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| Titel |
Exploiting Quartz to Constrain Pressure-Temperature-time-Deformation Histories in Metamorphic Rocks Through Recent Innovations in Thermobarometry and Geospeedometry |
| VerfasserIn |
Kyle Ashley, Richard Law, Jay Thomas, Mark Caddick, Donald Stahr III |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072031
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| Zusammenfassung |
| Despite the abundance of quartz in continental crust, it has only recently been exploited for
thermobarometric purposes. We are using trace element content, cathodoluminescence (CL)
characteristics, fabric properties, extent of recrystallization, elastic properties and chemical
diffusivities of quartz to better understand the pressure-temperature-time-deformation
(P - T - t - D) histories of metamorphic rocks. The Ti-in-quartz thermobarometer has
significant potential for unveiling important information on the metamorphic history of rocks,
since quartz is commonly present in multiple microstructural settings (e.g. matrix, veins,
inclusions) and zoning may be present in single crystals that reveal information about the
reequilibration, recrystallization and growth histories of quartz. CL imaging provides a
qualitative way to obtain such information, and provides a domainal framework for targeted
quantitative analyses. We illustrate such analyses with examples from Vermont, India and
Greece.
A recent study in metapelites from central-eastern Vermont revealed crystals that have
low Ti cores (interpreted to be preserved early prograde growth), with mantles that grade to
higher Ti, attributed to temperature increase during fabric development and liberation of Si
during crenulation cleavage development in the micaceous matrix. Low-Ti overgrowth rims
that form sharp boundaries with these graded mantles may be later retrograde overgrowths.
Forward modeling the expected volume of quartz present in the rock in P - T space
may be implemented to confirm periods of quartz production/precipitation and
dissolution. Rocks from the Sutlej Valley (north-west India) have matrix quartz grains
with triple junction grain boundaries indicating extensive recovery. CL imaging,
however, reveals high Ti ribbons that may be indicative of chemically-preserved
paleo-microstructures. At the temperatures and metamorphic rates experienced by these
samples, grain boundaries during recovery may not redistribute impurities, but rather behave
passively.
A recent advance in implementing Raman spectroscopy on quartz inclusions in
porphyroblasts facilitates accurate geobarometry for the time of porphyroblast growth. The
method requires that elastic parameters of the host and inclusion are determined, but is
independent of the composition of these phases. A recent application of this technique to
garnets from blueschists in Sifnos, Greece, resulted in P - T paths for garnet growth from
19.5 kbar at 460 Ë C to 21.5 kbar at 550 Ë C. These results are in good agreement with recent
studies attempting to constrain peak metamorphic histories for these rocks through
equilibrium assemblage diagrams and trace element thermobarometry. Preliminary Raman
work from the Sutlej Valley samples suggests peak pressures of c. 6.3 kbar approaching the
Main Central Thrust. It may be advantageous to couple this geobarometer with
Ti-in-quartz on inclusions in garnet to elucidate thermobarometric information relative
to microstructural context and deformation history. An additional benefit to this
technique is that pressure constraints needed for Ti-in-quartz thermobarometry may
now be independently constrained and requires no knowledge of stable mineral
assemblages, component activities or mixing parameters. Titanium diffusion from host
(garnet) to included quartz may also be implemented to obtain geospeedometry
information to estimate the duration of metamorphism. The integration of microstructural
analysis with these various techniques may allow for a better understanding of the
P - T - t - D histories of rocks than previously obtained using conventional techniques. |
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