Petrologic and age constraints on the origin of a low‐pressure/high‐temperature metamorphic complex, southern Alaska

Abstract

The interrelationships between metamorphism, deformation, magma intrusion, and40Ar/39Ar geochronology were determined for a low‐pressure/high‐temperature metamorphic complex which formed from an accretionary prism in the Chugach Mountains, southern Alaska. Compressional deformation, which first produced south verging folds and associated thrusts, was followed by magma intrusion and development of north verging folds. Synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall, as documented by the distribution of mineral assemblages and by pressure‐temperature modeling of garnet growth. The initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust. Regionally developed amphibolite facies metamorphism followed intrusion of felsic sills. The peak metamorphic conditions derived from geothermobarometry, mineral assemblages, and fluid inclusions ranged from 400° to 600°C at a depth of ∼10 km. The increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite‐bearing migmatites. Subsequent cooling was initially rapid (≈55°C/Ma) to ∼350°C based on40Ar/39Ar dates of 53 Ma for hornblende and 50 Ma for biotite and may have slowed to ∼11°C/Ma until 200°C based on an40Ar/39Ar date of 35 Ma for plagioclase. Intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds. This suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism.