THERMOBAROMETRIC CONSTRAINTS ON HIGH-PRESSURE EXHUMATION HISTORY OF THE DINKELMAN DECOLLEMENT, NORTH CASCADES, WASHINGTON: A RECORD OF DECOMPRESSION IN AMPHIBOLITE GARNET CORONAS

Approximately 2.7 kbars of high-temperature, near-isothermal decompression (ca. 8 km of exhumation) is recorded in mineral chemistry of garnet amphibolite from within the Dinkelman decollement. Deeply embayed, highly calcic garnet (Grs 0.26-0.30) is surrounded by coronas of plagioclase ± euhedral hornblende in a matrix of strongly foliated hornblende+titanite+ilmenite+quartz+plagioclase. Coronas are interpreted to result from garnet resorption during high-temperature decompression. Plagioclase zoning in coronas is systematic, from ~An.36 at the outer edge (preresorption garnet rim?) to ~An.25 at the inner edge adjacent to garnet. Hornblende in coronas is generally lower in Al and Mg than hornblende inclusions in garnet. Pressure and temperature were estimated for garnet+plagioclase+hornblende with activity models and procedures in Dale, Holland, & Powell (2000) and THERMOCALC 3.1 (Powell & Holland 2001). Preresorption assemblages (high-Grs garnet, low-An plagioclase, high-Al hornblende) suggest peak or near-peak metamorphism at ~778° ± 81° C, 13.1± 1.9 kbar. Postresorption assemblages (low-Grs garnet, high-An plagioclase, low-Al hornblende) suggest that corona development ceased at ~729° ± 76° C, 10.4± 1.7 kbar. Estimates using calibrations of Graham & Powell (1982) and Holland & Blundy (1994) also suggest near-isothermal decompression, but at temperatures below 700° C. Although absolute uncertainties are large, differences between pre- and postresorption pressure estimates are more certain and are interpreted to reflect substantial high-temperature decompression.

Exhumation from ~40 km to ~32 km depth with cooling of only ca. 50° C likely resulted from rapid tectonic exhumation or from decompression accompanied by continual heat input. Magmatic heat input may have resulted from emplacement of the numerous sheet-like tonalite bodies within and adjacent to the Dinkelman decollement.