Garnet amphibolites within the Clearwater Metamorphic Core Complex (CMCC) of northern Idaho represent dikes of tholeiitic basalt that were metamorphosed under high-pressure and high-temperature conditions. These rocks are exposed as lenses within uplifted Proterozoic and Archean basement that represent fragments of the ancient Laurentian margin. This study integrates petrography, geochemistry, and phase equilibria modeling to constrain the pressure–temperature (P–T) conditions and tectonic context of metamorphism. Four samples were selected for detailed study.

Garnet amphibolite samples consist of amphibole (magnesiohornblende, tschermakite, grunerite) + plagioclase (An_23-64) + almandine-rich garnet + quartz + zoisite + rutile ± titanite ± ilmenite.  Kelyphite, consisting of orthopyroxene, Fe-rich amphibole, and An-rich plagioclase, adjacent to garnet poikiloblasts was observed in one sample.  Another sample underwent partial melting, as evidenced by wispy leucosomes of tonalite.

To assess the origin of the basaltic protoliths, major and trace element data were obtained via ICP-OES and LA-ICP-MS. Low K₂O in the garnet amphibolite samples are suggestive of a tholeiitic protolith (similar to MORB or IAT).  Chondrite-normalized REE patterns are slightly LREE-enriched ([La/Yb]N=2.0-7.1) and exhibit virtually no Eu-anomaly.

Phase equilibria modeling was performed using Theriak-Domino (de Capitani and Petrakakis, 2010).  Peak metamorphic conditions of 8–11 kbar and 680–760 °C were estimated by comparison of the observed mineralogies and the modeled equilibrium mineral assemblages.  Garnet-hornblende-plagioclase thermobarometry yielded comparable results in one sample (~10.3 kbar, 700 °C). Garnet compositional zoning and pseudosection modeling document a prograde P–T path from garnet core formation at ~5 kbar and 550 °C to peak rim growth at ~8–11 kbar and 680–760 °C.  Peak metamorphism was followed by isothermal decompression, as evidenced by the presence of kelyphite.

Pressure-temperature estimates in this study are identical to those obtained by Grover et al. (1992) on aluminosilicate-bearing mica schists in the same area.  These findings suggest that peak metamorphism occurred during the Late Cretaceous M2 event of Grover et al. (1992), which was caused by deep burial from tectonic collision as well as heating from emplacement of magmas that crystallized to form the Idaho batholith (Grover et al., 1992; Vervoort, 2009; Gébelin et al., 2011). This study provides new constraints on the burial, metamorphism, and exhumation history of the CMCC and contributes to understanding the evolution of the western margin of Laurentia.