The Origin of the Continents

The origin of Earth's first continents and the onset of plate tectonics

Authors:

Hastie et al

Abstract:

The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth's mantle, hydrosphere, atmosphere, and biosphere for ∼4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ∼30–45-km-thick Eoarchean oceanic crust, modified Earth's mantle and Eoarchean environments and ecosystems.

The Recipe for Atmospheric Oxygen on Earth

Earth scientists from Rice University, Yale University and the University of Tokyo are offering a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere.

Based on a new model that draws from research in diverse fields including petrology, geodynamics, volcanology and geochemistry, the team's findings were published online this week in Nature Geoscience. They suggest that the rise of oxygen in Earth's atmosphere was an inevitable consequence of the formation of continents in the presence of life and plate tectonics.

"It's really a very simple idea, but fully understanding it requires a good bit of background about how the Earth works," said study lead author Cin-Ty Lee, professor of Earth science at Rice. "The analogy I most often use is the leaky bathtub. The level of water in a bathtub is controlled by the rate of water flowing in through the faucet and the efficiency by which water leaks out through the drain. Plants and certain types of bacteria produce oxygen as a byproduct of photosynthesis. This oxygen production is balanced by the sink: reaction of oxygen with iron and sulfur in the Earth's crust and by back-reaction with organic carbon. For example, we breathe in oxygen and exhale carbon dioxide, essentially removing oxygen from the atmosphere. In short, the story of oxygen in our atmosphere comes down to understanding the sources and sinks, but the 3-billion-year narrative of how this actually unfolded is more complex."

Lee co-authored the study with Laurence Yeung and Adrian Lenardic, both of Rice, and with Yale's Ryan McKenzie and the University of Tokyo's Yusuke Yokoyama. The authors' explanations are based on a new model that suggests how atmospheric oxygen was added to Earth's atmosphere at two key times: one about 2 billion years ago and another about 600 million years ago.

link.

Evidence of the First Continental Kernels From Archean China

U-Pb age and Hf isotopes of detrital zircons from the Southeastern North China Craton: Meso- to Neoarchean episodic crustal growth in a shifting tectonic regime

Auhtors:

Liu et al

Abstract:

The North China Craton (NCC) represents one of only a few cratonic nuclei on the globe with a geological history extending back to the Eoarchean. However, extensive ca. 2.5 Ga crustal reworking has destroyed a considerable portion of the pre-existing crustal record, hindering the investigation of tectonothermal evolution prior to 2.5 Ga. The Huoqiu Complex (HQC), located at the southeastern margin of the NCC, preserves the vestiges of crustal components that survived the ca. 2.5 Ga tectonothermal events, which provide the opportunity to investigate the Meso- to Neoarchean episodic crustal evolution of the NCC. Here we present results from in-situ detrital zircon U-Pb dating and Hf isotope analyses on zircons from three paragneisses in three drill cores that cut through the basement of the HQC. In combination with published data, the concordant age spectra of the detrital zircons in the paragneisses yield 207Pb/206Pb ages of 2343-3997 Ma that cluster into two principal age populations with peaks at 3015 and 2755 Ma. One zircon grain dated at 3997 ± 8 Ma with 98% concordance provides new evidence for 4.0 Ga components in the NCC. The εHf(t) values of all zircons range from -5.2 to + 6.5, with most of the spots (n = 31 of 47) showing positive values, indicating at least two episodes of juvenile continental crustal growth at 3.01 Ga and 2.75 Ga. The older episode is recorded only in few ancient cratons, suggesting limited crustal accretion occurred globally at a time of subdued mantle-derived magmatism. In contrast, the younger episode is coincident with a global rise in magmatic activity in the early Neoarchean. The geochemical and geochronological data suggest that the 3.01 Ga juvenile crust was likely generated in an island-arc subduction system, whereas the 2.75 Ga crustal rocks were probably formed during magmatic underplating and subsequent partial melting of lower crustal mafic rocks. Consequently, a tectonic transition is suggested from a compressive to an extensional setting along the southeastern margin of the NCC between 3.01 and 2.75 Ga. This sequence of events heralds a shift, from a mixture of net crustal growth and crustal reworking during multiple short-lived magmatic pulses, to fragmentation and dispersal of the early continental nucleus within 260 Ma.

