Circa 900 Ma low δ18O A-type rhyolite in the northern Yangtze Block: Genesis and geological significance

Abstract An integrated study of zircon U–Pb geochronology and O–Hf isotope compositions, and whole-rock geochemistry was carried out for Dashimen rhyolites from the Tongbai orogen in the northern Yangtze Block. Zircon grains from two rhyolite samples show oscillatory zoning and moderate U (498–1612 and 216–668 ppm) and Th (319–1144 and 161–452 ppm) contents with relatively high Th/U ratios (0.57–0.75 and 0.50–1.31), indicating their magmatic origin. Zircon U–Pb dating yielded consistent U–Pb ages of 906 ± 7 Ma (2σ; MSWD = 0.48) and 908 ± 4 Ma (2σ; MSWD = 0.68), representing the formation ages of the Dashimen rhyolites. The Dashimen rhyolites have relatively high SiO 2 (74.85–76.56 wt%), K 2 O (5.40–6.56 wt%) and Fe 2 O 3 T (1.61–2.33 wt%), and moderate Al 2 O 3 (11.48–12.32 wt%) and Na 2 O (1.76–2.47 wt%), but low MgO (0.15–0.24 wt%) and CaO (0.08–0.10 wt%) contents. They show strong enrichments of almost all incompatible elements, except for the notable depletions in Ba, Sr, Eu, and P. The volcanic rocks have high zircon saturation temperatures of 846 to 876 °C with an average of 862 °C. These features suggest that the Dashimen rhyolites are high-temperature, ferroan, A-type magmatism. The zircons from the two studied samples show δ 18 O values of 3.6 to 5.0‰ and 3.9 to 4.5‰ with weighted averages of 4.30 ± 0.18‰ and 4.23 ± 0.10‰, respectively. These δ 18 O values are obviously lower than those of the normal mantle zircons and thus suggest the zircons crystallized from low-δ 18 O magmas. The rhyolites have negative whole-rock e Nd (t) (−2.11–−0.28) and subtly positive zircon e Hf (t) (0.01–3.35) values, with corresponding two stage depleted mantle model ages of 1.73–1.58 Ga and 1.69–1.66 Ga, respectively. The low-δ 18 O magmas were therefore suggested to be generated by re-melting of meteoric-hydrothermally altered continental crustal rocks. The finding of the high-temperature, ferroan, and low-δ 18 O A-type Dashimen rhyolites indicates that there existed a back-arc rift of a Grenvillian accretionary orogen in the northern part of the Yangtze Block.