Geochronology and geochemistry of the fossil-flora-bearing Wuda Tuff in North China Craton and its tectonic implications

Abstract

The Wuda Tuff Flora in the North China Craton (NCC) is a “vegetational Pompeii” and one of the most completely preserved marsh ecosystems of the late Paleozoic Northern Hemisphere. However, the precise age, geochemistry, and petrogenesis of the thick tuff containing this peat-forming flora have not been established. We conducted a comprehensive petrographic, mineralogical, whole-rock geochemical, and zircon UPb and HfO isotopic investigation of five samples from a vertical section through the tuff bed. Our results confirm that the material that buried the flora is a volcanic tuff bed that has undergone intensive post-deposition alteration. Whole-rock Al2O3/TiO2 ratios (65.8–103.5) and negative Eu/Eu* values (0.27–0.38) further suggest a felsic volcanic origin. Secondary ion mass spectrometry zircon UPb dating of the altered tuff bed yielded a weighted mean 206Pb/238U age of 295.9 ± 1.4 Ma (1σ, MSWD = 0.94, n = 39), constraining the age of the Wuda Tuff Flora to the earliest Permian. Depletions of Nb and Ta in both the whole-rock and zircon compositions indicate an arc-related setting for the volcanism. The positive εHf(t) values (0.0 to +5.0) and δ18O values (5.05‰–6.17‰) of zircon grains from the Wuda Tuff suggest a juvenile crust origin of the volcanic magma. Geochemical comparisons with contemporaneous igneous rocks suggest that the volcanic eruption that led to the formation of the Wuda Tuff and associated vegetational Pompeii was most probably caused by arc volcanism along the western margin of the NCC related to subduction of Paleo-Asian Ocean oceanic crust, implying that the Alxa Terrane had not yet amalgamated with the NCC during the early Permian.