ABSTRACT The Seve Nappe Complex in the Scandinavian Caledonides records a range of peak metamorphic conditions and timings. To better understand pressure-temperature-deformation-time differences throughout the complex and possible tectonic scenarios, metamorphosed mafic rocks within the Tarfala Valley of the Kebnekaise Massif (Sweden) were investigated using integrated petrologic and geochronologic techniques. Thermodynamic modeling of two samples using domainal and whole-rock compositions integrated with mineral chemistry, mineral textures, and titanite and zircon U-Pb geochronology constrained a portion of the pressure-temperature ( P-T ) path. Peak metamorphic conditions of 590–660 °C and 9.7–10.5 kbar were followed by near-isothermal decompression or a subsolidus clockwise P-T path. Amphibolite units in the valley record retrograde conditions at 450–550 °C at less than 7.5 kbar, although mineral modes and textures are most consistent with pressures <4 kbar. The majority of titanite growth occurred due to the introduction of hydrous fluids during cooling and following exhumation to midcrustal levels. U-Pb ages of retrograde titanite define a spread from ca. 480 to 449 Ma, and the oldest age is interpreted to constrain the timing of retrogression following exhumation. This interpretation is supported by a U-Pb zircon crystallization age of 481 ± 7 Ma for a metamorphosed intermediate to felsic synkinematic dike hosted in one of the amphibolite units. These results indicate that the Kebnekaise region records Early Ordovician deformation and metamorphism that was of lower grade compared to other Seve Nappe Complex locations to the south. The tectonic history of these rocks includes metamorphism and exhumation during the Cambrian–Ordovician pre-Scandian event, followed by thrusting of the Seve Nappe Complex and neighboring rocks onto Baltica during the Silurian Scandian orogeny.

Abstract A transition from gabbro to eclogite has been investigated at Vinddøldalen in south-central Norway, with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct U–Pb zircon age of the metamorphic process. In the different rocks of the transition zone zircon occurs as (I) igneous prismatic grains, (II) metamorphic polycrystalline rims and pseudomorphs after baddeleyite, and (III) as tiny (<10 µm) bead-like zircon grains. Textural relations suggest that type II zircon formed by breakdown of baddeleyite in the presence of silica, whereas Fe–Ti oxides were the main Zr source for the type III zircon. Subsolidus liberation of Zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid-assisted metamorphism, and by resorption of Fe–Ti oxide in rock domains that were completely recrystallized to eclogite. SIMS (secondary ion mass spectrometry) and TIMS (thermal ionization mass spectrometry) dating provides comparable U–Pb ages of magmatic zircon and baddeleyite. Baddeleyite (TIMS) yielded an age of 1457±11 Ma for the gabbro emplacement. Bead-type metamorphic zircon from eclogite gave 425±10 Ma (TIMS) dating the metamorphic transition from gabbro to eclogite in the upper basement of the Lower Allochthon in the south-central Scandinavian Caledonides.

Abstract Dating of detrital zircons from well cuttings is a useful technique to constrain stratigraphic ages and structural interpretations in complexly deformed terranes. This technique was applied in the Helena Salient of the Montana Disturbed Belt to determine whether the Norcen, Kimpton Ranch 1-11 and Buckhorn, Federal 2-24 wells penetrated Phanerozoic strata beneith the allochthonous Precambrian rocks carried on the Lombard thrust. Since some Phanerozoic strata have reservoir potential, their presence below the Lombard thrust has important implications for the oil and gas potential of this structural province.

