Kretz, R; Loop, J; Hartree, R
The Prosperous granite (Rb/Sr age 2520 +- 25 m.y.) occurs as several plutons in a thick succession of metamorphosed greywacke- mudstone of the Yellowknife supergroup. The average mineral content of the Sparrow pluton (in vol.%) is quartz 32, plagioclase 31, K-feldspar 24, muscovite 9, biotite 3, and apatite 1. Average trace-element concentrations (in ppm) are Li 140, Be 4, B 28, Zn 47, Rb 250, Sr 76, Zr 75 and Ba 360. The central portion of the pluton is slightly richer in K, Sr, and Ba than the margin. Li is concentrated in mica, and Be and B in muscovite and plagioclase. Countless pegmatite dykes occur in the Sparrow pluton and in schist-hornfels to the E; the outer limit is marked by the cordierite isograd, 9 km from the granite contact. Dykes vary greatly in size, mineral content, and in major- and trace-element composition. Compared to Prosperous granite, the pegmatite bodies are richer in P and Rb, and poorer in Ti, Fe, Mg, Zr and Ba. Dykes rich in tourmaline, beryl and spodumene occur in overlapping zones situated progressively farther from the centre of the Sparrow pluton. The composition of tourmaline is related to the host rock; the highest concentrations of Fe and Zn occur in crystals from pegmatite and the highest Mg and V in crystals from tourmalinized schist, while those from granite and quartz veins occupy an intermediate position. Complex compositional zoning is present in some tourmaline crystals in pegmatite. T-P estimates (500-600 degrees C) and (2-4 kbar) of granite emplacement, based on the distribution of andalusite and sillimanite in the contact rocks, suggest that the final stage of granite emplacement occurred at sub-solidus conditions. A vapour-transport model is proposed to explain the widespread distribution of pegmatite dykes and their extreme compositional variability. Some of the pegmatite constituents, including Li, Be and B, were possibly derived from Yellowknife greywacke and mudstone. [Authors' abstract]
metamorphism; petrology; chemical composition; metals; domes; Yellowknife Northwest Territories; chain silicates; ring silicates; mineral composition; Canada; Western Canada; pyroxene group; Precambrian; metagraywacke; beryl; geochemistry; tourmaline group; alkali metals; plutonic rocks; beryllium; two-mica granite; emplacement; granites; metasedimentary rocks; pegmatite; metasomatism; P-T conditions; clinopyroxene; boron; Archean; country rocks; alkaline earth metals; Mackenzie District Northwest Territories; spodumene; lithium; Yellowknife Group; metamorphic rocks; dikes; intrusions; silicates; igneous rocks; Northwest Territories