GEOLOGY OF THE CARIBOU CREEK AREA, BIG DELTA NORTHEAST C4 AND SOUTHEAST D4 QUADRANGLES, EAST-CENTRAL ALASKA
Contacts, faults, and mappable units were identified and constrained in part using geophysical data. A high-angle fault, oriented 235° and steeply dipping NW, separates greenschist facies (NW) and amphibolite facies rocks (SE). Greenschist facies rocks include a bimodal greenstone/metarhyolite assemblage, quartzite, phyllite and metagrit. Amphibolite facies rocks include amphibolite, schist, sediment-derived quartzo-felspathic semi-schist, and orthogneiss. Quartz diorite bodies (108 Ma) and granodiorite plutons (92 Ma) regionally intrude both units. Tourmaline-muscovite granite, similar to a nearby body previously dated at 114 Ma (U-Pb), intrudes amphibolite-facies rocks.
Mapping and petrography delineate 10-20 km2 blocks, bounded by high-angle faults, which contain contrasting rock units and mineral assemblages. A central block contains abundant post-kinematic andalusite. Blocks east and west contain post-kinematic sillimanite; and along with the displacement of plutonic bodies, suggest the central block was down-dropped. Most blocks (depending on bulk composition) contain syn-kinematic garnet and staurolite and partly replaced kyanite. We conclude that the amphibolite-facies rocks experienced moderate-pressure (kyanite-stable) conditions followed by a collisional event associated with low-P and variable-T post-kinematic recrystallization.