U-Th Dating of Zircons from a Holocene Volcanic Eruption (Dayingshan Volcano, Tengchong Volcanic Field): Insights into Magma Chamber Storage

Abstract

The Tengchong volcanic field is located along the southeastern margin of the Tibetan Plateau and has shown active volcanism throughout the Cenozoic. Only three volcanoes, however, have erupted more recently in the Holocene. The source of this volcanism is poorly understood due to several unique eruptive stages. The Holocene volcanic rocks derive from a potassic melt with high SiO2 content (62 wt. %) and an abundance of zircon grains. Zircon is renowned for its important role in geochronology and crustal evolution studies. This research utilizes U-Th dating methods and secondary ion mass spectrometry to date zircons from the Dayingshan volcano, which can be used to provide constraints on magma chamber storage time. Whole-rock major and trace element analyses from Dayingshan indicate high Zr concentrations and a calc-alkaline suite. Trace elements plotted on a spider diagram reflect magma generation involving hydrous fluids in the mantle overlying a downgoing lithospheric slab. Measured zircon cores yielded an age of 87.5 ± 6.5 ka, while zircon rims yielded an age of 58 ± 13 ka. These different ages represent two separate magmatic events. The older age population is interpreted as antecrysts remobilized from an earlier magmatic episode. The calculated magma chamber storage time for the most recent Dayingshan eruption is 48 ka. Trace element and isotopic data suggest hydrous melting of a stagnant slab from earlier subduction as the likely cause for the continental crust signatures of the magma.