Paleoproterozoic Orogen Research Articles

Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic field gradients: Upper Granite Gorge, Grand Canyon, USA

The Paleoproterozoic orogen of the southwestern United States is characterized by a segmented, block-type architecture consisting of tens of kilometer-scale blocks of relatively homogeneous deformation and metamorphism bounded by subvertical highstrain zones. New fi eld, microstructural, and petrologic observations combined with previously published structural and geochronological data are most consistent with a tectonometamorphic history characterized by a clockwise, looping pressure-temperature (P-T) path involving: (1) initial deposition of volcanogenic and turbiditic supracrustal rocks at ca. 1.75‐1.74 Ga, (2) passage from 750 °C). Notable variations along the transect are also primarily thermal in nature and include differences in the temperature of the prograde history (i.e., early andalusite versus kyanite), equilibrium pressures recorded at peak temperatures, and intensity of late-stage thermal spikes due to local dike emplacement. High-precision ΔPT “relative” thermobarometry confi rms lateral temperature variations on the order of 100‐250 °C with little to no variation in pressure. The Upper Granite Gorge thus represents a subhorizontal section of lowermost middle continental crust (~0.7 GPa). Results imply that the entire ~70-km-long transect decompressed from ~0.7 to ~0.3‐ 0.4 GPa levels as one large coherent block in the Paleoproterozoic. The transect represents a 100% exposed fi eld laboratory for understanding the heterogeneity and rheologic behavior of lowermost middle continental crust during orogenesis. Hot blocks achieved partial melting conditions during penetrative subvertical fabric development. Although these blocks were weak, large-scale horizontal channel fl ow was apparently inhibited by colder, stronger blocks that reinforced and helped preserve the block-type architecture. Development of dramatic lateral thermal gradients and discontinuities without breaks in crustal level is attributed to: (1) spatially heterogeneous advective heat fl ow delivered by dense granitic pegmatite dike complexes and (2) local transcurrent displacements along block-bounding high-strain zones over an ~15‐20 m.y. time interval. Exhumation of the transect from 25 to 12 km depths is interpreted to refl ect erosion synchronous with penetrative development of steeply dipping NE-striking foliations and steeply plunging stretching lineations, consistent with an orogen-scale strain fi eld involving NW-SE subhorizontal shortening and subvertical extension during crustal thickening.

Evolution of paleoproterozoic magmatism: Geology, geochemistry, and isotopic constraints

Evolution of tectono-magmatic processes in the Paleoproterozoic is divisible into four stages each controlled by its own type of endogenic activity. The critical moment in geological history was at the third stage onset 2.05 Ga ago, when the Archean style of evolution gave way to subsequent one. Magmas of siliceous high- Mg (boninite-like) series (SHMS) dominant during the first stage (2.5-2.3 Ga) had the mixed mantle-crust gen- esis. They originated in highly depleted ultramafic reservoir of asthenospheric mantle being contaminated by the Archean crustal material during ascent to the surface. Typical of the SHMS magmas were the high content of silica, Al, Mg, Cr and elevated concentrations of Ni, Co, Cu, V, PGE and incompatible elements, LREE included, while concentrations of Fe, Ti, Nb and alkalies, especially K, were at a relatively low level. Magma- tism of K-granitoid type was of a limited significance. The second stage, especially its second substage (2.3- 2.05 Ga), was distinct because of mass eruptions of picritic and basaltic magmas enriched in Fe, Ti, Mn, P and incompatible elements, LREE in particular, which also had elevated Cr, Ni, Co, Cu and Ba concentration being relatively depleted in Mg and Al. This change in geochemistry of magmatism was independent of tectonic pro- cesses, which retained the former style. The third stage (2.05-1.8 Ga) was marked by opening of first oceans (Jormua, Purtuniq and other ophiolites) and by formation of orogens comparable with Phanerozoic orogens that was associated with development of subduction zones and back-arc basins with relevant magmatism. High Mg, Fe, Ti, Cr, Ni, Cu, V but low Th concentrations were typical of intense picrite-basaltic volcanism marking onset of the third stage. Magmas of suprasubduction genesis had considerably higher Si, Al, P, Zn, Th concentrations and very low e‡e/Al 2 O 3 ratios. Development of orogens came to the end 1.82-1.80 Ga ago. Appearance of giant belts of intraplate, usually silicic high-K volcanism juxtaposed on stabilized Paleoproterozoic orogens with abnormally thick crust was confined to the fourth stage spanning besides the initial Mesoproterozoic (1.8- 1.5 Ga). Anorthosite-rapakivi granitoid batholiths, which originated above mantle plume heads, corresponded to intermediate magma chambers of relevant magmatic systems. As is suggested, juvenile basaltic melts of this stage retained within the crust provoking a large-scale melting of sialic material. The high K, Ti, Zn, Pb, Zr and elevated Be, Sn, Y, Nb, Rb, F, W, Mo, Li and U contents characterized geochemistry of magmas, which origi- nated at the fourth stage

Th–U–Pb monazite geochronology of the Lüliang and Wutai Complexes: Constraints on the tectonothermal evolution of the Trans-North China Orogen

