scholarly journals 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

Tectonics ◽  
1997 ◽  
Vol 16 (4) ◽  
pp. 609-622 ◽  
Author(s):  
Daniel K. Holm ◽  
Peter S. Dahl ◽  
Daniel R. Lux
Keyword(s):  
North America ◽  
South Dakota ◽  
Black Hills ◽  
Slow Cooling ◽  
Early Proterozoic ◽  
Middle Proterozoic

Evidence of heterogeneous crustal sources: the Harney Peak Granite, South Dakota, U.S.A.

1996 ◽  
Vol 87 (1-2) ◽  
pp. 331-337 ◽  
Author(s):  
Eirik J. Krogstad ◽  
Richard J. Walker
Keyword(s):  
South Dakota ◽  
Magma Mixing ◽  
Black Hills ◽  
Single Source ◽  
Pb Isotope ◽  
Early Proterozoic ◽  
Metasedimentary Rocks ◽  
Isotopic Variations ◽  
Granite Suite ◽  
Different Sources

ABSTRACT:The Early Proterozoic (1715 Ma) Harney Peak Granite (Black Hills, SD, U.S.A.) is a complex of hundreds of dykes and sills. Earlier studies of Nd, O and Pb isotope variations demonstrated that the complex was not derived from a single source, or even different sources of a single age. Instead, the granites can be divided into a group with sources probably dominated by Early Proterozoic sediments and a group with sources probably dominated by Archean sediments. New results on the Nd isotopic variations of many additional samples indicate that there is considerable overlap between Nd isotopic compositions within the complex. Values of εNd (1715 Ma) of the Harney Peak Granite suite (n = 20) range from −2·0, indicating an Early Proterozoic (2300-2200 Ma) crustal source, to −13·4, indicating a Middle to Late Archean (3200-3100 Ma) protolith. These results suggest that the Early Proterozoic source may have included rocks such as the c. 2200-1900 Ma metasedimentary rocks that occur in the southern Black Hills. The Archean sources might have included rocks such as those exposed on the periphery of the Black Hills. The range in Nd model ages negates the usefulness of the concept of the ‘average’ age of the crust in this part of the craton. Because such heterogeneity is present in the magmatic compositions of the Harney Peak Granite, it can be inferred that at least as much heterogeneity was present in the sources. In this granite system, melts were evidently derived from isolated, heterogeneous zones and did not have the opportunity to coalesce into large magma bodies. In systems where coalescence does occur, the evidence for such highly heterogeneous sources may be lost. These results emphasise that inferences drawn from a few samples of plutonic rocks in which magma mixing and homogenisation occurred can lead to erroneous conclusions about the age and nature of protoliths and, consequently, the development of continental crust.

Trilobites, biostratigraphy, and lithostratigraphy of the Crepicephalus and Aphelaspis zones, lower Deadwood Formation (Marjuman and Steptoean stages, Upper Cambrian), Black Hills, South Dakota

2000 ◽  
Vol 74 (2) ◽  
pp. 199-223 ◽  
Author(s):  
James H. Stitt ◽  
Patrick J. Perfetta
Keyword(s):  
New Species ◽  
North America ◽  
South Dakota ◽  
Shallow Water ◽  
Black Hills ◽  
Strong Preference ◽  
Upper Cambrian ◽  
Shallow Subtidal ◽  
Intrashelf Basin ◽  
One New Species

Trilobites assigned to 25 genera and 39 species are reported from the Crepicephalus Zone (Marjuman Stage) and Aphelaspis Zone (Steptoean Stage) in the lower part of the Deadwood Formation in the Black Hills of South Dakota. Six taxa are left in open nomenclature, and one new species, Glaphyraspis newtoni, is described.Analysis of the lithologies for this interval from the best exposed measured sections on a southeast-northwest transect reveal a nearshore, shallow subtidal, siliciclastic dominated environment to the southeast, succeeded offshore by a shallow subtidal to lowest intertidal carbonate shoal environment, and then a transitional shaly limestone interval into a more shaly distal intrashelf basin to the northwest.Specimens of species of Coosia, Crepicephalus, Tricrepicephalus, Kingstonia, Pseudagnostina, and Coosina comprise more than 75 percent of the fauna of the Crepicephalus Zone. Coosina ariston, Crepicephalus snowyensis, Tricrepicephalus tripunctatus, Arcuolimbus convexus, and some species of Blountia had a strong preference for the shallow-water siliciclastic facies present in the southeastern sections closest to the paleoshoreline. Crepicephalus rectus, Tricrepicephalus coria, Agnostogonus, cf. A. incognitus and the genera Coosella and Uncaspis preferred the farther offshore, deeper-water, shaly intershelf basin located in the northern Black Hills. Trilobites from the Crepicephalus Zone are used to correlate the lower part of the Deadwood Formation with coeval strata elsewhere in North America.

