Donald W. Hyndman

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.

Controls on source and depth of emplacement of granitic magma

Large granitic batholiths emplaced in high-grade metamorphic rocks at mid-crustal levels were relatively hydrous compared with major batholiths emplaced at very shallow levels. Those emplaced at mid-crustal levels were probably generated by partial melting of crustal rocks at only slightly deeper levels. Those emplaced at shallow levels are associated with penecontemporaneous volcanic ejecta and were probably generated at deep crustal or possibly mantle depths.

Terrestrial Maria: The Origins of Large Basalt Plateaus, Hotspot Tracks and Spreading Ridges

Large lava plateaus, which form abruptly within plates without apparent tectonic cause, appear to be the terrestrial equivalents of lunar maria. Deep erosion of lava plateaus reveals that their magma chambers are gabbro and granophyre complexes, the classic lopoliths. The close association of the Deccan Plateau with the timing of the terminal Cretaceous boundary clay suggests that it is an impact crater large enough to cause pressure relief melting in the asthenosphere. Basalt then flooded the crater to form a lava lake, the terrestrial equivalent of a lunar mare. Part of the crater rim survives in the Amirante Ridge. The Carlsberg oceanic ridge and Chagos-Laccadive hotspot track formed simultaneously with the Deccan Plateau. Several other Mesozoic and Cenozoic lava plateaus are parts of similar arrays of simultaneous features. One of those is the southern Oregon part of the Columbia Plateau, with its associated continental rifting in the northern Basin and Range, and the Yellowstone hotspot track along the Snake River Plain. Impact craters apparently start hotspots by initiating pressure relief melting, which develops into a persistent low pressure cell within the mantle. They appear to form oceanic ridges and continental rifts by initiating a crack in stressed lithosphere, which then propagates in a direction transverse to that of maximum tensional stress. Pressure relief melting below the propagating fracture develops a persistent low pressure cell within the mantle.

The Role of Tonalites and Mafic Dikes in the Generation of the Idaho Batholith

Formation of the Idaho batholith is thought to have been promoted by prolonged injection of high-temperature mantle magmas that caused partial melting of continental crustal rocks to form granitic magma. The mafic magmas are now represented at the surface by numerous synplutonic mafic dikes in the batholith and by early tonalite and quartz diorite plutons forming the western part of the batholith and scattered elsewhere around its margins. Variable degrees of mixing between the mafic magmas and the granites produced quartz diorite complexes, intermediate and composite dikes, and mafic-rich, inhomogeneous parts of the main-phase granites and granodiorites.

Thermal and fluid regimes in the Bitterroot lobe–Sapphire block detachment zone, Montana: Evidence from 18O/16O and geologic relations

In western Montana, a 15-km-thick sequence of allochthonous Belt Series metasedimentary rocks is thought to have moved eastward off the Northern, or Bitterroot, lobe of the emergent Idaho batholith. The zone of detachment south of Missoula, defined by mylonitic rocks ∼500 m thick, is largely in the batholith but traverses the granite-metasediment boundary. Later chloritic breccias, characterized by intense transgranular fracturing, overlie and locally transect the mylonites. In granite and pegmatite mylonites, quartz (δ18O = 10.4 to 11.1) and coarse muscovite (8.4‰) yield isotopic temperatures of 550 ± 50 °C, interpreted as the temperature of ductile deformation, and would have involved fluids of 9.5 ± 0.5‰, which is close to the magmatic or high-temperature metamorphic range. K-feldspar (7.4 to 1.3‰), fine muscovite (8.3 to 6.8‰), and biotite (2.0 ± 0.2‰) have been shifted by variable magnitudes to low δ18O, which reflects post-mylonite, meteoric-water incursion resetting, less retentive minerals, an effect also observed in undeformed precursors to mylonites. Mylonitic metasedimentary rocks 25 km from the Bitterroot Lobe have not been isotopically disturbed by exchange with meteoric water but retain isotopic concordancy for coexisting quartz, k-feldspar, and muscovite, corresponding to estimated temperatures of 450 to 500 °C the ambient conditions of ductile deformation. In chloritic breccias, quartz and feldspar have undergone shifts of about −10‰ relative to their values in mylonitic granites and exhibit disequilibrium fractions (Δ = 5.3 to 9.6‰), due to preferential exchange of feldspar down to lower temperatures. Albite and chlorite yield temperatures of 370 °C (Δ = 4.5‰) to 250 °C (Δ = 6.3‰), and calculated δ18O of fluids in equilibrium with albite is −7 (370 °C) to −11.8 (250 °C). This temperature range is corroborated by fluid-inclusion data. High deduced temperatures and fluid isotopic compositions in the mylonites are commensurate with a pluton roof-zone environment at near magmatic conditions, providing enhanced ductility in mylonites. The chloritic breccia is regarded as a structural domain that accommodated late movement of the overlying rocks subsequent to removal of the main cover and provided conduits for incursion of low-temperature meteoric waters. The structural sequence reflects the change from high-temperature ductile deformation of mylonites, under conditions of crust-equilibrated fluids at low water/rock ratio, to a regime of brittle fracturing at diminished temperatures and lower confining stress. In the latter structural environment, hydrological communication to the surface promoted elevated water/rock ratios in the domains of fracturing.

