A new barometric technique measuring stored stress in quartz inclusions via laser
Raman microspectrometry was employed in an attempt to elucidate the extent of highpressure
(HP) metamorphism in the Llano Uplift of central Texas. Rare lithologies within
the Llano Uplift contain mineralogical evidence of HP metamorphism (pressures from 1.4 to
2.4 GPa at temperatures from 650 to 775°C), but much of the uplift is composed of felsic
gneisses lacking any HP signature; these felsic gneisses may never have transformed to HP
assemblages, or they may have been thoroughly overprinted by later low-pressure events.
Barometry via laser Raman microspectrometry computes entrapment pressure for a quartz
inclusion in garnet from measurement of the displacements of its Raman peak positions
from those of a quartz standard at atmospheric pressure. Quartz inclusions in garnets that
grew in felsic gneisses under HP conditions should retain HP signatures, despite later
overprinting. Application of the Raman microspectrometry technique should therefore allow
barometry of previously uncharacterizable rocks.
For two localities in the Llano Uplift, entrapment pressures from Raman barometry
(0.6-0.7 GPa and 0.2-0.3 GPa) were substantially lower than pressures expected based on
conventional barometers (1.4 GPa and 1.6-2.4 GPa). This absence of any HP signatures in
the Llano rocks contrasts with more successful applications of the Raman technique by
previous workers in high P/T blueschist-facies rocks. A key difference in the Llano rocks is
that they reached peak temperatures at which intracrystalline diffusion in garnet, driven by
compositional gradients produced during growth, had noticeable effects: complete
homogenization of growth zoning had occurred in the locality that produced the greatest
discrepancies between Raman and conventional pressures, and modest relaxation of zoning
occurred in the locality with the smaller discrepancies. The failure of the Raman technique
to recover pressures consistent with conventional barometry in the Llano Uplift is therefore
attributed to relaxation of stress on the quartz inclusions as the result of intracrystalline
diffusion within the garnet. This conclusion suggests that use of the Raman barometric
technique must be restricted to rocks whose time-temperature histories produce only very
limited intracrystalline diffusion in garnet, typically those rocks whose peak metamorphic
temperatures fall at or below upper amphibolite-facies conditions. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/26901 |
| Date | 24 October 2014 |
| Creators | McDowell, Emily Allen 1985- |
| Source Sets | University of Texas |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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