Zircon REE patterns and geochemical characteristics of Paleoproterozoic anatectic granite in the northern Tarim Craton, NW China: implications for the reconstruction of the Columbia supercontinent

摘要

Because Archean basement rocks are sparsely distributed around the Tarim Basin, little is known of the relationship between the Tarim Craton and the Paleo- to Mesoproterozoic Columbia supercontinent. Zircon U-Pb dating of a Paleoproterozoic gneissic granite in the northern Tarim Craton yielded a crystallization age of 1915 ± 13 Ma, consistent with global Paleoproterozoic collisional events (ca. 2.1-1.8 Ga) recorded in most cratons. Despite the fact that some zircons display discordant U-Pb ages with a distinct loss of radiogenetic Pb, all grains have similar 176Lu/ 177Hf and 176Hf/ 177Hf(t) values. Zircons from this granite have high Th/U ratios (0.15-0.95), but contrasting rare earth element (REE) patterns. The REE patterns show that the incorporation of most REE elements correlates with increasing age discordance. The discordant zircons, unlike typical magmatic zircons, were probably formed in an open system with the participation of aqueous fluids or hydrous melts during late- to post-magmatic recrystallization. Rocks of this granite pluton have high contents of SiO 2 (75.45-78.49 wt.%) and total alkali (6.38-9.00 wt.%), and moderate Al 2O 3 (10.31-13.80 wt.%), showing peraluminous characteristics. Their high contents of high field strength elements (HFSE) and significant negative Eu anomalies are consistent with the geochemical characteristics of A-type granites. The low e{open} Hf(t) values of the granite suggest a derivation through anatexis of late Neoarchean tonalite-trondhjemite-granodiorite (TTG) rocks of the basement, indicating that the geochemical characteristics of A-type granites may originate from partial melting of Archean TTGs. In combination with previously published age data, the regional anatexis provides evidence of a late Paleoproterozoic orogenic event on the northern margin of the Tarim Craton. Because the Paleoproterozoic and Mesoproterozoic sedimentary rocks in the northern Tarim Craton show characteristics of sediments deposited on a passive continental margin, the craton is suggested to have occupy an outboard position in the Columbia supercontinent. © 2011 Elsevier B.V. All rights reserved.