EARLY PROTEROZOIC GRANITOIDS IN THE JUNCTION ZONE OF THE OLEKMA GRANITE-GREENSTONE BELT AND THE ALDAN GRANULITE-GNEISS TERRANE, ALDAN SHIELD: AGE, SOURCES, AND GEODYNAMIC ENVIRONMENTS

Abstract

The geological evolution of the junction zone between the Olekma granite-greenstone belt and Aldan granulite-gneiss terranes (OGGB and AGGT, respectively) of the Aldan Shield involved at least four episodes of Early Proterozoic granitoid magmatism. The first of them was responsible for the origin of large massifs of anorogenic granitoids of the Nelyuki Complex at 2398 ± 4-2522 ± 2 Ma. The sources of the parental melts were tonalite-trondhjemite gneisses of the Olekma and Western Aldan stratigraphically undifferentiated complexes, which were tectonically combined during the collision of the Olekma and Aldan microcontinents at 2.6-3.0 Ga and the origin of the Olekma-Aldan microcontinent. The second and third episodes of the Early Proterozoic granitoid magmatism gave rise to granitoid intrusions of the Verkhnii Chugino (1993-1966 Ma) and Dzhaltunda complexes (1966 ± 4 Ma), which are collisional with respect to the collision of the Olekma-Aldan microcontinent and the Fedorovskii island arc (origin of the Olekma-Aldan-Fedorovskii microcontinent). The parental magmas of the granitoids of the Dzhaltunda Complex were most probably derived from metapelites of the Aldan granulite megacomplex and/or metapelites of the Bulgunnyakhtakh greenstone belt. The fourth episode of Early Proterozoic granitoid magmatism in the junction zone of OGGT and AGGT produced granitoid intrusions of the Amut Complex (1899 ± 6 Ma) in response to the collision of the Olekma-Aldan-Fedorovskii and Sunnagin (?) microcontinents. The parental magmas were derived from rocks compositionally similar to the metapelites of the Aldan granulite-gneiss megacomplex and/or the tonalite-trondhjemite gneisses of the Olekma undifferentiated complex. The Sm-Nd isotopic systematics of the rocks indicate that the Early Proterozoic granitoids of different ages in the junction zone of the Olekma granite-greenstone belt and Aldan granulite-gneiss terrane of the Aldan Shield were produced by the recycling of continental crustal rocks of Late Archean and Early Archean age. No traces of juvenile granite formation of Early Proterozoic age were detected in the zone.