Mineral inclusions in zircon and pyrochlore from cumulate quartz-feldspar rocks, Katugino alkaline granite massif, Transbaikalia, Russia

Sharygin V.V.¹, Vladykin N.V.²

¹ V.S. Sobolev Institute of Geology and Mineralogy SD RAS, Novosibirsk, Russia

² A.P. Vinogradov Institute of Geochemistry SD RAS, Irkutsk, Russia

 

Mineral inclusions (10-70 µm) were studied in zircon and pyrochlore from quartz-feldspar rocks of the Katugino massif, containing cumulative zircon and pyrochlore. Besides the above minerals, these rocks may bear abundant annite (up to 2 wt% ZnO, 1.5 wt% F), arfvedsonite, magnetite, ilmenite (up to 1 wt% ZnO, 1.6 wt% Nb₂O₅, 5.0 wt% MnO), sulfides (galena, sphalerite) and REE-carbonates (bastnäsite-(Ce), parisite-(Ce)). Fergusonite-(Y), exotic pyrochlore-group minerals (with REE, Y, Th, U, Sn, Pb), columbite, samarskite, Fe-hydromica (hydrotetraferriannite), thorite, fluocerite-(Ce), tveitite-(Y), gagarinite-(Y) with Yb₂O₃ (up 6.5 wt%), cassiterite, monazite-(Ce), rutile (4.2-6.8 wt% Nb₂O₅, 3.8 wt% Fe₂O₃), chalcopyrite, bismuth and bismuthinite occur as minor and accessory phases. Zircons are zoned and contain variable HfO₂ (1.0-3.4 wt%). Composition of pyrochlores is strongly different in discrete rocks. Mineral from rocks with cumulative zircon is zoned: rim is richer in Ta₂O₅ and UO₂ than interior (10.4-10.9, 7.7-8.2 and 2.6-3.1, 2.8-3.0 wt%, respectively). In contrast, mineral from rocks with cumulative pyrochlore does not show pronounced zonality and chemical variations; it approaches ideal pyrochlore (Ta₂O₅ - 2-3; F – up to 5 wt%).

Polymineral and individual mineral inclusions in zircon are mainly represented by quartz, K-feldspar and albite; ilmenite, fergusonite-(Y), fluocerite-(Ce), tveitite-(Y), gagarinite-(Y), chiolite, bastnäsite-(Ce), annite, fluorite, galena, bismuth, bismuthinite and cassiterite occur rarely. All inclusions are commonly confined to the central zones of the host zircons. Unlike zircon, mineral inclusions in pyrochlore are scarce and mainly represented by quartz, K-feldspar and albite, rarely by galena, cassiterite, rutile, columbite and unidentified Sn-Nb oxide phase.

Study of crystal inclusions in minerals of cumulate rocks of the Katugino massif has shown that zircon and pyrochlore were crystallized from mineral-forming media, which was enriched in volatiles (CO₂, F, S, possibly Cl) and trace elements (Zr, Nb, Ta, REE, Y).

This work was supported by integration project IPP47 (SD, UrD and FED RAS).