Mineralogy and Petrology

, 88:243 | Cite as

Franciscan eclogite revisited: Reevaluation of the PT evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A.

  • T. Tsujimori
  • K. Matsumoto
  • J. Wakabayashi
  • J. G. Liou


High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote ± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite (∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry and THERMOCALC average-PT calculations yield peak eclogite-facies PT conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-PT of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin PT trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, PT pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism. Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of ∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes.


Titanite Kyanite Contrib Mineral Petrol Garnet Amphibolite Franciscan Complex 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • T. Tsujimori
    • 1
  • K. Matsumoto
    • 2
  • J. Wakabayashi
    • 3
  • J. G. Liou
    • 1
  1. 1.Department of Geological and Environmental SciencesStanford UniversityStanfordU.S.A.
  2. 2.Department of Geology and Mineralogy, Graduate School of ScienceKyoto UniversityKyotoJapan
  3. 3.Department of Earth and Environmental SciencesCalifornia State UniversityFresnoU.S.A.

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