Late Precambrian Metamorphic Complexes of the Ulutau Massif (Central Kazakhstan): Age, Composition, and Formation Settings of Protoliths

Abstract

This work presents the results of a study of metamorphic rocks of the Bekturgan and Balazhezdy groups of the Precambrian Ulutau massif (Central Kazakhstan), which have been traditionally identified with the most ancient Early Precambrian complexes, comprising the massif’s basement. The protoliths of the metamorphic rocks, represented by lavas and tuffs of basalt and rhyolite–trachyrhyolite composition, sandstones, and tuffaceous sandstones, comprise a contrasted volcanic-sedimentary sequence. The obtained U‒Th–Pb zircon ages (LA–ICP MS, SHRIMP II) demonstrate that the formation of this sequence occurred during a narrow (762–788 Ma) interval during the second half of Tonian (Neoproterozoic). The metamorphic transformations of the rocks were completed before an intrusion of the alkaline syenites of the Karsakpai complex (673 ± 2 Ma), which were not subject to metamorphism. The whole-rock Nd isotopic compositions of the metamorphic rocks indicate the origin of their protoliths due to erosion and partial melting of the more ancient complexes of the Precambrian continental crust of the Ulutau massif. The Hf isotopic compositions of zircons indicate that the main stages of formation and transformation of the Ulutau continental crust occurred during the Neoarchean–Neoproterozoic and coincided with the main stages of the Precambrian magmatism in the western part of the Central Asian Orogenic Belt.