The application of glacial dispersal models to the interpretation of till geochemistry in Labrador, Canada

Abstract

In western Labrador, Canada, bedrock composition and glacial history are both reflected in till geochemistry. There, compositional contrasts between igneous and metamorphic bedrock of the Canadian Shield and comparatively unmetamorphosed bedrock of the Labrador Trough allow regional-scale (100s of km) definition of glacial dispersal trends through lithological and geochemical analyses. Compared to till derived from shield terrane, Labrador Trough till is depleted in chromium, but enriched in zinc, iron, and manganese. Further, the study area is situated near one or more dispersal centers of the Laurentide Ice Sheet and has been subjected to a complex sequence of ice flow, defined by striations and indicator erratic distributions. Three glacial events serve as the principal controls on drift composition, although their relative significance varies. Compositional profiles along the axis of two glacial dispersal trends show systematic changes in indicator erratic concentrations with distance along the path of ice flow. In the profile having one prominent glacial transport direction, indicator concentrations decrease exponentially with increasing distance of glacial transport. From dispersal models, Renewal Distances representing the compositional `half-life' tend to increase with decrease in grain size, i.e. 15 to 25 km for pebbles, and 30 to 40 km, or more, in silt- and finer-sized fractions. The differences between geochemical and lithological profiles reflect differences in: (1) the intensity or duration of subglacial process affecting the milling of indicator detritus to finer fractions; and (2) the physical and chemical properties of the rocks and minerals hosting the trace metals. Where regional glacial dispersal trains extend across contrasting geological terranes, indicator properties can be used to resolve till composition into `local' and `transported' components, providing a better approximation of bedrock composition for mineral exploration. The compositional expression of bedrock can be masked by transported overburden, with the extent of masking dependent on the trace metal examined. Linear relations between metals (Cr, Zn) and lithophile elements (e.g. K, Mg) indicate that Mg-bearing minerals, likely phyllosilicates, have a strong mineralogical control on till geochemistry, and metal : lithophile element ratios can be used to distinguish bedrock provenance in glacially transported materials of western Labrador.