Petrogenesis and Tectonic Implications of Paleoproterozoic Metapelitic Rocks in the Archean Kongling Complex from the Northern Yangtze Craton, South China

Abstract:

The Archean Kongling Complex in the northern Yangtze Craton is an ideal target to investigate the Precambrian accretion and evolution of continental crust in South China. This study aims to unravel the crustal evolution and tectonic setting of the Yangtze Craton during the Paleoproterozoic time, using integrated studies of petrography, zircon U-Pb and Hf isotopes and whole-rock geochemistry of Paleoproterozoic metapelitic rocks in the Kongling Complex. These rocks contain garnet, sillimanite, biotite, plagioclase, minor graphite and ilmenite. Zircons from the samples show nebulous sector-zoning and rim-core structure, suggesting both metamorphic origin and detrital origin with metamorphic overprints. The metamorphic zircons and metamorphic overprints have concordant ²⁰⁷Pb/²⁰⁶Pb age at ~2.0 Ga, while detrital grains yield three distinct concordant-age populations of >2.5 Ga, 2.4–2.2 Ga and 2.2–2.1 Ga. The age patterns indicate that the depositional age of the metasedimentary rocks was 2.1–2.0 Ga. Those 2.2–2.1 Ga detrital zircons with variable ε_Hf(t) values (-7.28 to 2.97) suggest the addition of juvenile materials from depleted mantle to the crust during 2.2–2.1 Ga. The 2.4–2.2 Ga zircons have Hf model ages (T_DM2) of ~2.6–3.5 Ga and >2.5 Ga zircons have T_DM2 ages varying from 2.9 Ga to 3.3 Ga. The new data suggest that the Kongling Complex was originally a Paleoarchean (old up to 3.5 Ga) continental nucleus, which experienced multiple episodes of growth and reworking events at 3.3–3.2 Ga, 2.9 Ga, 2.7–2.6 Ga, 2.4–2.2 Ga and 2.2–2.1 Ga. In combination with available data, the new results in this study suggest a continent–arc–continent evolution model to explain the tectonic evolution of the Yangtze Craton during the Paleoproterozoic time: the western margin of Yangtze Craton was originally an individual continent, which underwent a reworking event during 2.4–2.2 Ga and a crust growth event caused by continent–arc collision during 2.2–2.1 Ga; it subsequently collided with the Yangtze nucleus at ~2.0 Ga and experienced later ~1.85 Ga extension.