Abstract
The eclogites in California, U.S.A., have been regarded as tectonic blocks which formed 150–160 Ma ago and have been tectonically incorporated into younger rocks of the Franciscan Complex of 90–130 Ma. However, in Ward Creek, northern California, two different types of eclogites were identified: Type I is a typical tectonic block and Type II is a possible in-situ eclogite formed during the Franciscan metamorphism. Most eclogites in the Franciscan Complex are Type I eclogites. Type I eclogites in Ward Creek consist of garnet + omphacite + epidote + glaucophane + phengite + rutile ± barroisite ± quartz and have coarse-grained gneissic texture. They have precursors of the epidote-amphibolite facies and are overprinted in various degrees by blueschist facies retrograde metamorphism. Remnants of metamorphic rinds, which consist of actinolite ± talc ± chlorite and formed during metasomatic interaction with serpentinizing peridotite, occur at the margins of Type I eclogite blocks. Type II eclogites are medium-grained and consist of garnet + chloromelaniticclinopyroxene + epidote + glaucophane + paragonite + phengite + rutile + quartz. Petrochemical characters of Type II eclogites are similar to those of in-situ eclogitic schists in the Ward Creek metabasite sequence. Epidote-amphibolite f facies precursors, metamorphic rinds, and overprint of retrogressive blueschist facies metamorphism are not found in Type II eclogites. Garnets in Type II eclogites are medium-grained, unaltered and idioblastic and have higher spessartine contents (SP 14–25), whereas garnets in Type I eclogites are coarse-grained and chloritized and have lower spessartine contents (Sp 2–7). The PT-conditions of peak metamorphism for Type II eclogites are 380< T<400°C and 9.5< P<14 kb and those for Type I eclogites 480< T<540°C and P>10–11.5 kb. Type I eclogites follow a counter-clockwise PT-path from the epidote-amphibolite through eclogite to blueschist facies, whereas the presumable in-situ Type II eclogites follow a clockwise PT-path from the blueschist to eclogite facies.
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