High-REE Gabbroids and Hornblendites of the Ilmeny Mountains (Urals)

Abstract

Abstract —Chaotically localized isolated small bodies of metaultrabasic rocks have been found in the quartzite-schist strata of the Ilmeny metamorphic complex in the South Urals. These are metamorphosed rootless blocks and lumps of serpentinite melange within the so-called Urazbaevo olistostrome. Sometimes they contain lumpy inclusions of massive anorthite gabbroids with gabbro, ophitic, and cumulative textures, free of crystallization schistosity, and of different mineral compositions. The rocks have abnormally high contents of Al2O3, CaO, MgO, and REE and low contents of SiO2 and are characterized by weak secondary alteration. Seldom, inclusions of hornblendites, along with anorthite, spinel, apatite, enstatite, diopside, and rutile, are present. Some gabbroid and hornblendite bodies have abnormally high contents of REE, with a strong predominance of LREE (81–93% of the total REE). The maximum contents of REE have been established in zoisite amphibolites (170–850 ppm) and apatite–garnet hornblendites (up to 450 ppm). The conclusion has been drawn that the rocks formed in the basement of the Earth’s crust and got with protrusions of serpentinite melange to the surface.