Trilobites from the Cedarina dakotaensis Zone, lowermost part of the Deadwood Formation (Marjuman Stage, Upper Cambrian), Black Hills, South Dakota

1998 ◽  
Vol 72 (6) ◽  
pp. 1030-1046 ◽  
Author(s):  
James H. Stitt
Keyword(s):  
New Species ◽  
North America ◽  
South Dakota ◽  
Abundant Species ◽  
Black Hills ◽  
Basal Part ◽  
Upper Cambrian ◽  
Lowermost Part ◽  
One New Species

Trilobites assigned to 14 genera and 14 species are reported from basal part of the Deadwood Formation in the Black Hills of South Dakota. Six additional taxa are left in open nomenclature. One new species, Cedarina dakotaensis, is described. These trilobites are assigned to a new zone, the Cedarina dakotaensis Zone, named after the most abundant trilobite species.Species of Cedarina and Modocia are the most abundant in the Cedarina dakotaensis Zone, accompanied by less common specimens of species of Arapahoia, Menomonia, Hardyoides, Welleraspis, Kormagnostus, and Kingstonia. Cedarina dakotaensis and Modocia centralis are the most abundant species in the nearshore sandstone lithofacies, whereas Arapahoia spatulata is the predominate taxon in the offshore limestone lithofacies.The fauna of the Cedarina dakotaensis Zone (which lacks species of Cedaria) occupies the biostratigraphic interval of the traditional Cedaria Zone of the Marjuman Stage. Trilobites from the Cedarina dakotaensis Zone can be used to correlate the basal part of the Deadwood with coeval strata elsewhere in North America.

Depth of emplacement and tilting of the Middle Proterozoic (1470 Ma) Wolf River batholith, Wisconsin: 40Ar/39Ar thermochronologic constraints

1998 ◽  
Vol 35 (10) ◽  
pp. 1143-1151 ◽  
Author(s):  
Daniel Holm ◽  
Daniel Lux
Keyword(s):  
Country Rock ◽  
Slow Cooling ◽  
Early Proterozoic ◽  
Isostatic Uplift ◽  
Adjacent Country ◽  
Thickness Variations ◽  
Magmatic Suite ◽  
Miarolitic Cavities ◽  
Emplacement Depth ◽  
Middle Proterozoic

The 1470 Ma Wolf River batholith of northeastern Wisconsin is one of the oldest and largest intrusive sequences of the Middle Proterozoic magmatic suite that trends across the continent from Labrador to southern California. The presence of miarolitic cavities in portions of the batholith and in associated plutons (Wausau syenite complex) and equilibrium considerations suggest a shallow emplacement depth (i.e., <4 km). However, these data contrast with published Rb/Sr biotite ages, which suggest much of the batholith was uplifted through mid-crustal temperatures well after intrusion. To address this problem better, we have obtained four biotite 40Ar/39Ar dates from a transect across the southwestern portion of the batholith, one date from the ~1520 Ma Wausau syenite complex, and another from rocks metamorphosed during the Early Proterozoic Penokean orogeny. The two oldest dates are concordant (within error) with the crystallization age of the batholith, suggesting rapid cooling after intrusion and hence shallow emplacement depth. Cooling dates young to the south-southeast, however, and the youngest dates within the batholith (~1400 Ma) are concordant with previous Rb/Sr biotite dates and with the Ar/Ar biotite date from the adjacent country rock. The data suggest slow cooling and southeast-side-up tilting of this portion of the exposed batholith from mid-crustal temperatures (~300°C). Postemplacement uplift and tilting of the batholith appear related to differential isostatic uplift driven by substantial original thickness variations of the intrusion.

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