Zircon U-Pb age and Hf isotope of Paleoproterozoic mafic intrusion on the western margin of the Yangtze Block and their implications

A large number of mafic dykes occur in the Kang-Dian Axis of the western Yangtze Block, which had a great bearing on the early evolution of the Yangtze Block. In this paper, zircon U-Pb age, Hf isotope, and whole-rock geochemistry were studied for gabbros collected from Jiaopingdu area, which intruded into the Tongan Formation, with the purpose of constraining the age of the crystallization basement and the early evolution of the Yangtze Block. The zircons yielded a weighted mean 207Pb/206Pb age of 1694Ma±16Ma（MSWD=0.9）, which is interpreted as the intrusion age. The results constrain the depositional age of the Tongan Formation to 1694Ma, implying that the Tongan Formation is of Paleoproterozoic. Geochemical data show that these gabbros have N-MORB affinity. Initial Hf isotope ratios(176Hf/177Hf)i of the zircons range from 0.281881 to 0.281982, with corresponding εHf(t) values of 6.2~9.8. These zircons give the depleted mantle model ages(tDM1) of 1738~1883Ma(1803Ma on average), slightly older than the time of the zircon growth. All these data suggest that the Paleoproterozoic gabbro was derived from a depleted mantle. In combination with the presence of abundant Archean to Paleoproterozoic detrital zircons in the Mesoproterozoic strata, the newly identified 1694Ma mafic intrusion in the western Yangtze block supports the existence of an Archean-Paleoproterozoic crystalline basement beneath the western Yangtze block. The results obtained by the authors also show that the tectonic event, which resulted in the intrusion of large quantities of late Paleoproterozoic mafic dykes in the western Yangtze block, was obviously younger than the continental collision and post-orogenic extension of Columbia supercontinent recorded in Yangtze block, and thus most probably linked to the breakup of Columbia supercontinent.