Tectonothermal evolution of metamorphic rocks in the south-central Brooks Range, Alaska: Constraints from <sup>40</sup>Ar/<sup>39</sup>Ar geochronology

Abstract

To constrain the age of deformational/metamorphic events in the south-central Brooks Range, we analyzed 16 samples of white mica, amphibole, and biotite using the 40Ar/39Ar incremental heating technique. Metamorphic rocks in the study area (between 151°W and 148°W) occur in three principal east-west–trending faultbounded belts. These are, from south to north, the pumpellyite-actinolite–facies rocks of the Phyllite belt, the high-pressure/low-temperature (HP/LT) metamorphic rocks of the Schist belt, and the predominantly greenschist-facies metamorphic rocks of the Skajit allochthon. All three belts have been affected by two penetrative deformational/metamorphic events. The oldest of these (D1a) resulted in tight to isoclinal folding and was accompanied by pumpellyite-actinolite–facies metamorphism in the Phyllite belt, blueschist-facies metamorphism in the Schist belt, and blueschistto greenschist-facies metamorphism in the Skajit allochthon. Two white micas samples, one from the Schist belt, and one from the sodic-amphibole–bearing schists north of the Minnie Creek thrust (MCT) yielded convex-upward Ar-release spectra with maximum apparent ages of 142 and 129 Ma, respectively; we interpret maximum apparent ages as a minimum age for D1a deformation and HP/LT metamorphism in the south-central Brooks Range. A second synmetamorphic deformational event (D1b) affected all but the northernmost rocks of the Skajit allochthon, resulting in pervasive dynamic recrystallization accompanied by growth of metamorphic minerals; in the Schist belt and in sodicamphibole–bearing schists north of the MCT, D1b occurred under lower amphiboliteto greenschist-facies metamorphic conditions, and in the Phyllite belt, D1b occurred under lower greenschist(?)-facies conditions. Plateau, preferred, and isochron dates on white mica, amphibole, and biotite are Early Cretaceous in age, and range from 135 to 110 Ma. We argue that the predominance of Early Cretaceous dates result from the degassing of HP/LT metamorphic minerals during dynamic recrystallization associated with D1b folding and the pervasive growth of syn-D1b neoblasts. These ages are significantly older than Late Cretaceous to Tertiary(?) extensional structures in the southern Brooks Range, and support the interpretation that D1b structures formed during Brookian contractional deformation. Gottschalk, R. R., and Snee, L. W., 1998, Tectonothermal evolution of metamorphic rocks in the south-central Brooks Range, Alaska: Constraints from 40Ar/39Ar geochronology, in Oldow, J. S., and Ave Lallemant, H. G., eds., Architecture of the Central Brooks Range Fold and Thrust Belt, Arctic Alaska: Boulder, Colorado, Geological Society of America Special Paper 324.