Recent lithological, metamorphic and geochronological studies show that the basement of the North China Craton (NCC) formed by collision of two discrete Archean to Paleoproterozoic blocks (Eastern and Western Blocks) along a Paleoproterozoic orogen, named the Trans-North China Orogen. However, the timing of the collision is controversial and post-collisional exhumation has not been precisely dated. This study applies the electron microprobe Th–U–Pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low- to medium-grade Lüliang and Wutai Complexes in the Trans-North China Orogen. Electron microprobe Th–U–Pb monazite data for the Lüliang Complex reveal five ThO 2 */PbO age ranges: (1) 1940–1938 Ma, (2) 1880–1847 Ma, (3) 1795–1755 Ma, (4) 1720–1703 Ma, and (5) ∼1648 Ma. Of these age ranges, the first four have also been recorded in the Wutai Complex, which yields ThO 2 */PbO ages of (1) ∼1930 Ma, (2) 1838–1822 Ma, (3) ∼1793 Ma, and (4) ∼1719 Ma. The oldest age range of 1940–1930 Ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the Lüliang and Wutai Complexes. The age range of 1882–1822 Ma is in accord with the SHRIMP U–Pb ages of metamorphic zircons and mineral Sm–Nd and 40Ar/ 39Ar ages obtained for other complexes in the orogen, and is interpreted as the age of the major metamorphic event caused by amalgamation between the Eastern and the Western Blocks. The age range of 1795–1755 Ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800–1765 Ma, interpreted as the time of post-orogenic extension. The 1720–1703 Ma and ∼1648 Ma ages are considered to date later multiple stages of hydrothermal alteration, since monazites with such young ages only occur along fractures in older monazite grains and at the rims. These new monazite ThO 2 */PbO ages support the tectonic model for the evolution of the North China Craton that envisages discrete Eastern and Western Blocks colliding along the Trans-North China Orogen at ∼1.85 Ga and then undergoing post-collisional extension in the period 1795–1755 Ma.

Correlation Chart of the evolution of the Trans-Hudson Orogen — Manitoba–Saskatchewan segment

A correlation chart showing the evolution of the Manitoba–Saskatchewan segment of the Paleoproterozoic Trans-Hudson Orogen (THO) has been constructed based on available geochronological and isotopic data. Geological, petrological, geochemical, and structural data have also been incorporated into the correlation chart to provide finer detail and to allow comparison of specific tectonic events. The chart also emphasizes that the bounding Archean cratons (Superior, Hearne, Sask) have been significantly affected by Paleoproterozoic events, and it is hoped that this chart will also facilitate comparison with other Paleoproterozoic orogens in North America and beyond.

Introduction to special issue of Canadian Journal of Earth Sciences: The Trans-Hudson Orogen Transect of Lithoprobe

Dedication: Dr. John F. Lewry (1939–1999; see Saskatchewan Geological Survey 1999) dedicated his career to investigations of the Saskatchewan–Manitoba segment of the Trans-Hudson Orogen (THO), one of the principal Paleoproterozoic orogens associated with the assembly of Laurentia. Indeed, one can make a strong case that Lithoprobe's Trans-Hudson Orogen Transect (THOT) was designed to test the tectonic models proposed by John Lewry. He delineated the distinct tectonic provinces in the western part of the THO, predicted the presence of an Archean craton trapped within the THO, and recognized and interpreted the significance of the Pelican Thrust between the juvenile Paleoproterozoic volcanic arc complex of the western Flin Flon Domain and the Archean craton, now called the Sask craton. The research published in Lewry and Stauffer (1990), and many of his ideas, provided the framework for the design of the THOT geophysical and geological studies. John Lewry was co-leader of the THOT until he passed away in 1999 after a battle with cancer. This Special Issue of the Canadian Journal of Earth Sciences is dedicated to him.

Deformation style way back when: thoughts on the contrasts between Archean/Paleoproterozoic and contemporary orogens

In many important ways, Archean and Paleoproterozoic (`older') orogens differ structurally from contemporary examples. This essay examines the premise that contrasts between older orogens and contemporary orogens reflect long-term changes in the temperature of the continental crust, in the density of supracrustal sections, and in exhumation rates. For example, if continental crust were warmer and exhumation rates faster, earlier in Earth history, then higher grade rocks would occur closer to the surface of older orogens, and the orogens would be lower and wider. This situation might contribute to the formation of wide belts of high-grade gneiss found in ancient crust. If the high-strength layer of the crust were thinner and supracrustal sequences denser, earlier in Earth history, then regional extensional tectonism might lead to crustal-scale boudinage and diapirism. This situation might explain formation of the extensive dome-and-keel provinces found in ancient crust. Testing such speculations, through the application of structural analysis coupled with petrologic studies, dating, and rheological modelling, will constrain models of Earth's long-term physical evolution.

