TRACE ELEMENT GEOCHEMISTRY OF THE WARD CREEK METAMORPHIC SEQUENCE AND BIG OAT CREEK METABASALT, CAZADERO, CA

One of the most intensely studied metamorphic complexes in the world is exposed in Ward Creek, Sonoma County, California (Erickson, 1995). The Ward Creek (WC) metamorphic sequence is composed primarily of glaucophane and sodic pyroxene blueschist metabasalt with aragonite stringers, and abundant metachert. Protolith textures have been completely obliterated. In contrast, Big Oat Creek (BOC) metabasalts adjacent to the WC sequence across an unexposed contact have green flattened relict pillows with blue glaucophane margins. BOC metabasalts have interpillow aragonite and no metachert. Aragonite in WC and BOC is from pre-metamorphism interpillow pelagic carbonate, suggesting a pillowed WC protolith.

Three samples from the WC sequence and two samples from the BOC sequence were analyzed for major and trace elements. WC REE diagrams show N-MORB patterns with a weak positive Eu anomaly. WC samples show enrichment in water soluble elements (Sr, K, Rb, Ba) and deficiencies in Ta, Nb, and Ti on spidergrams (Pearce, 1983), indicating an arc origin. Ti-V plots (Shervais, 1982) place WC samples in the N-MORB field and Cr-Y plots (Pearce et al, 1981) place them within the SSZ-MORB boundary. WC samples plot in the supra-subduction zone (SSZ) field on a Ta-Th-Hf diagram (Wood, 1980) and in the MORB field on a [Zr-Y]-Zr diagram (Pearce, 1983).

The BOC sample nearest WC has a similar REE plot and Pearce spidergram to the WC samples, though 1.5X enriched. It also plots MORB on a [Zr-Y]-Zr diagram. The second, more distant BOC sample is, alternatively, enriched in LREE (La/Sm=1.9) and its Pearce spidergram shows an OIB signature. Both BOC samples plot separately from WC on Ti-V and Cr-Y graphs.

Contrasting outcrop textures in BOC and WC and absence of metachert in BOC also suggest separate units. All evidence suggests for WC an N-MORB source modified by water soluble elements from the subducting plate in an SSZ environment. Part of BOC has the same type of source as WC, but part has an OIB source.