Guang-Hui Wu

The tectonic setting of the widespread ca. 500–400 Ma tectono-thermal events in in the Tethyan domain remains controversial, with competing interpretations invoking either the waning stage of the Pan-African orogeny or subduction of the Proto-Tethyan Ocean. Here we present U–Pb isotopic ages and trace elemental compositions of zircon, along with major and trace elemental, and Sr–Nd isotopic compositions of whole-rock for granitic gneisses and their associated amphibolites and surrounding low-grade metasedimentary rocks from the Baoshan Block, a segment of the Sibumasu Block in southwestern China. The collected granitic gneisses yield zircon ages of 475–460 Ma. They have high SiO2 and K2O, but low MgO and TFe2O3 contents. Their A/CNK ratios range from 1.08 to 2.47, and εNd(t) values from −6.48 to −9.47 with TDM2 of 1.96–1.72 Ga. The detrital zircons from surrounding strata yield major and subordinate U–Pb age peaks at ~0.97 and ~2.42 Ga, respectively, with the youngest age of 554 Ma. The εNd(t) values of metasedimentary rocks and amphibolites respectively vary from −11.39 to −13.55, and from +1.60 to +0.69. Major oxides exhibit pronounced compositional gaps between invested granitic gneisses and their associated amphibolites. We propose that peraluminous S-type granitic melts in early Paleozoic were derived mainly from partial melting of deeply buried ancient metasedimentary materials, heated by underplated mantle-derived mafic magmas. Comparative geochemical proxies based on zircon P contents, Eu anomalies, U/Yb ratios and Ce/(U×Ti)1/2, reveal that 500–450 Ma granitic magmas are characterized by sediment-dominated, shallower sources, weaker arc affinity, and lower oxygen fugacity, relative to those of Pan-African orogens during 600–500 Ma. The ca. 500–400 Ma tectono-thermal activities can be attributed to the subduction of the Proto-Tethys oceanic plate.