GEOLOGIC MAPPING, STRUCTURAL ANALYSIS, AND GEOCHRONOLOGY OF THE BENDELEBEN MOUNTAINS METAMORPHIC COMPLEX, SEWARD PENINSULA, ALASKA

Mapping at 1:24,000 scale in the central Bendeleben Mountains, Seward Peninsula, Alaska covered ~500 km² in one month during summer 2006. Map units include upper-amphibolite facies biotite-garnet-sillimanite-schist, biotite quartzite, marble and minor calc-silicate rocks. Igneous rocks include highly strained granitic orthogneiss, foliated granitic dikes, folded garnet-bearing granitic pegmatite dikes, minor diabase dikes, abundant undeformed granitic dikes, and the undeformed biotite granite Pargon and Bendeleben plutons. Some evidence of partial melting in metasedimentary units was present on the north side of the Pargon pluton but large migmatite zones were not observed.

There are two styles of pre-Late Cretaceous deformation: 1) E-W trending dome-like structural culminations of metasedimentary rocks cored by undeformed plutons with radially symmetric meter-scale parasitic folds; and 2) NW-trending, NE-verging, moderately inclined, tightly to isoclinally-folded map-scale layers of marble and schist. Two steeply dipping mylonitic shear zones trend E-W and are found on the north and south flanks of the domal structure. North of the dome, NE-verging folds predominate. The south side of the dome is cut by a south-dipping normal fault. Crosscutting relationships of granite and pegmatite dikes suggest the tight NE-verging folds occurred before the doming.

New SHRIMP U/Pb ages from zircons of 870 ± 6 Ma (MSWD = 1.07) from granitic orthogneiss previously mapped as Early Cretaceous age represent the oldest rocks yet dated on Seward Peninsula. Spots from high-U rims on orthogneiss zircons indicate mid-Cretaceous metamorphic zircon growth around the Proterozoic cores. Monazite ages of 86 ± 1 Ma from the same orthogneiss constrain onset of cooling from regional heating. Previously published U/Pb geochronology data from the nearby Kigluaik gneiss dome yielded monazite cooling ages of 91 Ma and a pluton crystallization age of 90 Ma.