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TRACE-ELEMENT EVIDENCE FOR VOLATILE-INFLUENCED DIFFERENTIATION IN A FLOW OF ALKALI BASALT, PENG HU ISLAND, TAIWAN

John D. Greenough^1, and Brian J. Fryer²

¹ Department of Earth and Environmental Sciences, University of British Columbia – Okanagan, 3333 University Way, Kelowna, British Columbia V1V 1V7, Canada
² Great Lakes Institute for Environmental Research, University of Windsor, Windsor, Ontario N9B 3P4, Canada

E-mail address: john.greenough{at}ubc.ca

A 20-m-thick alkali basalt flow on the Peng Hu Island, Taiwan, shows three well-developed segregation veins in its upper 8.5 meters. Each pegmatitic and "vesicular" (ocelli-bearing?) segregation displays distinct whole-sample chemical traits that are shared with the enclosing basalt. Augite, plagioclase and olivine exhibit major-element (electron-microprobe data) and trace element compositions (laser ablation microprobe – inductively coupled plasma – mass spectrometry) that reflect this whole-rock chemical stratification. Thus the chemical stratification is a product of igneous processes and is not the result of secondary alteration. Elements defining the stratification (K, Rb, Li, Na, Zn in minerals and whole-rock data, but Cl, S, As, Pb and Sr are also important based on whole-rock data) tend to be complexed and moved by volatiles in various geological environments. Conventional crystal-fractionation models do not reproduce the observed variations in these elements up through the flow. The patterns of data suggest that rising plumes of vesicles carried volatile-complexed (scavenged) elements to high levels in the flow at the time the segregation veins were forming and the interior of the flow was largely molten.

Keywords: alkali basalt, trace-element geochemistry of minerals, differentiation, layering, volatiles, segregation veins, laserablation ICP–MS, Peng Hu Island, Taiwan.