Metamorphism of the Wales Group and Moira Group on Prince of Wales and Dall Islands, southeastern Alaska

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] This study examines the metamorphic history of the Proterozoic to Early Ordovician Wales Group and the newly-named Ordovician to Silurian Moira Group exposed on Prince of Wales, Dall, and surrounding islands in southeastern Alaska. Three metamorphic events (M1, M2, and M3) affected the Wales Group and a single event (M3) occurred within the Moira Group. 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. The Precambrian M1 event is newly identified with this study and is restricted to the oldest Wales Group rocks. M3 is newly recognized within the Wales Group rocks, but has long been recognized in the Moira Group. Pressure-temperature (P-T) calculations were completed on Wales Group samples that reflect peak M2 conditions and P-T pseudosections were completed on Wales and Moira Group samples that reflect M3. All calculations were completed using THERMOCALC v. 3.1 (Powell and Holland, 1988, plus upgrades) and its dataset (Holland and Powell, 1985, 1990, 1998; Powell and Holland, 1985, 1988). Interpretation of the M2 results constrains a path from about 6 to 8 kbar near 500 to 525°C prior to achieving peak conditions near 590 to 650°C at about 9.7 to 10.8 kbar. Results from the M3 P-T pseudosections all suggest greenschist facies conditions. Further interpretation proposes conditions near 400°C at 2-4 kbar to nearly 500-550°C at 10 kbar. These calculated P-T conditions suggest regional metamorphism for the M2 and M3 events.--From public.pdf.