Early Permian A‐type Granites and Their Tectonic Implications, Zhangdaqi Area, Inner Mongolia (China)

There is a controversy regarding the amalgamation of Xing'an and Songnen Blocks along the Hegenshan-Heihe Suture (HHS) in the eastern Central Asian Orogenic Belt (CAOB). To solve this problem, we performed detailed study on the granites from the Zhangdaqi area, adjacent to the north of the HHS in the northern part of the Great Xing'an Range, NE China. Geochemically, the granites in the study area are metaluminous-weak peraluminous and high-K calc-alkaline series. Trace elements of the granites show that LREEs are relatively enriched, while HREEs are relatively deficient and obvious REE fractionation. The granites are characterized by obvious negative Eu anomalies, meanwhile, they are relatively enriched in Rb, K, Th and depleted in Ba, Nb, Sr, P, Ti. All the geochemical features suggest that the granites in the Zhangdaqi area are aluminum A-type granites. The zircon LA-ICP-MS U-Pb ages of these granites are 294-298 Ma, indicating that they formed in the Early Permian. These granites also have positive epsilon(Hf) (t) values (8.4-14.2) and a relatively young two-stage model age between 449 Ma and 977 Ma, implying that the magma was derived from the re-melting of the Early Paleozoic-Neoproterozoic juvenile crust. Combined with geochemical characteristics (Nb/Ta ratios of 9.0-22.2, and Zr/Hf ratios of 52.3-152.0), we believe that the magmatic source area is a mixture of partial melting of the lower crust and depleted mantle. A-type granites and bimodal volcanic rocks along the Hegenshan-Heihe Suture formed during the Late Carboniferous-Early Permian, indicating that the HHS between Xing'an and Songnen Blocks closed in the late Early-Carboniferous. Subsequently, the Zhangdaqi area was in a post-orogenic extensional environment from Late Carboniferous to Early Permian and resulted in the formation of the A-type granites.