Strontium and oxygen isotopic evidence for strike/slip movement of accreted terranes in the Idaho Batholith

Abstract

The oxygen and strontium isotope compositions of granitic rocks of the Idaho Batholith provide insight into the magma source, assimilation processes, and nature of the suture zone between the Precambrian craton and accreted arc terranes. Granitic rocks of the Idaho Batholith intrude basement rocks of different age: Triassic/Jurassic accreted terranes to the west of the Salmon River suture zone and the Precambrian craton to the east. The age difference in the host rocks is reflected in the abrupt increase in the initial 87Sr/ 86Sr ratios of granitic rocks in the batholith across the previously defined 0.706 line. Initial 87Sr/ 86Sr ratios of granitic rocks along Slate Creek on the western edge of the batholith jump from less than 0.704 to greater than 0.707 along an approximately 700 m transect normal to the Salmon River suture. Initial 87Sr/ 86Sr ratios along the Slate Creek transect do not identify a transition zone between accreted arcs and the craton and suggest a unique tectonic history during or after suturing that is not documented along other transects on the west side of the Idaho Batholith. The lack of transition zone along Slate Creek may be a primary structure due to transcurrent/transpressional movement rather than by contractional thrust faulting during suturing or be the result of post-imbrication modification.