Metamorphism at the northwest contact of the Stanhope Pluton, Quebec Appalachians: Mineral equilibria in interbedded pelite and calc-schist
✍ Scribed by Philippe Erdmer
- Book ID
- 104746803
- Publisher
- Springer
- Year
- 1981
- Tongue
- English
- Weight
- 637 KB
- Volume
- 76
- Category
- Article
- ISSN
- 0010-7999
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✦ Synopsis
The interbedding of pelite and calc-schist in part of the contact aureole of a Devonian (biotite K-Ar age 346_+ 4 Ma) granite pluton that straddles the Quebec-Vermont border offers an opportunity to compare metamorphic conditions prevailing in both rock types, and to test for internal consistency among several different geothermometers and geobarometers.
Microprobe analyses and recent thermodynamic data are used with simple activity-composition models to estimate P-T-Xflu~a conditions near the sillimanite zone. Calculations yield the following results: P=2 to 3 kb, and T=400 ~ to 600 ~ C in the inner one kilometre of the aureole; Xu2o was around 0.8. Estimates are based on the calc-silicate isobaric invariant assemblages tremolite-K-feldspar-plagioclase-clinozoisite-phlogopitecalcite-quartz and diopside-tremolite-plagioclase-clinozoisite-calcite-quartz, and the paragonite-andalusite-sillimanite-albite-quartz equilibrium. The solid-solid reaction phlogopite + 2 diopside + 4 quartz=tremolite+K-feldspar in the calc-schist (combined with the andalusite-sillimanite equilibrium), and phase relations in the granite yield apparently inconsistent results.
The implied 6 to 10 km of cover at the time of intrusion may have been provided by subsequently eroded thrust sheets.