Origin of volcanic ash beds across the Permian-Triassic boundary, Daxiakou, South China:Petrology and U-Pb age, trace elements and Hf-isotope composition of zircon

摘要

Abundant volcanic ash layers, largely altered to clay, characterize the Permian-Triassic boundary beds of the Daxiakou section, South China. Volcanism may have had an important role in the Permian-Triassic mass extinction, so it is important to understand the origin of these ash beds. The lithology of ash bed 260, as constrained by modes of crystal fragments (66.7% plagioclase, 7.2% alkali feldspar, and 26.1% quartz), suggests that the ash represents dacitic volcanism. The U-Pb ages, trace elements and Hf-isotope compositions of zircons from ten ash beds have been analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons can be divided into two groups:magmatic zircons (~97%, 227-279Ma) and inherited zircons (~3%, 678-2424Ma). Magmatic zircons have Y, Hf, Th and U contents and Nb/Ta ratios typical of zircons from silicic calc-alkaline volcanism. εHf(t) values of magmatic zircons vary from ?10.9 to +5.3, implying magmatic mixing between juvenile crustal material (probably Neoproterozoic in age) and ancient crustalmaterial (probably Archean). Beds 252-259-b have relatively depleted Hf-isotope compositions with ε_(Hf)(t) values of ?10.9-+5.3 and averages of ?5.0-?2.5, while other beds have ε_(Hf)(t) values of ?10.8-+4.4 (mostly ?10.8-?3.5) and averages of ?8.9-?6.8, implying more input of juvenile crustal material in the volcanism of Beds 252-259-b. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggests that the volcanismproducing the ashes took place along the convergent continent margins during the formation of the Pangea supercontinent. This intense silicic volcanism may have played an important role in causing the Permian-Triassic mass extinction in South China.