Archean Enriched Mantle Beneath the Baltic Shield: Rare-Earth-Element Evidence from the Burakovsky Layered Intrusion, Southern Karelia, Russia

Abstract

In order to better understand the origin and character of late-Archean mantle beneath the Baltic Shield, we have analyzed mafic-ultramafic rocks from one of the best-preserved, least-metamorphosed regions of Karelia, Russia. Trace-element data for samples from the ultramafic and gabbronorite zones of the large (700 km2) Burakovsky layered intrusion (BLI) are presented. Samples from the ultramafic zone are LREE enriched, indicating that they formed from a LREE-enriched parental magma. Indeed, a calculated parental magma for the ultramafic zone has a (Ce/Yb)n ratio of 2.6, a (Nd/Sm)n ratio of 1.1, and a (Dy/Yb)n ratio of 1.6. The LREE enrichment in the parental magma suggests either that the source region was LREE enriched or that the melt was contaminated by crust en route to the BLI magma chamber. Samples from the gabbronorite zone also are LREE enriched and indicate two distinct parental magmas. Group-I magmas, from the lower part of the gabbronorite zone, have (Ce/Yb)n ratios of 6.9 to 13.9, whereas Gro...