CHARACTERIZATION OF YUKON-TANANA TERRANE, KANTISHNA HILLS, WEST-CENTRAL ALASKA

The Kantishna Hills (KH) is underlain mainly by four regionally metamorphosed rock units that range in age from Proterozoic? to Mississippian: a Basement Schist (BS), the Spruce Creek Sequence (SCS), the Keevy Peak Formation (KPF), and the Totatlanika Schist (TS). All four units have been regarded as western exposures of the Yukon-Tanana terrane (YTT). BS consists of quartzite, schist, garnet amphibolite, and small bodies of orthogneiss that have been metamorphosed to the amphibolite facies and subsequently retrograded to greenschist facies conditions. The structurally higher SCS, KPF, and TS contain variable amounts of metavolcanic rocks, and carbonaceous and calcareous metasedimentary units that underwent one, well defined, greenschist facies event. The SCS, KPF, and TS are tectonically juxtaposed against the BS; e.g., the higher rank BS is thrust over the lower rank SGS in the southern KH. Cretaceous-Tertiary (K-T) plutonic rocks intrude the YTT.

Zircon fractions from three samples have been dated by the U-Pb TIMS method. Quartz monzonite orthogneiss in the BS has an upper intercept age of 374 +/- 11 Ma, whereas a sample of felsic metavolcanic rock in the SGS have upper intercept ages of 369 +/- 4 Ma. Zircon fractions from a sample of metamorphosed rhyolite porphyry in the TS yielded an upper concordia intercept age of 336 +/- 9 Ma; however, there is evidence of Pb loss superposed on this discordia array containing inherited zircons of Early Proterozoic age, which suggests this is a minimum age.

Stratiform, volcanogenic massive sulfide-barite (VMS) deposits from both the SGS and TS contain del 34S values in sulfides that range from +9.0 to +14.4, indicative of a submarine volcanogenic setting. Pb isotope data from the Lloyd deposit in the SCS plot in a field defined by Alaskan, mid-Paleozoic VMS deposits. The productive Kantishna Sb-Ag-Au-polymetallic veins, which are hosted in the YTT, exhibit radiogenic Pb-Pb values and Ar/Ar ages indicating association with K-T plutonic rocks.

Metabasite from the BS can be classified as LREE-enriched island arc tholeiite, whereas the felsic metavolcanics in both the SCS and TS are mildly peralkaline, which is compatible to generation in a rift environment. The Devonian-to-Mississippian? plutonic and volcanic rocks in the KH are similar in character to magmatism observed in pericratonal terranes described elsewhere in Alaska and Canada.