Mid-Neoproterozoic amphibolite facies metamorphism at the northern margin of the Yangtze craton

Abstract

The tectonic setting of the continental margin of the Yangtze craton during the Neoproterozoic has long been a subject of debate. The Douling Complex is a small segment of crystalline basement in the northern Yangtze craton; the study of which may provide useful insights into the tectonic evolution of this area. In this paper we report medium-grade regional metamorphism that occur in the Douling Complex. Petrographical textures and mineral compositions suggest that amphibolite facies rocks from the Douling Complex underwent a progressive metamorphic evolution from staurolite–kyanite grade to sillimanite grade. Conventional geothermobarometry defines P–T conditions to be 490–560 °C and 6–7.5 kbar for prograde metamorphic stage and 630–720 °C and 8–11 kbar for peak metamorphic stage. The post-peak evolution is characterized by decompression and thermal relaxation. Metamorphic zircons from garnet amphibolites and metapelites and magmatic zircons from a syn-deformed pegmatite pod yield U–Pb ages between 820 ± 5 Ma and 779 ± 3 Ma, with two populations at ca. 820–815 Ma and ca. 790–780 Ma, respectively. Biotites from metapelites also give two 40Ar/39Ar plateau ages of 814 ± 4 Ma and 790 ± 5 Ma. It is inferred that peak metamorphism occurred at ca. 820–815 Ma and subsequent cooling took place at ca. 790–780 Ma. Considering the distinct Precambrian crustal history of the Douling Complex, together with medium-grade amphibolite facies metamorphism accompanied by a clockwise P–T path, we propose that these observations provide evidence for the collision of the small Douling block with the northern margin of the Yangtze craton during the Mid-Neoproterozoic. Immediately after this collision, a newly accreted Andean-type active continental margin may have formed along the northern Yangtze craton.