UPPER MANTLE AND LOWER CRUSTAL XENOLITHS FROM THE UPPER HYLAND RIVER REGION, SOUTHEAST YUKON TERRITORY

Xenoliths of spinel-bearing lherzolite and granulite facies paragneiss were recovered from basaltic dykes that intrude Neoproterozoic-Palaeozoic metasedimentary rocks in the upper Hyland River region of southeast Yukon Territory. The dykes are typically 1-5 metres in diameter, and commonly exhibit columnar jointing along their margins. The age of the dykes is poorly constrained, but they post-date Early Cretaceous regional metamorphism and deformation. Whole rock geochemical data indicates the dykes are nepheline-normative alkali basalts, that plot within the basanite field on a TAS diagram. The xenoliths occur as discrete subrounded to subangular fragments up to 8 cm in maximum dimension. The lherzolite samples consist of olivine (~Fo90), orthopyroxene (Mg# of 0.90), clinopyroxene (Mg# of 0.88-0.90), brown spinel (Mg# of 0.76; Cr# of 0.1), and Fe-Ni sulphide, and exhibit a protogranular texture. Orthopyroxene locally contains fine exsolution lamellae of clinopyroxene. Pressure conditions are constrained through the calculation of isochemical P-T phase diagrams in the Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-Cr₂O₃ system for spinel lherzolite samples. Estimated pressures range from 13-16 kbar (43-53 Km). Temperature estimates from 3 samples and 27 mineral pairs using two-pyroxene geothermometry give equilibrium temperature estimates between 930 – 1100˚C at 14 kbar, with a pressure dependency of less than 5˚C/kbar. Granulite facies paragneiss samples contain plagioclase, K-feldspar, orthopyroxene, garnet, quartz, graphite, monazite, and zircon. One sample also contains sillimanite and 1-8 mm rounded domains of patchy, symplectic intergrowths of plagioclase, spinel, and orthopyroxene. These domains are interpreted as pseudomorphs after garnet and consist of three distinct textural and compositional zones, which may record various stages of post-peak metamorphic decompression. Future geochronology is planned for both suites of rocks.