GARNET HORNBLENDITE BLOCKS IN SERPENTINITE-MATRIX MELANGE IN THE BLUE MOUNTAINS PROVINCE, NE OREGON

Garnet hornblendite blocks on Mine Ridge occur as inclusions in serpentinite-matrix mélange and Tertiary intrusives along the Conner Creek fault, which marks the boundary between the Baker and Olds Ferry terranes. Thirteen blocks, ranging from 30 to 1000 meters in length, consist of coarse-grained, foliated hornblende, garnet (Alm_53-56, Py_12-15, Gr_26-28, Sp_4-7), epidote, sphene, apatite, and rutile. Notable are the lack of plagioclase and clinopyroxene in the hornblendites. Quartz and feldspathic veins cross-cut the hornblendite. The garnet hornblendites are broadly basaltic in composition and have slightly LREE- depleted chondrite-normalized patterns similar to N-MORB, and to basalts and gabbros from the Canyon Mountain ophiolitic complex, 30 km to the west. The lack of plagioclase in these rocks is reflected in their low Na₂O contents, relative to N-MORB and Canyon Mountain basalts.

The garnet hornblendites in this area may represent: 1) a high-pressure residual assemblage from a partial melting event, exhumed from a deep crustal source, 2) a high-pressure cumulate from an evolving water-rich magma, or 3) high-grade equivalents of oceanic crust or ophiolitic affinity. In an effort to address these hypotheses, we estimated the melt composition that would have been in equilibrium with garnet and hornblende, using the garnet/liquid and clinopyroxene/liquid partition coefficients, respectively, of Klein et al. (2000, GCA, v.64, pp.99-115). The results suggest that a melt composition in equilibrium with these phases would have been LREE-depleted, very different to that expected for a high-Al₂O₃ tonalitic/trondhjemitic magma with a hornblende + garnet residual or cumulate assemblage (hypotheses 1 and 2). A hornblendite from Mine Ridge yielded a K-Ar age of 260 Ma (Hooper et al., 1995, USGS Prof. Paper 1438), similar to ages for the Canyon Mountain complex. Therefore, we suggest that the garnet hornblendites represent high-grade fragments of the Canyon Mountain ophiolite, perhaps from the metamorphic sole. The absence of plagioclase and clinopyroxene in these rocks further suggests that the protolith experienced water-saturated partial melting and melt loss.