Relationships between crustal partial melting, plutonism, orogeny, and exhumation: Idaho–Bitterroot batholith

Abstract

The northern Idaho–Bitterroot batholith region is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the Cordilleran orogen. The relative timing of metamorphism, partial melting, intrusion, and deformation in this area may provide an analogue for magmatic and deformation processes active at mid-crustal depths in modern orogenic belts. Crustal thickening in this part of the Cordilleran began before 100 Ma, but the early stages of this process are poorly constrained. U–Pb zircon dates indicate that high-grade metamorphism was coincident with intrusion of syntectonic quartz diorite plutons at ca. 75–80 Ma, in the Bitterroot metamorphic core complex, where the deepest crustal levels are exposed. Metamorphic conditions reached ∼0.65–0.75 GPa and ∼600–750 °C in the northeastern part of the core complex. Upper amphibolite facies conditions caused widespread partial melting in quartzofeldspathic gneiss facilitated by muscovite breakdown in underlying semi-pelitic schist. New and previously published U–Pb zircon ages indicate that major partial melting of the lower and middle crust occurred between ca. 65 and 53 Ma, leading to the intrusion of the voluminous “main-phase” granitic plutons as thick (3–4 km) sills. Intrusion was accompanied by renewed upper amphibolite facies metamorphism and partial melting forming migmatites at ∼0.65 GPa pressure. The youngest mid-crustal granitic intrusions are about the same age as initial collapse of the orogen and extension at ca. 52–50 Ma. Extension was accommodated mainly on the Bitterroot mylonite zone that deforms the younger intrusions as well as the older high-grade rocks. Therefore, large-scale partial melting of the middle and lower crust followed crustal thickening by as much as 15–35 Ma, but pre-dated extension and exhumation by only 1–3 Ma. Collapse in this sector of the Cordilleran orogen appears to have been focussed where partial melting and plutonism were most intense and long-lived. Exhumation is revealed by the transition from amphibolite facies mylonitization, to greenschist-facies shearing, to brittle faulting, to inactivity of the shear zone that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 51–38 Ma, based on U–Pb and Ar–Ar results. Alkali-feldspar granites were emplaced during the onset of exhumation, but were intruded only into the shallowest Eocene crustal levels. Their generation may have been linked to decompression of the lithospheric column during crustal thinning.