THE ORIGIN OF RARE MINERALS IN THE KIPAWA SYENITE COMPLEX, WESTERN QUEBEC

The Kipawa Syenite Complex, a concordant, folded sheet of mildly peral kaline kataphorite-aegirine syenite less than 200 m thick, can be trac ed for more than 50 km in the Grenville Province of western Quebec. Se veral lenses of biotite-aegirine nepheline syenite up to 200 long by 3 0 m thick occur within the complex. Along its lower margin. a region 1 300 m long by 5 m thick consists of diopside and magnesiorichterite-ri ch schist with large amounts of eudialyte, agrellite and other rare mi nerals. This skarn-like lens is fringed on its upper side by alkali gr anite. The Kipawa Syenite Complex lies entirely within granitic gneiss derived from an Archean protolith and emplaced al 1247 +/- 47 Ma (Guo & Dickin 1994). The complex, emplaced about 1240 Ma ago, consists mai nly of deformed alkaline igneous rocks (plutonic, volcanic, or both) a s shown by its geochemical signature. Marble-bearing sedimentary rocks were thrust-imbricated with the complex during northwest-directed tec tonic transport at amphibolite-facies (690 degrees C, 9.5 kilobars) me tamorphic conditions. Mechanical mixing during this process, followed by alkali metasomatism and local anatectic melting, produced skarn-lik e rocks (contents of CaO and MgO of 12 and 10 wt.%) containing the rar e minerals. U-Pb dating of zircon from the rare mineral occurrence sho ws that rare mineral formation occurred at 994 +/- 2 Ma. Alkalis, Zr, REE, Be and other elements were redistributed on scales ranging from m eters to kilometers by a combination of fluid flow and anatectic melti ng in the presence of F-rich brines.