Daniel Holm

New constraints on the timing of Early Proterozoic tectonism in the Black Hills (South Dakota), with implications for docking of the Wyoming province with Laurentia

A question regarding the 1900–1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40 Ar/ 39 Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes (F 1 ) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest–trending upright F 2 folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40 Ar/ 39 Ar hornblende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is ∼50–60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810–1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780–1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

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.

Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia

Bedrock mineral cooling dates in the southern Lake Superior region establish a sharp ca. 1630 Ma thermal front that separates basement having typical post-Penokean (1750–1700 Ma) cooling ages to the north from basement having thermally reset (≤ 1630 Ma) ages to the south. The thermal front coincides spatially with an apparent deformational front in overlying post-Penokean quartzites. Subhorizontal quartzite north (Barron, northwest Wisconsin) and west (Sioux, Minnesota) of the front and highly folded quartzite south of the front (Flambeau, northwest Wisconsin) all yield ion microprobe (single-grain, single-spot) 207 Pb/ 206 Pb detrital zircon ages as young as 1750 Ma (but none younger than 1714 Ma). The new detrital zircon dates and the thermal-deformational front together with other geologic information suggest that these quartzites were all deposited between 1750 Ma and 1630 Ma. This study provides the first good structural evidence that deformation accompanied the long-recognized but enigmatic 1630 Ma low-grade, thermal-metamorphic event in the southern Lake Superior region. We interpret the strong compressional deformation exhibited in the Early Proterozoic quartzite bodies throughout much of Wisconsin to reflect foreland deformation associated with the assembly of southern Laurentia during the ca. 1650 Ma Mazatzal orogeny.

Core complex model proposed for gneiss dome development during collapse of the Paleoproterozoic Penokean orogen, Minnesota

Gneiss domes preserved throughout the Paleoproterozoic (1870–1820 Ma) Penokean orogenic belt are commonly viewed as collisional features. However, 1700 Ma mica 40Ar/39Ar ages from Archean rocks of the McGrath gneiss dome, Minnesota, are in marked contrast with 1750–1760 Ma mica ages obtained everywhere outside the dome, suggesting the dome may have formed well after compression had ceased. We hypothesize that the bimodal age pattern is a result of crustal excision between Archean basement and Paleoproterozoic cover during extensional collapse of the overthickened orogen. Removal of structural section and progressive unroofing may have led to the development of a domed basement-cover shear zone. In this respect, the McGrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex. Postorogenic collapse of the Penokean orogen was apparently long-lived, with significant pulses occurring at ∼1755 Ma and at ∼1700 Ma The sudden and widespread crustal fusion at 1770–1760 Ma that preceded the proposed collapse suggests that rapid subcrustal lithospheric thinning may have triggered collapse as has been proposed recently for several Phanerozoic collapsed orogens.

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.

Determining the Extent and Nature of Mazatzal-Related Overprinting of the Penokean Orogenic Belt in the Southern Lake Superior Region, North–Central USA

Abstract Twenty-one hornblende and mica 40Ar/39Ar dates from central and northwest Wisconsin, USA, provide important information on the timing, spatial extent, and intensity of Mazatzal-age metamorphism and deformation which overprinted the Paleoproterozoic (1870–1820 Ma) Penokean orogenic belt in the southern Lake Superior region. 1760–1750 Ma mica plateau ages from bedrock beneath undeformed 1750–1630 Ma quartzites are interpreted as the time of lower temperature (∼300–350°C) cooling and crustal stabilization after the Penokean orogeny. Six mica ages from bedrock underlying deformed Paleoproterozoic quartzites cluster around 1600 Ma (1576–1614 Ma). The complete absence of 1760–1750 Ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300–350°C during Mazatzal-related deformation and metamorphism. The ∼1600 Ma mica ages are interpreted to date the cooling phase of this metamorphism. Two anomalously young biotite dates are interpreted to indicate partial resetting associated with Mesoproterozoic rifting at 1100 Ma. Ten hornblende 40Ar/39Ar dates obtained in this study address the higher-temperature overprinting effects in the southern Lake Superior region. One latest Archean age of 2503±18 Ma and two ages of 1853 and 1830 Ma are interpreted as remnant evidence of Archean and Penokean age amphibolite metamorphic events, respectively. The majority of hornblende ages are younger than the Penokean orogeny, scattering between 1796 and 1638 Ma. Microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages. This suggests that shearing and recrystallization did not play an important role in the retention or loss of argon. The 1638 Ma hornblende age is concordant with the Mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid-related resetting associated with that event. Six other post-Penokean ages scatter over a 70 million year interval (1796–1723 Ma) and probably reflect variable retention of radiogenic argon. They are interpreted to indicate variable degrees of partial intermediate-temperature (350–500°C) resetting of hornblende argon systematics at ∼1650–1630 Ma. Collectively, these data suggest that the effects of the Mazatzal orogeny in the southern Lake Superior region involved 350–500°C metamorphism and penetrative deformation.

