United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia

This paper presents a plate-scale model for the Precambrian growth and evolution of the North American continent. The core of the North American continent (Canadian shield) came together in the Paleoproterozoic (2.0–1.8 Ga) by plate collisions of Archean continents (Slave with Rae-Hearne, then Rae-Hearne with Superior) as well as smaller Archean continental fragments (Wyoming, Medicine Hat, Sask, Marshfield, Nain cratons). The resulting Trans-Hudson orogen was a collisional belt similar in scale to the modern Himalayas. It contains mainly reworked Archean crust, but remnants of juvenile volcanic belts are preserved between Archean masses. The thick, buoyant, and compositionally depleted mantle lithosphere that now underlies North America, although dominantly of Archean age, took its present shape by processes of collisional orogenesis and likely has a scale of mantle heterogeneity similar to that exhibited in the overlying crust.

In marked contrast, lithosphere of southern North America (much of the conti nental United States) was built by progressive addition of a series of dominantly juvenile vol canic arcs and oceanic terranes accreted along a long-lived southern (present coordinates) plate margin. Early juvenile additions (Pembine-Wausau, Elves Chasmarcs) formed at the same time (1.84–1.82 Ga) the core was assembling. Following final assembly of the Archean and Paleoproterozoic core of North America by 1.8 Ga, major accretionary provinces (defined mainly by isotopic model ages) were added by arc-continent accretion, analogous to present-day convergence between Australia and Indonesia. Also similar to Indonesia, some accreted terranes contain older continental crustal material [Archean(?) Mojavia], but the extent and geometry of older crust are not well known. Accretionary provinces are composed of numerous 10 to 100 km scale terranes or blocks, separated by shear zones, some of which had compound histories as terrane sutures and later crustal-assembly structures. Major northeast-trending provinces are the Yavapai province (1.80–1.70 Ga), welded to North America during the 1.71–1.68 Ga Yavapai orogeny; the Mazatzal province (1.70–1.65 Ga), added during the 1.65–1.60 Ga Mazatzal orogeny; the Granite-Rhyolite province (1.50–1.30 Ga), added during the 1.45–1.30 Ga tectonic event associated with A-type intracratonic magmatism; and the Llano-Grenville province (1.30–1.00 Ga), added during the 1.30–0.95 Ga broader Grenville orogeny. During each episode of addition of juvenile lithosphere, the transformation of juvenile crust into stable continental lithosphere was facilitated by voluminous granitoid plutonism that stitched new and existing orogenic boundaries. Slab roll back created transient extensional basins (1.70 and 1.65 Ga) in which Paleoproterozoic quartzite-rhyolite successions were deposited, then thrust imbricated as basins were inverted. The lithospheric collage that formed from dominantly juvenile terrane accretion and stabilization (1.8–1.0 Ga) makes up about half of the present-day North American continent. Throughout (and as a result of) this long-lived convergent cycle, mantle lithosphere below the accretionary provinces was more hydrous, fertile, and relatively weak compared to mantle lithosphere under the Archean core.

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Tectonic model for the Proterozoic growth of North America

The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O₂) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O₂ levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O₂ concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

https://science.sciencemag.org/content/sci/346/6209/635.full.pdf

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

High-pressure granulites are characterised by the key associations garnet-clinopyroxene-plagioclase-quartz (in basic rocks) and kyanite-K-feldspar (metapelites and felsic rocks) and are typically orthopyroxene-free in both basic and felsic bulk compositions. In regional metamorphic areas, two essential varieties exist: a high- to ultrahigh-temperature group and a group representing overprinted eclogites. The high- to ultrahigh-temperature type formerly contained high-temperature ternary feldspar (now mesoperthite) coexisting with kyanite, is associated with garnet peridotites, and formed at conditions above 900 °C and 1.5 GPa. Clinopyroxene in subordinate basic rocks is Al-rich and textural evidence points to a high-pressure–high-temperature melting history. The second variety contains symplectite-like or poikilitic clinopyroxene-plagioclase intergrowths indicating former plagioclase-free, i.e. eclogite facies assemblages. This type of rock formed at conditions straddling the high-pressure amphibolite/high-pressure granulite field at around 700–850 °C, 1.0–1.4 GPa. Importantly, in the majority of high-pressure granulites, orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure. In contrast to low- and medium-pressure granulites, which form at conditions attainable in the mid to lower levels of normal continental crust, high-pressure granulites (of nonxenolith origin) mostly represent rocks formed as a result of short-lived tectonic events that led to crustal thickening or subduction of the crust into the mantle. Short times at high-temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene. High-pressure granulites of both regional types, although rare, are known from both old and young metamorphic terranes (e.g. c. 45 Ma, Namche Barwa, E Himalaya; 400–340 Ma, European Variscides; 1.8 Ga Hengshan, China; 1.9 Ga, Snowbird, Saskatchewan and 2.5 Ga Jianping, China). This spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the Archean.

