Oxygen and hydrogen isotope studies of the Cordilleran batholiths of western North America

Abstract

Isotopic analyses are now available for large portions of the Idaho, Boulder, Sierra Nevada, and Southern California batholiths in the United States and for the Coast Range, Okanagan, and Nelson batholiths of British Columbia. The primary δ D values of the biotites and hornblendes in all the batholiths are relatively constant at about -50 to -85‰. However, many of the individual plutons have interacted at high temperatures with circulating meteoric ground waters, producing broad areas of very low D/H ratios (δ D as low as -¹⁸O‰.) as well as more localised zones of low ¹⁸O/¹⁶O ratios (δ¹⁸O as low as - 7‰). In part because of latitudinal isotopic variations in ancient meteoric waters, as one moves north through the batholiths of western North America an increasingly higher percentage of the plutonic igneous rocks has OH-bearing minerals with δ D values lower than -120‰: Southern California (0%), Sierra Nevada (5%), Boulder (25%), Idaho (40%), Coast Range, B.C. (50%). Okanagan (55%), Nelson (75%), Coast Range, Alaska (60%). The combination of low δ D values and "normal" δ¹⁸O values that is commonly observed over wide areas in the more northerly batholiths indicates meteoric water/ rock ratios of less than 0.10. The lowering of δ D values commonly correlates with resetting of K-Ar ages and with proximity to plutons of early Tertiary age. In the Idaho batholith, for example, two broad zones (termed the Atlanta and Bitterroot lobes, totalling 10000 km² in area) can be defined where δ D biotite < -100‰, and K-Ar "ages" have all been reset to values less than 60 m.y. , suggesting that the Ar loss was caused by the meteoric-hydrothermal convective circulation systems. Plutonic igneous intrusions have been emplaced throughout the Cordilleran orogenic belt during most of Mesozoic and Cenozoic time. but this igneous activity is compressed into a much narrower belt in Idaho, Washington, and British Columbia than in the south-western U.S.A. The meteoric-hydrothermal effects are much more prominent in the northern batholiths than in the Sierra Nevada and Southern California batholiths because the latter have been much less affected by shallow-level batholithic intrusions during the early and middle Tertiary.

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