Petrography of the Mesozoic Alkaline Rocks of the Taezhniy Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

Abstract

The article is concerned with the petrographie compositions of the Mesozoic alkaline igneous rocks of the Taezhniy massif and its small bodies (dikes). The Taezhniy massif is located in the central part of the Nimnyr block (Aldan-Stanovoy shield). According to our field observations, it was first determined that the Taezhniy massif has a two-phase structure, its rocks of contain syenite-porphyry and leucocratic syenites. The rocks of the massif intrude the Archean granites, and are themselves intruded by late dikes of bostonites and vogesites. Based on petrographic studies, we also identified two phases of intrusion in the Taezhniy massif – syenite-porphyry and leucocratic syenites. The difference of them is as follows: in the syenite-porphyry rocks, plagioclase predominates over K-feldspar, and the content of dark-colored minerals reaches 10%. The rocks of the second phase are characterized by the predominance of K-feldspar over plagioclase, with the content of dark-colored minerals up to 5%. The order of intrusion of the massif formations is determined by the presence of xenoliths of syenite-porphyry rocks in leucocratic syenites. The rocks of the bostonite dike cutting the massif are characterized by the absence of amphibole and an increased content of pyroxene. This is most likely due to the fact that, the rock contains xenoliths of the host rocks of biotite-pyroxene composition. The dikes also conyain xenoliths of the second phase rocks, which indicates a later age of this dike. The bostonite dike located in the immediate vicinity of the massif is almost identical to the dike found in the massif, except an amphibole in the composition of the rocks. The only dike of vogesites that intersects the body of the massif is characterized by the presence of two varieties of amphibole: common hornblende and a sufficient amount of barkevikite, which phenocrysts stand out clearly against the background of the total microcrystalline mass. Also, in the course of crystal-optical studies, it was noted that in the structure of the massif there is an increase in the leucocratic magmatism from the early phase to the late, but with a decrease in this indicator, taking into account the introduction of vogesite dikes at the late stages of magmatism development. Based on this, the conclusion is made about the antidromic development of the rocks of the Taezhniy massif. When comparing the evolution of magmatism of the Taezhniy massif with the Ryabinoviy massif, a possible gold-ore specialization of the studied object is suggested.