High‐pressure granulites: formation, recovery of peak conditions and implications for tectonics

Supplementary Materials for Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

High-temperature, high-pressure eclogite and garnet pyroxenite occur as lenses in garnet peridotite bodies of the Gfohl nappe in the Bohemian Massif. The high-pressure assemblages formed in the mantle and are important for allowing investigations of mantle compositions and processes. Eclogite is distinguished from garnet pyroxenite on the basis of elemental composition, with mg number 0.75 wt.%, Cr2O3 < 0.15 wt.% and Ni < 400 ppm. Considerable scatter in two-element variation diagrams and the common modal layering of some eclogite bodies indicate the importance of crystal accumulation in eclogite and garnet pyroxenite petrogenesis. A wide range in isotopic composition of clinopyroxene separates [eNd, +5.4 to −6.0; (87Sr/86Sr)i, 0.70314–0.71445; δ18OSMOW, 3.8–5.8%o] requires that subducted oceanic crust is a component in some melts from which eclogite and garnet pyroxenite crystallized. Variscan Sm-Nd ages were obtained for garnet-clinopyroxene pairs from Dobesovice eclogite (338 Ma), Uhrov eclogite (344 Ma) and Nove Dvory garnet pyroxenite (343 Ma). Gfohl eclogite and garnet pyroxenite formed by high-pressure crystal accumulation (±trapped melt) from transient melts in the lithosphere, and the source of such melts was subducted, hydrothermally altered oceanic crust, including subducted sediments. Much of the chemical variation in the eclogites can be explained by simple fractional crystallization, whereas variation in the pyroxenites indicates fractional crystallization accompanied by some assimilation of the peridotite host.

Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere

Red, supermature quartzites of the Baraboo interval of the Lake Superior region contain detrital zircon that ranges in age from 1782 to 1712 Ma. Deposition clearly occurred after the geon 18 Penokean orogeny. These late Paleoproterozoic sedimentary rocks consist largely of quartz, kaolinite or pyrophyllite, and hematite; detrital feldspar and muscovite are rare or absent. Their Chemical Index of Alteration ranges from 96.8 to 98.6, among the most chemically mature clastic sediments in the geological record. The quartzites are underlain by mature, feldspar‐free paleosols, accounting for the absence of feldspar in the overlying sediments and indicating the presence of first‐cycle quartzose detritus. Such physical and chemical characteristics imply that late Paleoproterozoic deposition in the Lake Superior region occurred in a stable tectonic setting with subdued topographic relief in a warm, humid climate. Folding and low‐grade metamorphism of the quartzites is thought to reflect ∼1630 Ma foreland ...

http://www.geology.wisc.edu/~bsinger/Publications/2003.MedarisetalJGeol/Medaris_Singer_et_al_J_Geol_2003.pdf

Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites

The Wolf River Batholith is a large anorogenic plutonic complex in central and northeastern Wisconsin that is similar lithologically to the rapakivi massifs of Finland. Quartz monzonite, the most abundant rock type present, can be subdivided into three major lithologic units, one of which commonly has well-developed rapakivi texture. Other rock types include syenite, granite, quartz and feldspar porphyries, monzonite, and anorthosite; the anorthosite exists as totally isolated masses within the quartz monzonite and may or may not be genetically related to the main complex. Whole-rock Rb-Sr data yield an age of 1,468 ± 34 m.y., with initial Sr 87 /Sr 86 = 0.7048 ± 0.0017 (λ (Rb 87 ) = 1.39 × 10 −11 yr −1 ). U-Pb data from cogenetic zircon fractions define a concordia intercept age of 1,510 ± 15 m.y. for traditionally used values of the U 235 and U 238 half-lives or an age of 1,485 ± 15 m.y. based on recently redetermined values. Rb-Sr data also show that the syenite complex of the Wausau area is approximately, if not exactly, the same age as the Wolf River Batholith. The Wolf River Batholith is similar in age to other granites in southeastern Missouri and the southwestern United States and is apparently a representative of widespread anorogenic igneous activity that existed throughout the southern part of the North American continent 1,400 to 1,500 m.y. ago.

Geology and Age of the Wolf River Batholith, Wisconsin

Penokean deformation in central Wisconsin

Two Paleoproterozoic (Statherian) Siliciclastic Metasedimentary Sequences in Central Wisconsin: The End of the Penokean Orogeny and Cratonic Stabilization of the Southern Lake Superior Region

L. G. Medaris

Papers

Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere

Late Paleoproterozoic climate, tectonics, and metamorphism in the southern Lake Superior region and Proto-North America: Evidence from Baraboo interval quartzites

Geology and Age of the Wolf River Batholith, Wisconsin

Penokean deformation in central Wisconsin

Two Paleoproterozoic (Statherian) Siliciclastic Metasedimentary Sequences in Central Wisconsin: The End of the Penokean Orogeny and Cratonic Stabilization of the Southern Lake Superior Region