Lamproitoides of the Magnitogorsk megazone (South Urals): petrologic-geochemical features and geodynamic conditions of their formation

Abstract

The material composition and petrographic features of lamproitoide dikes of the Magnitogorsk Megazone of the Southern Urals are described. Their younger age (197–240 million years) in comparison with lamprophyres developed in the same megazone (304–308 million years) is established. It is shown that these formations are a single petrographic series (from alkaline-ultrabasic to orendites). The mineralogical features of rocks are characterized and the conclusion is made about their proximity to lamproites by mineralogical criteria, but there are also significant differences from the latter. The petrogeochemical features of rocks are studied. According to their chemistry, they are divided into three groups: ultrabasic (rare), basic (the absolute majority of the analyzed rocks) and medium (orendites). The average compositions of the selected groupsare calculated, the features of their normative composition are described and the geochemical characteristic is given. The conclusion about the similarity of the studied rocks with postcollisional igneous formations is made. A possible mechanism of the petrogenesis of lamproitoides is reconstructed. The most probable mantle source for the primary melt of lamproitoides is established as phlogopite peridotite, which underwent partial melting in the presence of a substantially aqueous fluid with a sharply subordinate role of carbon dioxide in its composition. The crystallization of the melt occurred according to the scheme corresponding to the experimentally established phase relationships in the presence of an aqueous fluid. The geodynamic position of dikes was clarified and the described rocks were compared with “classical” lamproites of different regions. Their similarity to the lamproites of Aldan and Spain, the formation of which is associated with ancient subduction zones, is shown. Taking into account the geological situation of lamproitoides, we propose distinguishing them into a “postcollisional” subtype. The conducted research allows us to confirm the earlier conclusion that the lamproitoides of the Magnitogorsk Megazone are not the primary sources of Ural diamonds.