Southern Urals lamproites: the problems of terminology, age, and geodynamic interpretation

Abstract

We have studied mineral and chemical composition of lamproites from Kalymbaevsky Complex the Middle Trias. These lamproites were developed in Magnitogorsk and Eastern-Urals megazones of the Southern Urals. We have established the presence of olivine, phlogopite, diopside phenocrysts. We have registered the presence of globular structures, consisting of sanidine and interstitial glass. We have also shown the presence in the base of rocks microlites of aluminous diopside-augite and alkaline pyroxenes of aegirin-augite series, which previously were taken for alkaline and sub-alkaline amphiboles. We have established high sulfur concentration in the apatite which, without magmatic sulfides in the rocks, witnesses for oxidation of lamproite magmas. For the first time precise geochemical data for microelemental and isotopic Sr, Nd composition of rocks are given. It was found that the South Urals rocks have a intermediate composition between lamproites and potassium alkaline basalts. Their source was the enriched mantle with the value εNdi = +0.7-+3.9. We have shown uncertainty of geochronological data, according to which lamproite magmatism could be initiated 197-240 or 300-310 Ma