Open Access Kent State (OAKS)

The Paleoproterozoic crust in the north-central U.S. represents intact juvenile terranes accreted to the rifted Archean Superior craton. A new tectonic province map, based on the interpretation of a new aeromagnetic compilation, published geologic maps, and recent geochronologic data, shows progressive accretion of juvenile arc terranes from ca. 1900–1600Ma. Contrary to earlier models, geon 18 Penokean-interval crust is primarily confined to a ∼2100Ma tectonic embayment of the rifted Superior craton. The newly defined Spirit Lake tectonic zone, characterized by a sharp magnetic discontinuity that marks the southern limit of Archean and Penokean-interval rocks, is here interpreted to represent an eastern analog of the Cheyenne belt suture zone in southern Wyoming. South of this boundary, geon 17 Yavapai-interval rocks form the basement upon which 1750Ma rhyolite and succeeding quartzite sequences were deposited. Substantial portions of the Penokean and Yavapai terranes were subsequently deformed during the 1650–1630Ma Mazatzal orogeny. The northern boundary of the Mazatzal terrane is obscured by abundant 1470–1430Ma “anorogenic” plutons that stitched the suture with the older Yavapai terrane rocks. These data reveal a progressive tectonic younging to the south as the Laurentian craton grew southward and stabilized during the Proterozoic. Late Mesoproterozoic rift magmatism produced pronounced geophysical anomalies, indicating strong, but localized crustal modification. In comparison to the western U.S., little tectonism has occurred here in the last 1 billion years, providing a uniquely preserved record of the Precambrian evolution of the continental U.S. lithosphere.

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.

Polygenetic monazite grains in diverse Precambrian crystalline rocks from the Black Hills, South Dakota, have been analyzed in situ by ion and electron microprobe methods (SHRIMP and EMP), to evaluate the accuracy and precision of EMP ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections. Parallel evaluations were conducted by comparing EMP chemical and SHRIMP isotopic ages at regional-, rock-, and grain-scales. The monazite data set includes 354 EMP chemical analyses from 26 grains in six metamorphic rocks, which resolve into 54 age-composition domains, and 31 SHRIMP isotopic ages from 13 grains in one of the rocks, with six grains microanalyzed in common by the two methods. The data set also includes monazite-bearing garnets in two of the rocks, whose isotopic compositions were analyzed using Pb stepwise-leaching (PbSL) methods. Both the EMP and SHRIMP data sets reveal a continuum of apparent monazite ages spanning a ∼1790-1680 Ma timeframe, with a relatively high probability of ages at ∼1755 and ∼1715 Ma that correspond spatially to core and rim domains. PbSL ages of ∼1742 and ∼1734 Ma obtained from monazite-bearing garnet in two rocks are intermediate compared to the corresponding EMP ages, and are thereby interpreted as mixed ages. EMP data for two grains in the structurally deepest of the six rocks record ∼1785 and ∼1755 Ma ages in the cores and (higher-Y and lower-Th) rims, respectively, and these results are duplicated by SHRIMP ages in these and/or other grains from the same rock. Overall, the EMP, SHRIMP, and PbSL ages are internally consistent at the various scales of observation, which serves to validate EMP chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains.

The EMP data set is interpreted geologically as reflecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the Black Hills. Taking the most precise EMP data at face value, it is possible to resolve the timing of the two older events at ≤1784 ± 4 Ma (or ≤1786 ± 6 Ma) and 1756 ± 3 Ma (or 1753 ± 4 Ma), with 95% confidence. These events are considered to be related to sequential episodes of N-directed thrusting and ∼E-W compression associated with Paleoproterozoic crustal assembly in the mid-continent. A younger metamorphism, related to granite intrusion known to have occurred at 1715 ± 3 Ma, is dated independently at 1717 ± 2 Ma from the EMP monazite ages.

A metapelite from the easternmost Wyoming craton (Black Hills, South Dakota) has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800-1700 Ma assembly of southern Laurentia. Three deformational fabrics are recognized in oriented thin sections: an ENE-trending S1 fabric, preserved as oblique inclusion trails in garnet porphyroblasts; a NNW-trending S2 fabric, preserved as microlithons in the rock matrix; and a flattening fabric, S3, which transposed S1-S2 and dominates the matrix. A complex monazite porphyroblast has been analyzed in situ with the electron microprobe (Ultrachron) to constrain the timing of S1-S3 fabric formation associated with monazite growth. The core of this grain uniquely preserves the S1-S2 fabrics as sigmoidal inclusion trails. The mean total-Pb age of this domain is 1750+ or -10 Ma (all dates reported at 95% confidence; n = 39 spots), which is equivalent to the published 207Pb/206Pb age for the same domain. These results validate the total-Pb dating method in general and the Ultrachron in particular, for reliable age determination in low-Th monazite, and are interpreted as 1750 Ma minimum ages for the S1-S2 fabrics and sequential, D1-D2 collisional events that imposed them (∼N-directed arc accretion and ∼E-W continental collision, respectively). A higher-Th, Y rim of this same "Rosetta" grain truncates the S1-S2 sigmoid, and is associated with resorption textures in garnet porphyroblasts, coupled release of Y, and an S3 fabric that pervasively overprinted S1-S2 in the rock matrix. The mean Ultrachron date of this domain is 1692+ or -5 Ma (n = 17 spots), which is slightly younger that the published isotopic age for all monazite rims combined. These results support a approximately 1715-1690 Ma timeframe for localized doming (D3) related to granite magmatism, the onset of which has been dated independently at 1715+ or -3 Ma. The timing of post-D3 cooling through 350 and 300 °C is constrained by 40Ar/39Ar dates of approximately 1610 and approximately 1480 Ma obtained for separates of D3 matrix muscovite and biotite, respectively, which are interpreted as closure ages.

This study shows that fabrics in poly-deformed rocks can be dated by linking monazite spot ages to key microtextures. Further, the results of this micrometer-scale study enhance previous knowledge of local thermotectonism (Black Hills) and regional terrane assembly (Laurentia).