Evidence From South Africa Supporting PaleoProterozoic Continent Vaalbara

Source and age of upper Transvaal Supergroup, South Africa: Age-Hf isotope record of zircons in Magaliesberg quartzite and Dullstroom lava, and implications for Paleoproterozoic (2.5–2.0 Ga) continent reconstruction

Authors:

Zeh et al

Abstract:

The Magaliesberg Formation quartzite of the upper Transvaal Supergroup in north-central South Africa represents the biggest Paleoproterozoic arenite deposit on Earth, with a present-day lateral extent of more than 600 km. It therefore has a great potential to provide detailed information about Archean to early Paleoproterozoic crust–mantle evolution in the hinterland of the Kaapvaal Craton and for global plate tectonic reconstruction. Detrital zircons in Magaliesberg Formation quartzites provide evidence for magmatic activities in the source region at 2080–2130 Ma (11%), 2220 Ma (35%), 2350–2450 Ma (8%) and, following an age gap of ca. 200 million year, at 2650–2780 Ma (11%), 2820–2910 Ma (25%), and at 3060–3100 Ma (6%). Minor zircon formation took place during Bushveld Complex contact metamorphism at 2055 Ma, and less than 3% underwent a post-Bushveld age reset. Detrital zircons in a sediment-rich Dullstroom lava yield similar ages between 2120 ± 12 Ma and 2865.2 ± 1.7 Ma, indicating that Magaliesberg and post-Magaliesberg/Dullstroom sediments were derived from the same sources, and deposited between 2080 and 2055 Ma. Distinct age spectra among the investigated quartzites indicate that the sandstone detritus was derived from different sources and rapidly deposited without complete homogenization, in a regressive shore line, braid-delta environment.

Combined Hf isotope-age data reveal two distinct groups of zircon. Zircons of group I have Archean ages (greater than 2.65 Ga), and define a crustal array that points to protracted crust reworking in the hinterland between 3.10 and 2.65 Ga. Likely sources for these zircons are granitoids of the Pietersburg Block, which is located at the northern edge of the Transvaal basin. Zircons of group II have Paleoproterozoic ages between 2.45 and 2.08 Ga. They are mostly subchondritic, and show highly variable εHft between +4 and −19. These large variations hint to their formation in a magmatic arc and/or collisional setting. As granitoids with ages between 2.65 and 2.06 Ga are absent on the Kaapvaal Craton, detrital zircons of such ages must have been derived from sources outside of the present-day craton. One potential source could be the Ophthalmia orogenic belt, which became amalgamated to the joined Kaapvaal–Pilbara and perhaps Zimbabwe cratons (Zim)vaalbara at 2.22–2.14 Ga, whereas coeval magmatic rocks in southern America (e.g., Minero Belt of the Sao Francisco Craton) or West Africa can be excluded, as they were mostly derived from depleted mantle sources in oceanic environments. Abundant occurrence of detrital zircons with ages of 2.5 Ga in the lower Pretoria Group (Duitschland Formation), but their complete absence in the upper Pretoria Group (Magaliesberg Formation), hint that (Zim)vaalbara was attached to an early Paleoproterozoic orogenic belt, which disappeared completely between 2.3 and 2.08 Ga, most likely by cratonic break up.

Evidence From Brazil of the PaleoProterozoic Paranapanema Paleocontinent

U–Pb age of detrital zircon from the Embu sequence, Ribeira belt, Se Brazil

Authors:

Duffles et al

Abstract:

The Embu Complex crops out in the southeastern part of São Paulo state in Brazil and consists mainly of Neoproterozoic metasedimentary rocks belonging to the interference zone between the Brasilia and Ribeira fold belts. We present U–Pb ages (LA-IPCMS) supported by cathodoluminescence images for detrital zircon from these rocks in order to constrain the formation and evolution of the source area of this complex.

