16 Petrology, Mineralogy, and Geochemistry

Tuesday, 23 May 2017
1:30 PM - 4:50 PM
Room 325A

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

Paper number: 16-3
Program: Technical Sessions
Time: 1:55 PM – 2:15 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 4

doi: 10.1130/abs/2017CD-290782

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