The rare element Little Nahanni Pegmatite Group, NWT : studies of emplacement, and magmatic evolution from geochemical and Li isotopic evidence

Abstract

Rare element pegmatites represent some of the last stages of igneous differentiation and are influential in element redistribution in the upper crust, leading to significant enrichment/depletion of various trace elements. Research into the processes that form these intrusions increases our understanding of the geochemical evolution of silicate earth and improves the potential for successful pegmatite exploration. This study focussed on the dikes comprising the rare element Little Nahanni Pegmatite Group (LNPG), Mackenzie Mountains, northern Canadian Cordillera. These peraluminous dikes have high concentrations of several rare elements, e.g., Li (up to 14,000 ppm), Cs (up to 500 ppm), Ta (up to 190 ppm), and Rb (up to 7,500 ppm). Orientation of the dikes was influenced during emplacement (2-3 kbar, ~400-500 °C) at ~90 Ma (apatite, U-Pb) by pre-existing foliation in the strongly deformed, stratified host rock of the Fork anticlinorium (axial planar cleavage and bedding). Differences in ⁴⁰Ar/³⁹Ar dates on pegmatite minerals (muscovite 77.1±3.6 and ~80 Ma and lepidolite 65.8±0.8 Ma) indicate the presence of an elevated paleogeothermal gradient (~60°C/km). Structural and contact metamorphic evidence identify a local heat source within the anticlinorium that may have been the source chamber for the dikes. Whole rock trace element concentrations and ratios, mineralogical and textural variations, and fractionation of Li isotopic ratios (δ7Li = -0.94‰ to +11.36‰) record a range of magmatic fractionation. Approximately 85% of the dikes are spodumene-rich, with discontinuous REEN patterns and low degrees of Li isotope fractionation, the remaining ~15% show greater magmatic fractionation, with little spodumene, and have flat or listric REEN patterns and strongly fractionated Li isotopic ratios. The replication of the REEN patterns by P and F saturation (mineral precipitation and fluid separation), illustrates the influence of flux components on the composition of late stage melts. The Li isotope composition of rapidly crystallized, co-precipitated mineral assemblages appears to show the retention of a kinetic isotopic fractionation signature; providing a potential method to assess the chemical equilibrium of the system. This integrated study advances our understanding of rare element pegmatite formation in several aspects, in particular the role of fluxes in their geochemical evolution.