THE YITT-B PEGMATITE SWARM AT BERNIC LAKE, SOUTHEASTERN MANITOBA - A GEOCHEMICAL AND PARAGENETIC ANOMALY

The YITT-B granitic pegmatite swarm, located in the Bird River Greenst one Belt of the Bird River Subprovince, western Superior Province of t hr Canadian Shield, consists of steeply dipping to subhorizontal dykes hosted by cordierite-bearing metaturbidites. Internal structure of th e 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 art best classified as peralumino us, LCT-family, beryl - columbite - phosphate subtype of the rare-elem ent class, but they are unique in many respects. Beryl and tourmaline are extremely rare, whereas Nb,Ta-oxide minerals and cassiterite are a bundant. Primary phosphates and the Nb,Ta,Sn-oxide minerals are very f ine-grained and dispersed in the albite-rich zones. With the exception of the phosphates, no other minerals show secondary alteration. Geoch emically, 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 ar e unique within thr Cat Lake - Winnipeg River pegmatite field. Thr div ersity of pegmatite categories in this field, accentuated by the exoti c features of the examined pegmatite swarm, reflects localized generat ion of granitic magmas from a broad spectrum of protoliths. Separate p aths of ascent and internal differentiation of the parent granites com pounded the effect of the heterogeneous sourer lithologies on the end- product pegmatites.