Late Neoproterozoic amphibolite-facies metamorphism of a pre-Caledonian basement block in southwest Wedel Jarlsberg Land, Spitsbergen: new evidence from U-Th-Pb dating of monazite

Abstract – Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic crustalblocks with differing metamorphic histories. Both blocks experienced Caledonian greenschist-faciesmetamorphism, but only the southern block records an earlier pervasive M1 amphibolite-faciesmetamorphism and strong deformational fabri c. In situ EMPA total-Pb monazite geochronologyfrom both matrix and porphyroblast inclusion results indicate that the older M1 metamorphismoccurred at 643 ±9 Ma, consistent with published cooling ages of c . 620 Ma (hornblende) and580 Ma (mica) obtained from these same rocks. This region thus contains a lithostratigraphic profileand metamorphic history which are unique within the Svalbard Archipelago. Documentation of apervasive 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 onthe Baltic and Laurentian passive margins. Instead, our new metamorphic age implies an exotic originof the pre-Devonian basement exposed in SW Spitsbergen and supports models of terrane assemblypostulated for the Svalbard Archipelago.Keywords: Svalbard, Caledonides, terranes, geochronology, tectonics.

A strike-slip terrane boundary in Wedel Jarlsberg Land, Svalbard, and its bearing on correlations of SW Spitsbergen with the Pearya terrane and Timanide belt

Abstract: Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic terranes with differing structural and metamorphic histories. The northern terrane experienced two Early Palaeozoic deformation events both accompanied by greenschist-facies metamorphism of similar grade. The southern terrane records a Neoproterozoic pervasive amphibolite-facies metamorphism and strong deformational fabric only locally retrogressed during a Caledonian greenschist-grade event. These terranes are separated by an important sinistral ductile shear zone defined as the Vimsodden–Kosibapasset zone, which comprises wrench- and contraction-dominated domains characteristic of strain partitioning in transpression zones; in this case apparently controlled by contrasting rheologies of the juxtaposed crustal domains. The northern terrane of Wedel Jarlsberg Land shares affinities with Pearya in northern Ellesmere Island of Arctic Canada whereas the southern one resembles the Timanide belt of NE Europe. A quantitative approach facilitated by a numerical plate model demonstrates that correlation with Pearya is feasible if sinistral displacement of c. 600 km occurred during the Caledonian orogeny. The correlation with the Timanides is valid if the southern terrane represents an outlier of the Timanide belt separated from Baltica by the opening of the Iapetus Ocean.

40Ar/39Ar evidence for Middle Proterozoic (1300–1500 Ma) slow cooling of the southern Black Hills, South Dakota, midcontinent, North America: Implications for Early Proterozoic P‐T evolution and posttectonic magmatism

40Ar/39Ar total gas and plateau dates from moscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600–1650 Ma) mica dates were obtained from metasedimentary rocks located in a synformal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and undeformed samples of the ∼1710 Ma Harney Peak Granite (HPG) yield Middle Proterozoic mica dates (∼1270–1500 Ma). Two samples collected between the synform and Bear Mountain dome yield intermediate total gas mica dates of ∼1550 Ma. We suggest two end-member interpretations to explain the map pattern of cooling ages: (1) subhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (∼1500 Ma) ∼300°C isotherm. According to the second interpretation, the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east-west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills published elsewhere, indicate that the ambient country-rock temperature at the 3–4 kbar depth of emplacement of the HPG was between 350°C and 500°C, suggesting that the average upper crustal geothermal gradient was 25°–40°C/km prior to intrusion. The thermochronologic data suggest HPG emplacement was followed by a ∼200 m.y. period of stability and tectonic quiescence with little uplift. We propose that crust thickened during the Early Proterozoic was uplifted and erosionally(?) thinned prior to ∼1710 Ma and that the HPG magma was emplaced into isostatically stable crust of relatively normal thickness. We speculate that uplift and crustal thinning prior to HPG intrusion was the result of differential thinning of the subcrustal lithosphere beneath the Black Hills. If so, this process would have also caused an increase in mantle heat flux across the Moho and triggered vapor-absent melting of biotite to produce the HPG magma. This scenario for posttectonic granite generation is supported, in part, by the fact that in the whole of the Black Hills, the HPG is spatially associated with the deepest exposed Early Proterozoic country rock.

Thermochronological evidence for late Proterozoic (Vendian) cooling in southwest Wedel Jarlsberg Land, Spitsbergen

Two Proterozoic terranes with different metamorphic histories are distinguished from geological mapping in southwestern Wedel Jarlsberg Land: a northern greenschist facies terrane and a southern amphibolite facies terrane which has been overprinted by greenschist facies metamorphism. To better characterize the tectonothermal history of these terranes we have obtained new 40 Ar/ 39 Ar mineral dates from this area. A muscovite separate from the northern terrane yielded a Caledonian plateau age of 432±7 Ma. The southern terrane yielded significantly older 40 Ar/ 39 Ar ages with three muscovite plateau dates of 584±14 Ma, 575±15 Ma, and 459±9 Ma, a 484±5 Ma biotite plateau date, and a 616±17 Ma hornblende plateau date. The oldest thermochronological dates are over 300 Ma younger than the age of amphibolite facies metamorphism and therefore probably do not represent uplift-related cooling. Instead, the Vendian dates correlate well with a regionally widespread magmatic and metamorphic/thermal resetting event recognized within Caledonian complexes of northwestern Spitsbergen and Nordaustlandet. The apparent Ordovician dates are interpreted to represent partial resetting, suggesting that late Caledonian greenschist facies overprinting of the southern terrane was of variable intensity.