New structural, metamorphic, and geochronologic data from the Clearwater complex, north-central Idaho, define the origin and exhumation history of the complex. The complex is divisible into an external zone bound by normal faults and strike-slip faults of the Lewis and Clark Line, and an internal zone of Paleoproterozoic basement exposed in two shear zone–bounded culminations. U-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of metamorphic zircon overgrowths from the external zone yield zircon growth at ca. 70–72 Ma and 80–82 Ma, during peak metamorphism and before tectonic exhumation of the external zone. U-Pb SHRIMP dating of metamorphic zircon rims from the internal zone record growth at ca. 64 and between 59 and 55 Ma. The older ages record pre-extension metamorphism. The younger rim ages were derived from fractured zircons in the Jug Rock shear zone, and they document the beginning of exhumation of the internal zone along deep-seated shear zones that transported the basement rocks to the west. The 40 Ar/ 39 Ar ages record quenching of the external zone starting ca. 54 Ma and the internal zone between 53 and 47 Ma by movement along the bounding faults and internal shear zones. After ca. 47 Ma, extension was accommodated via a west-dipping detachment that was active until after ca. 41 Ma. The Clearwater complex is interpreted as an Eocene metamorphic core complex that formed in an extensional relay zone between faults of the Lewis and Clark Line.

Abstract High–precision U–Pb TIMS, SHRIMP, and LA–ICPMS dating of magmatic and detrital zircons from the core of the Clearwater complex, northern Idaho, U.S.A., provide new ages and new tectonic interpretations for potential Precambrian basement rocks in this part of the Cordillera. The Boehls Butte anorthosite, which is exposed in lens–like masses within the core of the Clearwater complex, crystallized at 1787 ± 2 Ma. Amphibolites, which are intercalated with the anorthosite, crystallized during distinctly different magmatic episodes around 1587 Ma, 1467 Ma, and 1453 Ma. These dates better define the age of Precambrian basement in this region and document a new exposure of 1580 Ma igneous rocks along the western edge of the North American craton. Surrounding the anorthosite are metasedimentary rocks (Boehls Butte Formation) that have been interpreted as predating the anorthosite and the Mesoproterozoic BeltPurcell Supergroup. Detrital zircons from these metasedimentary rocks yield age populations that are predominantly Paleoproterozoic with some Archean grains. The youngest concordant 207 Pb/ 206 Pb ages are between 1597 and 1761 Ma, well after crystallization of the anorthosite. On this basis, we conclude that most of the rocks once assigned to the Boehls Butte Formation are better correlated with the lower part of the Belt–Purcell Supergroup. The only part of the Boehls Butte Formation that remains potential basement is the Al–Mg– rich schist that borders the masses of anorthosite. We propose that the anorthosite and bordering Al–Mg schists are displaced tectonic slivers that were juxtaposed against the metasedimentary rocks by shear zones that predate peak metamorphism. This zone of shear may be related to the basal decollement for the Rocky Mountain fold–and–thrust belt.

Abstract Mafic intrusions within the Mesoproterozoic Belt-Purcell basin of the Northern Rocky Mountains of Montana record important Mesoproterozoic and Neoproterozoic rifting episodes. Previous studies established four major Mesoproterozoic igneous intrusive events. This project focuses on sills and dikes that represent a fifth, Neoproterozoic intrusive event and establishes their regional extent and relationship to the Gunbarrel magmatic event and late Proterozoic Windermere rift activity. The sills and dikes of this study intruded Archean basement rock and the Mesoproterozoic formations of the Belt-Purcell Supergroup and are unconformably overlain by the Cambrian Flathead Sandstone. When restored on a palinspastic map, the thickest and deepest segments of the sills, along with dike swarms in western Montana and northern Wyoming, are colinear with the Belt-Purcell basin axis. The sills and dikes fall on the eastern margin of Windermere rock exposures along the margins of the Laurentian craton. The sills and dikes are tholeiitic diabase and have locally undergone low-grade metamorphism and alteration. Granophyre is associated with some sills, and Belt-Purcell xenoliths found in the granophyre show some signs of interaction with the magma. Geochemical analysis is compatible with a single intrusive event. A U-Pb zircon date of 777.5 ± 2.5 Ma obtained from granophyre in the Holland Lake sill supports argon dates of previous studies. Discordant U-Pb data from two other sills, exposed at Turah and near Rogers Pass, are consistent with this date, although it is also possible that these sills were emplaced at different times. The 777.5 ± 2.5 Ma date from the Holland Lake sill establishes the emplacement of this sill during the Gunbarrel magmatic event (ca. 780 Ma) and may reflect earliest Windermere rift activity.