Thermobarometric and geochronologic constraints on the emplacement of the Neoproterozoic Evate carbonatite during exhumation of the Monapo granulite complex, Mozambique

Abstract

Fluid inclusion and mineral thermobarometric study supplemented by U-Th-total Pb monazite dating has been carried out in country rocks of the Evate carbonatite, which is an example of a Neoproterozoic (~590 Ma) orogenic carbonatite formed within a post-collisional tectonic setting in the Monapo granulite complex of NE Mozambique. The fluid inclusion analysis of a 573 ± 13 Ma old fenite subjacent to the Evate carbonatite revealed primary, high-density to superdense CO2-rich fluids (>1 g.cm−3) with minor admixtures of N2 (0–17.5 mol%) and CH4 (0–1.1 mol%) accompanied by alkalic hydrocarbonate melts and aqueous fluids with minor sulphates. Isochores of CO2 inclusions intersected by isopleths calculated from Ti-in-quartz and Zr-in-rutile thermometers indicate fenite formation at 480–530 °C and 2.1–5.4 kbar. The CO2-rich gas of the fenite-forming fluids/melts is similar to the anhydrous CO2 gas with traces of N2 (<1.5 mol%) trapped in granulites and paragneisses of the Monapo Complex during granulite-facies metamorphism (~850 °C, ~9 kbar) and superimposed retrogression under amphibolite-facies conditions (~550 °C, ~5 kbar) within the time interval between 680 and 590 Ma (Cryogenian to Ediacaran). Peak metamorphic conditions within the Monapo Complex are diagnostic of a hot orogen similar to that in other Pan-African Neoproterozoic HT/UHT granulite terranes. The exhumation PT trajectory of the Monapo Complex diverted from a „near-isochoric“-shaped curve to near-isothermal (~500 °C) decompression at ~590 Ma coeval with the intrusion of the Evate carbonatite into the disrupted continental crust. The pressure fluctuations during the concomitant fenitization were reflected in densities of carbonic inclusions, ranging between 1.178 and 0.773 g.cm−3. Secondary, low-density N2-rich inclusions (82–99 mol% N2, 0.8–13.7 mol% CH4, 0–4.5 mol% CO2) homogenized to vapour at temperatures below −173 °C recorded the fenite cataclasis at temperatures below 400 °C and very low pressures. The probable maximum burial depth of the Evate carbonatite, ~20 km, is estimated from the maximum fluid pressure of 5.4 kbar attributed to lithostatic load of the crust with an average density of 2.75 g.cm−3.