REE Distribution in Corundum-Bearing and Other Metasomatic Rocks during the Exhumation of Metamorphic Rocks of the Belomorian Belt of the Baltic Shield

摘要

The paper reports data on the distribution of REE in metasomatic rocks,including those with corundum,that were formed during the exhumation of the rocks of the Belomorian Belt at 1.9-1.75 Ga.This process is thought to have occurred concurrently with the horizontal extension and tectonic denudation of the upper crust,which,in turn,induced the massive release of fluids.The latter formed two major groups of silicic metasomatic rocks.The deepest-sitting corundum-bearing and other mafic metasomatic rocks are enriched in REE,alkalis,and alumina compared to the host rocks.The coeval acid metasomatic rocks such as orthotectites are,conversely,depleted in REE (with positive Eu anomalies),mafic elements,and HFSE but are also enriched in alumina.These complimentary rocks are thought to have been produced early during the exhumation of deep rocks under the effect of reduced fluids whose genesis was related to decompression.The silicic metasomatic rocks of the second group (with muscovite) have elevated REE concentrations (with negative Eu anomalies) and were formed by already oxidized fluids at shallower depths.The fact that the corundum-bearing metasomatic rocks are enriched in REE (in spite of the ultrabasic composition of these rocks) suggests that they were generated in an extensional environment with the participation of deep fluids,which enriched the rocks in Al,Na,K,Ba,Sr,Zr,and LREE.