Abstract

Three types of subvertical pegmatitic leucogranites and pegmatites populate the Red Cross Lake pegmatite field, located in a greenstone belt of the Sachigo Subprovince, Manitoba, in the northwestern part of the Archean Superior Province of the Canadian Shield. Most of the leucogranites and pegmatites are geochemically rather primitive, but one of the leucogranites is lithium-enriched, and the most evolved lepidolite-rich pegmatites represent the most fractionated products of granitic differentiation known to date. The lepidolite pegmatites consist mainly of albite, quartz and muscovite to K-, Rb- or Cs-dominant lepidolite, subordinate Rb-rich K-feldspar and spodumene + quartz pseudomorphs after petalite, accessory spessartine, elbaite–rossmanite, beryl, pollucite, fluorapatite, amblygonite, oxide minerals of Ta, Nb, Ti, Sn, Sb and U, and rare zircon, lithiophilite and other phosphates, arsenopyrite, löllingite, stibarsen and bismuth. Along contacts with mafic metavolcanic rocks, a (Ca,Fe,Mg,Ti)- enriched assemblage carries, i.a., K-, Rb- or Cs-based annite to phlogopite dominant over holmquistite, Ca–Mg–Fe-rich tourmaline and garnet, and clinozoisite–epidote. Primary K-feldspar and secondary Rb-dominant feldspar show K/Rb as low as 1.9 and 0.25, respectively. Zoned muscovite-to-lepidolite crystals have K/Rb ≈ 0.09 and K/Cs ≈ 0.02 in the outer Li-rich parts. Spessartine, lithiophilite, elbaite, and (Nb,Ta) oxide minerals are almost devoid of Fe; Nb is locally extremely depleted relative to Ta. Zircon contains ~32.1 wt.% HfO2, but wodginite is the main carrier of Zr and Hf. The lepidolite pegmatites underwent extensive shearing leading to mylonitization that did not, however, affect the chemical composition of any minerals except the partially recrystallized and re-equilibrated pollucite.

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