ABSTRACT

Low-grade metamorphic Paleozoic sedimentary rocks are exposed along the northern margin of the Qinling-Dabie orogenic belt that formed by the collision of the North and South China blocks during the Early Mesozoic. Geological provenance and tectonic setting of these low-grade sedimentary rocks are strongly debated. Recent studies have shown that they contain Paleozoic, Late Proterozoic and Late Archean detrital zircons, which points to an origin from different sources including rocks from the South and North China blocks. This study presents single-grain mineral Rb-Sr results for detrital muscovites and biotites from Carboniferous sedimentary sequences of the Beihuaiyang low-grade metamorphic zone exposed along the northern margin of the Dabie Mountains. About 15% of detrital muscovite grains are phengitic in composition with high Si/Al ratios of >3.3. Single grain Rb-Sr isotopic analyses show that both muscovites and phengites give a pooled age of about 420 Ma, implying Middle Paleozoic thermal and high-pressure metamorphic events in the source area(s). Rb-Sr isotopic evidence from detrital biotites suggests at least two different sedimentary sources of old crustal rocks and juvenile material. The latter probably comprised magmatic rocks related to the Paleozoic convergence along the southern margin of the North China block. Whole-rock Sm-Nd isotopic composition of the sedimentary rocks also demonstrates a mixture of sedimentary sources of different ages that confirm conclusions proposed previously from evidence of detrital zircon ages. We conclude that the Paleozoic detrital phengites originate from Paleozoic subduction and subsequent collision proposed for the North Qinling terrane. Our study shows that, single-grain detrital mica Rb-Sr isotopic characteristics can be a potential indicator for the provenance of geological complex terranes.