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

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 X_Fe/(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.