Direction and shear sense during suturing of the Seven Devils-Wallowa terrane against North America in western Idaho

A northeast-dipping 1.5-km-thick mylonite near Dworshak Dam marks the suture zone between Precambrian North America and the Seven Devils-Wallowa terrane in western Idaho. The mylonite formed under amphibolite facies conditions from quartz diorite containing apparently synplutonic mafic and synkinematic pegmatite dikes of the Kamiah plutonic complex. Mylonitic lineations and fold axes have a mean plunge of 48° toward 056°, nearly down the dip of the mylonitic foliation. Shear sense, given by offset of late-stage crosscutting pegmatites, is consistently top-to-the-southwest, reverse-slip, parallel to the mylonitic lineation. Folds that formed by progressive folding of the mylonitic foliation approach sheath-fold geometry. Axial planes and fold limbs are nearly parallel to the mylonitic foliation. Mafic dikes that are apparently synplutonic in the undeformed quartz diorite immediately south of the mylonite zone and north of Kamiah have variable dips and azimuths. In the shear zone, however, these dikes lie nearly in the mylonitic foliation. Transposition of the dikes into near concordance with the foliation by simple shear requires high values of shear strain and suggests that cumulative top-to-the-southwest, reverse-slip displacement across the mylonite zone is at least 27 km, and likely more than 80 km. This displacement involves underthrusting of the Kamiah plutonic complex, emplaced within the Seven Devils-Wallowa terrane, beneath North America during Late Cretaceous docking with continental North America.

Boulder batholith: A result of emplacement of a block detached from the Idaho batholith infrastructure?

Rise of the Cordilleran infrastructure during the last half of Mesozoic time provided the gravity potential for lateral spreading and overthrust movements. Near the northeastern margin of the Idaho batholith a tectonic block, 100 by 75 km in size, became detached from the infrastructure in the later stages of its evolution and moved more than 25 km east. We suggest that the emplacement of the Boulder batholith is related to the eastward movement of the block of the tectonic suprastructure.

Mid-Mesozoic Multiphase Folding Along the Border of the Shuswap Metamorphic Complex

The high-grade regional metamorphism of the extensive Shuswap metamorphic complex of southeastern British Columbia has been considered by most geologists to be Precambrian in age. On the basis of structural analysis, it is now apparent that low-grade Triassic rocks of the nearby Slocan Group have undergone the same three generations of folding as the Shuswap, beginning with that accompanying regional metamorphism. Intense isoclinal recumbent folding of the first generation accompanied regional metamorphism and development of a schistosity parallel to the axial surfaces of these folds and a penetrative lineation parallel to their axes. Following regional metamorphism, a second deformation, accompanying formation of nearly upright macroscopic folds, formed a strain-slip cleavage by crenulation of the schistosity. Subsequent intrusion of granitic plutons developed mesoscopic folds having coplanar axes in any single domain. This third deformation refolded the nearly constant eastward plunge of earlier folds and lineations into small cones. Deformation accompanying regional metamorphism of the Shuswap complex must have occurred since the Triassic and probably during the Jurassic period.