THE ROLE OF MAGMATIC AND HYDROTHERMAL PROCESSES IN THE CHEMICAL EVOLUTION OF THE STRANGE LAKE PLUTONIC COMPLEX, QUEBEC-LABRADOR

The Strange Lake plutonic complex consists of three annular Mid-Proter ozoic arfvedsonite-aegirine-bearing alkali granites emplaced in the Ra e province of the Canadian Shield. The mineralogy, chemistry and struc tural setting of the complex are very similar to that of many peralkal ine central salic complexes associated with the development of the Gar dar rift in southern Greenland. The Strange Lake granites are highly f ractionated (Rb/Sr = 5 to 160 and K/Rb = 27 to 120) and carry unusuall y high abundances of HFSE and REE-bearing exotic minerals (e.g. pyroch lore, gittinsite, elpidite, gadolinite and kainosite) which are reflec ted in the elevated HFSE (e.g. Zr = 307 to 16800 ppm) and REE (e.g. La = 84 to 1337 ppm) contents of the granites. HFSE and REE increase fro m the oldest intrusive unit, which is hypersolvus and unaltered, to th e youngest, which is subsolvus and metasomatized. The unaltered granit es display a restricted range of deltaO-18 values (+ 8.2 to + 9.6 part s per thousand) and low deltaO-18 signatures for fresh arfvedsonite/ae girine (+ 4.8 to + 5.2 parts per thousand). Anomalously high CaO (0.7 to 3.2 wt%) and MgO (0.1 to 0.6 wt%) concentrations characterize the a ltered subsolvus granites. These rocks also have elevated whole rock d eltaO-18 values (+ 9.6 to + 11.9 parts per thousand), negative DELTAde ltaO-18quartz-alk.feld. (- 0.1 to - 1.6), and high deltaO-18 values of altered arfvedsonite (i.e. + 6.5 to 13.75 parts per thousand) that co rrelates positively with whole rock deltaO-18 values. The chemical and isotopic data are consistent with a model in which the least evolved alkali granites are formed through differentiation from trachytic (sye nitic) parents. Extreme HFSE and REE-enrichment may have been accompli shed by differentiation through fractional crystallization and heterog enous distribution of F-rich silicic residual melts in which the REE a nd HFSE are transported as fluorocomplexes. The O-isotopic values are consistent with the circulation of low temperature (< 200-degrees-C) h ydrothermal fluids in the youngest subsolvus intrusive unit which caus ed extensive Ca (Mg and Sr) metasomatism and fluorine leaching, widesp read hematization, and remobilization of the HFSE and REE.