^(87)Sr/^(86)Sr and ^(18)O/^(16)O isotopic systematics and geochemistry of granitoid plutons across a steeply-dipping boundary between contrasting lithospheric blocks in western Idaho

Abstract

A spatially abrupt geochemical boundary is preserved within four plutonic complexes along the western margin of the Cretaceous Idaho Batholith near McCall, Idaho. These intrusives ranging in composition from tonalite to granite were emplaced across a regional boundary between accreted oceanic-arc terranes and the continental margin, and their isotopic, major-element, and trace-element geochemistry provide detailed information about this change in crustal characteristics at depth, indicating that the boundary is nearly vertical and extends deep into the lithosphere. The Hazard Creek complex, emplaced west of the transition in wall-rock lithology, has initial ^(87)Sr/^(86)Sr (Ri) less than 0.7045 and δ^(18)O greater than 7.5, indicating little or no continental crust in its source region; however, elevated δ^(18)O requires some incorporation of rocks formed or altered at the earth's surface. A large shift in Ri and δ^(18)O is observed across the 5–8 km wide Little Goose Creek complex, which was emplaced across the wall-rock boundary. This is interpreted as mixing between: (1) a basaltic or andesitic magma with low K_2O and high Na_2O, Al_2O_3, and Sr, similar to that forming the Hazard Creek complex; and (2) materials similar to Precambrian sedimentary sedimentary rocks with low Sr, high δ^(18)O (+15) and high Ri (0.83 at 100 Ma). The Payette River complex, emplaced east of the wall-rock boundary, exhibits at least one additional component with low δ^(18)O (+6), moderate R_i (0.708) and mafic composition. This component is inferred to be old basaltic material in the lower crust or upper mantle similar to that inferred to be a minor part of the Peninsular Ranges Batholith in SE California (Silver et al. 1979; Hill et al. 1986). The easternmost complex in the Idaho transect is made up of granites that may contain a component of granitic cratonal basement. The entire west-to-east geochemical transition from oceanic-arc magmas to cratonal magmas takes place over a lateral distance of less than 20 km. Although the zone of transitional protolith dominated by metasedimentary rocks is unusually narrow and may have been in part tectonically removed, the striking geochemical similarities between this traverse and several other transects across much broader areas of Nevada and California suggest that the craton itself was not rifted apart, but that juxtaposition of the accreted oceanic-arc terranes occurred along the preexisting craton margin. The data confirm that the isotopic geochemistry of granitoid plutons can be used as a probe of deep lithospheric character, and that major lateral variations in the lithosphere on the order of one to two kilometers in width can be recognized in favorable circumstances.

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