Progressive metamorphism of schists recovered from a deep drill hole near Fairbanks, Alaska

Abstract

In 1965, a deep test hole drilled near Eielson Air Force Base, Fairbanks district, Alaska, penetrated 9,774 ft (2,979.1 m) into schists of the metamorphic complex of the Yukon-Tanana Upland. Cores recovered from the test hole show that the section is dominated by calc-magnesian rocks with subordinate pelitic schists. Pelitic mineral assemblages define a progressive increase in metamorphic grade with depth, from the garnet to the kyanite isograd. Diopside first appears in the calc-magnesian schists that were cored at a depth of 9,766 ft (2,976.5 m), indicating the onset of the reaction: Tremolite + 3 calcite -f- 2 quartz→5 diopside + 3CO 2 +H 2 O. Ubiquitous staurolite and rare andalusite occur in the kyanite-bearing pelitic schists. Andalusite appears to have crystallized under postkinematic conditions, and the staurolite is apparently of both synkinematic and postkinematic origin. Hornblende and biotite from calc-magnesian schists sampled at depths of 7,142½ ft (2,176.9 m) and 9,766 ft (2,976.5 m) gave 40 K/ 40 Ar ages of 140±8 and 57.3±1.9 m.y., respectively. The hornblende age is believed to represent the age of the latest synkinematic metamorphism, and the biotite age appears to be an anomalously young one related to the outgassing of argon from biotite at greater depth in the section. Recent experimental data on andalusite-kyanite-sillimanite and staurolite equilibria and the stability field of calcite+quartz+tremolite versus diopside in calc-magnesian rocks suggest that the rocks recovered from the 8,218- to 9,770-ft (2,504.7- to 2,977.7-m) interval were synkinematieally recrystallized at crustal depths of 17 to 19 km and at temperatures of 515° to 580°C; the present thermal gradient (31.5°C/km) is similar to that which accompanied metamorphism in Jurassic time.