GEOLOGY OF THE CARIBOU CREEK AREA, BIG DELTA NORTHEAST C4 AND SOUTHEAST D4 QUADRANGLES, EAST-CENTRAL ALASKA

The Caribou Creek area, located on the northern margin of an alleged sillimanite gneiss dome in the Yukon Tanana Terrane, is reported to contain a gradual S to N decrease in metamorphic grade. The higher-grade rocks are truncated against greenschist facies rocks by a variously-interpreted (thrust, detachment, high-angle) fault. These geotectonic speculations are based on reconnaissance-scale (1:250,000) regional mapping (Weber et al., 1978). This study presents new geologic mapping (1:25,000) of a 200 km² area, conducted over an 11 weeks period, and utilizes 400m line-spacing airborne magnetic, electromagnetic and radiometric surveys released by the State of Alaska in 2000. Follow up investigations include feldspar staining (200), ⁴⁰Ar/³⁹Ar dating (5), thin section examination (150), and major/trace element determination (30). Private exploration companies made available 1196 additional rock and soil analyses.

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.