RECOGNITION OF THREE METAMORPHIC EVENTS WITHIN THE WALES GROUP ON PRINCE OF WALES AND DALL ISLANDS, SOUTHEASTERN ALASKA

Three metamorphic events affected the Precambrian-Cambrian Wales Group on Prince of Wales and Dall Islands, southeastern Alaska. Radiometric dating and cross-cutting relationships define the M1, M2, and M3 events as Precambrian (pre-550Ma), Early Ordovician (480-470 Ma), and Early Devonian (410-400 Ma), respectively. M1 is a newly identified event in southeastern Alaska and M3 is newly recognized within the Wales Group rocks, and is structurally expressed in the Wales Group and younger rocks.

Metabasites are the most diagnostic rocks in the Wales Group, indicating that M1 and M2 achieved greenschist to amphibolite facies, and M3 achieved greenschist facies conditions. The majority of petrographically determined mineral assemblages are attributed to M2, with extensive M3 alteration. Garnet chemistry and observed microstructures and textures suggest that relict M1 porphryoblast cores may still be present, although clear identification is difficult due to the proposed similarities between M1 and M2 conditions.

Pressure-temperature (P-T) calculations were completed on samples interpreted to represent peak M2 conditions, using THERMOCALC v. 3.1 with the updated thermodynamic database (Holland and Powell, 1998) in average P-T mode, with a pure water fluid phase and activities defined by the ax program (Holland and Powell, 1998). The P-T results range from approximately 610 to 690 °C and 8.4 to 10.1 kbar. The incorporation of X_CO2 into the fluid phase decreases the resultant pressures and temperatures, and is a reasonable assumption due to the abundance of metamorphic carbonates. Application of conventional geothermobarometers yield similar P-T results.

Pseudosections developed for the metabasites, also constructed in THERMOCALC v. 3.1 using the thermodynamic dataset and activity models of Holland and Powell (1998), help compensate for the scarcity in texturally identifiable mineral reactions and suitable assemblages for P-T calculations. They are modeled in a simplified NCFMASH system, using excess H₂O to simulate prograde mineral reactions related to pressure and temperature. The input bulk composition and variable values are derived from mineral and whole rock chemistry analyses. The pseudosections are compatible with our interpretation of the metamorphic history of the Wales Group.