REE DISTRIBUTION IN SOME LAYERED MIGMATITES - CONSTRAINTS ON THEIR PETROGENESIS

The REE distributions in mesosomes, neosomes, leucosomes and melanosom es of four layered migmatites have been investigated. In one example ( Arvika migmatites) the REE patterns in adjacent paragneisses, the pres umed parent rock of the migmatites, were also determined. REE patterns of neosomes and mesosomes of Arvika migmatites are similar to the fin e-grained layers and coarse-grained layers, respectively, observed in the adjacent paragneiss. This is in agreement with the layer by-layer paragneiss-migmatite transformation model. The REE patterns of mesosom es and neosomes indicate that these lithologies may have been closed s ystems (for REE) during the formation of the migmatites. No indication of metasomatic reactions, melt segregation or injection could be dete cted. Within the neosomes, leucosomes are depleted and melanosomes enr iched in REE contents. This is interpreted to be due to separation and concentration of accessory minerals (monazite, epidote, allanite, zir con, sphene, apatite, garnet) into the melanosomes. The behaviour of a ccessory minerals during migmatite formation is closely allied to that of biotite, which is also concentrated in the melanosomes.