GEOCHEMISTRY AND INTERPRETATION OF GARNET AND MANGANESE-RICH METASEDIMENTARY ROCKS (‘COTICULES’) IN THE CHIWAUKUM SCHIST, NORTH CASCADE MOUNTAINS, WASHINGTON

The term ‘coticule’ has long been used to refer to extremely garnet-rich metasedimentary rocks. However, the term was originally linked to a specific lithologic unit in Belgium, and later to usage as whetstone. Subsequently, the term became associated with geochemically similar but non-industrial lithologies, and then was again restricted to Belgian occurrence and utilization accompanied by the introduction of the petrologically overlapping term ‘pseudocoticule’. Due to inconsistent use over time and geographic and industrial connotations, we suggest abandoning ‘coticule’ and instead substituting the non-genetic, geography-free, and broader ‘garnet- and manganese-rich metasedimentary rock’ (GMMR), a phrase preserving these key characteristics: 1) the rock is garnet-rich, typically composed of >50% Fe and/or Mn-rich garnet; 2) the rock is anomalously Mn-rich compared to ordinary sedimentary rocks; and 3) the rock has a sedimentary protolith.

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.