skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Kilbuck terrane: Oldest known rocks in Alaska

Abstract

The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2,070 {plus minus}16 and 2,040 {plus minus}74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite ({epsilon}{sub Nd}(T) = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton ({epsilon}{sub Nd}(T) = {minus}5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several continental terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of themore » Kilbuck terrane. Conversely, provinces with Nd model ages of 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded.« less

Authors:
 [1]; ;  [2];  [3]
  1. (U.S. Geological Survey, Spokane, WA (USA))
  2. (U.S. Geological Survey, Menlo Park, CA (USA))
  3. (U.S. Geological Survey, Anchorage, Alaska (USA))
Publication Date:
OSTI Identifier:
5679914
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geology; (USA); Journal Volume: 18:12
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; ALASKA; GEOLOGIC FORMATIONS; AGE ESTIMATION; GEOCHEMISTRY; CONTINENTAL CRUST; EARTH MANTLE; GEOLOGIC FAULTS; GEOLOGY; ISOTOPE DATING; ISOTOPE RATIO; LEAD ISOTOPES; MAGMA; NEODYMIUM ISOTOPES; PLUTONIC ROCKS; PRECAMBRIAN ERA; SUBDUCTION ZONES; URANIUM ISOTOPES; ZIRCON; ACTINIDE ISOTOPES; CHEMISTRY; EARTH CRUST; FEDERAL REGION X; GEOLOGIC AGES; GEOLOGIC FRACTURES; GEOLOGIC STRUCTURES; IGNEOUS ROCKS; ISOTOPES; MINERALS; NORTH AMERICA; OXYGEN COMPOUNDS; ROCKS; SILICATE MINERALS; SILICATES; SILICON COMPOUNDS; TRANSITION ELEMENT COMPOUNDS; USA; ZIRCONIUM COMPOUNDS; ZIRCONIUM SILICATES; 580000* - Geosciences

Citation Formats

Box, S.E., Moll-Stalcup, E.J., Wooden, J.L., and Bradshaw, J.Y.. Kilbuck terrane: Oldest known rocks in Alaska. United States: N. p., 1990. Web. doi:10.1130/0091-7613(1990)018<1219:KTOKRI>2.3.CO;2.
Box, S.E., Moll-Stalcup, E.J., Wooden, J.L., & Bradshaw, J.Y.. Kilbuck terrane: Oldest known rocks in Alaska. United States. doi:10.1130/0091-7613(1990)018<1219:KTOKRI>2.3.CO;2.
Box, S.E., Moll-Stalcup, E.J., Wooden, J.L., and Bradshaw, J.Y.. 1990. "Kilbuck terrane: Oldest known rocks in Alaska". United States. doi:10.1130/0091-7613(1990)018<1219:KTOKRI>2.3.CO;2.
@article{osti_5679914,
title = {Kilbuck terrane: Oldest known rocks in Alaska},
author = {Box, S.E. and Moll-Stalcup, E.J. and Wooden, J.L. and Bradshaw, J.Y.},
abstractNote = {The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2,070 {plus minus}16 and 2,040 {plus minus}74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite ({epsilon}{sub Nd}(T) = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton ({epsilon}{sub Nd}(T) = {minus}5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several continental terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of the Kilbuck terrane. Conversely, provinces with Nd model ages of 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded.},
doi = {10.1130/0091-7613(1990)018<1219:KTOKRI>2.3.CO;2},
journal = {Geology; (USA)},
number = ,
volume = 18:12,
place = {United States},
year = 1990,
month =
}
  • Electrical conductivity of a water-saturated quartz-mica-garnet-schist, collected from a surface outcrop near the Denali Fault Zone in the Yukon-Tanana terrane of east central Alaska, increases slightly with pressure to about 200 MPa. This behavior is unlike that exhibited by other Yukon-Tanana samples or by most rocks from other locations. Detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 {mu}m thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation. The stringer is probably responsible for the anomalous conductivity change withmore » pressure, making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature. The carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast. The authors interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer, possibly during the main phase of metamorphism and deformation, and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts. Brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer, explaining in part why the rock does not exhibit anomalously high conductivity at 0.1 MPa (1 atm) pressure. The brittle deformation is interpreted to have been caused by unloading due to uplift. The observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic, throughgoing fractures. 42 refs., 9 figs., 1 tab.« less
  • Since the discovery of Kepler-10, the system has received considerable interest because it contains a small, rocky planet which orbits the star in less than a day. The system's parameters, announced by the Kepler team and subsequently used in further research, were based on only five months of data. We have reanalyzed this system using the full span of 29 months of Kepler photometric data, and obtained improved information about its star and the planets. A detailed asteroseismic analysis of the extended time series provides a significant improvement on the stellar parameters: not only can we state that Kepler-10 ismore » the oldest known rocky-planet-harboring system at 10.41 ┬▒ 1.36 Gyr, but these parameters combined with improved planetary parameters from new transit fits gives us the radius of Kepler-10b to within just 125 km. A new analysis of the full planetary phase curve leads to new estimates on the planetary temperature and albedo, which remain degenerate in the Kepler band. Our modeling suggests that the flux level during the occultation is slightly lower than at the transit wings, which would imply that the nightside of this planet has a non-negligible temperature.« less