In this paper we choose to individualize within the Embu Complex a Paleoproterozoic basement, a metasedimentary sequence, the Embu Sequence, and Neoproterozoic intrusive bodies, now largely orthogneisses. Six distinct rock types were recognized in the Embu Sequence: biotite gneiss, biotite schist, quartzite, amphibolite, calc-silicate rock and talc schist. The sequence is strongly deformed by several deformation phases and metamorphosed into amphibolite facies. An important shear zone, the Buquira Shear zone, separates the Embu Complex from the Socorro-Guaxupé Nappe, to the north, considered to be part of the upper plate of the southern Brasília belt collision. However, our mapping showed that the shear zone cuts obliquely into the Embu Complex, diminishing considerably its regional importance. Three samples were separated for dating, a schist north of the shear zone and two quartzites south of the shear zone. The results show that the ages of detrital zircon of these samples are mainly Paleoproterozoic, with a minor Archean component. The youngest igneous zircon grain from the schist produced an age of about 1000 Ma, indicating the maximum sedimentation age for the sequence. No significant difference appears between the samples north and south of the shear zone, reforcing the interpretation that this zone is only of local importance. Important metamorphic overgrowth of zircon in the age range 670–640 is present in all three samples; it is interpreted as related to the installment of a continental magmatic arc in the Socorro-Guaxupé Nappe and in the Embu Complex. A second metamorphic pulse between 621 and 599 Ma is only apparent in one sample. According to the literature this was the period of continental collision between the Paranapanema and São Francisco paleocontinents leading to the formation of the southern Brasília belt. Possibly the fact that the analyzed samples are localized some 30 km away from the suture, in the upper plate, explains that the increase in temperature during the collision in this area was insufficient to grow significant new zircon. A third metamorphic pulse with ages of 576–574 Ma, possibly related to collision in the Ribeira belt, is only detected in three analyses of metamorphic rims in the schist sample. Earlier reported ages of about 790 Ma, both for igneous crystallization and metamorphism in a more westerly part of the Embu Complex, are only recognized in one analysis of metamorphic overgrowth (794 ± 24 Ma) from the schist sample, and two probably hybrid analyses of 830 ± 19 Ma and 767 ± 14 Ma from the northern quartzite sample. It is concluded that the results show that the detritic zircon is probably derived from the Paranapanema Paleocontinent.

Pondering the Precambrian #3

Proterozoic:

NeoProterozoic:

The Royal Tyrrell Museum has a lecture on the Ediacaran.

There is stronger evidence for the biomarkers for sponges being present starting in the Cryogenian.

MesoProterozoic:

The shallow seawaters of the Calymmian MesoProterozoic were very hypoxic, showing evidence of having .1% of the current oxygen in seawater.

PaleoProterozoic:

There is evidence of intracontinental rifting from China during the Rhyacian Paleoproterozoic.

Was there a 'Mawson continent' during the Paleoproterozoic?

Archean:

The SETI Institute has a lecture on volcanism during the Archean.

PaleoArchean:

At least part of the trace fossils from Australia are pseudofossils (not real fossils).

EoArchean:

There is evidence of atmospheric oxygen from the EoArchean, 800 million years earlier than ever detected (or suspected) before.

Hadean:

In an almost extraordinary claim, the Siberian Craton may have bits of the crust from the Hadean!

Pilbara Craton Controversy: Were There or Were There NOT Hadean Continents?

