Detrital zircon U–Pb ages, Hf isotope, and geochemistry of Devonian chert from the Mianlue suture: Implications for tectonic evolution of the Qinling orogen

Abstract

The Mianlue suture marks the southern boundary of the Qinling–Dabie orogen and divides the South China and North China continental blocks. Here we present in-situ U–Pb geochronology and Lu–Hf isotopic data on detrital zircons in chert beds intercalated with continental detritus from the Sanhekou Group in Wenxian along this suture. Our data suggest that deposition of these sediments in the Mianlue suture zone occurred later than ca. 412Ma. The data define four main age peaks with more than 25% of the detrital zircons yielding ages between 801 and 999Ma with εHf(t) ranging from −26.9 to 12.2. The age data are comparable with those zircons in the Neoproterozoic granites and volcanic rocks in the Bikou Block which has an affinity to the South China Block. Zircons with ages from 2310 to 2595Ma comprise 14.1% of the detrital population, and were mainly sourced from the South China Block as inferred from their mostly negative εHf(t) values compared to the North China Block. Zircons with ages ranging from 412 to 587Ma constitute 12.6% and show markedly negative εHf(t) values ranging from −0.6 to −25.5 (the majority ranging from −4.4 to −19.6), correlating with Palaeozoic magmatic intrusions in the South China Block. Another age peak of 1548–1686Ma with εHf(t) values varying between −6.5 and −14.8 is comparable to the zircons in magmatic intrusions of the South China Block. Our results suggest that the major source of zircons in the continental detritus mixed with chert within the Mianlue suture zone is Proterozoic and Palaeozoic magmatic intrusions in the South China Block. Combined with the geochemical features of the chert in Wenxian and other outcrops in the Mianlue suture zone, we infer that these sediments were deposited in the Mianlue sea basin adjacent to the continental margin, and that the opening of the Mianlue suture was initiated in the eastern section of the northern margin of South China Block, with subsequent migration to the western part, in a zipper-type opening of the continental rift from the east to the west of the Mianlue suture zone.