Tectonic conversion on the northern margin of the North China Craton in the Early Triassic: Constraints from geochronology and petrogenesis of the Datan plutonic complex, middle of Inner Mongolia, China

Abstract

Abstract In the Datan area, located 60\u202fkm northeast to Hohhot, an undeformed plutonic complex has intruded into the Panyangshan thrust system on the northern margin of the North China Craton (NCC). The plutonic complex consists of the predominant earlier Datan pluton and the scattered later Hanqinggou plutons. The Datan pluton has relatively high eHf(t) (−7.4 to 0.2), low (87Sr/86Sr)i (0.704280 to 0.706508), and negative eNd(t) (−14.9 to −10.6), and the core phase with the lowest SiO2 content has features of C-type Adakite-like granite (high Sr/Y, (La/Yb)N, and Sr; low Y and Yb; no Eu anomaly; and HREE depletion). This suggests that the Datan pluton was probably derived from the metamorphic mafic rocks of the thickened continental lower crust that was heated and injected with juvenile mantle-derived melt. The Hanqinggou plutons which have similar geochemical features but are more depleted in Ba, Ti, and P may represent the residual phase of the Datan pluton. Crystal fractionation predominated by feldspar, biotite, apatite, zircon, titanite, ilmenite, apatite, allanite, titanite, and zircon took place during the magmatic evolution. The dating results suggested that the zircon U Pb ages for the undeformed Datan pluton intruding into the Panyangshan thrust system ranged from 252\u202f±\u202f1\u202fMa to 248\u202f±\u202f1\u202fMa. Additionally, the youngest detrital zircon U Pb age of the autochthon recorded in the literature is 274\u202f±\u202f4\u202fMa. These indicate that the Panyangshan thrust system representing the crustal thickening occurred from ca. 274\u202fMa to ca. 252\u202fMa and may be due to the subduction of the Paleo-Asian Ocean and the collision between the Mongolian arcs and the NCC. This contraction was followed by asthenospheric upwelling and initial extension represented by Datan complex, indicating a significant tectonic conversion in ca. 250\u202fMa.