ELEMENTAL AND O-SR-ND ISOTOPE GEOCHEMISTRY OF THE GLEN DESSARRY SYENITE, NW SCOTLAND

Elemental, radiogenic isotope (Sr, Nd) and stable isotope (O) data from samples of the 456 Ma Caledonian Glen Dessarry syenite are all compatible with a two-stage crystal-fractionation model for the evolution of the magmas, locally modified by contamination with a partial melt of the surrounding Moine metasediments. The early-crystallizing phases which controlled melasyenite-leucosyenite evolution were biotite, calcic clinopyroxene plus minor apatite and titanite, whilst in-situ crystal-liquid separation also involved alkali feldspar and amphibole with minor allanite, zirconolite and zircon. The diverse accessory mineral suite controlled significant variations in many of the 'incompatible' elements, including inordinate Zr/Hf fractionation. Crustal contamination led to severe disturbance in abundances and inter-element ratios of U, Th; Zr, Hf, Nb, Ta; heavy REEs, Sr and Ba. The elemental characteristics of the magmas are typical of shoshonitic subduction-related suites, with high LILEs, relatively low HFSEs and pronounced negative Nb-Ta anomalies. Inferred isotopic characteristics of the parent magmas (Sr-87/Sr-86 almost-equal-to 0.7040, SIGMA-Nd = + 2.5 and delta-O-18 almost-equal-to + 7 parts per thousand) are probably derived from a multicomponent source region involving a substantial input from contemporary depleted mantle.