CHEMISTRY AND TECTONIC SETTING OF AMPHIBOLITES FROM THE NASON TERRANE, NORTH CASCADE MOUNTAINS, WASHINGTON

Amphibolites from the Nason Terrane fall into two main groups (G1 & G2). G1 consists of thick, concordant sheets of amphibolite 10s of m thick. Some are extremely homogeneous amphibolite, but they may also contain lenses or thin ribbons of calc-silicate. Nearly all have at least one epidote-group mineral and lack garnet and ilmenite. G2 is more diverse, ranging from pillows to thin concordant layers; epidote group minerals are uncommon but garnet and ilmenite are common. Some are coarse-grained and extremely garnet rich (>50%); others are finely layered. A third ‘group’ (G3) consists of a unique Al-rich gt-ky-staur-ged-chl amphibolite.

Both groups average ~47-48% SiO₂, but G2 has higher Ti, Fe, Mn, and P, and lower CaO, MgO, and Mg#. Among the trace elements, G2 has notably much higher Zn, Rb, Nb, Zr, Cs, Ba, Ta, Th, and U. REEs cleanly separate the groups; G1 has a strongly N-MORB pattern whereas G2 shows N-MORB abundances for the HREEs, rising smoothly to 10x N-MORB abundances for La. G3 has a HREE pattern akin to G2, but is extremely depleted in the LREE down to about 0.2x N-MORB. It appears that minor calc-silicate segregations or seafloor contamination did not affect the REE patterns.

TE discriminant diagrams generally indicate a MORB origin for G1 rocks, and possibly a back-arc setting, whereas G2 data are more scattered. Sr isotopic data show mostly regular behavior, but some G2 samples suggest an increase of the ⁸⁷Sr/⁸⁶Sr probably due to ocean floor processes. Nd isotopic data are fairly regular for G1 samples, and together with Sr data and data from G3, suggest maximum model ages of 0.3-0.4 Ga.

The similarity of G2 REE patterns, and their elevation above G1 (N-MORB) values, given the diversity of rocks including pillow cores, is intriguing. G2-type protolith may have formed from a more Si & Ti-rich, Mg-poor magma, and volcaniclastic rocks of a broadly basaltic origin. The latter are not, however, simple mixtures of N-MORB with either Chiwaukum sediment or with coticule-forming hydrothermal deposits, but most likely formed largely from seafloor weathering processes and possibly mixtures with small amounts material of a more evolved origin. G3 may be derived from an entirely different, highly depleted mantle source, and it shows a very different metamorphic evolution including possible UHT and HP phases.