Significant Li isotope fractionation in geochemically evolved rare element-bearing pegmatites from the Little Nahanni Pegmatite Group, NWT, Canada

摘要

Lithium isotope signatures of whole rock pegmatite samples and mineral separates from the rare element-bearing Little Nahanni Pegmatite Group. NWT, and whole rock samples from nearby granitic intrusions were measured. Correlation of the Li isotopic values from the pegmatite dikes with whole rock trace element geochemistry, mineralogy and primary textural evidence reflect mechanisms of Li isotopic fractionation during pegmatite formation. The heavier δ~7Li signatures within the broad range measured from whole rock LNPG samples ( -0.94%o to -h 11.36%o) are related to the consolidation of the final ～15% melt fraction of a volatile-rich peraluminous magma in the late stages of magmatic fractionation. Rock-forming minerals (quartz, albite, spodumene and mica) display δ~7Li signatures that indicate consolidation of the dikes under variable, non-equilibrium conditions. Lithium isotope signatures of relatively cool, highly evolved peraluminous magmas reflect the build-up of fluxes (e.g.. H2O and F) and provide a qualitative assessment of the state of mineral/ melt chemical equilibrium.