Geochemistry of trachybasalt-trachyandesibasalt-phonolite formation rocks of the Talysh zone

This article examines the geochemistry of rocks in the Talysh zone. It was determined that the structural folds that make up the structural zones of Talysh are composed of Paleogene and Neogene sedimentary and volcanic formations. Two complexes are distinguished within the volcanic formation: early-middle Eocene absarokite-shoshonite-alkaline basalt and late Eocene trachyandesibasalt–phonolitic. Petrographic characteristics of these complexes have been studied. The phenocrysts of several minerals and their distribution within mineral parageneses have not been fully investigated. At the same time, the typomorphic features of trace elements in the evolution of subalkaline olivine basalt melt have not been studied. This article attempts to address this gap. In all the samples collected for this purpose, iron (Cr, Ni, Co, Ti, V), metallogenic (Cu, Zn), alkaline (Rb, Cs), alkaline earth (Ba, Sr), radioactive (U, Th), and other (Nb, Ta, Y, Hf) elements were analyzed by X-ray fluorescence, atomic absorption, and colorimetric methods. The analyses were performed in the laboratories of the Institutes of Geochemistry and Analytical Chemistry, Mineralogy, Geochemistry, and Crystallochemistry of Rare Elements of the Russian Federation. Specific standards were used to ensure the accuracy of the analyses. It was observed that iron-group elements predominate among the elements described. It has been established that, according to petrochemical composition, the rocks of the studied formation correspond to the differentiates of the subalkaline and alkaline series. The rocks of this formation are characterized by normative nepheline, leucite, olivine, and clinopyroxene. In this regard, the rocks belong to the nepheline–olivine normative type. The initial melt was enriched in alkaline and subalkaline (K, Rb, Na) and alkaline-earth (Sr, Ba) elements. The distribution of rare-earth elements in the rocks of the trachybasalt–trachyandesibasalt-phonolite formation shows that the initial subalkaline olivine basalt melt underwent a process of crystallization differentiation under the conditions of the earth’s crust. As a result, under high thermobaric conditions, olivine, clinopyroxene, phlogopite, spinel, and other minerals that are carriers of heavy rare-earth elements crystallized and separated from the melt, and therefore these elements became depleted in the rocks of the formation. The subalkaline olivine basalt melt was able to enter into mutual thermal contact with the earth’s crust. As a result, the concentrations of barium, potassium, and rubidium among lithophile elements in the Astara uplift area partially increased. Due to the sharp decrease in heavy rare-earth elements, light rare-earth elements became significantly enriched.