Zircon U-Pb and Hf isotopic study of gneiss and granodiorite from the southern Beishan orogenic collage: Mesoproterozoic magmatism and crustal growth

Abstract

The distribution of Precambrian basement rocks and their tectonic attributes are of key importance for understanding the tectonic architecture and the crustal growth model of the Central Asian Orogenic Belt (CAOB). We present LA-(MC)-ICP-MS zircon U-Pb and Hf isotopic data for the biotite-plagioclase gneiss and granodiorite from the southern Beishan orogenic collage, on the southern margin of the CAOB. The magmatic zircon grains from the gneiss yielded a protolith age of 1408±4 Ma, indicating the occurrence of Mesoproterozoic basement rocks in the southern Beishan. Furthermore, these zircon grains have high positive e Hf( t ) values (4.1-9.9), and young Hf model ages (1.50-1.72 Ga) close to the crystallization age, suggesting that the protolith of this gneiss was derived from the depleted mantle or the juvenile crust. This indicates the involvement of an important Mesoproterozoic (~1.4 Ga) event in juvenile crust growth in the southern Beishan. Conversely, the granodiorite intruding the biotite-plagioclase gneiss yielded a crystallization age of 257±3 Ma, and numerous ~1.4 Ga inherited zircon grains were found in this rock. These inherited zircon grains exhibit crystallization ages and Hf isotope compositions consistent with those of the biotite-plagioclase gneiss, further confirming the occurrence of Mesoproterozoic crustal basement rocks beneath the southern Beishan. By comparing our new data with previous results from the Precambrian basement in the Dunhuang Block (part of the Tarim Craton) and the Central Tianshan Block (microcontinent in the Chinese Tianshan), we postulate that the Precambrian microcontinents in the southern Beishan have similar crustal evolutionary histories to the Central Tianshan Block, and they may not have been derived from the Tarim Craton. The Mesoproterozoic basement rocks investigated here provide new clues for examining the accretionary orogenic processes of the southern CAOB.