ABSTRACT

The apatite fission track (FT) analysis of Jurassic sandstones from the Beipiao basin in the eastern China indicates a large variation in FT age peaks. The sandstone of the Beipiao Formation has two peak ages at 178.8 and 40.0 Ma, while the sandstone of the Tuchengzi Formation has three age peaks at 152.0, 77.5 and 32.5 Ma. This implies that the provenance of the Beipiao basin in the early Jurassic and later Jurassic changed obviously. According to the Mesozoic regional thermo-tectonic evolution of the Yan-Liao orogenic belt, these apatites with different FT ages possibly represent different source components, although partial annealing had occurred to these apatites. The apatites of the oldest peak age (178.8–152.0 Ma) possibly originated from the pre-Mesozoic sedimentary covers of the North China Block (NCB), while the apatites of the intermediate peak age (77.5 Ma) and the younger peak age (40.0–32.5 Ma) came from the underlying Archean basement rocks and the Mesozoic volcanic detritus respectively. It is recognized that the basin sediments in the early Jurassic are composed of the Mesozoic volcanic detritus and pre-Mesozoic sedimentary detritus eroded from the uplifted regions around the basin. The existence of the intermediate peak age in the Tuchengzi Formation implies that the Archean basement rocks of the NCB might have become an important source of the upper Jurassic, which responded well to the south-southeast-trending thrust faulting in the northern of the NCB. Apatite FT ages of both the lower and upper Jurassic sediments in the Beipiao basin have a major peak age (30–40 Ma), which perhaps recorded an important Cenozoic thermo-tectonic event accompanied by the intensive rifting, basin subsidence and strong basaltic magmatism in the North China Block. Given a paleo-thermal gradient of 30°C/km, it can be deduced that the Liaoxi area has uplifted about 3 km at an average rate of about 0.1 mm/a since 30–40 Ma.