THE ANACONDA METAMORPHIC CORE COMPLEX: CRETACEOUS DUCTILE FLOW AND EOCENE DETACHMENT FAULTING FACILITATED BY LITHOSPHERIC MAGMATISM

The Anaconda metamorphic core complex was exhumed by Eocene detachment faulting between 53 and 38 Ma. The core complex exposes a cross section through the middle to upper crust in a part of the Cordilleran Orogen where arc-like plutonism protruded eastward into the foreland during Late Cretaceous shortening. Regionally, Mesoproterozoic Belt Supergroup through Cretaceous strata exceeds 12,000m in thickness. In the footwall these units underwent amphibolite-facies metamorphism, extreme attenuation, and recumbent-isoclinal folding during quartz diorite–granodiorite plutonism at ca. 78–74 Ma. Thermobarometry gives pressures of 4.6–6.0 kbar for peak metamorphism. Late Cretaceous plutons intruded as laccolith-shaped bodies within strata and along thrust faults at deep and shallow levels. Magmatic and sub-solidus fabrics in shallow plutons (e.g., Boulder batholith) are weak, whereas deeper level plutons show strong fabrics (e.g., Storm Lake stock) oriented parallel to high temperature fabrics in wall rocks. Regional-scale isoclinal folds within Belt metasediments are accompanied by penetrative axial planar foliation. Extreme thinning is prominent in metapelites and calc-silicates. Upright, west-verging folds deform the isoclinal folds, and may have developed at the interface between a magma-inflated mid-crustal channel and a rigid lid (back folds). Isotopic data suggest that the quartz diorite-granodiorite suite is largely the product of partial melting of ca. 1.4 Ga mafic rocks within part of Belt rift that formed along an Archean and Proterozoic suture. Limited magmatism in Paleocene time was felsic and derived from Archean crust. Footwall extension in Eocene time initiated at upper greenschist facies conditions 5-10 km shallower than Late Cretaceous deformation. Extension was accompanied by voluminous mafic to felsic plutons (Hurst Lake suite) with deeper Archean to Mesoproterozoic crustal sources mixed with juvenile magma associated with mantle upwelling. The unusual combination of features in the Anaconda complex are the result of unique Precambrian basement and basin structures, the position of major thrust ramps at the hinterland-foreland transition of the Helena salient, and nexus of Paleogene magmatism and rifting.