Age and tectonic setting of the early Paleozoic magmatism of the Mamyn Terrane, Central Asian Orogenic Belt, Russia

Abstract

This paper presents new geochemical, U–Pb geochronological, and Sm–Nd isotopic data for early Paleozoic granitoids and acidic volcanic rocks within the Mamyn Terrane that constrain the early Paleozoic tectonic evolution of the eastern central Asian orogenic belt (CAOB). The Mamyn Terrane is usually considered part of the Argun Massif, although our new geochronological data indicate the presence of two magmatic events within the terrane that occurred at the Ediacaran–Cambrian boundary (∼541Ma) and the late Cambrian–Early Ordovician boundary (507–488Ma). Field observations indicate that all of the late Ediacaran–Early Cambrian (∼541Ma) volcanic rocks are deformed whereas the Late Cambrian–Early Ordovician (507–488Ma) intrusive and volcanic rocks are either deformed or undeformed. The ∼541Ma magmatic event in the study area produced rhyodacite, trachyrhyodacite, and trachyrhyolite units that are either high-K calc-alkaline or shoshonitic. These units have εNd(t) values from −7.4 to −8.7, tNd(DM) ages of 1.9–1.8Ga, and formed from primary magmas generated by the partial melting of Mesoproterozoic continental crustal material in a suprasubduction zone setting. The Late Cambrian–Early Ordovician (507–488Ma) magmatic event in this area formed gabbrodiorite, diorite, granodiorite, granite, trachyrhyodacite, and rhyodacite units that are medium-K and calc-alkaline, and have arc-like trace element compositions that are enriched in the large ion lithophile elements (LILEs) and depleted in the high field strength elements (HFSEs). These units have initial 87Sr/86Sr(i) ratios and εNd(t) values that range from 0.7048 to 0.7067 and from −3.3 to −0.2, respectively, yielding tNd(DM) ages of 1.6–1.1Ga. These features indicate that the magmas that formed these units were generated in a subduction zone setting, most likely by the partial melting of Mesoproterozoic crustal material with the addition of some younger juvenile material. In addition, the Late Cambrian–Early Ordovician trachyrhyodacite and rhyodacite units in this area are high-K and calc-alkaline or are shoshonitic and are enriched in K, Rb, the rare earth elements (REE), Y, Nb, Ta, and Zr, and are depleted in Sr and have A-type granite affinities. However, these units do not contain alkali amphibole or pyroxene and have high FeOtot/MgO ratios (6.7–12.7) that indicate they formed from highly fractionated magmas. Therefore, these volcanic rocks represent extreme melt differentiates that also formed the coeval granites. The early Paleozoic magmatic events within the Mamyn Terrane have also been identified in the Argun, Bureya, and Jiamusi areas of the eastern CAOB, suggesting that all of these areas have a common geodynamic history.