Zircon age of metarhyodacite of the aleksandrovsk suite of the mykhailivka series (megablock KMA)

Abstract

The LA-ICP-MS U-Pb method has been used to define the Palaeoarchaean (3264 ± 7 Ma) age of zircon from metarhyodacite of the Aleksandrovsk Suite of the Mykhailivka Series (the southeastern part of the Tim-Yastrebov structure of the Kursk Magnetic Anomaly (KMA)). According to the geochemical data, the primary melt probably developed in a crustal magmatic source due to melting of older felsic rocks. The metarhyodacite closely resemble in composition the felsic, igneous A-type granites, formed in continental rift or hot spot settings. The studied metarhyodacite of the Aleksandrovsk Suite was probably formed in association with plume emplacement into the tonalitic basement during the initial stage in the development of greenstone belts in the KMA megablock. It has high potassium content (up to 6.20 weight %), high XFe ratio (up to 0.67), low Sr content, and high content of HFSE and REEs. They are enriched in light REEs, while heavy REEs are undifferentiated. The metarhyolite shows a highly negative Eu anomaly (Eu/Eu* = 0.5). In contrast, the younger, Mesoarchean (ca. 3.12 Ga) felsic metavolcanic rocks of the Dichnya Suite of the greenstone belts of the KMA, which are close in their chemical composition to the TTGs, were generated via partial melting of mafic rocks at the vanishing stage of the mantle plume.