Lamprophyres of the Alexandrinsky ore area (Southern Urals): petrogenesis and geodynamic conditions of formation

Abstract

The material composition, petrographic and mineralogical features of dyke lamprophyres in the sulfide-bearing Alexandrinsky ore area of the Southern Urals are described. The previously published isotope-geochronological data on lamprophyres of the Southern Urals are summarized, showing more ancient (304-308 million years) their age compared with lamproitoids in the Magnitogorsk iron ore area developed (197-240 million years). It is shown that the rocks are represented by a continuous petrogenetic series: monchikites – (camptonites, sannaites) – minettes – kersantites. Intensive carbonatization of most rock varieties was pointed out. It is concluded that the compositions of primary magmatic clinopyroxenes that form two distinct generations evolve according to two trends, each of them corresponds to its own generation. The composition of the first generation varies in the range from Ca40Mg53Fe7 to Ca48Mg45Fe7, the composition of the second generation-from Ca47Mg50Fe3 to Ca46Mg35Fe19. Petrogeochemical features of rocks are studied. According to their chemistry, they are divided into three groups: ultrabasic (monchikites, rare), basic (most of the analyzed rocks) and medium (kersantites). The average compositions of the selected groups are counted, the features of their normative composition are described, and the geochemical characteristics are given. It is concluded that they differ significantly from intraplate structures and have a certain similarity with collision-type rock formations. A possible mechanism of lamprophyre petrogenesis is reconstructed. It is confirmed that the primary lamprophyric melt was a product of partial melting of a metasomatically enriched mantle substrate represented by phlogopite-bearing harzburgites or lherzolites. Crystallization of the melt at the initial stages occurred according to the scheme corresponding to the experimentally established phase relationships in the presence of an aqueous fluid, and at the later stages – in the presence of an aqueous-carbon dioxide fluid. Change of composition of the fluid component due to the delay of the melt in the transitional chamber , it leads to a drastic change in phase system configuration due to sudden expansion of orthopyroxene crystallization field and termination of crystallization of phlogopite and accessory oxide phases. The geodynamic formation environment of dikes is clarified and their geochemical similarity to "Autonomous" lamprophyres of collision regions is revealed. It is proposed to establish a new lamprophyre complex.

Keywords:

lamprophyre, Southern Urals, petrogenesis, geodynamic setting