NATURE AND ORIGIN OF AN LCT-SUITE PEGMATITE WITH LATE-STAGE SODIUM ENRICHMENT, BRAZIL LAKE, YARMOUTH COUNTY, NOVA SCOTIA. II. IMPLICATIONS OF STABLE ISOTOPES ( 18O, D) FOR MAGMA SOURCE, INTERNAL CRYSTALLIZATION AND NATURE OF SODIUM METASOMATISM

Abstract

The 395 Ma Brazil Lake granitic pegmatite, a rare occurrence of an LCT-type (Li–Cs–Ta) pegmatite in the Meguma Terrane of Nova Scotia, Canada, is hosted by metasedimentary and metavolcanic rocks of the Silurian White Rock Group. Previous mineralogical studies showed the pegmatite formed in two distinct stages, an early spodumene – K-feldspar – quartz ± muscovite stage that was followed by a pervasive metasomatic stage in which albite and secondary muscovite grew at the expense of earlier K-feldspar. The economically important oxide phases (Ta–Nb) are intimately associated with secondary albite. Determination of the stable isotopic (δ18O, D) values for silicate phases (quartz, K-feldspar, spodumene, albite, muscovite, garnet, tourmaline) provide insight into the origin of the pegmatite-forming melt and its subsequent internal evolution. A primary δ18O signature for the melt of +8‰ is calculated using the δ18Ovalues of quartz and Δquartz – whole-rock values from studies on other granitic suites. Interpreted in the context of the geological setting, the δ18O data preclude a dominantly metasedimentary source and suggest, instead, that the pegmatite-forming melt originated via partial melting of a mixed igneous ( i.e., volcanic)- sedimentary source. The calculated δ18OH2O–δDH2O values, at 500°C, correspond to the field for magmatic waters, but some isotopic data, including δD for fluid-inclusion extracts, suggest late-stage incursion of metamorphic water that had equilibrated with the surrounding metasedimentary rocks. Mineral–mineral fractionation ( e.g., ΔQtz–Ms) does not reflect equilibrium within the pegmatite, which may have resulted from several processes, including non-equilibrium crystallization, variable degrees of isotopic exchange during protracted cooling, or interaction with an external reservoir(s). The data for albite (δ18O = +9.1‰, n = 6) are consistent with a magmatic origin, albeit late in the evolution of the pegmatite, on the basis of textural arguments. A model is proposed for the origin of the extensive albite-enriched zones, which involves introduction of a sodic melt derived at depth after protracted fractionation of the same parental melt from which the Li-rich parts of the pegmatite originated.