Franciscan eclogite revisited

Reevaluation of the P-T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A

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

Research output: Contribution to journalArticle

63 Citations (Scopus)

Abstract

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-P-T calculations yield peak eclogite-facies P-T conditions of P = 2.2-2.5-GPa and T = 550-620-°C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P-T of 1.8 ± 0.2-GPa at 490 ± 70-°C for the pre-peak stage. The inferred counterclockwise hairpin P-T 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, P-T 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.

Original languageEnglish
Pages (from-to)243-267
Number of pages25
JournalMineralogy and Petrology
Volume88
Issue number1-2
DOIs
Publication statusPublished - Sep 2006
Externally publishedYes

Fingerprint

eclogite
Garnets
peninsulas
Tectonics
garnets
tectonics
epidote
garnet
blueschist facies
omphacite
phengite
paragonite
glaucophane
albite
exhumation
Minerals
metamorphism
Amphibole Asbestos
minerals
Quartz

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Franciscan eclogite revisited : Reevaluation of the P-T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A. / Tsujimori, T.; Matsumoto, K.; Wakabayashi, J.; Liou, J. G.

In: Mineralogy and Petrology, Vol. 88, No. 1-2, 09.2006, p. 243-267.

Research output: Contribution to journalArticle

Tsujimori, T. ; Matsumoto, K. ; Wakabayashi, J. ; Liou, J. G. / Franciscan eclogite revisited : Reevaluation of the P-T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A. In: Mineralogy and Petrology. 2006 ; Vol. 88, No. 1-2. pp. 243-267.
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