Late Paleozoic Age and Nature of Protolith of Metamorphic Rocks of the Djagdagle Formation, Bureya Terrane, Central Asian Fold Belt

Abstract

The results of geological, geochemical, Sm–Nd isotopic geochemical studies of metamorphic rocks of the Djagdagle Formation and U–Pb geochronological studies of its zircons are presented. In correlation schemes of geological complexes of the Bureya Terrane, this formation is the oldest (conditionally, Late Archean) amid other formations of the terrane; thus, it is considered its basement. The biotite and two-mica garnet-bearing gneisses of the formation formed owing to transformation of primary sedimentary rocks. The maxima on the relative probability curve of zircon age correspond to 487, 541, 690, 778, and 896 Ma. Thus, the Djagdagle Formation is Paleozoic rather than Early Precambrian, as was considered before. The lower age boundary of the accumulation of the protolith is identified by the age of the youngest zircon population of ~487 Ma. Similar ages of some cores and rims of zircons indicate that the rocks of the protolith underwent structural metamorphic transformations under PT conditions of amphibolite facies. The results of Sm–Nd and Lu–Hf isotopic studies indicate that the geological complexes which originated by means of reworking of the Mesoproterozoic crust could have been the source of primary sedimentary rocks. Taking into account the lack of Mesoproterozoic (and older) rocks in the structure of the Bureya Terrane, it can be suggested that the sources included some amount of ancient recycled material. Similar geological evolution of the Bureya and Jiamusi terranes is suggested on the basis of similar age ranges of metasedimentary rocks of the Djagdagle Formation and Turan Group of the Bureya Terrane and the Kimkan Formation of the Jiamusi Terrane. This is also evident from their synchronous Neoproterozoic magmatism.