Hf isotopes in detrital and inherited zircons of the Pilbara Craton provide no evidence for Hadean continents

Authors:

Kemp et al

Abstract:

Predictions of large volumes of stabilized continental crust by the early Archaean stand in stark contrast to the actual amount of pre-3.5 Ga rocks presently exposed on Earth's surface. The Pilbara Craton of Western Australia, one of the best preserved Paleoarchean crustal blocks on Earth, is believed to have developed on a cryptic, possibly ≥3.8 Ga continental basement. If substantiated, this could support the notion of a widespread and enduring Hadean (ca. 4.5–4.0 Ga) felsic–intermediate crust. To test this, and to elucidate the earliest evolution of the Pilbara Craton, we report Hf isotope data from previously dated detrital zircon grains, and inherited zircon crystals hosted by granitic gneisses, the crystallization ages (3.80–3.55 Ga) of which substantially exceed those of the oldest exposed igneous rocks of the craton (∼3.52 Ga). The Hf isotope compositions of the ancient zircons analyzed in this study are consistent with most of the earliest components of the Pilbara Craton being extracted from near chondritic mantle between ∼3.7 and 3.6 Ga, with little or no input from significantly older crust. These new data suggest either that the Pilbara Craton developed remote from the isotopic influence of the putative Eoarchean to Hadean continental masses, or that the stabilized volumes of the earliest continents have been overestimated. The latter scenario would be consistent with the extreme scarcity of greater than 3.9 Ga rocks and minerals, and the dominantly chondritic Hf isotope composition of the oldest continental rocks in Earth's most ancient Archaean cratons.

Evidence of Continental Growth in the North China Craton From Neoarchean (MesoArchean?) to Statherian Paleoproterozoic

Neoarchean to Paleoproterozoic continental growth in the southeastern margin of the North China Craton: Geochemical, zircon U-Pb and Hf isotope evidence from the Huoqiu complex

Authors:

Liu et al

Abstract:

The Huoqiu complex in the southeastern margin of the North China Craton (NCC) is dominated by Neoarchean grey gneisses, amphibolites and voluminous metasediments. Here we report the occurrence of Neoarchean to Paleoproterozoic rocks from drill core samples. The gneisses are similar to TTG (tonalite-trondhjemite-granodiorite) in composition and show close spatial association with amphibolites. Geochemical characteristics such as high Sr/Y and (La/Yb)N with steep REE patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta-basalts (amphibolites) at the base of a thickened mafic continental crust, leaving a rutile-bearing eclogite residue. LA-MC-ICP-MS U-Pb age data from magmatic zircon grains show protolith emplacement ages of 2.76-2.71 Ga. Subsequently, widespread migmatization took place at 1.91-1.82 Ga, generating voluminous migmatites and high-K granites. Hf isotopic compositions of zircon grains from the amphibolite and gneiss show εHf(t) values of 2.4-15.5 and − 3.0-1.5, respectively. The tDM2(Hf) model ages of the gneisses range from 2.87 to 3.14 Ga, and are identical to the tDM1(Hf) ages of amphibolites (2.84-3.16 Ga) within analytical uncertainty, suggesting that the gneisses formed by partial melting of amphibolite, and attest to large-scale reworking of the ancient continental crust during Neoarchean. The zircon grains from the granites define two groups with regard to their Hf isotopic composition. The older group (1916 ± 42 Ma) has εHf(t) values and tDM2(Hf) ages of − 10.5-2.4 and 2.40-3.20 Ga, respectively, whereas the younger one (1823 ± 41 Ma) shows a large variation in εHf(t) values ranging from − 18.1 to 12.5, with tDM2(Hf) model ages of 1.70-3.59 Ga. A couple of zircon grains from the younger group display consistent U-Pb ages and tDM2, indicating accretion of juvenile crust from depleted mantle sources during 1.82 to 1.91 Ga. However, the dominant Hf isotope features are consistent with the reworking of preexisting continental crust. We therefore infer that only limited accretion of juvenile crust occurred during this time, and that the Paleoproterozoic (1.82 to 1.91 Ga) tectonics in the southeastern margin of the NCC witnessed extensive reworking of older continental crust.

