Origin and evolution of peraluminous granite and rare-element pegmatite in the Dryden Area, Superior Province of northwestern Ontario.

摘要

Rare-element pegmatites in the Superior Province of northwestern Ontario occur at the boundaries between high grade gneissic belts (English River (ERS) and Winnipeg River (WRS) Subprovinces) and low to medium grade metavolcanic/sedimentary belts (Uchi and Wabigoon Subprovinces).;The GLG exhibits more complex mineral zoning: columbite ;A five-stage genetic model is proposed for the Dryden Pegmatite Field. Rare elements, boron fluorine were concentrated during the development of sedimentary basins in the ERS and WRS. High grade Abukuma-regional metamorphism at 2.68 Ga led to fluid-absent breakdown of muscovite and biotite producing S-type peraluminous granitic melt with concentrated rare elements and volatiles; restite unmixing and crystal-melt fractionation in the GLB led to vertical compositional zonation characterized by repletion of Na;Fluxing components (e.g B;The Dryden pegmatite field has been studied by geological mapping (1:63,360 to 1:60), petrography and mineral and rock geochemistry. It comprises two distinct rare-element pegmatite clusters, the Mavis Lake Group (MLG) and Gullwing-Tot Lakes Group (GTC). The MLG is genetically linked with the peraluminous Ghost Lake Batholith (GLB), a late-tectonic, composite plutonic complex emplaced discordantly to regional metamorphic zones. The eastern lobe of the GLB contains sporadic beryl, columbite and cassiterite. The adjacent MLG reveals systematic rare-element mineral zoning (beryl ;Rare element concentration was increased by late- to post-magmatic albitization and greisenization. This resulted in muscovitization of a Li-F-Fe-enriched biotite and primary muscovite which allowed release of Li into an exsolved fluid phase enriched in Li, Rb, Cs, B and F. A holmquistite alteration zone in mafic metavolcanics may represent the missing fractionation link between the GLB (198 ppm Li) and the nearest spodumene pegmatite (8500 ppm).;The post-magmatic stage of rare-element pegmatite evolution led to the development of aureoles characterized by elevated concentrations of rare elements, F and B.