Abstract
The Alxa Block is considered part of the North China Craton, but the unambiguous Archean basement has not been reported. In this study, we present the first evidence of the Neoarchean rocks in the Beidashan area of the western Alxa Block. The petrographic and geochemical data show that these rocks are granodiorite with TTG (tonalite-trondhjemite-granodiorite) characteristics. Zircon U-Pb dating gave an age of 2522±30 Ma for the magmatic core and 2496±11 Ma for the metamorphic recrystallized rim. The near-identical age between the Latest Neoarchean magmatism and the high-grade metamorphism shows that these features were related to the same Latest Neoarchean-Earliest Paleoproterozoic tectonothermal event. The age-corrected ɛHf(t) value is mainly between 0.4 and 4.9. The two-stage zircon Hf model age ranges from 2.7 to 3.0 Ga, suggesting that the Mesoarchean-Neoarchean (2.7–3.0 Ga) juvenile crust was reworked at the end of the Neoarchean in the western Alxa Block. These data suggest that the western Alxa Block experienced a Mesoarchean-Neoarchean crust growth and Latest Neoarchean-Earliest Paleoproterozoic tectonothermal event similar to the North China Craton.
Highlights
The Alxa Block is considered part of the North China Craton, but the unambiguous Archean basement has not been reported
We present the first evidence of the Neoarchean rocks in the Beidashan area of the western Alxa Block
In this paper, based on zircon dating, we report the first ca. 2.5 Ga TTG rock outcropped in the Beidashan area of the western Alxa Block, indicating the existence of a Neoarchean metamorphic basement in the Alxa block
Summary
The Precambrian basement of the western Alxa Block is mainly exposed in the Longshoushan, Beidashan and Yabulaishan areas, which lie to the north of the Hexi Corridor and is separated from the eastern Alxa Block by the Tenggeli Desert. The Beidashan, which extends in a NWW-SEE trend, is bounded by the Yabulaishan and the Badanjilin Desert to the north, by the Tengeli Desert to the east, and is separated from the parallel Longshoushan by the Chaoshui basin to the south (Figure 1(b)). The high-grade metamorphic rocks in the Beidashan area are intruded by widely-distributed granitic rocks. These rocks were first called “Beidashan Group” by the Regional Geological Surveying Team of the Gansu Province in 1971 [21]. Given that the exposed basement in the Beidashan area contains both metamorphosed pluton and supracrustal sequences that were strongly deformed, we renamed “the Beidashan Group” as “the Beidashan Complex”
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