Re-Os isotopic evidence for a lower crustal origin of massif-type anorthosites

Massif-type anorthosites are large igneous complexes of Proterozoic age. Th ey are almost monomineralic, representing vast accumulations of plagioclase with subordinate pyroxene or olivine and Fe-Ti oxides-the 930-Myr-old Roga land anorthosite province in southwest Norway(1) represents one of the youn gest known expressions of such magmatism. The source of the magma and geody namic setting of massif-type anorthosites remain longstanding controversies in Precambrian geology, with no consensus existing as to the nature of the parental magmas or whether these magmas primarily originate in the Earth's mantle or crust. At present, massif-type anorthosites are believed to have crystallized from either crustally contaminated mantle-derived melts that have fractionated olivine and pyroxenes at depth(2) or primary aluminous ga bbroic to jotunitic melts derived from the lower continental crust(3). Here we report rhenium and osmium isotopic data from the Rogaland anorthosite p rovince that strongly support a lower crustal source for the parental magma s. There is no evidence of significantly older crust in southwest Scandinav ia and models invoking crustal contamination of mantle-derived magmas fail to account for the isotopic data from the Rogaland province. Initial osmium and neodymium isotopic values testify to the melting of marc source rocks in the lower crust with an age of 1,400-1,550 Myr.