Paper No. 3
Presentation Time: 1:30 PM-5:30 PM
PRESSURE-TEMPERATURE PATHS AND THORIUM-LEAD MONAZITE DATING IN THE HANGING WALL OF A MAJOR THRUST IN THE HINTERLAND OF THE SEVIER OROGENIC BELT, ALBION MOUNTAINS, SOUTH-CENTRAL IDAHO
The Basin-Elba fault in the Albion Mountains, Idaho, juxtaposes a >2.8 km
package of Cambrian and Proterozoic metasedimentary rocks over Paleozoic
metasedimentary rocks and is the only thrust preserved through Tertiary
extension in the region. Garnets
were collected from garnet schist and garnet-staurolite schist within the schist
of Willow Creek middle member in the hanging wall.
In the garnet schist, garnet grew by the reaction Chl + Pl=Grt + Bt ±
Qtz ± Ms + H2O, and in the garnet-staurolite schist by the reaction
Bt + St + Pl=Grt + Ms ± Qtz + H2O.
Garnet-biotite thermometry indicate that garnet growth in the garnet
schist began between 506 ± 25 ˚C and 540 ± 25 ˚C and ended between
578 ± 25 ˚C and 600 ± 25 ˚C based on 3 garnets from 3 samples.
In the garnet-staurolite schist, growth began between 546 ± 25 ˚C
and 575 ± 25 ˚C and ended between 595 ± 25 ˚C and 598 ± 25 ˚C
based on 3 garnets from 3 samples. Pressures
calculated using muscovite-biotite-plagioclase-garnet barometry on the rims of
the 6 garnets range from 4.5 to 9.5 kb. Garnet
growth zoning profiles were numerically simulated using the Gibbs method based
on Duhem's theorem to determine pressure-temperature (P-T) paths.
Total pressure increases associated with the growth of any one garnet
range from 1 to 2 kb. All garnet
profiles contain a distinct boundary between an inclusion-rich core and an
inclusion-free rim, correlating with a change to a steeper decline in Mn toward
the rim. This boundary can be
explained by an initial growth phase that produced an inclusion-rich garnet, a
second phase that consumed the garnet rim, and a final growth phase that formed
the inclusion-free rim under increased nucleation density.
Hanging wall P-T paths are complex and different from footwall paths.
Hanging wall paths show episodes of increasing and decreasing pressure
with at least one discontinuity in each path.
Th-Pb dating of 24 monazite grains from two samples shows no demonstrable
difference between matrix and garnet inclusion ages.
10 monazites from one sample yielded a weighted mean age of 144.6 ± 3.5
Ma and 8 analyses from the other sample yielded a weighted mean age of 120.5 ±
3.4 Ma, which we interpret as maximum ages of garnet growth, consistent with
Sevier orogenesis. The P-T paths
suggest repeated episodes of thrust burial and exhumation by faults inferred to
lie above the current level of exposure.