Middle Paleozoic Basites of the Nakyn Kimberlite Field

Abstract Within the Nakyn kimberlite field, kimberlites with a unique diamond tenor are spatially associated with different-aged basites of various composition and explosive breccias. The paper presents a detailed analysis of the specific material composition of the basites, the sequence of their formation, the relationship with kimberlites, and the age of formation of magmatites in the region. Intrusions of the Vilyui paleorift were formed first 368.5 - 374.4 myr ago (Tomshin et al. 2018). The most common are basites formed during prevailing extension of magma-controlling faults within the rift structures. These are typical basites of the Middle Paleozoic paleorifts of the eastern part of the Siberian platform. They are composed mainly of prismatic-ophitic gabbro-dolerites. According to the total content of potassium and sodium, compositions of dolerites vary within the limits of the tholeiitic and subalkaline series. Paleomagnetic data indicate that the extension mode of rift valleys was repeatedly replaced by compression conditions. This resulted in the closure of magma permeable zones and formation of local conditions for the origin of deep-seated intermediate chambers. Fractionation of a tholeiitic melt at depths of 35–40 km promoted protocrystallization of basic plagioclase and the formation of horizons and lenses of anorthositic gabbro-dolerites. Bytownite, chrysolite, moissanite, garnet, and chrome spinel are identified as the early magmatic minerals. Next manifestation of the deep evolution of the basite melt is represented by a complexly differentiated two-phase dike, which includes monzonite-porphyry. Such bodies are formed when the processes of crystallization differentiation of the tholeiite melt in a shallow-depth (no more than 20 km) intermediate chamber are combined with those in intra-chamber conditions. The dike is located in a kimberlite pipe and divides it into two bodies. At the turn of Devonian and Carboniferous (363-364 Ma), kimberlite bodies were formed in the Nakyn kimberlite field. The kimberlite-controlling faults (338.2-345.6 Ma) were pathways for ascending magma that produced post-kimberlite basites near the kimberlite pipes. High contents of potassium and titanium and low SiO2 values are distinctive features of post-kimberlite basites. Formation of the bodies of this group of basites was followed by eruptive activity with the formation of explosive breccias. This is a characteristic feature of the intrusions of post-kimberlite alkaline magmatites. Most of the intrusions are composed of fine-grained almond-shaped rocks with slightly crystallized glassy mesostasis. Magmatic activity completed within the Nakyn kimberlite field 331-324,9 myr ago with the formation of explosive breccias after bodies of alkaline basites and their host sedimentary rocks. The disintegration of rocks was followed by the manifestation of active Mg-K-metasomatism. Alkaline-type basites together with accompanying explosive breccias are good indicators of fluid-permeable zones, thus locating the kimberlite-controlling structure. Their temporal and spatial proximity to kimberlites makes it possible to use intrusions of alkaline basites as an indicator of primary diamond deposits.