KYANITE AFTER ANDALUSITE IN THE CONTACT AUREOLE OF AN EPIDOTE-BEARING PLUTON NEAR PETERSBURG, ALASKA: CONFIRMATION OF RAPID TECTONIC BURIAL IN SOUTHEAST ALASKA?

Douglass and Brew (1984) mention the occurrence of kyanite after andalusite in the contact aureole of epidote-bearing Mid-Cretaceous tonalite and granodiorite plutons near Petersburg, southeast Alaska. These plutons are part of the Mid-Cretaceous Coast Plutonic Complex that extends for >1000 km along the North American Cordillera. Also near Petersburg, Himmelberg et al. (2004) describe and document the replacement of andalusite by kyanite ± staurolite ± sillimanite but were not able to find this texture associated with epidote-bearing plutons. In this contribution, we show that magmatic epidote is present in Mid-Cretaceous plutons on Zarembo and Wrangell islands, and that the unusual occurrence of kyanite replacing andalusite is present in the contact aureole of an epidote-bearing pluton on Wrangell Island. The presence of magmatic epidote suggests a crystallization pressure in excess of ~600 MPa for this pluton, and the Al-in-hornblende barometer and hornblende-plagioclase thermometer yield 804 MPa and 668°C. Geothermobarometry from the contact aureole containing kyanite after andalusite pseudomorphs yields 661 MPa and 614°C.

The shape preferred orientation of hornblende in the pluton, and the kyanite after andalusite porphyroblasts in the county rocks are sub-parallel, and form well-defined mineral lineations plunging 40° toward ENE. The concordance of fabrics in the pluton and country rocks and the lack of sub-solidus textures in the pluton support the conclusions of Himmelberg et al. (2004) that intrusion of the Mid-Cretaceous plutons were synchronous with regional NE-SW shortening. In addition, our confirmation of kyanite after andalusite in the contact aureoles of epidote-bearing plutons supports the hypothesis of rapid magma transport to shallow crustal levels (<380 MPa, the maximum stability of andalusite), followed by rapid tectonic burial before magmatic epidote could be resorbed into the melt, and before final crystallization of the pluton as recorded by Al-in-hornblende barometry.

References: Douglass, S.L., and Brew, D.A., 1985, USGS Circular 967, p. 89-92; Himmelberg, G.R., Haeussler, P.J., and Brew, D.A., 2004, Can. J. of Earth Sci., v. 41, p. 87-102.