Compositional variations of zirconolite from the Evate apatite deposit (Mozambique)

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Marco E. Ciriotti
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Compositional variations of zirconolite from the Evate apatite deposit (Mozambique)

Messaggio da Marco E. Ciriotti » mer 08 nov, 2017 8:38

Prossima pubblicazione.

▪ Hurai, V., Huraiová, M., Gajdošová, M., Konečný, P., Slobodník, M., Siegfried, P.R. (2017): Compositional variations of zirconolite from the Evate apatite deposit (Mozambique) as an indicator of magmatic-hydrothermal conditions during post-orogenic collapse of Gondwana. Mineralogy and Petrology, 111, (in press).

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 M2++M5+→Ti + M3+ (M2+ = Fe2++Mg, M3+ = Fe3+, M5+ = Nb5++Ta5+). In addition, REE-zirconolite is typical of the REE + M2+ → Ca + M3+ substitution (M2+ = Mg, M3+ = 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)ZrFe3 +  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.
Marco E. Ciriotti

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