, as are the abundances of Sr, Ba and Pb. Chondrite-normalized REE patterns are typical for carbonatite, except for dolomite showing a significant positive Eu-anomaly. The carbon (δ ¹³ C) and oxygen (δ ¹⁸ O) isotopic compositions of dolomite and magnesite fall within the primary carbonatite field, indicating negligible hydrothermal overprinting. Δ ¹⁸ O _Dol−Mgs isotope thermometry yielded equilibration temperatures in the range of 430–500°C suggestive of final equilibration during the retrograde path of the last metamorphic event. Monazite U-Th-Pb ages agree with this multi-stage evolution: the dominant age population clusters at the age of the last, amphibolite-facies metamorphic overprint at 900–940 Ma, whereas an older upper discordia intercept U-Pb age of 1910 ± 194 Ma (2σ) supports a Paleoproterozoic formation age. Our results are in favor of a primary magmatic origin of dolomite and magnesite, and a complex later reworking during metamorphism.