Restites, Eu anomalies and the lower continental crust

Abstract

Dehydration partial melting of the lower continental crust is the main process responsible for granite genesis in the post-Archean. If this melting occurs to undifferentiated mantle-derived rocks (i.e., basaltic protoliths), it should result in a granitic upper crust having a negative Eu anomaly and a mafic restitic lower crust having a positive Eu anomaly. However, most post-Archean high-pressure granulite facies terranes are not chemically complementary to granites because they themselves have bulk granitic compositions. If one restricts the discussion to mafic granulites from terranes, it is apparent that although their bulk composition is complementary to granite, most do not have positive Eu anomalies. Granite residues may, however, be present in granulite facies terranes if the parent rock giving rise to the granite had a negative Eu anomaly and a significant melt fraction were left behind with the residue. Such restitic granulites will have either no Eu anomaly or a negative one and “normal” LILE ratios due to the dominance of the incompatible element-rich melt. Granulite xenoliths do have the requisite compositions to balance the composition of the upper crust (i.e., mafic bulk composition and positive Eu anomalies); however, these samples are generally interpreted as melts or cumulates derived from underplated basaltic magmas rather than restites. Intracrustal differentiation may therefore proceed in several steps: intrusion and fractionation of basaltic magmas in the lower crust serves to 1. (1) create mafic and ultramafic cumulates (having positive Eu anomalies), 2. (2) produce evolved magmas (having negative Eu anomalies) 3. (3) melt the preexisting (evolved) crust to produce granites and evolved restitic granulites. The magmas produced by differentiation of the basalt may remain in the lower crust where they may be subsequently melted in a later underplating event or may mix with the crustal melts and intrude the upper crust.