Geochemistry, zircon U-Pb and Lu-Hf systematics of high-grade metasedimentary sequences from the South Muya block (northeastern Central Asian Orogenic Belt): Reconnaissance of polymetamorphism and accretion of Neoproterozoic exotic blocks in southern Siberia

Abstract

High-grade metasedimentary rocks of the South Muya block (Baikal-Muya Foldbelt) have been studied with respect to whole-rock elemental and Nd isotopic compositions, as well as the U-Pb and Lu-Hf systematics of detrital zircon and in-situ grown metamorphic grains in order to reveal the initial source provenance for sedimentation and the age of major metamorphic modification. Based predominantly on trace-elemental compositions, the sedimentary protolith is most likely a poorly sorted and immature greywacke, such as those formed in island arc or active continental margin settings. The island arc-related chemical signature coupled with zircon U-Pb age data supports a Neoproterozoic (>760 Ma) sedimentation age, whereas Nd-Hf isotope data of precursor sediments are linked to a relatively immature Tonian (∼940–780 Ma) continental arc involving Paleoproterozoic crustal basement. The limited provenance information and metamorphic history of the Muya block suggests it was exotic to Siberia and originally related to another continent, e.g. – subduction-accretion complexes of South China. The polyphase metasediments experienced high-grade metamorphism in the Tonian during the 764–754 Ma arc accretion to an enigmatic continental block and then in the Ediacaran during the 622–608 Ma accretion of the Baikal-Muya belt to the arc situated at the southern Siberia margin. Metamorphic overprinting during the introduction of supracrustal rocks to deeper-crustal levels led to protracted recrystallization of primary igneous zircon population and growth of new granulite-facies grains that did not involve notable juvenile input. Tonian sedimentary provenance and two metamorphic events suggest a protracted evolution for the Baikal-Muya belt associated with the formation of precursor complexes beyond the southern margin of Siberia and their later accretion to Late Cryogenian – Ediacarian arc complexes of the southern Siberian, which proceeded only after the opening of the Paleoasian Ocean (670–630 Ma).