Isotopic evidence for meteoric-hydrothermal alteration of plutonic igneous rocks in the Yakutat Bay and Skagway areas, Alaska

Abstract

Oxygen and hydrogen isotope ratios were measured on coexisting minerals from quartz diorites and quartz monzonites from a section across the Coast Range batholith in the Skagway area, Alaska, including a variety of outlying plutons west of the batholith in the Yakutat Bay-Mt. St. Elias region (latitudes 59–60°N). The extremely low and variable δ18Oand δD results indicate widespread meteoric-hydrothermal alternaiton of the Coast Range batholith, and to a lesser extent, of the Yakutat Bay plutons as well. In the Yakutat Bay area, the plutons with K—Ar ages younger than 50 m.y. have widely varying δD values of −72 to −148, compared to δD = −69to−90 for all but one sample in the 50–225 m.y. age grouping (one biotite has δD = −109). This suggests that the major meteoric-hydrothermal episodes in this area occurred during the Eocene and Miocene. This involved relatively small meteoric water/rock ratios(<0.1), as none of the δ18O values show any clear-cut evidence of alteration (δ18Oquartz= 7.4 −11.8; δ18Ofeldspar= 5.7−10.0). However, in the section across the Coast Range batholith, 85% of the plutonic rocks have very low δD values of −100 to −167, and the δ18O values are extremely variable δ18Ofeldspar= + 10.3to−4.0 and Δ18Oquartz-feldspar= 0.4−10.5. These data indicate that a major portion of the batholith, particularly the quartz monzonite-rich eastern part, but also including many of the quartz diorite plutons as well, interacted with meteoric-hydrothermal convective systems that involved water/rock ratios of about 0.3–1.4. The quart diorite plutons are most depleted in18O near their northeast contacts against the younger quartz monzonite intrusions. The primary igneous δ18O values of the quartz diorites were apparently higher than those of the quartz monzonites; they are also unusually high in18O compared to most other analyzed quartz diorites, suggesting derivation from, exchange with, or assimilation of high-18O metasediments or altered volcanic rocks. These data and conclusions are very similar to those reached previously on a similar isotopic study of the Coast Range batholith in British Columbia, 700 km to the southeast at latitudes 54–55°N, except that in the Skagway area an even greater proportion of the batholith was apparently depleted in deuterium. This implies that deep (≉5km?) circulation of meteoric groundwaters is probably a characteristic of the later stages of emplacement of the Cordilleran batholiths of western North America, suggesting that the eastern sections of these batholiths in particular were emplaced at relatively shallow depths.

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