The newly discovered Attu carbonatite of West Greenland: A Mesoproterozoic dyke intrusion enriched in primary Sr-Ba-REE minerals

A newly discovered ca. 1.5 Ga old carbonatite dyke in central West Greenland is characterized by very high contents of rare earth elements (REE), Sr and Ba (up to 12.4 wt% total REE, 14.6 wt% SrO and 12.4 wt% BaO). The dyke occurrence is named ‘Attu carbonatite’ after the nearby village Attu. The carbonatite is primarily composed of carbonate minerals such as Sr-rich calcite (CaCO₃) and dolomite (CaMg(CO₃)₂), huntite (Mg₃Ca(CO₃)₄), strontianite (SrCO₃), alstonite (CaBa(CO₃)₂), burbankite ((Na,Ca)₃(Sr,Ba, Ce)₃(CO₃)₅) and daqingshanite ((Sr,Ca,Ba)₃(Ce, La)(PO₄)(CO₃)_3−x(OH, F)_x). In addition to the wide range of carbonate minerals, the carbonatite contains coarse-grained monazite([sbnd]Ce), apatite and magnetite. Barite occurs as discrete crystals together with the rock-forming carbonates. Texturally, the carbonatite rock displays abundant intimately intergrown fine- to medium-grained Ca, Sr, Ba and REE carbonate minerals, which exhibit prominent exsolution textures within calcite, burbankite, strontianite and dolomite hosts, as well as in the apatite crystals. Several different exsolution textures are observed: 1) alstonite in calcite; 2) daqingshanite in calcite; 3) daqingshanite in burbankite; 4) Mg[sbnd]Ba carbonate in strontianite; 5) Mg[sbnd]Ba carbonate in calcite and strontianite in dolomite; 6) strontianite in apatite; and 7) monazite([sbnd]Ce) in apatite. The carbonatite dyke is foliated and exsolution textures are observed internally in the foliation-defining minerals indicating that exsolution occurred after the main deformation event. The carbonatite magma intruded into Archaean basement gneisses that had been affected by the Nagssugtoqidian tectono-metamorphic event at approximately 1850 Ma. Magmatic monazite([sbnd]Ce) crystals from the carbonatite yield a U[sbnd]Pb age of 1565 ± 53 Ma, which is the current best estimate of dyke emplacement. Monazite([sbnd]Ce) that exsolved from apatite and calcite yields a U[sbnd]Pb age of 1492 ± 33 Ma, which is within the analytical uncertainty of the primary magmatic monazite U[sbnd]Pb age. However, the U[sbnd]Pb age determinations suggest that mineral exsolution occurred a few million years after carbonatite magma emplacement, in response to further cooling of the deep-seated dyke (tectonically-induced uplift?). Dyke emplacement may have occurred within an active ductile shear zone, which helps to explain the foliation of the carbonatite rock, predating cooling-related mineral exsolution. Country rock fenitization by fluids that emanated from the carbonatite dyke intrusion is recorded by the increasing abundance of mafic silicates such as Ba-rich phlogopite at the contact zone.