Origin of Late Archean and Early Proterozoic rocks and associated mineral deposits from the Zhongtiao Mountains, east-central China

Abstract

Abstract Late Archean and Early Proterozoic supracrustal rocks in the Zhongtiao Mountains of east-central China include five successions separated by unconformities or intrusive contacts. The oldest successions, the Shushui Complex (⩾ 2500 Ma), and the Jiangxian (2400-2200 Ma) and Zhongtiao Groups (∼ 2100 Ma) are composed chiefly of metapelites, quartzites and marbles with small amounts of bimodal felsic-mafic volcanics in which felsic ash flow tuffs dominate. The overlying Danshanshi Group includes chiefly quartzites and conglomerates and the Xiyanghe Group (∼ 1830 Ma) comprises bimodal volcanics and red beds. Granitoids range from Late Archean to Early Proterozoic in age and vary from tonalite to granite in composition. Precambrian igneous rocks show a subduction zone geochemical component (depletion in Nb-Ta relative to LILE), and Early Proterozoic granitoids range in composition from I to S types. Geochemical and isotopic constraints favor an origin for Early Proterozoic Zhongtiao felsic magmas either by melting of sediments or tonalites in the crust or by melting of amphibolite/eclogite in a descending slab. Zhongtiao pelites are similar in composition to Phanerozoic shales and exhibit negative Eu anomalies on chondrite-normalized graphs. Stratiform copper deposits in the Zhongtiao Proterozoic rocks are of three types: volcanic-hosted deposits of hydrothermal or exhalative origin; primary sedimentary deposits; and remobilized sedimentary deposits. The Jiangxian and Zhongtiao lithologic assemblages of quartzite-pelite-carbonate together with primary textures and structures in these rocks suggest deposition in shallow-marine, tectonically stable basins, whereas coarse terrigenous sediments in the Danshanshi and Xiyanghe Groups appear to reflect near-shore marine to fluvial deposition in tectonically active basins. Most data are consistent with deposition of the Early Proterozoic supracrustal rocks in a continental-margin back-arc basin.