Assimilation of Crustal Material by Basaltic Magma: Strontium Isotopic and Trace Element Data from the Edgecumbe Volcanic Field, SE Alaska

Abstract

The Edgecumbe volcanics, which range from basalt through rhyodacite, have Sr contents between 125 and 370 p.p.m., Rb contents of 1 to 70 p.p.m., Ba ranging from 50-550 p.p.m. and initial l7Sr/**Sr ratios between 0-70291 and 0-70404. No simple correlation exists between these components and silica. The highest Sr/^Sr values occur in a group of intermediate lavas (55-60 wt. per cent SiO2) while the rhyodacites have initial ratios between 0-7035 and 0-7038. With increasing silica, Sr increases to a maximum in the andesites and then steadily decreases; Ba and Rb increase over the same compositional range. The highest l7Sr/86Sr lavas have major and trace element concentrations which depart from trends defined by most of the lavas. The variation in strontium isotopic compositions suggests interaction between parental basaltic magma and crustal material. Attempts to model the assimilation process using fixed end-member assimilation and assimilation-fractional crystallization models have failed to produce the observed chemical trends. Because the parental basaltic liquid underwent little fractionation, the variability in hybrid lavas is attributed to variation in contaminant composition. Initial melts were low in CaO, Al2Oj, MgO and Sr and enriched in SiO2. K2O, Na2O, Rb and Ba. As melting progressed, melts became enriched in the more refractory components. Because hybrid strontium isotopic composition is a function of Sr concentration as well as isotopic compositions, the Sr content of the assimilant strongly influences resultant isotopic systematics. The development of the assimilants suggests plagioclase was a residual phase during early melting. This model of crustal assimilation represents one end-member in the spectrum of processes responsible for the generation of continental volcanic suites.