New chronological constrains on the tectonic affinity of the Alxa Block, NW China

The origin and tectonic affinity of the Alxa Block in NW China is currently debatable. This study reports new stratigraphically-constrained LA-ICP-MS zircon U-Pb data from Precambrian rocks exposed in the westernmost part of the Alxa Block, to constrain their ages and provenance. Ages obtained for a granitic gneiss from the Longshoushan Group are mainly discordant yielding an upper intercept near 1200 Ma and a lower intercept between ∼350 and ∼310 Ma, indicating the protolith for the granitic gneiss intruded during the Mesoproterozoic, and the gneissic fabric formed in late Paleozoic associated with the accretion of the Central Asian Orogenic Belt. Detrital zircons of a meta-sandstone from Pre-Sinian strata yielded predominant Mesoproterozoic ages with bimodal peaks at ∼1166 Ma and ∼1375 Ma. These ages are interpreted as tectonic events associated with the long-lived accretion and collision processes coeval with the amalgamation of the Rodinia supercontinent. The youngest concordant zircon age of 968 ± 70 Ma constrains its maximum depositional age to the Stenian-Tonian boundary. Two metasedimentary rocks from the Sinian strata share similar age patterns, yielding an earliest Paleoproterozoic peak at ∼2469-2460 Ma, and two Tonian peaks at ∼992-978 Ma and ∼826-807 Ma. The youngest detrital zircon ages constrain their maximum depositional age to be latest Neoproterozoic. The detrital age patterns, as well as zircon morphology, indicate that the protoliths for these metasedimentary rocks were probably deposited in collision-related foreland basins. Our new data combined with published data indicate that the Alxa Block is not a uniform early Precambrian craton like the North China Craton, but a Precambrian composite ribbon continent which originally detached from a main cratonic accretionary and collisional margin. The Alxa Block, like the South China Craton, underwent a multi-stage tectono-thermal evolution related to the assembly of Rodinia and Gondwana supercontinents during global plate margin reorganization. Current data support a close affinity of the Alxa Block to the SCC during the Precambrian. During the Paleozoic, the Alxa ribbon continent behaved as a backbone to the Paleo-Asian Ocean to the north, on which early Paleozoic-Permian magmatic arcs were built forming the southernmost segment of the Central Asian Orogenic Belt.