The discovery of the Paleoproterozoic syenite in Helishan, Gansu Province, and its implications for the tectonic attribution of the Alxa Block

The tectonic affinity of the Alxa block has long been in debate. It may be part of the North China Craton (NCC), or independent from the NCC during the Precambrian. The comparison of basements between the Alxa block and the NCC would be helpful to solving this dispute, but the Alxa basement is relatively poorly studied due to limited outcrops, with most of available data reported in eastern Alxa. Recently, a syenite that intruded into the Longshoushan Group has been sampled in Helishan area, western Alxa, and both of them are unconformably covered by Sinian strata. The Helishan syenite is characterized by extremely enriched K₂O (13.77%) and LREE(La/Yb)_N=46.62, and shows distinct negative and positive anomalies of Nb-Ta and Pb-Zr-Hf, respectively, with EM-I type Sr-Nd isotope features (ε_Nd(t)=-5.05), implying partial melting of basaltic lower crust. Moreover, LA-ICP-MS zircon U-Pb data indicate that this syenite was formed during Paleoproterozoic (1872 ±12) Ma and display records of~2.7 Ga crustal growth and~2.5, 2.1 and 1.95 Ga magmatic activities. According to data from this study and previously published data, the Neoarchean-Pelaoproterozoic basements and tectono-thermal events of the Alxa block and the NCC are geochronologically consistent, indicating very close affinity between them.