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< r<540°C, P< 10-11-5 kb.Medium-grained eclogites occur as individual blocks in Ward Creek; they are different from Type IV tectonic eclogites but are very similar to in situ eclogitic schists. They have unaltered idioblastic garnet with high almandine and spessartine content (Alm47Sp23Gr20Py10), and they have chloromel-anitic clinopyroxene and quartz but no barroisite. Paragonite is also stable in these eclogites. The blocks formed at 380 °C< r<400°C, and 9-5 kb<P< 14 kb. They are presumably in situ eclogites formed at the highest-temperature part of the Ward Creek metabasite unit and may be younger than Type IV tectonic eclogites.Such low-temperature occurrences of eclogitic assemblages are due to the compositional effect on reactions between blueschist and eclogite that are insensitive to pressure and shift towards lower temperatures as bulk-rock MnO content and XFe/(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.
ASJC Scopus subject areas
- Geochemistry and Petrology