Apatite (U–Th)/He thermochronometry of iron deposits in the central Ningwu volcanic basin, eastern China: Implications for exhumation and preservation

Abstract

Apatite (U–Th)/He thermochronometry (AHe) was used to constrain the exhumation history of the Taocun and Washan iron oxide deposits, located in the central Ningwu volcanic basin of the Lower Yangtze River Belt (LYRB) in eastern China. The porphyritic gabbro–diorite host rock yields zircon U–Pb ages of ~130 Ma, which are different to the apatite (U–Th)/He ages from this study. The Taocun iron deposit was emplaced below the apatite helium partial retention zone (PRZ) and its AHe ages (17.8 ± 1.0 to 19.5 ± 1.2 Ma) reveal a rapid exhumation-induced cooling event that began at ~23 Ma. The Washan iron deposit, which is hosted within a cryptoexplosion breccia, was probably emplaced in the apatite helium PRZ. It yields two groups of AHe ages: an older group (58.5 ± 3.3 and 57.8 ± 3.5 Ma) and a younger group (27.0 and 22.8 Ma). The youngest AHe age from the Washan iron deposit (22.8 Ma) records the same rapid exhumation-induced cooling event that is recorded in the Taocun deposit AHe ages. This event was related to structural inversion in eastern China that began during the early Miocene (~23 Ma). This inversion marked the end of widespread extension that started in the Early Cretaceous, and it was accompanied by the formation of structures related to Neogene and Quaternary regional compression. Given the relatively recent timing of the exhumation, the overall amount of exhumation of the central Ningwu Basin is relatively minor. We integrated our results with the porphyrite iron deposit model and existing information on the Xuejin detachment fault. It seems that iron deposits of the deepest (Gushan) sub-type of the porphyrite iron deposit model may still be preserved at depth in the central Ningwu Basin.