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

Abstract

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.