TRANSPRESSIONAL MODIFICATION OF A CORDILLERIAN BATHOLITH: MODIFICATION OF THE 87SR/86SR ISOTOPIC TRANSITION IN WESTERN IDAHO

Isotopic analyses provide insight into the processes involved in the magmatic evolution of batholiths. In particular, geochemical studies demonstrate that the 87Sr/86Sr ratio tracks the amount of oceanic crust vs. continental crust that have been assimilated into a pluton. Examination of the spatial distribution of the 87Sr/86Sr ratio throughout a batholith have subsequently been interpreted in terms of igneous processes – i.e. the spatial distribution of melt sources at depth are inferred from these data. All batholiths in the North American Cordilleria, however, are deformed to a greater or lesser degree. The deformation recorded by a particular pluton occured after the emplacement of that pluton. Thus deformational processes overprint magmatic processes. In some cases, deformation is much smaller in scale than the batholith as a whole, and can be safely ignored. However, if deformation is at the same spatial scale as the batholith itself, the effects of deformation cannot be ignored. The western edge of the Idaho Batholith is an example of the redistribution of 87Sr/86Sr values by deformation. The Idaho Batholith is anomalous, containing only a narrow (< 10 km) band of mid-Cretaceous plutonism on its extreme western margin. The transition from island-arc lithosphere to continental cratonic lithosphere occurs in <10 km, as recorded by the Sr isotopic ratios. Although originally interpreted as plutons intruded along a vertical continental margin, the present isotopic gradient is the result of subsequent deformation. The 87Sr/86Sr transition is spatially coincident with the western Idaho shear zone, a Late Cretaceous transpressional plate boundary. Restoration of the contractional component of Late Cretaceous deformation results in a mid-Cretaceous magmatic arc in western Idaho similar in width (~85-100 km) to other mid-Cretaceous Cordilleran magmatic arcs. Integrated analysis of the structural and geochemical data suggests the presently sharp 87Sr/86Sr isotopic transition in western Idaho represents structural modification of the batholith after emplacement, not intrusion of the batholith along a sharp boundary.