Detrital zircon U-Pb dating in the southern Hefei Basin: Evidence for exhumation of HP-UHP rocks of the Dabie Orogen

Abstract

The Hefei Basin of eastern China developed in response to uplift of the Dabie Orogen, and zircon dating can be used to assess the exhumation history of the orogen. Zircons were collected from samples of the Lower Jurassic Fanghushan Formation and Middle Jurassic Sanjianpu Formation in the southern Hefei Basin, and mica-quartz schist and biotite granite gneiss from the Susong Complex of the Dabie Orogen. The zircon U-Pb dating was undertaken using laser ablation-inductively coupled plasma-mass spectrometry. The detrital zircons from conglomerates of the Fanghushan Formation and from clasts within the conglomerates have age-frequency distributions with the main clusters between 2.0 and 1.8 Ga, similar to age data of the Susong Complex. On the other hand, the zircons of the Fanghushan Formation do not show the age cluster at 1000-900 Ma that characterizes zircons in the underlying metasediments of the lower Paleozoic Foziling Group. A cluster of Triassic zircon ages also appears in the arkosic sandstones of the Fanghushan Formation. These data indicate that the provenance of the Fanghushan Formation was a mixture of high-pressure (HP) and ultrahigh-pressure (UHP) Triassic metamorphic rocks, Paleozoic magmatic rocks, and the Susong Complex, but not the lower Paleozoic Foziling Group even though it directly underlies the sediments of the Hefei Basin. Two samples from the Sanjianpu Formation show zircon age clusters at 797 and 791 Ma (middle Neoproterozoic) and 226 Ma (Triassic), and again, these are markedly different from the age clusters that characterize the Foziling Group. It seems, therefore, that despite the Foziling Group being at the surface in the underwater depositional area of the Hefei Basin, it was not exposed in the source area of the Hefei basinal sediments during the Jurassic, and there are two possible reasons for this. First, the exhumation of the Dabie Orogen was directed partly towards the north, in the process of which some of the Foziling Group was covered. Second, the Susong Complex rocks became involved in the development of an accretionary wedge, thus covering some of the Foziling Group during the process of subduction.