Low-temperature eclogites and eclogitic schists in mn-rich metabasites in ward creek, California; Mn and Fe effects on the transition between blueschist and eclogite

Chang W. Oh, J. G. Liou, S. Maruyama

Research output: Contribution to journalArticle

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Abstract

In situ eclogitic schist lenses occur in the coherent low-grade epidote-zone Ward Creek metabasite unit of the Central Franciscan belt. They contain almandine garnet, clinopyroxene, and rutile. They have slightly higher Mn content (0-5-1-0 wt.%) than the coexisting Type III metabasites (0-12-0-25 wt%) which contain epidote + glaucophane + actinolite + chlorite + omphacite + quartz + sphene ± aragonite± lawsonite ± pumpellyite + albite. The in situ eclogitic schists (130-140 Ma) can be distinguished from older tectonic eclogites (150-160 Ma) in Ward Creek as follows: (1) they are medium grained, whereas Type IV tectonic eclogites are coarse grained; (2) they have unaltered spessartine-rich idioblastic (0-4-10 mm) garnets, whereas Type IV tectonic eclogites have larger xenoblastic to hypidiomorphic spessartine-poor garnets which were corroded and chloritized along the rim during retrograde metamorphism; (3) clinopyroxenes are chloromelanite in in situ eclogitic schists but omphacite in Type IV tectonic eclogites; (4) barroisitic amphiboles occur both as inclusions in garnets and as matrix minerals in Type IV tectonic eclogites but not in in situ eclogitic schists; (5) albite is present in in situ eclogitic schists but not in Type IV tectonic eclogites; and (6) the estimated P-T condition of in situ eclogitic schists is 290 °C <T<350 °C, P = 8-9 kb, whereas that of Ward Creek Type IV tectonic eclogites is 500°C<r47Sp23Gr20Py10), and they have chloromel-anitic clinopyroxene and quartz but no barroisite. Paragonite is also stable in these eclogites. The blocks formed at 380 °C<rFe/(Fe+Mg) increase. The Mn/(Mn + Fe) ratio of bulk rock is an important factor in controlling the P-T positions of these reactions at temperatures below 450 °C, whereas the Fe/(Fe + Mg) ratio of bulk-rock becomes important at temperatures higher than 450 °C.

Original languageEnglish
Pages (from-to)275-302
Number of pages28
JournalJournal of Petrology
Volume32
Issue number2
DOIs
Publication statusPublished - Apr 1991
Externally publishedYes

Fingerprint

eclogite
Quartz
schist
metabasite
blueschist
Tectonics
tectonics
Amphibole Asbestos
Garnets
Temperature
Calcium Carbonate
garnets
garnet
Lenses
Minerals
spessartine
omphacite
epidote
albite
clinopyroxene

ASJC Scopus subject areas

  • Molecular Biology
  • Statistics and Probability
  • Computational Mathematics
  • Computer Science Applications
  • Computational Theory and Mathematics
  • Geophysics
  • Geochemistry and Petrology

Cite this

Low-temperature eclogites and eclogitic schists in mn-rich metabasites in ward creek, California; Mn and Fe effects on the transition between blueschist and eclogite. / Oh, Chang W.; Liou, J. G.; Maruyama, S.

In: Journal of Petrology, Vol. 32, No. 2, 04.1991, p. 275-302.

Research output: Contribution to journalArticle

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abstract = "In situ eclogitic schist lenses occur in the coherent low-grade epidote-zone Ward Creek metabasite unit of the Central Franciscan belt. They contain almandine garnet, clinopyroxene, and rutile. They have slightly higher Mn content (0-5-1-0 wt.{\%}) than the coexisting Type III metabasites (0-12-0-25 wt{\%}) which contain epidote + glaucophane + actinolite + chlorite + omphacite + quartz + sphene ± aragonite± lawsonite ± pumpellyite + albite. The in situ eclogitic schists (130-140 Ma) can be distinguished from older tectonic eclogites (150-160 Ma) in Ward Creek as follows: (1) they are medium grained, whereas Type IV tectonic eclogites are coarse grained; (2) they have unaltered spessartine-rich idioblastic (0-4-10 mm) garnets, whereas Type IV tectonic eclogites have larger xenoblastic to hypidiomorphic spessartine-poor garnets which were corroded and chloritized along the rim during retrograde metamorphism; (3) clinopyroxenes are chloromelanite in in situ eclogitic schists but omphacite in Type IV tectonic eclogites; (4) barroisitic amphiboles occur both as inclusions in garnets and as matrix minerals in Type IV tectonic eclogites but not in in situ eclogitic schists; (5) albite is present in in situ eclogitic schists but not in Type IV tectonic eclogites; and (6) the estimated P-T condition of in situ eclogitic schists is 290 °C <T<350 °C, P = 8-9 kb, whereas that of Ward Creek Type IV tectonic eclogites is 500°C23Gr20Py10), and they have chloromel-anitic clinopyroxene and quartz but no barroisite. Paragonite is also stable in these eclogites. The blocks formed at 380 °C",
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