U-Pb Silurian age for a gabbro of the Platinum-bearing belt of the Middle Urals (Russia): evidence for beginning of closure of the Uralian Ocean

Abstract

AbstractThe Platinum-bearing Belt of the Urals consists of a series of zoned ultramafic bodies obducted onto the passive continental margin of the East European Craton during the Palaeozoic. Conventional U-Pb and secondary ionization mass spectrometry U-Th-Pb analyses of single zircon grains from a pegmatitic gabbro of the Kumba massif provide Mid-Silurian ages of 425 ± 3 Ma and 419 ± 10 Ma, respectively, interpreted as dating crystallization of the gabbroic magma. This contrasts with the c. 360 Ma age of the neighbouring Kytlym massif and indicates that the Uralian Platinum-bearing Belt is best interpreted as remnants of an island-arc oceanic lithosphere that started forming in Mid-Silurian times and had a protracted lifetime of about 70 Ma. The Uralian Platinum-bearing Belt is thus coeval with the Sakmara arc of the Southern Urals, and is of similar age to other ultramafic bodies such as the Kempersai and Mindyak massifs. Ophiolitic fragments, now preserved along the Main Uralian Fault, may thus represent the assemblage of contemporaneous oceanic and arc terranes brought together during the final stages of the evolution of the Uralide orogen. The age of the Kumba massif, in addition, indicates that the Uralian Ocean underwent contractional events as early as the Mid-Silurian, which suggests a possible link with the pivotal rotation of Baltica.