Oxygen isotope evidence for large-scale interaction between meteoric ground waters and Tertiary Granodiorite Intrusions, Western Cascade Range, Oregon

Abstract

^(18)O data have been obtained on 7 of the 17 intrusive centers of diorite and granodiorite (Miocene?) that lie along a N-S belt about 50 km west of the High Cascades. Whole-rock δ values (per mil relative to SMOW) in the intrusive stocks are: +2.3 to +4.9 (South Umpqua River), −0.7 to +3.8 (Bohemia mining district), +4.3 to +4.6 (Vida stock), −0.2 to +5.5 (Nimrod stock), −1.7 to +0.5 (Detroit stock), −2.1 to +0.7 (Laurel Hill stock, Mt. Hood area), and +2.9 to +5.5 (Shellrock, Columbia River). The volcanic country rocks (mainly andesites) in the vicinity of the intrusions are also depleted in ^(18)O, with δ=−5.6 to +4.5. The rocks collected more than 3 stock diameters from an intrusive contact have ‘normal’ δ values of +5.8 to +8.2. Approximately 1200 km^2 (8% of the area of the Western Cascades) thus appears to be underlain by propylitically altered igneous rocks that have suffered an average ^(18)O depletion of about 5 to 7 per mil. These δ^(18)O effects are similar to those previously discovered in the Tertiary intrusive centers of western Scotland and the San Juan Mountains, Colorado. They are typically associated with (1) pervasive epidote and chlorite alteration; (2) ‘turbid’ feldspars; (3) granophyric textures and miarolitic cavities; and (4) young, jointed, flat-lying, volcanic country rocks that are known to be highly permeable to ground-water flow. Convective circulation of heated ground waters in the vicinity of the stocks must have occurred throughout a large part of their crystallization and cooling history. The amounts of H_2O involved are estimated to be about equal in volume to that of the exchanged rock. Hence, much so-called ‘deuteric’ alteration of igneous rocks is probably caused by such meteoric-hydrothermal waters rather than by H_2O released during magmatic crystallization.