Lithospheric thinning and reworking of Late Archean juvenile crust on the southern margin of the North China Craton: evidence from the Longwangzhuang Paleoproterozoic A-type granites and their surrounding Cretaceous adakite-like granites

In the Longwangzhuang area along the southern margin of the North China Craton (NCC), a layer of Cretaceous K-feldspar granites surrounds Paleoproterozoic granites. The Paleoproterozoic granites are enriched in sodic ferrogedrite and show low Al₂O₃ (12.3-13.0wt%) and aluminum saturation index (ASI) (0.93-1.09) with high Zr (609-966ppm), Y (47.2-96.7ppm) and Nb (58.7-97.7ppm) concentrations, resembling A-type granites. The Cretaceous K-feldspar granites have relatively high Sr (145-419ppm), Ba (1252-1660ppm), Sr/Y (19-127), La/Yb (36-56) ratios, but low Y (4.01-8.88ppm) and Yb (0.46-0.99ppm), geochemically resembling adakite-like rocks. However, these Cretaceous adakite-like K-feldspar granites have rather low MgO (0.08-0.26wt%), Al₂O₃ (13.6-14.5wt%), Mg# (11-23), Cr (2.30-4.61ppm) and high initial ⁸⁶Sr/⁸⁷Sr (0.7098-0.7118), as well as abundant K-feldspar minerals, and are different from typical adakite. Zircon LA-ICP-MS geochronology shows that the two types of granites investigated in this study formed at 1616±20Ma and 140±1Ma, respectively. However, zircons from both the Paleoproterozoic and Cretaceous granites yield similar two-stage Hf model ages at ca. 2.5-2.6Ga, suggesting the reworking of Late Archean juvenile crust. The Paleoproterozoic A-type granites show enriched Nd-Hf isotopic features but high whole-rock oxygen isotopes. These rocks may have formed from partial melting of restitic crustal material during lithospheric thinning, along with extension and experienced crystal fractionation of plagioclase, apatite and magnetite. The Cretaceous adakite-like K-feldspar granites were formed by local anatexis of TTG rocks from the Taihua Group, with amphibole as the major residual phase. The local anatexis and accompanying migmatisation suggest crustal compression and possible transpression. Therefore, the lithospheric thinning in the NCC should have taken place after 140Ma on the southern margin of the NCC.