A Single Superplume is Splitting Africa

Africa is splitting in two. The reason: a geologic rift runs along the eastern side of the continent that one day, many millions of years in the future, will be replaced with an ocean. Scientists have argued for decades about what is causing this separation of tectonic plates. Geophysicists thought it was a superplume, a giant section of the earth's mantle that carries heat from near the core up to the crust. As evidence, they pointed to two large plateaus (one in Ethiopia and one in Kenya) that they said were created when a superplume pushed up the mantle. Geochemists were not able to confirm that theory. Instead they thought there might be two small, unrelated plumes pushing up the plateaus individually. The theories did not align, says David Hilton, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “There was a mismatch between the chemistry and the physics.”

So in 2006 and 2011 Hilton headed to East Africa to see whether he could lay the argument to rest. He and his team decided to use gases emanating from the rift to determine how it was created. Donning gas masks, they hiked to the tops of volcanoes in Tanzania and Ethiopia and climbed into mazuku (the Swahili word for “evil wind”)—geothermal vents and depressions where deadly gases accumulate and often kill animals. At these locations, the team collected samples of rocks deposited during eruptions, including olivines, crystals that trap volcanic gases like a bottle.

Back home in California, Hilton crushed the rocks inside a vacuum to release their gases. He was looking for helium 3, an isotope of helium present when the planet was forming that was trapped in the earth's core. Hilton figured that if rocks around both the Ethiopian and Kenyan plateaus contained this primordial gas, that would at least confirm that underground mantle plumes created them. The readings showed that, indeed, both plateaus contained helium 3. But Hilton and his group still had to wonder: Was one superplume behind it all? Or were there a couple of lesser plumes?

link.

Well, James, there's no need to go to the Moon!  Let's strip mine the Olduvai Gorge for Helium-3!

Komatiite Locations Controlled by Continental Formation

Archean komatiite volcanism controlled by the evolution of early continents

Authors:

Mole et al

Abstract:

The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50–30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean–Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits.

Evidence of a Neoarchean Basement in South Australia

U-Pb and Hf isotopic evidence for Neoarchean and Paleoproterozoic basement in the buried northern Gawler Craton, South Australia

Authors:

Reid et al

Abstract:

The northern Gawler Craton is buried beneath tens to hundreds of meters of largely Phanerozoic sediment. Recent drill holes into crystalline basement have recovered gneissic rocks that were dated by SHRIMP zircon U-Pb and their Lu-Hf isotopic characteristics determined by LA-ICPMS. These new data reveal an orthogneiss with a magmatic crystallization age of 2526 ± 7 Ma. The age and Hf isotopic composition of this ca. 2525 Ma rock are similar to Neoarchean rocks exposed in the central Gawler Craton, which suggests that the ca. 1790–1740 Ma volcano-sedimentary rocks of the northern Gawler Craton are probably underlain by a latest Neoarchean basement and argues against a fundamental boundary between these domains. A second sample recorded a minimum magmatic crystallization age of 1914 ± 8 Ma; a magmatic event not previously reported for the Gawler Craton. The Hf isotopic composition suggests magma derived from a juvenile source mixed with older crust, of ca. 1920–1970 Ma. Previous studies on the Hf and O isotopes in zircons from the Musgrave Province and Rudall Province of central Australia have proposed a major crust forming event at ca. 1900–1950 Ma; however, no zircons of this age have been located in these terranes. The discovery of ca. 1920 Ma granitic magmatism in the northern Gawler Craton is the first evidence of rock formation in southern Australia at this time and may indicate geodynamic connection between the Gawler Craton and central and western Australian terranes.

Increased Continental Weathering Made the Great Oxygenation Event Possible?

Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering

Authors:

Mills et al

Abstract:

A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered—contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500–500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.

