Compositional variations of zirconolite from the Evate apatite deposit (Mozambique) as an indicator of magmatic-hydrothermal conditions during post-orogenic collapse of Gondwana

Abstract

Zirconolite is documented from the Evate apatite-magnetite-carbonate deposit in the circular Monapo Klippe (eastern Mozambique)—a relic of Neoproterozoic nappe thrusted over the Mesoproterozoic basement of the Nampula block. Zirconolite enriched in rare earth elements—REE = Y + Lu+ΣLa–Yb (up to 24.11 wt% REE₂O₃, 0.596 apfu REE) creates thin rims around spinel and magnetite grains, whereas zirconolite enriched in U and Th (up to 18.88 wt% ThO₂ + UO₂, 0.293 apfu Th + U) replace the Late Ediacaran (~ 590 Ma) zircon and baddeleyite along contacts with pyrrhotite and magnetite. Both types of zirconolite contain locally increased Nb and Ta concentrations (up to 7.58 wt% Nb₂O₅ + Ta₂O₅, 0.202 apfu Nb + Ta). Typical substitutions in zirconolite from Evate involve REE + U,Th → Ca, and M ²⁺+M ⁵⁺→Ti + M ³⁺ (M ²⁺ = Fe²⁺+Mg, M ³⁺ = Fe³⁺, M ⁵⁺ = Nb⁵⁺+Ta⁵⁺). In addition, REE-zirconolite is typical of the REE + M ²⁺ → Ca + M ³⁺ substitution (M ²⁺ = Mg, M ³⁺ = Fe³⁺+Al³⁺). Hence, Fe³⁺ predominates over Fe²⁺ in all types of zirconolite, thus enabling the high REE content in Nb-poor zirconolites to be stored in locally dominant REEZrTiFe³⁺O₇ component known so far only as a synthetic analogue of natural zirconolite. Other types of zirconolite from Evate are dominated by the common CaZrTi₂O₇ end member, but the aforementioned “synthetic” REEZrTiFe³⁺O₇ accompanied by another ‘synthetic’ (U,Th)ZrFe^3 + ₂O₇ component are also abundant. The U,Pb,Th concentrations in U,Th-zirconolites plot discordantly to theoretical isochrons, thus indicating ~ 440 ppm of non-radiogenic excess lead in earlier Nb-rich zirconolite contrasting with secondary Pb loss from later Nb-poor zirconolite. The non-radiogenic Pb-corrected age of the early zirconolite corresponded to 485 ± 9 Ma, within uncertainty limit identical with the 493 ± 10 Ma age of the associated uranothorianite. The variegated chemical composition of zirconolites reflects the complex history of the Evate deposit. Compositional and substitution trends of the REE-zirconolite overlaps that genetically linked with carbonatites, syenites and mafic igneous rocks, whereas the U,Th-zirconolite is reminiscent of hydrothermal-metasomatic deposits. The predominance of trivalent iron in zirconolite most likely reflects strongly oxidizing parental fluids that percolated during episodic Late Ordovician to Late Cambrian rifting of Gondwana.