Late Precambrian tectonic affinity of the Alxa block and the North China Craton: Evidence from zircon U-Pb dating and Lu-Hf isotopes of the Langshan Group

摘要

The Langshan area is located in the northeastern margin of the Alxa block, which is the westernmost part of the North China Craton (NCC). The Langshan Group has been divided into four formations, of which the 1st formation deposited in the period of 1.83-1.66 Ga, the 2nd and 3rd formations formed between 1.16 Ga and 0.80 Ga and the 4th formation deposited after 1.17 Ga, whereas part of the previously believed Neoarchean Alxa "Group" deposited in the period of 1.65 Ga and 1.37 Ga. U-Pb ages of detrital zircons from the Langshan Group and the early Mesoproterozoic Alxa "Group" yielded age peaks of 2.51-2.48 Ga, 2.38-2.33 Ga, 1.89-1.81 Ga, 1.79-1.62 Ga, 1.58-1.56, 1.36-1.31 and 1.18-1.17 Ga. The late Neoarchean to middle Paleoproterozoic detrital zircons are mainly documented in the 1st formation and are derived from the ca. 2.5 Ga Guyang, Xi Ulanbulang and Beidashan granitoids, ca.1.9 Ga Xuwujia, Halaqin igneous rocks and the S-type granites, and the ca. 2.32 Ga Bayanwulashan orthogneisses. The late Paleoproterozoic (1.79-1.62 Ga) detrital zircons with negative epsilon(Hf)(t) values in the Alxa "Group" are interpreted to be sourced from the coeval rift-related granitic plutons and volcanic rocks in the northern NCC. On the other way, the late Paleoproterozoic to Mesoproterozoic detrital zircons with positive epsilon(Hf)(t) values from the upper Langshan Group probably sourced from the Fennoscandian shield of the Baltica craton. Based on the similarities in the stratigraphic sequence and detrital zircon age patterns of the Changcheng, Jixian and Qingbaikou systems, we infer that the Alxa Block was still an integrated part of the NCC in the early Neoproterozoic time.