Early Paleoproterozoic tectono-magmatic and metamorphic evolution of the Yuanmou Complex in the southwestern Yangzte Block

Abstract

Paleoproterozoic rocks are rare and essential for understanding the early evolution of the Yangtze Block. We present the newly defined Early Paleoproterozoic gneisses from the Yuanmou Complex at the western boundary of the Yangtze Block by a combined study of the structure, petrology, U-Pb age and Hf isotopic composition of zircons, and whole-rock geochemistry. The magmatic origin of inherited and detrital zircons, underlined by high Th/U ratios, of an orthogneiss and a paragneiss yield four main cluster ages of ca. 2500–2300, 2200–1770, 1580–1200, and 1100–1000 Ma. Zircon rims yield two main age groups, 970–840 and 780–750 Ma, which document the timing of overprinting metamorphic/partial melting events. Geochemical data of the orthogneiss show S-type granite characteristics. The Neoarchean to Early Paleoproterozoic ages (2500–2300 Ma) have positive εHf(t) values and two-stage model (TDM2) ages of 2760–2688 Ma, indicating a magmatic source from the depleted mantle with no crustal material involved. The Early to Middle Paleoproterozoic (2200–1770 Ma) zircon cores of mainly negative εHf(t) values and corresponding TDM2 ages (2800–2400 Ma) indicate magmatic materials predominantly from ancient (Neoarchean) crustal material with a small amount of depleted mantle material. The Early to Middle Mesoproterozoic (1580–1200 Ma) zircons indicate a magma originating from ancient crust, with minor input of mantle-derived material. The εHf(t) values of zircon rims indicate reworking of ancient crust based on the Neoproterozoic geological events (two major age groups of ca. 970–840 and 780–750 Ma), which were associated with the post-collision and partial melting events. The 2500–2300 Ma age group corresponds to the Neoarchean magmatism underneath the southwestern Yangtze Block, and the 2200–1770 Ma age group corresponds to the assembly of the Columbia supercontinent with related continental collision. The wide range of zircon cores with the Lu-Hf isotopic data provide strong evidence regarding the assembly and breakup of different stages of the supercontinent, as well as critical information on the evolution of the Yangtze Block in South China.