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.

中文翻译：

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Ulutau地块（哈萨克斯坦中部）的晚前寒武纪变质复合体：原生岩的年龄，组成和形成背景

摘要—

这项工作提出了对前寒武纪乌鲁陶断块（哈萨克斯坦中部）的Bekturgan和Balazhezdy组变质岩研究的结果，这些变质岩传统上被认为是最古老的早寒武纪复合体，包括该断层的基底。变质岩的原岩以玄武岩和流纹岩的熔岩和凝灰岩为代表，流纹闪石成分，砂岩和凝灰质砂岩构成了对比鲜明的火山-沉积序列。获得的U‒Th-Pb锆石年龄（LA-ICP MS，SHRIMP II）表明，该序列的形成发生在Tonian（新生代）后半期的狭窄（762-788 Ma）间隔内。岩石的变质转变在Karsakpai复合体（673±2 Ma）的碱性正长岩侵入之前完成，没有变质。变质岩的全岩Nd同位素组成表明其原石的起源是由于Ulutau断层的前寒武纪大陆壳更古老的复合物的侵蚀和部分融化。锆石的f同位素组成表明，乌鲁陶陆壳的形成和转变的主要阶段发生在新古宙—新元古代，并且与中亚造山带西部的前寒武纪岩浆作用的主要阶段一致。