POSTERUPTIVE ALTERATION OF SILICIC IGNIMBRITES AND LAVAS, GRAN-CANARIA, CANARY-ISLANDS - STRONTIUM, NEODYMIUM, LEAD, AND OXYGEN ISOTOPIC EVIDENCE

Isotopic analyses of Miocene comenditic, pantelleritic, and trachyphonolitic ignimbrites and lavas from Gran Canaria, Canary Islands, provide evidence for posteruptive mobility of Rb, Sr, and O. Calculated initial Sr-87/Sr-86 ratios in whole-rock samples from basaltic lavas and feldspar mineral separates from ignimbrites define a magmatic trend in the stratigraphic section, from ratios of 0.70340 at the base of the Mogan Formation to 0.70305 in the lower Fataga Formation. However, calculated apparent initial Sr-87/Sr-86 ratios in hydrated vitrophyre and devitrified matrix separates range from 0.7035 to 0.7090. DeltaO-18 ratios in basalts and feldspars vary little, from +5.7 to +6.1, yet range from +6,5 to +15.0 in the ignimbrite matrices. In contrast to the Sr and O isotope ratios, Pb and Nd isotope ratios are identical within analytical error in feldspars and their silicic ignimbrite matrices. Sequential leaching experiments and the oxygen data suggest that low-temperature, posteruptive interaction with meteoric water, perhaps containing a small seawater component, has modified Rb and Sr concentrations in the matrices, such that calculated apparent initial Sr-87/Sr-86 ratios are not those of the magmas when they were erupted. Mobilization of Rb and Sr must occur significantly after eruption. Nd and Pb isotope systems appear to be unaffected by this process. Therefore, Sr-87/Sr-86 ratios determined by whole rock analysis of silicic rocks from hotspot-type oceanic islands are suspect and should not be incorporated into mantle tracer studies, although analysis of phenocrysts may produce useful data.