Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 14
Presentation Time: 8:00 AM-5:00 PM


LESSARD, Richard R.1, NEWBERRY, Rainer J.2, KESKINEN, Mary J.2, LAYER, Paul W.3 and WERDON, Melanie B.4, (1)Geology and Geophysic, Univ of Alaska, Fairbanks, 900 Yukon Drive, PO Box 757320, Fairbanks, AK 99775, (2)Geology and Geophysics, Univ of Alaska Fairbanks, Fairbanks, AK 99775, (3)Geophysical Institute, Univ of Alaska, Natural Sciences Building, 900 Yukon Drive, P.O. Box 755780, Fairbanks, AK 99775-5780, (4)Alaska Div of Geol & Geophysical Surveys, 3354 College Road, Fairbanks, AK 99709-3707,

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.