Transect across the Salmon River suture, South Fork of the Clearwater River, western Idaho : rare earth element geochemical, structural, and metamorphic study

Graduation date: 1987In the South Fork of the Clearwater River area, the Salmon River suture (Lund and others, in prep.) is defined by an abrupt change in lithology, structure, initial strontium isotope ratios (Fleck and Criss, 1985), and rare earth element signatures. These changes occur across a narrow, structurally complex zone (less than 1.5 mi wide)\ud that is complicated by intrusion of plutons and by later mylonitic deformation. The suture separates amphibolite fades island arc metasedimentary rocks on the west (western tectonostratigraphic terrane) from amphibolite facies continental metasedimentary rocks on the east (eastern tectonostratigraphic terrane). Structures in rocks of the two terranes were formed during suturing and show opposing sense of\ud vergence; folds in the western terrane show northwest vergence, thrust faults show southeast-over-northwest movement, and thrust faults in the eastern terrane show west-over-east movement. Syntectonic plutons (foliated tonalites) intruded the western\ud terrane and were metamorphosed. Post-tectonic plutons intruded both the suture zone (porphyritic granodiorite/quartz-monzodiorite orthogneiss unit) and the eastern terrane (tonalite/granodiorite and biotite- and muscovite-biotite granite units). Mylonitic deformation that was concentrated in the porphyritic orthogneiss unit along the suture zone also overprinted structures in both terranes. An abrupt change from initial strontium isotope ratios of 0.7043 to 0.7065 (Fleck and Criss, 1985) occurs from west to east within the porphyritic orthogneiss unit. Rare earth element (REE) variations in plutonic and metamorphic rocks indicate that neither anatexis of metasedimentary wall rocks now exposed in the region, nor contamination of primary magmas by the wall rocks was important in the formation of the plutons. Differentiation\ud and crystal fractionation processes were also not important. Instead, magmas that formed rocks in the different plutonic units originated in different source regions. Magmas that formed the foliated torialites in the western terrane were probably derived by partial melting of oceanic crust or island arc basement at low to\ud intermediate pressures, leaving a residual mineral assemblage that probably consisted of amphibole, pyroxene, plagioclase, and no garnet. Magmas that formed rocks in the porphyritic granodiorite/quartz-monzonite orthogneiss unit were probably derived by\ud partial melting of a mixed source at medium to high pressures that left a probable residual mineral assemblage of pyroxene, amphibole, minor or no plagioclase, and garnet. The REE signatures in the porphyritic orthogneiss indicate the source was heterogeneous; these rocks probably formed by melting of the complex zone of mixed rocks along the suture zone. Magmas that formed rocks in the tonalite/granodiorite and biotite granite units were derived by partial melting of continental crustal rocks at medium to high pressures leaving a probable residual mineral assemblage of amphibole,\ud pyroxene, minor or no plagioclase, and garnet. The biotite granite parent magmas probably had a contribution from older (Proterozoic?) continental crust and the tonalite/granodiorite parent magmas may have had a contribution from younger (Paleozoic?) continental crust or rubidium depleted crust. The change from rocks that represent magmas generated at lower pressures to rocks that represent magmas generated at higher pressures represents the change from oceanic to continental magmatic\ud source regions. Both syn-tectonic plutons in the western terrane and post-tectonic plutons in the suture zone and the eastern terrane are thought to reflect the crustal source regions that were formed by the suturing event. Morphologically, the Salmon River Suture is probably a narrow, steep, structurally complex zone of mixed rocks from both oceanic and continental crust