Open Access Kent State (OAKS)

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 40Ar/39Ar 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.

New geophysical analysis of the Precambrian basement in Minnesota–Iowa–Wisconsin indicates that an Archean–Proterozoic boundary (Spirit Lake trend) previously recognized in NW Iowa can be continued eastward into central Wisconsin and farther east as the Spirit Lake tectonic zone (SLtz). To test the age of Paleoproterozoic crust south of this structure, several subsurface samples of Precambrian basement from the north-central United States have been analyzed or re-examined using modern techniques of U–Pb, Sm–Nd, and 40Ar/39Ar geochronology. The results fill in a major data gap for the region and show that all U–Pb crystallization ages for samples south of the SLtz are geon 17 (1700–1800 Ma). Bedrock core samples from eastern Nebraska are ca. 1760–1800 Ma, and two samples from SE South Dakota, immediately south of the SLtz, yield ages of 1762 ± 28 Ma (Vermillion) and 1733 ± 2 Ma (Elk Point). Xenoliths from impact breccia in the Manson structure in north-central Iowa yield a similar age of ca. 1705 ± 30 Ma and metagabbro from SE Minnesota yields an age of 1760 ± 9 Ma. Farther to the northeast, zircons from Paleoproterozoic gneiss in the basement of Manitoulin Island, only a few km south of the Superior craton in Ontario, also yield a geon 17 age (1714 ± 10 Ma). Sm–Nd model ages (TDM) for samples immediately south of the SLtz fall in the range 1.9–2.2 Ga, indicating limited involvement of Archean crust. In contrast, Sm–Nd TDM ages for samples north of the SLtz typically range from 2.5 to 3.0 Ga, for both Paleoproterozoic plutons and Archean gneisses. Ion microprobe analyses of zircons from the Elk Point and Manson samples also show the presence of geon 16 overgrowths, indicating a strong regional thermal overprint during geon 16 accretion. This is supported by mid-geon 16 hornblende 40Ar/39Ar ages for samples from SE South Dakota and SE Minnesota. Although no U–Pb ages are available for juvenile basement beneath the ca. 1760 Ma granite–rhyolite suite of southern Wisconsin, south of the SLtz, Sm–Nd model ages are typically ca. 1.9–2.0 Ga, consistent with basement to the rhyolites being geon 17 in age. Collectively, the data require that most, if not all, of the Paleoproterozoic crust immediately south of the SLtz formed during geon 17 and probably represents eastward continuation, from Colorado, through Nebraska, of the Yavapai crustal province in the SW United States. Penokean (geon 18) crustal rocks are limited mainly to northern and central Wisconsin, east-central Minnesota, and northern Michigan. These results also show that medium grade (>500 ◦C) tectonothermal effects of the subsequent geon 16 (≈Mazatzal) orogeny to the south continue into the north-central United States. Both terranes probably also continue eastward into Ontario, Canada and farther east into protolith of the Grenville Province.

Metamorphism along the southern margin of the Archean Superior Province has been historically attributed to the Penokean orogeny. A narrow corridor of amphibolite facies rocks north of the main suture does record 1.83–1.80 Ga metamorphic monazite U–Th–Pb ages that mark the culmination of arc accretion. However, subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest Penokean crustal thickening: the tectonically buried Archean–Proterozoic continental margin. In Minnesota, new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the ∼1.775 Ga East-central Minnesota batholith at moderate depths. In northern Wisconsin and upper peninsula Michigan metamorphic monazite growth at 1.78–1.745 Ga (and far from geon 17 intrusions) reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca. 1.76 Ga Yavapai-interval accretion, not solely Penokean induced crustal collapse. South of the Penokean suture, Penokean terrane rocks were twice metamorphosed to upper greenschist facies; first during Yavapai accretion and again during geon 16 Mazatzal accretion. Geon 16 overprinting also affected a small part of the continental margin in the northeast orogen, the Peavy metamorphic node. South-directed basement thrusts there likely accommodated substantial Mazatzal foreland shortening, suggesting thick-skinned deformation. Mazatzal amphibolite facies metamorphism occurred throughout Iowa and southernmost Wisconsin (south of the Baraboo quartzite). 40Ar/39Ar mineral cooling ages from eastern Wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1.47 Ga Wolf River batholith, in part reflecting its rapid emplacement at shallow crustal levels. A local area of anomalously young

Metamorphic geochronology and thermochronology of the deformed Southern Province north of Lake Huron in Ontario, Canada reveals a complex and protracted tectonothermal history for the Proterozoic southern Laurentide margin that is distinct from that of the Lake Superior region to the west. While Penokean-interval deformation is well-constrained in the Lake Superior area, a well-defined geon 18 geochronometric signature is lacking in the Huronian vicinity. The oldest metamorphic ages revealed by in situ monazite U-Th-total Pb electron microprobe analyses are ca. 1800Ma, with a strong geon 17 (Yavapai-interval) and local geon 14/15 magmatic influences. Furthermore, conventional 40Ar/39Ar incremental step-heating of hornblende indicates Paleoproterozoic heating through 500°C occurred only adjacent to geon 17 intrusions; regional analysis of mica cooling ages suggests that Mesoproterozoic thermal overprinting of primary geon 17 magmatism and metamorphism is limited to south of the Murray fault. The Penokean orogeny, long considered the dominant Paleoproterozoic event in the Great Lakes region, is now recognized as only the first of several accretionary events that impinged on the southern Laurentide margin. However, the Penokean orogen is restricted to the 2100Ma embayment of the Superior Province rifted margin. Moreover, subsequent tectonothermal pulses (Yavapai, Mazatzal, and ca. 1450Ma) recorded in the rocks of the Southern Province in Ontario mimic that documented across much of the central and western United States. Based on the distinct lack of Penokean-interval ages and the strong geon 17 and geon 14 total-Pb and 40Ar/39Ar geochronometric signature, the tectonothermal history of Southern Province more closely resembles that of the southwest United States than the Lake Superior region.

