Publication: 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
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Publication Date
2011
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Elsevier Science B.V., Amsterdam
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Abstract
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 theWrangellia
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 maximumpressures at 16 kbar and 250–280 °C for the Lawsonite-bearing
blueschists, and 15 kbar and 400–500 °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
fromca. 9 kbar to 5 kbar. The Liberty Creek schists can be correlated with the Seldovia blueschist belt on the Kenai
Peninsula.Metamorphismin both terranes took place in the Early Jurassic (191–192 Ma), recording an early stage
of subduction beneathWrangellia. In the nearby terranes of the same margin, the age ofmetamorphismrecords
an early stage of subduction at 230 Ma. Based on this difference in age, a maximum of 40Ma 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.