Home – Home – Jornals – Russian Geology and Geophysics 2004 number 9

In this paper, we report the chemical composition and contents of REE, Cr, and Ni of amphibolites making up olistoliths in aposedimentary matrix and exotic blocks in metaultrabasic rocks. The latter are large fragments of ancient metamorphosed serpentinite melange, which form chaotic accumulations (olistostrome) in the Riphean-Vendian quartzite-schist strata of the Ilmeny complex in the Urals. The amphibolites are extremely depleted in SiO₂ (<40-45%), rich in Al₂O₃ (>17-22%) and CaO (9-20%), and have noticeable amounts of alkalies (Na₂O = 0.6-2%, K₂O = 0.1-0.7%) and extremely high contents of SrO in the hosted plagioclases. In these features the amphibolites have no chemical analogs among the known igneous rocks. Moreover, they bear finest zircon grains and have abnormally high contents of REE (REE = 40-1246 ppm) and elevated contents of Ni and Cr (30-225 and 35-655 ppm, respectively). Such amphibolites are lacking in the other strata of the Riphean-Vendian Ilmeny metamorphic complex. The specific composition of the studied amphibolites might be explained by the fact that their protolith was the disintegration products of ancient weathering crust developed over basic and ultrabasic rocks.