%0 Journal Article
%A López Carmona, Alicia
%A Kusky, Timothy M.
%A Santosh, M.
%A Abati Gómez, Jacobo
%T P–T and structural constraints of lawsonite and epidote blueschists from LibertyCreek and Seldovia: Tectonic implications for early stages of subduction along thesouthern Alaska convergent margin
%D 2011
%@ 0024-4937
%U https://hdl.handle.net/20.500.14352/44187
%X The southern Alaska convergent margin contains several small belts of sedimentary and volcanic rocksmetamorphosed to blueschist facies, located along the Border Ranges fault on the contact between theWrangelliaand Chugach terranes. These belts are significant in that they are the most inboard, and thus probably contain theoldest record of Triassic–Jurassic northward-directed subduction beneath Wrangellia. The Liberty Creek HP-LTschist belt is the oldest and the innermost section of the Chugach terrane.Within this belt lawsonite blueschistscontains 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 fromfour representative samples constrain maximumpressures at 16 kbar and 250–280 °C for the Lawsonite-bearingblueschists, and 15 kbar and 400–500 °C for the epidote-bearing blueschists, suggesting a initial subduction stageof 50–55 km depth. The growth of late albite porphyroblasts in all samples suggests a dramatic decompressionfromca. 9 kbar to 5 kbar. The Liberty Creek schists can be correlated with the Seldovia blueschist belt on the KenaiPeninsula.Metamorphismin both terranes took place in the Early Jurassic (191–192 Ma), recording an early stageof subduction beneathWrangellia. In the nearby terranes of the same margin, the age ofmetamorphismrecordsan early stage of subduction at 230 Ma. Based on this difference in age, a maximum of 40Ma were necessary tosubduct the protoliths of the Seldovia and Liberty Creek blueschists to depths of circa 50–55 km, suggesting aminimum vertical component of subduction of 1.2–1.5 cm/year.
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