Rare Earth Element Behaviour During Evolution and Alteration of the Dartmoor Granite, SW England

Abstract

The distribution of rare earth elements (REE) within the compositionally zoned Dartmoor pluton is used to constrain models of granite evolution and to assess the effects of pervasive hydrothermal alteration on REE mobility. The main process of magma evolution was crystal fractionation of early plagioclase, biotite, and accessory minerals (apatite, monazite, zircon, and xenotime). Concentrations of REE (particularly LREE and Eu) and other elements (Fe2O3t, MgO, CaO, TiO2, Zr, Ba, and Sr) decrease strongly with evolution of the pluton from 71 to 74% SiO2. These trends, and the inward zoning of the pluton, are compatible with differentiation by crystal fractionation at the level of emplacement, a process that gave rise to a marginal cumulate granite (CGM) modified by country rock assimilation, a body of inner granite (PM), and a late-stage evolved granite (FG) that intruded the earlier types. REE modelling of the Dartmoor granite types by fractional crystallization of REE-enriched accessory minerals from a parent PM-granite shows that the FG-granite cannot have formed from a residual liquid left by crystallizati on of the CGM-granite. Two discrete stages of crystallization occurred; side-wall cumulate CGM-granite crystallization dominated by LREE-enriched monazite fractionation followed by a late-stage mobile residual FG-granite in which fractionation was dominated by HREE-enriched apatite and zircon. Modelling supports the idea that largescale assimilation of country rock was not the dominant process during Dartmoor granite evolution. Pervasive hydrothermal alteration locally affected all Dartmoor granite types, altering primary plagioclase, biotite, apatite, monazite, and, to a lesser extent, zircon and xenotime. During pervasive sericitization, chloritization, and tourmalinization, REE were mobilized over distances of centimetres only and redistributed into the secondary alteration products seridte, chlorite, tourmaline, allanite, and sphene. Whole-rock REE abundances were not affected