P-T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

The southern Alaska convergent margin contains several small belts of sedimentary and volcanic rocks metamorphosed to blueschist facies, located along the Border Ranges fault on the contact between the Wrangellia and Chugach terranes. These belts are significant in that they are the most inboard, and thus probably contain the oldest record of Triassic-Jurassic northward-directed subduction beneath Wrangellia. The Liberty Creek HP-LT schist belt is the oldest and the innermost section of the Chugach terrane. Within this belt lawsonite blueschists contains an initial high-pressure assemblage formed by lawsonite + phengite + chlorite + sphene + albite +/- apatite +/- carbonates and quartz. Epidote blueschists are composed of sodic, sodic-calcic and calcic amphiboles + epidote + phengite + chlorite + albite + sphene +/- carbonates and quartz. P-T pseudosections computed from four representative samples constrain maximum pressures at 16 kbar and 250-280 degrees C for the Lawsonite-bearing blueschists, and 15 kbar and 400-500 degrees C for the epidote-bearing blueschists, suggesting a initial subduction stage of 50-55 km depth. The growth of late albite porphyroblasts in all samples suggests a dramatic decompression from ca 9 kbar to 5 kbar. The Liberty Creek schists can be correlated with the Seldovia blueschist belt on the Kenai Peninsula. Metamorphism in both terranes took place in the Early Jurassic (191-192 Ma), recording an early stage of subduction beneath Wrangellia. In the nearby terranes of the same margin, the age of metamorphism records an early stage of subduction at 230 Ma. Based on this difference in age, a maximum of 40 Ma were necessary to subduct the protoliths of the Seldovia and Liberty Creek blueschists to depths of circa 50-55 km, suggesting a minimum vertical component of subduction of 1.2-1.5 cm/year. (C) 2010 Elsevier B.V. All rights reserved.