The Geochemical Characteristics, Genesis, and Geological Significance of Early Paleozoic Granites in the South Altun Orogenic Belt of Western China

The Altun Orogenic Belt (AOB) has undergone multiple complex subduction–collision events. However, there are numerous disagreements regarding the Early Paleozoic tectonic–magmatic evolution of the AOB, primarily due to differing interpretations of magmatic rock types and their sources. As a result, we conducted detailed geochemical analyses of granite samples obtained from several exploration wells in the Dongping area (DPA) of the South Altun Orogenic Belt (SAOB) at the western boundary of the Qaidam Basin. This approach differs from previous studies that mainly relied on outcrop samples. The granites in the study area are metaluminous and have high alkali contents (avg. 7.63%) and high TFe2O3/MgO ratios (avg. 4.50). Their rare-earth elements are enriched in light REEs and show weak to moderate negative Eu anomalies (δEu = 0.49–1.11). These geochemical signatures indicate an affinity to A-type granites. Through comprehensive diagram analysis, the rocks plot near the upper crustal composition in a Ta/Yb-Th/Yb diagram, indicating that they primarily originated from a mixed source of recycled and juvenile crustal material. A comprehensive analysis of the regional tectonic background shows that the Early Paleozoic granites in the SAOB formed in post-collisional extensional environments and syn-collisional volcanic arc tectonic settings. The majority formed in post-collisional extensional thinning environments, whereas a minority formed in syn-collisional volcanic arc tectonic settings, closely related to the subduction and collision of the Qaidam Block beneath the Central Altun Block.