The crustal tongue melting model and the origin of massive anorthosites

Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently linked with weakness zones in the crust which favour their emplacement at mid-crust levels. Recent experimental data have also shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but rather are produced by melting of gabbronoritic sources. In the Sveconorwegian province terrane boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths of 40-50 km. We therefore suggest that the parent magmas of anorthosite massifs are produced by melting of gabbronoritic rocks from the lower crust that had been thrust into the mantle by collision of terranes.