Abstract

The YITT-B granitic pegmatite swarm, located in the Bird River Greenstone Belt of the Bird River Subprovince, western Superior Province of the Canadian Shield, consists of steeply dipping to subhorizontal dykes hosted by cordierite-bearing metaturbidites. Internal structure of the pegmatites is highly variable and, most significantly, is marked by mutual textural transitions and virtual paragenetic identity of zones that would normally be distinguished as layered albite-rich aplite and saccharoidal albite. The pegmatites are best classified as peraluminous, LCT-family, beryl-columbite-phosphate subtype of the rare-element class, but they are unique in many respects. Beryl and tourmaline are extremely rare, whereas Nb, Ta-oxide minerals and cassiterite are abundant. Primary phosphates and the Nb, Ta, Sn-oxide minerals are very fine-grained and dispersed in the albite-rich zones. With the exception of the phosphates, no other minerals show secondary alteration. Geochemically, the pegmatites are enriched in (Ta>Nb) and Sn, but are very poor in Li, Be, B, and F. The level of Fe-Mn fractionation is moderate, but that of Nb-Ta is very advanced. The YITT-B pegmatites have only a limited number of approximate analogs on a global scale, and they are unique within the Cat Lake-Winnipeg River pegmatite field. The diversity of pegmatite categories in this field, accentuated by the exotic features of the examined pegmatite swarm, reflects localized generation of granitic magmas from a broad spectrum of protoliths. Separate paths of ascent and internal differentiation of the parent granites compounded the effect of the heterogeneous source lithologies on the end-product pegmatites.

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