SUPERPOSED MAGMATIC BELTS IN THE GLACIER BAY NATIONAL PARK REGION, SOUTHEASTERN ALASKA--A PRELIMINARY REPORT ON GEOLOGY AND POTASSIUM-ARGON AGES OF INTRUSIVE ROCKS

Glacier Bay National Park, Alaska, contains spectacular mountain peaks as well as spectacular glaciers; many of those peaks, including the highest one--Mount Fairweather (15,300 feet), are underlain by intrusive igneous rocks. The intrusive rocks also form many of the remarkably steep shorelines in Glacier Bay proper.

Intrusive rocks in the Glacier Bay region are in three main regional magmatic belts related to Cretaceous and Tertiary subduction. Altogether the intrusives have been classified into fifteen different chronometric-compositional units that range in age from Jurassic through middle Cretaceous to late Tertiary and in composition from gabbro to granite and syenite. Almost all samples are calc-alkalic and metaluminous. In general, the middle Cretaceous units intrude rocks of the Alexander and Wrangellia lithotectonic terranes, the units of mid-Tertiary age intrude rocks of the Chugach, Alexander, and Wrangellia terranes, and those of Neogene age are found in the Alexander terrane.

The middle Cretaceous rocks are dominantly foliated tonalite, quartz diorite, diorite, and granodiorite; the mid-Tertiary ones are gabbros, granodiorites, and granites; and the Neogene rocks are dominantly quartz syenite and quartz monzonite. More specifically, the middle Cretaceous intrusive rocks are grouped into biotite-hornblende granodiorite, biotite-hornblende tonalite and quartz diorite, and hornblende diorite units. The mid-Tertiary intrusive rocks are grouped into two suites, a gabbroic and related granitic rock suite and a granitic suite. The gabbroic and related granitic rock suite includes layered hornblende and pyroxene gabbro, layered magnetite-bearing gabbro, layered pyroxene gabbro, and layered pyroxene-hornblende gabbro units, together with some granitic rocks. The granitic rock suite includes garnet-biotite granodiorite, hornblende-biotite granodiorite, hornblende-biotite granite and granodiorite, and biotite-hornblende quartz diorite units. The Neogene intrusive rocks include hornblende quartz monzonite, porphyritic (pyroxene-)quartz syenite, and hornblende andesite units.