The Cascade Mountains revisited: A re-evaluation in light of new lead isotopic data

Abstract

Lead isotopic analyses have been repeated using silica gel for several samples from the Cascade Mountains which were previously analyzed by lead sulfide. The improved precision indicates that some of the scatter in the original data was due to thermal fractionation; however, the bulk of the data have not changed significantly. Two-point mixing lines are demonstrated for main cone-satellitic cone pairs from Glacier Peak, Mt. Baker and Mt. Shasta. Comparison with data on oceanic basalts from the Juan de Fuca and Gorda Ridge area indicates that hypothesis of mixing of mid-ocean ridge (MOR) basalt lead and “alkali basalt-like” lead from the oceanic crust is not tenable. Lead isotope analyses of pre-Astoria Fan sediments from DSDP Leg 18 sites and from the Eocene Tyee Formation indicate that the sedimentary continental detritus from the North American continent has the correct lead isotopic composition to be the continental component necessary to account for the Cascade Mountains lead isotopic array by mixing with Juan de Fuca-Gorda Ridge MOR basalts. However, from recent work on the structure of oceanic trenches by Karig and Sharman (1975), it does not appear that subduction of sediments is the rule. A model of crustal contamination and/or assimilation at the crust/mantle interface is the preferred explanation for the lead isotopic data from the Cascade Mountains.