GEOCHEMISTRY AND INTERPRETATION OF GARNET AND MANGANESE-RICH METASEDIMENTARY ROCKS (‘COTICULES’) IN THE CHIWAUKUM SCHIST, NORTH CASCADE MOUNTAINS, WASHINGTON
In the Chiwaukum Schist of N-central Washington, GMMR occurs in mm to several dm-scale red to red-brown to dark-brown layers interbedded with schist, most commonly at amphibolite-schist contacts, likely yielding a novel tops indicator. Some specimens are directionless with submicroscopic garnet, whereas others, typically more quartz rich, show relict bedding with preserved layers down to ~20 mm. Garnet ranges from 100 mm to submicroscopic, and is commonly inclusion-rich. Nearly all examples are gar+qtz+amph, ±sulfides, chl, bio, rut, and apatite.
Bulk chemical analysis has been carried out on 17 specimens from 7 locations. Most examples have low SiO2 (24-47 wt%), with K and Na ~0, MnO is 2.7-11.3 wt%, Fe2O3 is 10.7-45.8 wt%, and P2O5 is 0.04-5.4 wt%. Except for one locality, the rocks are S-poor, and C is generally <0.15 wt%. Cu is highly variable, from 8 to 2290 ppm. REEs patterns generally show strong negative Eu and Ce anomalies, and REE+Y values average 478 ppm. REE patterns display a negative slope, generally 100-1000x chondritic values. The Mn/Fe (molar) ratio ranges from 0.22-0.54.
Based on SiO2, P2O5, and S content, and relationships to amphibolites, there may be multiple geochemical processes responsible for their formation, but hydrothermal processes involving S-poor fluids following seafloor basaltic volcanism appears a dominant mechanism.