Mafic alkaline ring Yllymakh massif , Aldan shield, Yuakutia, geochemistry and geochronology

The Yllymakh massif belongs to a number of ring massifs of the Mesozoic age typical for the Aldan shield. Being formed in an intraplate environment, the Yllymakh massif is characterized by specific features of intraplate rocks in general and a number of coeval intrusions of the Aldan magmatic province in particular. These include a potassium-sodium slope, an extremely low eNd value (-13--14), a specific distribution of rare-earth elements with almost or with a weakly developed Eu minimum.

The range of rocks composing the Yllymakh massif is very wide. It consists of up to 20 species of rocks. The most melanocratic are olivine schonkinites. The most widespread and diverse are the rocks average in SiO2 content. These include feldsparless syenites, feldspar syenites with nepheline, feldspar syenites with quartz, syenites with phoid (nepheline or leucite) in various quantities, alkaline granites. Of course, the question arises of the processes that led to such diversity.

Previous geochronological studies of the Ar-Ar method [Vasyukova et al, 2020] three stages of the massif formation were determined: 140 ± 1.9 Ma, 130 ± 1.9 - 131 ± 2.4 Ma and 125 ± 1.9 Ma. And geochemical studies showed that the Yllymakh massif was formed in several stages due to the pulsed introduction of successive portions of magma.

The analysis of petrochemical and geochemical diagrams showed the impossibility of the formation of the rock spectrum by fractionation of the melt. The critical fractionating phases were different: pyroxene in first and plagioclase in the second group. So they gave different trends in the coordinates CaO-, Na2O-, Al2O3-SiO2 and MgO-, Fe2O3-SiO2. apatite also plays an important role in the formation of the spectrum of rocks, as can be seen in P2O5-SiO2. However, it is not a rock-forming mineral, but it is a good marker of the fluid and geochemical conditions of the melt.

The isotopic composition of oxygen showed the predominance of mantle material in the source. The Nd and Sr isotopic data show that the rocks of the Yllymakh massif were formed in an enriched source. Sharply negative values of the eNd of the studied rocks fit into the overall picture of the region – similar characteristics are determined for other objects of similar age (Inagli, Ryabinovy, etc.).

Supported by RFBR grant 19-05-00788

Vasyukova E.A., Ponomarchuk V.A, Doroshkevich A.G. Petrology and age boundaries of Yllymakh massif. Russian Geology and Geophysics, 2020 in press