Petrology and geochronology of Paleoproterozoic garnet-bearing amphibolites from the Dunhuang Block, Eastern Tarim Craton

Abstract

The Dunhuang Block, located in the eastern margin of the Tarim Craton, possesses a wealth of information about the geological evolution of the Tarim Craton. In this study, we conducted detailed petrological, geochemical and geochronological investigations on garnet-bearing amphibolites as well as granites in the Dunhuang Block, which record important metamorphic and magmatic events that are poorly constrained. Based on field occurrence, geochronological and geochemical characteristics, the garnet-bearing amphibolites can be divided into two groups. Group I garnet-bearing amphibolite displays typical features of island arc basalts, which may formed at ~1.83Ga in a collisional setting during the assembly of the Columbia supercontinent, whereas group II has geochemical characteristics of ocean island basalt which may formed at ~1.61Ga in an ocean island setting. The ca. 1.83Ga amphibolite has positive e{open}Hf(t) values (+4.4 to +8.9), with TDM1 model ages of 2.02-1.83Ga, indicating an important addition of juvenile mantle-derived materials during the formation of its protolith. The ca. 1.61Ga amphibolite has e{open}Hf(t) values ranging from -0.6 to +5.6 and corresponding TDM1 model ages varying from 2.03Ga to 1.84Ga, which suggest that juvenile mantle-derived materials were dominantly contributed to the formation of its protolith but were later contaminated by crustal materials. The U-Pb ages obtained from the rims of the zircon grains in amphibolites and granites recorded a tectonothermal event at ca. 400Ma, which is coeval with the widespread magmatism in the Beishan orogen. We thus propose that these two groups of garnet-bearing amphibolites were possibly not related to each other during Paleoproterozoic time, and a late Paleozoic orogenic event brought them together during which both of them underwent metamorphism. © 2014 Elsevier B.V.