The south margin of the central Altyn is an early paleozoic tectonic unit confirmed by Zircon dating evidence

The south margin of Central Altyn (SMCA) has long been considered as Precambrian stratigraphic unit of the Altyn Orogen. However, their strata attribution is subjected to challenge by the new finding of Siluran-Devonian metamorphic rocks. Aiming to further understand the geological evolution of the SMCA and its relations with the adjacent tectonic units, here we investigate the metasedimentary, metamafic and metagranitic rocks and in turn to re-examine the tectonic characteristics of the region. Detrital zircons from six metasedimentary rocks hold a sharp peak of ~ 452 Ma and moderate peak of 800-1600 Ma, whose spectra characteristics are similar to those of extensional basins formed under subduction setting of active continental margins. Two metamafic rocks and one metagranitic rock obtained crystallization ages for their protolith of c.456 Ma, c.462 Ma and c.457 Ma, which fell in the range of the 462-451 Ma magmatism of South Altyn (SA). These ages are widely distributed from west to east along the SMCA, and congruously indicate its stratum should formed in the Middle- Late Ordovician sedimentary environment under the extensional background of the SA subduction-early exhumation. All rocks record three stages metamorphism by zircons of ~ 430 Ma, ~ 400 Ma and ~ 377 Ma, consistent with previously reported HP pelitic gneiss. These protolith and metamorphic studies suggest that the SMCA should be a Paleozoic subduction-collision zone, rather than Mesoproterozoic sedimentary strata, and it should be separated from the Central Altyn block. Moreover, the similar protolith and metamorphic age have been also widely reported in the North Qaidam (NQ) and Dunhuang (DH) orogen, suggesting they may be the response of the same geological event in different tectonic units.