STRUCTURAL EVOLUTION, TRANSITION FROM ANATECTIC TO MANTLE-DERIVED MAGMATISM, AND TIMING OF EXHUMATION: BENDELEBEN METAMORPHIC COMPLEX, SEWARD PENINSULA, ALASKA

Structural, petrologic, geochronologic studies, and 1:24,000-scale mapping of the Bendeleben metamorphic complex indicate episodes of Mesozoic deformation, granitic magmatism, and gneiss dome development. Low-grade white-mica–chlorite schists of the Nome Group exposed south of Kingsley Creek have tight upright fold axes that trend NNW. Metamorphic grade in structurally deeper schists increases from staurolite grade to second sillimanite grade near the Pargon pluton. Kilometer-scale, upright, open folds SW of Mount Bendeleben and 100-meter-scale tight, NE-vergent folds of structurally deeper metasedimentary rocks NW of the Bendeleben pluton both have NW-trending fold axes. Units between the Bendeleben pluton and Mt. Bendeleben exhibit late-stage doming along an E-W trend and are intruded by the Pargon pluton. Local migmatization, pure-shear boudinage, limited microtextural asymmetric shear sense, and cordierite-bearing schists indicate a transition to extensional deformation.

SHRIMP U-Pb zircon dating of deformed and undeformed intrusive rocks temporally constrains the magmatic history. A hornblende-biotite foliated granite concordant with metamorphic foliation yields an age of 847±25 Ma (all uncertainties 2-sigma), within error of an earlier date of 870±7 Ma, and represents the oldest known intrusive event on Seward Peninsula. A foliated biotite granite yields 390 Ma and low Th/U 91 Ma metamorphic zircons. Other foliated biotite granites and pegmatites yield Cretaceous ages: the 104±1 Ma Bendeleben pluton, a 101±1 Ma orthogneiss, a 99±2 Ma garnet-bearing pegmatite, a 98±1 Ma sillimanite and garnet-bearing dike, and an 88±1 Ma mylonitic orthogneiss. Detrital zircons from a cordierite-schist have low Th/U rims dated at 87±2 Ma. Undeformed granites yield ages of 87±1 Ma from a dike, 86±1 Ma from the sphene-bearing Pargon pluton, and 82±1 Ma for a hypabyssal porphyritic dike. Anatectic garnet- and sillimanite-bearing melts formed during Late Albian-Cenomanian time yielded to biotite-sphene melts around 88-85 Ma, synchronous with migmatization. Exhumation of the metamorphic complex was nearly complete by 82 Ma, as evidenced by miarolitic cavities and hypabyssal intrusions.