Evidence of Sedimentation From a Continent From NeoArchean Greenland

The Neoarchaean Storø Supracrustal Belt, Nuuk region, southern West Greenland: An arc-related basin with continent-derived sedimentation

Authors:

Szilas et al

Abstract:

We present new major and trace element data, as well as Sm–Nd isotope compositions for Archaean supracrustal rocks and a gabbro-anorthosite complex from the island of Storø in Godthåbsfjord, southern West Greenland. We also provide new U–Pb isotope data for zircon extracted from these rocks. The Storø rocks have experienced amphibolite facies metamorphism, and the entire sequence forms an east-dipping frontal thrust ramp in tectonic contact with Mesoarchaean (‘Nûk’) gneisses to the west and Eoarchaean (‘Itsaq’) gneisses to the east. These orthogneisses record a complex regional accretion history, as established by previous work. The present study aims at explaining the geochemical features of the Storø Supracrustal Belt (SSB), which comprises metavolcanic and metasedimentary rocks that are situated within this collisional zone. The SSB has a maximum age of ca. 2800 Ma, constrained by the youngest detrital zircon population found in a metasedimentary unit. The minimum age of the SSB is 2707 ± 8 Ma, defined by previously published Re–Os isotope data. The metavolcanic rocks have a tholeiitic basaltic composition, with relatively flat primitive mantle-normalised trace element patterns and generally negative Nb-anomalies (Nb/Nb* 0.30–0.90). They plot above the mantle array in Th/Yb-Nb/Yb space, consistent with a subduction zone affinity, as also proposed by previous studies. A thin fault contact separates the SSB from a gabbro-anorthosite complex (formally defined here as the ‘Storø Anorthosite Complex’), which has an age of ca. 3050 Ma. The Sm–Nd isotope data of the SSB suggest, that these metavolcanic rocks experienced contamination by a crustal source that was isotopically similar to the Storø Anorthosite Complex (SAC). This in turn suggests that the SAC could have formed the basement for the younger volcanic sequence of the SSB or alternatively that the mantle source of the SSB was contaminated by melts derived from SAC-aged crust or sediments. The metasedimentary rocks of SSB show a mixed mafic–felsic component with variable degrees of maturity. Highly mature metasediments of the SSB contain several age populations of regionally well-known magmatic events, and thus support a significant local crustal provenance. Furthermore, the youngest documented detrital zircon is found in a thin metasedimentary unit within the metavolcanic rocks, and thereby shows that the SSB formed in close proximity to subaerially exposed continental crust (i.e. a back-arc environment) and not a distal island arc setting as previously proposed. Finally, this suggests, that relatively cool lower continental crust, which was capable of supporting subaerial mountains, existed at least locally during the Meso- Neoarchaean.

Keweenawan Midcontinent Rift: the Attempt to Destroy North America in the Stenian MesoProtoerozoic

Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

Authors:

Swanson-Hysell et al

Abstract:

The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian time. U/Pb dates from extrusive and intrusive rocks of the western Lake Superior Basin suggest a latent stage of reduced magmatic activity from ca. 1106 to 1100 Ma that places constraints on the dynamics of rift development and the record of plate motion. However, it has remained unclear whether this stage is a feature of the entire >2500-km-long rift. The succession of picritic and basaltic lava flows at Mamainse Point in the eastern Lake Superior Basin may be the most continuous and best exposed record of rift-related volcanism and magnetic reversals, but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates. We present a weighted mean 206Pb/238U date of 1100.36 ± 0.25 Ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the Great Conglomerate. This date indicates that eruptive activity at Mamainse Point continued during the interval of diminished magmatic activity in the western Lake Superior Basin. This result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at Mamainse Point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change.

Ectasian MesoProterozoic Continental Configuration: 1270 Million Years ago

Ectasian MesoProterozoic Continental Configuration: 1380 Million Years ago

Calymmian MesoProterozoic Continental Configuration: 1450 Million Years ago

Calymmian MesoProterozoic Continental Configuration: 1470 Million Years ago

Calymmian MesoProterozoic Continental Configuration: 1500 Million Years ago

Calymmian MesoProterozoic Continental Configuration: 1380 Million Years ago