Paleogeographic affinity of the Alxa Block across the Archean–Proterozoic: Insights from metamorphosed Archean basement

Abstract

The Alxa, Quanji, and Dunhuang blocks are microblocks that link the North China craton (NCC) and the Tarim craton (TC). Their paleogeographic affinities during the late Archean–early Paleoproterozoic remain poorly constrained due to a paucity of reliable paleomagnetic data and less exposed Archean basement, especially in the Alxa Block. This study presents new geochronological and Hf isotopic data from Archean granitic gneisses in the northern Alxa Block. LA-ICP-MS zircon U-Pb dating results indicate that the protoliths of these granitic gneisses were emplaced at 2.67–2.65 Ga and subsequently metamorphosed at ca. 2.5 Ga and 1.84–1.83 Ga. In addition, in-situ zircon Hf isotopic compositions suggest that the protoliths were likely derived from reworking of Mesoarchean crust. These newly obtained data, together with previous data across the Alxa Block, indicate that Mesoarchean crustal materials were principally reworked to generate the 2.7–2.3 Ga crystalline basements of the Alxa Block. Given the recently reported ca. 2.6–2.5 Ga orthogneisses elsewhere in the Alxa Block, there should have existed a Neoarchean basement in the block, which experienced two episodes of magmatism at 2.7–2.5 Ga and 2.3–2.0 Ga, along with a metamorphic event at ca. 2.5 Ga. A further review of field geology and data across the Alxa Block and its adjacent blocks constrain a late Archean–early Paleoproterozoic crustal evolutionary history of the Alxa Block that was analogous to those of the Kuruktag Block of TC, the Khondalite Belt of NCC and the Quanji Block, indicating a close paleogeographic affinity of these blocks at this time.