Geochronology of the alkaline-ultra-basic carbonatite complex arbarastakh (aldan shield, yakutia): new ar-ar and u-pb data

This paper is devoted to a new geochronological data for the main rocks that make up the Arbarastakh alkaline ultramafic carbonatite complex, and includes information on the mineralogical and petrographic features of these rocks. The Arbarastakh complex is located in the Republic of Sakha (Yakutia) in the southeast of Russia, within the basin of the Arbarastakh River, in the Lena River basin. Tectonically, the complex is located in the eastern part of the Aldan shield on the southeastern margin of the Siberian craton. The main structural feature of the Arbarastakh complex is its concentric-zonal structure, expressed in the regular alternation of rocks with different compositions and different occurrence conditions. The main phase of the complex composes a stock-like body of alkaline pyroxenites, which has a rounded-ellipsoid shape in plan view. Between the body of pyroxene rocks and the ring fault there is a continuous zone of fenites - contact-metasomatic rocks. Dikes of alkaline syenites, ijolite-urtites and melteigites, are found in the area of the alkaline complex. Dikes of alkaline syenites are localized in the endocontact zone of the complex. The carbonatites of the Arbarastakh complex are concentrated within the core of pyroxene rocks and occur as a series of incompletely conical dike-like bodies. The most common are calcite varieties of carbonatites with pyrochlore and phlogopite, as well as silicocarbonatites. Veins of dolomitic and ankeritic carbonatites occur much less frequently in pyroxenites. Geochronological studies of alkaline rocks of the Arbarastakh alkaline-ultramafic carbonatite complex reflect a rather long history of the formation of the complex and characterize the multi-stage intrusion of rocks. The results of Ar-Ar dating showed the time of crystallization of the main phase of the complex - pyroxenites - 632.5 +/- 6 Ma; alkaline syenites - 645.9 +/- 6.4 Ma. The emplacement time of silicocarbonatites is 642.6 +/- 6.6 Ma; the formation of carbonatites occurred in the ranges of 651 +/- 6 and 657.8 +/- 6.5 Ma. U-Pb dating of pyroxenites showed an age of 638.2 +/- 3.1 Ma; time of intrusion of carbonatites is 650.3 +/- 9.8 Ma; and the crystallization age of alkaline syenite is 641.7 +/- 5.6 Ma. Thus, a certain age range of formation of rocks of the Arbarastakh complex is similar to 657-636 Ma, which is within the range of values for other alkaline carbonatite complexes, which structurally gravitate towards the marginal parts of the Siberian Craton and are characterized by similar types of rare-metal deposits.