The Late Archaean bonanza: metallogenic and environmental consequences of the interaction between mantle plumes, lithospheric tectonics and global cyclicity

The Late Archaean records periods of intense magmatism and the development of prodigious metallogenic provinces of Ni, Fe, CuZn and Au deposits. In particular, the period 2.74-2.66 Ga represents one of the most widespread episodes of ultrabasic and basic volcanism preserved in the geological record, as well as anomalously widespread granitoid magmatism. Extensive assemblages of this age, which comprise komatiites and komatiitic basalts derived from mantle plumes, together with tholeiites and calc-alkalic volcanic rocks, are preserved on most Late Archaean cratons. Intense submarine volcanism in plume-like environments resulted in rich komatiite-hosted Ni mineralization in continental-margin basins, and CuZn sulphide mineralization in extensional volcanic arcs. Peak submarine magmatism was accompanied by marine transgression and thereby flooding of previously exposed continental crust. Elevated hydrothermal activity and widespread su☐ic conditions in submarine basins are reflected by sulphide-rich carbonaceous sedimentary rocks, that contain the organic remains of bacterial communities, and banded iron formations (BIF). A major episode of mesothermal gold mineralization accompanied accretionary tectonics, as metal-and carbon-rich submarine volcanic and sedimentary successions were subducted or incorporated into nascent continental crust by 2.59 Ga.Comparison with Neoproterozoic and Phanerozoic tectonic and metallogenic patterns indicates that the period 2.78-2.59 Ga represents the first half of an ∼ 360 m.y. global tectonic cycle. This period records the breakup of a supercontinent and the opening and closing of marginal basins along long-lived convergent margins of the external ocean to that supercontinent. Enhanced magmatic events between 2.74 and 2.66 Ga were most likely the result of intrabasinal mantle plumes and a subsequent global plume-breakout event. Together, the plume events were responsible for the extreme environmental conditions during the Late Archaean relative to both the preceding and succeeding periods of Earth history. Interactions between mantle plumes and long-lived convergent margins of a Pacific-type ocean were responsible for the prodigious metal inventory of Late Archaean granitoid-greenstone terranes. Extensive convergent-margin and plume magmatism during that period, coupled with episodic periods of low-angle subduction underplating by oceanic lithosphere, may also have been the cause of development of the buoyant, continental mantle lithosphere that is responsible for the preservation of these highly mineralized cratons. It is also likely that the bonanza metallogenic provinces in the Witwatersrand basin and Paleoproterozoic orogens of West Africa and Laurentia-Baltica reflect interactions of mantle plumes with long-lived convergent margins of the external ocean.

Lithotectonic elements of the Grenville Province: review and tectonic implications

The Grenville Province formed the southeastern limit of Proterozoic Laurentia and is composed of lithologic units that range in age from Archean to Late Mesoproterozoic. Reworked continuations of Archean, Early and Middle Paleoproterozoic orogens extend at the surface and at depth well into the southwestern and central Grenville Province, but are absent in the northeast. There is evidence for the existence of an active margin, with subduction-accretion and arc formation, along southeastern Laurentia for >400 Ma from the Late Paleoproterozoic to the Late Mesoproterozoic. Major juvenile crustal additions to the Laurentian margin, comprising Andean-style, calc-alkaline magmatic arcs and coeval inboard backarc deposits, occurred between ∼ 1.71 and 1.61, 1.51 and 1.42, and 1.40 and 1.23 Ga. Backarc magmatic and (or) sedimentary products document variable degrees of backarc extension and basin formation. Examples include the coeval oceanic and continental backarc basin settings of Hastings/Frontenac and Wakeham/Seal Lake groups respectively during geon 12, and the incipient continental backarc extensional settings of the Michael-Shabogamo dyke swarm during geon 14 and of alkali granite and anorthosite complexes during geons 13 and 12. Arc magmatism was followed by accretionary orogenesis during the Labradorian (∼ 1680-1660 Ma), Pinwarian (∼ 1500-1450 Ma) and Elzevirian (∼ 1250-1190 Ma) orogenies, respectively, resulting in substantial growth of Laurentia. A result of this growth is that the ages of most major units tend to young towards the southeast of the Grenville Province, except for those that formed inboard of the continent margin in a backarc setting. The continent-continent Grenvillian Orogeny took place between ∼ 1.19 and 0.98 Ga and comprised three distinct pulses of crustal shortening at ∼ 1.19-1.14, 1.08-1.02 and 1.00-0.85 Ga, separated by periods of extension. The loci of the earlier (Shawinigan and Ottawan) pulses of crustal shortening were in the hinterland of the orogen, whereas the latest (Rigolet) pulse caused northwesterly propagation of the orogen into its foreland. Periods of crustal extension during the Grenvillian Orogeny were coeval with emplacement of mafic magmas and anorthosite complexes, implying that large quantities of mantle magma and heat had access to the base of the previously thickened orogenic crust. This scenario is compatible with extensional collapse of the orogen following delamination or convective removal of the lower lithosphere, as suggested previously by others. An inference from these conclusions is that anorthosite complexes formed in two contrasting extensional tectonic environments in southeastern Laurentia during the Mesoproterozoic, that is, in areas of backarc extension inboard from an active continental-margin magmatic arc and within a collisional orogen during periods of tectonic collapse and rising isotherms.