Ca. 2.5 Ga TTG rocks in the western Alxa Block and their implications

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 Hf (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.

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 petro- graphic 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 enf（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 Paleoprotero- zoic tectonothermal event similar to the North China Craton.