CRYSTALLIZATION DEPTHS OF CRETACEOUS PLUTONS ALONG THE WRANGELL TRANSECT, SOUTHEAST ALASKA

Cretaceous granodiorite, tonalite, and quartz diorite plutons intrude rocks of the Alexander terrane and Western Metamorphic Belt across a >100 km wide zone along the western margin of North America at the latitude of Wrangell, Alaska (56°N). Metamorphic grade of the country rocks increases from west to east, nearly perpendicular to the NW strike of the predominately east-dipping foliation and thrust faults in the metamorphic rocks of the Western Metamorphic Belt. Al-in-hornblende barometry and phase equilibria from plutons that intrude the Alexander terrane consistently yield low pressures (250 to 400 MPa) while plutons that intrude the Western Metamorphic Belt yield higher pressures (730-780 MPa). One sample from a Paleocene pluton east of the Coast shear zone yielded a pressure of 600 MPa.

The lower pressures recorded by the plutons in the west are consistent with the lack of obvious contact aureoles in the country rocks, suggesting that these plutons intruded relatively cool rocks at shallow (<15 km) depth. The higher pressures recorded by the plutons that intrude the Western Metamorphic Belt are consistent with the presence of magmatic epidote in the plutons. However, the presence of andalusite pseudomorphs in the contact aureoles of some Cretaceous plutons (e.g. Woronkofski Island) contradicts the high pressures recorded by hornblende barometry and magmatic epidote. This apparent conundrum suggests that the application of the Al-in-hornblende barometer and phase equilibria in these rocks is problematic. Alternatively, these rocks record a complicated intrusion, crystallization, and deformation history for the Western Metamorphic Belt.