Anorthoclase, sanidine and associated megacrysts in Scottish alkali basalts: high-pressure syenitic debris from upper mantle sources?

Abstract Megacrysts of high temperature alkali feldspar are common in alkali basalt minor intrusions and vents, mainly of upper Palaeozoic age, in south, central and western Scotland. Discrete megacrysts, composite megacrysts and multicrystal feldspathic xenoliths are found. The dominant phase is anorthoclase but sodic plagioclases and sanidines also occur. Whereas individual crystals are usually homogeneous, suites of megacrysts from individual localities commonly define extended compositional ranges forming roughly linear trends within the An-Ab-Or diagram. Associated minerals occur as discrete megacrysts or as accompanying phases in composite megacrysts and xenoliths and include clinopyroxene, biotite, kaersutite, magnetite, apatite, zircon, corundum and exotic oxide minerals. The precise megacryst assemblages tend to be specific for particular localities. The alkali feldspar crystals did not crystallize from their host magmas and, in some cases, can be shown to have been in isotopic disequilibrium with them. They are, however, deduced to be fragments of high-pressure syenitic veins crystallized from trachytic magmas within the upper mantle. It is suggested that volatile fluxes, rising through the lithospheric mantle, led to enriched partial melt fractions. The latter produced biotite (ikaersutite) pyroxenite veining; more extreme differentiated fractions crystallized alkali feldspar-bearing assemblages as syenitic residues. Basanitic magmas, arising independently from sub-lithospheric sources were responsible for violent disaggregation of uppermost lithospheric wall-rocks and entrained the syenitic debris as accidental xenoliths and megacrysts.