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

The Edgecumbe volcanics, which range from basalt through rhyodacite,have Sr contents between 125 and 370 p.p.m., Rb contents of1 to 70 p.p.m., Ba ranging from 50–550 p.p.m. and initial⁸⁷Sr/⁸⁶Sr ratios between 0.70291 and 0.70404. No simple correlationexists between these components and silica. The highest ⁸⁷Sr/⁸⁶Srvalues occur in a group of intermediate lavas (55–60 wt.per cent SiO₂) while the rhyodacites have initial ratios between0.7035 and 0.7038. With increasing silica, Sr increases to amaximum in the andesites and then steadily decreases; Ba andRb increase over the same compositional range. The highest ⁸⁷Sr/⁸⁶Srlavas have major and trace element concentrations which departfrom trends defined by most of the lavas. The variation in strontiumisotopic compositions suggests interaction between parentalbasaltic magma and crustal material. Attempts to model the assimilationprocess using fixed end-member assimilation and assimilation-fractionalcrystallization models have failed to produce the observed chemicaltrends. Because the parental basaltic liquid underwent littlefractionation, the variability in hybrid lavas is attributedto variation in contaminant composition. Initial melts werelow in CaO, A1₂O₃, MgO and Sr and enriched in SiO₂. K₂O, Na₂O,Rb and Ba. As melting progressed, melts became enriched in themore refractory components. Because hybrid strontium isotopiccomposition is a function of Sr concentration as well as isotopiccompositions, the Sr content of the assimilant strongly influencesresultant isotopic systematics. The development of the assimilantssuggests plagioclase was a residual phase during early melting.This model of crustal assimilation represents one end-memberin the spectrum of processes responsible for the generationof continental volcanic suites.