Abstract Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic crustal blocks with differing metamorphic histories. Both blocks experienced Caledonian greenschist-facies metamorphism, but only the southern block records an earlier pervasive M1 amphibolite-facies metamorphism and strong deformational fabric. In situ EMPA total-Pb monazite geochronology from both matrix and porphyroblast inclusion results indicate that the older M1 metamorphism occurred at 643 ± 9 Ma, consistent with published cooling ages of c. 620 Ma (hornblende) and 580 Ma (mica) obtained from these same rocks. This region thus contains a lithostratigraphic profile and metamorphic history which are unique within the Svalbard Archipelago. Documentation of a pervasive late Neoproterozoic Barrovian metamorphism is difficult to reconcile with a quiescent non-tectonic regime typically inferred for this region, based on the occurrence of rift-drift sequences on the Baltic and Laurentian passive margins. Instead, our new metamorphic age implies an exotic origin of the pre-Devonian basement exposed in SW Spitsbergen and supports models of terrane assembly postulated for the Svalbard Archipelago.

The nature and style of mid-crustal assembly and exhumation during continental collision has been investigated in the Tatra Mountains of the Western Carpathians. The pre-Alpine basement of the Western Carpathians represents the easternmost exposure of the Variscan orogen in Europe, which marks the collision of Laurasia with Gondwanian-affiliated terranes during the Palaeozoic. The Tatric crystalline unit of the Western Tatra in northern Slovakia displays an inverted metamorphic sequence where a high-grade unit comprising migmatites with relicts of eclogite has been thrust over a lower-grade mica schist unit. New geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the Western Tatra. The Upper Unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1.6GPa to 1.0-1.2GPa at 750-800^oC which lead to intensive re-equilibration at high-pressure granulite facies conditions, comparable to the peak metamorphic conditions of the host migmatite. Both eclogite and migmatite shared a retrograde P-T path following the insertion of the eclogite assemblage into the migmatites. The metamorphic evolution of the Lower Unit mica schist is constrained to peak P-T conditions of 0.6-0.8GPa and 640 and 660^oC followed by retrogression. This suggests that different rock types of the Western Tatra metamorphic core shared only their exhumation path from mid-crustal levels. ID-TIMS Sm-Nd dating of garnet from eclogite yields a whole rock-garnet isochron age of 337+/-10Ma, with an initial εNd isotopic composition of +8.3. In situ U-Pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c. 380Ma whereas monazite from a migmatite away from the eclogite preserves a robust 340+/-11Ma age which is indistinguishable from Sm-Nd garnet age and U-Pb age of zircons in the anatectic leucosome of the migmatite (347+/-7Ma). A younger monazite age population from the migmatite of 300+/-16Ma is consistent with ^4^0Ar/^3^9Ar mica ages of c. 310Ma. This argues for a contemporaneous, and likely shared, exhumation path of the assemblage pair. In situ monazite total-Pb analyses from the Lower Unit mica schists yields xenocrystic and c. 370Ma ages, but no geochronologic evidence for peak Variscan tectonism. Exhumation of the deep crustal root occurred most probably in a two-stage process. The timing of the high-pressure, eclogite facies metamorphism before the onset of exhumation into the mid crust, was likely between c. 380Ma and 360Ma. Subsequent exhumation into the middle crust was coeval with migmatite generation at c. 340Ma and garnet diffusion modeling suggest ~30^oC/Ma cooling rates. The exhumation was likely tectonically forced by the action of a rigid indentor, which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

The crystalline core of the southern Black Hills, South Dakota, exposes an extensive, low-P–high-T aureole of garnet- to second-sillimanite-zone schists centered on the plutonic core of the 1715 Ma Harney Peak Granite (HPG). This paper demonstrates regional patterns of apparent ages observed for 52 40Ar/39Ar dates of muscovite and biotite in diverse rocks from across the ∼1000 km2 metamorphic aureole and its plutonic center. About 20 biotite dates, sampled mostly near faults, are influenced by excess 40Ar and obscure the regional trends. The remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite, with previously unrecognized, elliptical age zones centered on the main HPG pluton and its outliers. The regional pattern of 40Ar/39Ar cooling ages indicates non-uniform slow cooling of the mid-crust between ∼1600–1250 Ma. This scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite (Rb/Sr) and apatite (U/Pb), which range from 1690 to 1550 Ma and from 1700 to ∼1500 Ma, respectively. To explain these results, it is likely that ambient pre-granite temperatures of the country rocks were ≥350 °C at the ∼10–14 km depth of granite emplacement, as previously proposed, and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years. Alternatively, or in addition, the HPG and inner aureole were not uplifted until ∼1480–1330 Ma, whereupon they finally cooled through ∼300–350 °C.

This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a mineral's unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength. Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (TC) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including TC and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z ≈ 29%), titanite (∼ 34%) and apatite (∼ 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a TC-Zscale “calibrated” with field-based TC data for titanite (≥ 680 ± 20°C) and apatite (∼ 500°C). Extrapolation of this scale yields TC estimates for the following minerals: staurolite (TC ≥ 1060°C, Z ≈ 25%); garnet (≥ 1010°C, ∼ 26.5%); zircon (≥900°C); monazite, xenotime, and epidote (≥ 750°C, ∼ 32%); and Ca-clinopyroxene (≥ 670 ± 30°C, ∼ 34 ± 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature.