Archean-Paleoproterozoic magmatism in the Xishui Complex, South China: Implications for crustal evolution and amalgamation of the Yangtze Block

The Xishui Complex is a recently discovered Archean unit in the northern margin of the Yangtze Block in South China. We report the discovery of Neoarchean and Paleoproterozoic rocks within this complex. Three samples of granitic gneiss in the complex yielded protolith ages of 2.51, 2.49, and 2.51 Ga. The 2.5 Ga granites are per-aluminous to strongly peraluminous, and were derived from partial melting of metasedimentary rocks, indicating crustal stabilization at that time. One pyroxene-plagioclase gneiss and one granodioritic gneiss both have a protolith age of 2.0 Ga. The protolith of the 2.0 Ga pyroxene-plagioclase gneiss was Nb-enriched basaltic rocks with strong negative epsilon Hf (t) values, which may have originated from mantle wedge metasomatized by slab melts. The 2.0 Ga granodiorite belongs to A-type granite derived from high-temperature dehydration melting of K-rich mafic crustal rocks. Archean-Paleoproterozoic magmatic zircons of the granitic and granodioritic gneisses all have negative epsilon Hf (t) values, suggesting that they were derived from reworking of ancient crust, with a main period of crustal growth at 3.8-3.3 Ga. From a comprehensive analysis, we may suggest that the Xishui Complex represents one of the oldest nuclei of the Yangtze Block during the Paleoarchean. Three independent terranes can be recognized in the southwestern, central, and northern parts of the Yangtze Block. Terranes or complexes within the block simultaneously underwent subduction at 2.15-2.00 Ga and collision at 2.00-1.93 Ga. Therefore, we may suggest that the unified Yangtze Block developed at 2.00-1.93 Ga and constituted an important part of the Columbia supercontinent.