David A Schneider

U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis

We propose that the late Paleoproterozoic igneous and deformational history preserved in the northern midcontinent United States can be explained by a change in subduction-polarity from geon 18 south-dipping subduction during Penokean accretion to geon 17 north-dipping subduction as convergence continued after Penokean orogenesis. New U-Pb zircon ages indicate that late to post-Penokean magmatism occurred at ca. 1800, 1775, and 1750 Ma and generally migrated southeastward across the newly accreted Penokean terrane. We suggest that geon 17 Yavapai slab rollback caused continental arc magmatism to step southeastward between 1800 and 1750 Ma. As the slab steepened, reduced compressional stresses and magma-induced thermal weakening allowed for collapse of the overthickened portions of the Penokean crust. Postcollapse crustal stabilization (the 1750-1650 Ma Baraboo interval) was followed by geon 16 Mazatzal arc accretion further south. The 1900-1600 Ma tectonic history of the north-central United States, not surprisingly, records events related to the southward growth and tectonic development of the southern Laurentian margin. New and published 4 0 Ar/ 3 9 Ar mineral ages delineate the northern and western extent of geon 16 Mazatzal deformation. Interestingly, only little exhumed crust intruded by a small volume of shallow-level ca. 1750 Ma plutons (and associated rhyolites) was deformed significantly during geon 16. In contrast, more deeply exhumed crust and crust pervasively invaded by a large volume of post-Penokean magma (i.e., East-Central Minnesota Batholith) were largely unaffected by Mazatzal deformation and reheating. We suggest that posttectonic intrusions and crustal thinning were an important step in strengthening and stabilizing the crust in the southern Lake Superior region.

Thermochronology of the west Sudetes (Bohemian Massif): Rapid and repeated eduction in the eastern Variscides, Poland and Czech Republic

The Sudete Mountains of northeastern Bohemian Massif were amalgamated during the closure of the Rheic ocean, culminating with Variscan orogenesis, and contain occurrences of high-pressure granulite and small relict ultrahigh-pressure eclogite formed during subduction. We performed 40Ar/39Ar thermochronometry on primarily amphibolite-facies gneisses and schists from two crustal blocks within the Sudetes: Góry Sowie and Orlica-Snieznik massifs. Hornblende and mica plateau ages from the mountainous portion of the Góry Sowie reflect relatively rapid cooling between 382 ± 1 Ma and 373 ± 0.5 Ma, following peak conditions at ca. 400 Ma. Kyanite-sillimanite and cordierite-garnet mineral growth textures denote near isothermal decompression during eduction. Concordant hornblende and biotite cooling ages obtained from the eastern, topographically flat region adjacent to the Niemcza shear zone indicate markedly younger cooling at 337 ± 0.8 Ma. 40Ar/39Ar results from the Snieznik Mountains, 50 km to the south, yield plateau cooling ages for white mica and biotite between 341 ± 1 to 337 ± 0.6 Ma and 342 ± 1 to 334 ± 0.6 Ma, respectively, and are remarkably similar in age to an amphibolite-facies overprinting episode. Mica analyses from the Orlica Mountains also yield cooling ages between 338 ± 0.9 to 335 ± 0.5 Ma. Thermochronometry illustrates temporally disparate cooling histories but similar metamorphic evolutions between the Góry Sowie Block and the Orlica-Snieznik Complex. In both terranes, it took ∼35 m.y. from the time of peak high-pressure conditions to residence in the upper crust, and possibly as little as 10 m.y. between high-pressure and mid-crustal supra-Barrovian events. These results in light of regional geochronology illustrate the rapid, localized and repetitious nature of eduction/exhumation of the Sudetic orogenic root including the eclogite and HP-granulite assemblages during protracted subduction.

Exhumation and metamorphism of an ultrahigh-grade terrane: geochronometric investigations of the Sudete Mountains (Bohemia), Poland and Czech Republic

The Sudete Mountains, NE Bohemian Massif (Czech Republic and Poland), preserve abundant eclogitic and granulitic centimetre- to decimetre-scale boudins enveloped in a predominantly migmatitic matrix. Published geochronometry and thermobarometry from the UHP and UHT rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units; however, the timing of stage 2 metamorphism and associated unroofing is less well constrained. New in situ ion microprobe Th–Pb monazite results, together with complementary U–Pb zircon and electron microprobe analyser total-Pb monazite results, on 11 amphibolite-facies gneissic to migmatitic samples, place important temporal constraints on the second stage of UHP and UHT metamorphism–exhumation. The Orlica–Snieznik Dome records UHP metamorphism occurring at 375 Ma and subsequent exhumation to midcrustal levels in supra-Barrovian conditions at c. 345–330 Ma. In contrast, the western Góry Sowie Block preserves evidence of HP-granulite conditions at c. 400 Ma, and exhumation to mid-crustal levels at 380–370 Ma, revealing a c. 30 million years difference in exhumation events between the neighbouring terranes. The eastern Góry Sowie Block preserves ages similar to the Orlica–Sneiznik Dome, suggesting that different preserved metamorphic–cooling histories are juxtaposed across the Sudetic Marginal fault. The bounding Niemcza shear zone yields preliminary Th–Pb dates that range from 380 ± 8 Ma to 283 ± 2 Ma, preserving a protracted metamorphic record that spans the exhumation history of the region. The distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low-viscosity crust under supra-Barrovian conditions near the mid-crustal high-strength lid during oblique (transpressional) convergence.

Miocene calibration for calcareous nannofossils from low-latitude Ocean Drilling Program sites and the Jamaican conundrum

Prior work with marine sedimentary sections exposed on Jamaica had been interpreted as evidence for a curious pattern of diachrony between the tropics and subtropics in the ages of certain biostratigraphic zonal boundaries of middle to late Miocene age. We offer an alternative to the Jamaican low-latitude nannofossil calibration, using a combination of paleomagnetic, biostratigraphic, and lithostratigraphic data derived from tropical sections recovered by the Ocean Drilling Program (ODP) in the eastern Pacific and western Atlantic Oceans. Our nannofossil calibration is based on magnetic polarity stratigraphy obtained from the Pacific Ocean and is consistent with the notion that Milankovitch periodicities governed cyclic changes in the lithology of sediments from the Ceara Rise, a sea-floor high in the Atlantic Ocean located offshore of the Amazon delta. Our results thus appear to be representative of the tropics and so call into question the ages that had been assigned to nannofossil zonal boundaries based on results from Jamaican sections.