IGNEOUS ALBITITE DIKES IN OROGENIC LHERZOLITES, WESTERN PYRÉNÉES, FRANCE: A POSSIBLE SOURCE FOR CORUNDUM AND ALKALI FELDSPAR XENOCRYSTS IN BASALTIC TERRANES. II. GEOCHEMICAL AND PETROGENETIC CONSIDERATIONS

Abstract

Dikes of pegmatitic albitite intrude serpentinized lherzolite at Urdacb and Espéchère, two ultramatic Alpine-type bodies from the western Pyrénées, France. The rocks exhibit mineralogical features matching those of xenoliths containing alkali feldspar ± zircon ± corundum and associated megacrysts, found in many alkaline basaltic provinces. Geochemical and isotopic data document a mantle origin for these albitites. Their Si-Al-Na-dominated bulk-composition is similar to that of certain glass inclusions observed at a microscopic scale within peridotitic xenoliths in alkali basalts. On the basis of a comparison with experimental data, these dike rocks are interpreted as products of very-low-degree (<1%) partial melting of a harzburgite source previously enriched by carbonatite-related metasomatism. Local variations in volatile components (H2O and CO2) at the source may control the relative solubility of Al2O3 and SiO2 in mantle fluids, and may well account for the crystallization of corundum in some batches of felsic magma and its absence in others. The breakup of upper mantle domains veined by such felsic pegmatites, during ascent of later-stage basic magma generated at greater depth, might well explain the presence of xenocrysts of sodic or Na-K-rich feldspar, corundum, and zircon in alkali basalt lavas or tuffs and associated alluvial deposits.