OXYGEN AND STRONTIUM ISOTOPIC EVIDENCE FOR THE ABRUPT AND STEEPLY-DIPPING SUTURE BETWEEN LITHOSPHERIC PLATES IN THE IDAHO BATHOLITH

Granitic rocks of the Cretaceous Idaho batholith intrude both sides of a major structural and geochemical crustal boundary. Basement rocks of the batholith are Mesozoic accreted arc terranes and the Precambrian craton. The age difference in the basement rocks is reflected in the abrupt increase of initial ⁸⁷Sr/⁸⁶Sr of granitic rocks in the batholith across the previously defined 0.706 line. The Salmon River suture zone and the 0.706 line are roughly coincident.

This study contains samples from a west-to-east trending transect along Slate Creek on the west side of the Idaho batholith. Granitic rocks on the west side of the transect have initial ⁸⁷Sr/⁸⁶Sr ratios less than 0.706 and d¹⁸O(magma) approximately 8‰. Over a distance of less than 2.0 km, granitic initial ⁸⁷Sr/⁸⁶Sr ratios jump discontinuously from 0.704 to an average of 0.708. Across this transect, d¹⁸O(magma) increases with a near linear and continuous trend from 8‰ to 11‰.

The transition to the craton in the Slate Creek transect is significantly more abrupt than other transects studied in the Idaho batholith due to the lack of Precambrian metasediments in the suture zone. Mesozoic rocks of the North Fork block are entrained in the suture zone at Slate Creek whereas transects north and south of Slate Creek contain metasediments of the Belt Supergroup. The d¹⁸O of the Mesozoic rocks indicate the accreted arcs have been variably altered by seawater. The batholith is estimated to have intruded at approximately 10 km depth, or greater. The granitic rocks obtain their geochemical signature at the magma source and during ascent through the crust, so the vertical nature of the suture zone must extend at least 10 km. This study suggests that the transition from oceanic lithosphere to continental lithosphere is near vertical at significant depths in the crust.