Evidence of a Continental Collision Between Laurentia & Rodinia From Stenian MesoProterozoic Africa?

U–Pb Zircon (SHRIMP) ages of granite sheets and timing of deformational events in the Natal Metamorphic Belt, southeastern Africa: Evidence for deformation partitioning and implications for Rodinia reconstructions

Authors:

Mendonidis et al

Abstract:

This study provides constraints on the ages of deformation events and fabric development in deformed rocks of the Margate Terrane of the Natal Metamorphic Province. The Margate Terrane forms the southernmost of three terranes considered to represent multiple arc accretion onto the southern margin of the Kalahari Craton, and geochronological data indicate that the Margate Terrane has a long history of sporadic magmatism from ∼1180 to ∼1025 Ma. Two granite sheets of differing structural age, as revealed by deformational fabric and cross-cutting relationships, were sampled for U–Pb (SHRIMP) dating from coastal outcrop at Southbroom (30°54′43.61″S, 30°20′1.61″E). The older sheet contains fabrics related to both D1 and D2 events, whereas the younger shows evidence for syn-D2 emplacement and contains an S2 fabric. For both intrusive sheets, zircon core domains showing magmatic zoning yielded ages that are statistically identical at 1075 ± 6 Ma. This is interpreted to represent the intrusion age of the older sheet, and to sampling of a xenocrystic population in the cross-cutting sheet due to assimilation. Zircons from both sheets show metamorphic rim zones with a mean age of 1042 ± 10 Ma, which is attributed to rim growth during development of the S2 foliation, close to the intrusion age of the younger sheet. The 1075 ± 6 Ma age is comparable to the ages of other granitic units in the Margate Terrane that intruded between ∼1091 and 1070 Ma and implies that all of these predated the D1 deformation, which is considered to record the accretion of the terrane onto the Kalahari Craton. The D2 event is characterized by northward-verging folds with a pervasive southward-dipping axial planar fabric S2, which largely overprinted the S1 fabric. Previously, the pure-shear D2 deformation was considered to be older than the narrow sinistral shear zones occurring within the Mzumbe and Margate Terranes. However, dating of the D2 event at 1042 ± 10 Ma indicates that these deformations are coeval, and represent deformation partitioning during a transpressional event at ∼1050–1025 Ma that may have been related to collision with Laurentia during the amalgamation of Rodinia.

Evidence of the Separation of Laurentia From Australia During the Rifting of the Coulmbia Sueprcontinent found?

Rifting of Columbia to form a deep-water siliciclastic to carbonate succession: The Mesoproterozoic Pinguicula Group of northern Yukon, Canada

Authors:

Medig et al

Abstract:

The Mesoproterozoic Pinguicula Group (less than 1.38 Ga) is exposed in the Wernecke and Hart River inliers in northern Yukon, Canada. The Pinguicula Group records deposition of non-cyclic siliciclastic and carbonate strata on low-energy slopes affected by rare high-energy deposits in a tectonically active epicratonic setting. The succession is ∼1.4 km thick at its measured type sections and comprises three newly formalised formations: the Mount Landreville, Pass Mountain, and Rubble Creek formations (formerly units A, B, and C, respectively). The Mount Landreville Formation is a predominantly siltstone succession with minor conglomerate and sandstone deposited below storm wave-base on a relatively gentle slope. The Pass Mountain Formation is a wispy- to planar-laminated carbonate succession deposited on a low-energy slope mostly below storm wave-base and is punctuated by rare high-energy gravity-flow deposits including debrites, grain-flows, turbidites, and micro-turbidites. The Rubble Creek Formation is dominated by repetitive centimetre- to decimetre-scale lime mudstone beds; it is distinguished from the Pass Mountain Formation by abundant zebra texture (an alternating dark grey and white banding caused by late diagenetic or hydrothermal fluid influx) and a lack of sediment gravity-flow deposits.

The Pinguicula Group is the middle of five, unconformity-bounded, Proterozoic stratigraphic successions deposited on the northwestern margin of Laurentia (ancestral North America). The Pinguicula basin was epicratonic and deepened to the south (present coordinates). The basin formed during an amagmatic extensional event that contributed to the break-up of Columbia and the separation of Laurentia from Australia.

Whereas most preserved Mesoproterozoic basins are dominated by shallow-water lithofacies deposited in rift and epicratonic settings, with few deep-water lithofacies preserved, the carbonate strata of the Pinguicula Group provide a rare insight into deeper-water carbonate environments.

Evidence From Tasmania East Antarctica and Laurentia Connected in the Columbia (Nuna) Supercontinent

Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna

Authors:

Mulder et al

Abstract:

Most recent paleogeographic reconstructions of the supercontinent Nuna juxtapose the North Australian craton, Mawson continent (South Australia–East Antarctica), and Laurentia between 1.6 Ga and 1.3 Ga but differ in their relative positioning. The greater than 10-km-thick siliciclastic Rocky Cape Group of Tasmania was deposited in an opening marine basin on the margin of East Antarctica during Nuna breakup. Based on a similar detrital zircon signature and depositional age, the Rocky Cape Group has been correlated with the upper Belt-Purcell Supergroup in Laurentia, thus representing a key tie point within Nuna. Here the detrital zircon age signature of Mesoproterozoic Rocky Cape Group quartzites is investigated by comparing new detrital zircon U-Pb-Hf isotopic data to an extensive compilation of zircon isotopic data from Australia, East Antarctica, and Laurentia. Our analysis demonstrates that the Rocky Cape Group is unlikely to have been sourced from any geological terrane exposed in presentday Australia. Instead, zircon U-Pb-Hf isotopic data from basement terranes in Laurentia and East Antarctica show striking similarities to the Rocky Cape Group detrital signature. Paleocurrent data indicate that the majority of sediment in the Rocky Cape Group was sourced from Laurentia, which was to the southeast (present-day coordinates) of Tasmania, supporting a SWEAT-like (southwest United States–East Antarctica) configuration for Nuna. We suggest that rifting left a thinned continental connection between East Antarctica and Laurentia onto which the lower-middle Rocky Cape Group was deposited between 1.45 and 1.30 Ga.

Cambrian Radiation Linked to an Iapetus-Pacific Oceanic Connection?

Cambrian transgression and radiation linked to an Iapetus-Pacific oceanic connection?

Author:

Dalziel

Abstract:

The geologically abrupt appearance in the fossil record of almost all animal phyla is referred to as the Cambrian radiation or "explosion" of life on Earth. Also known as "Darwin's dilemma," because it seemingly posed a major problem for his theory of gradual evolution, it coincided with the initiation of the first of the two principal global marine transgressions of the Phanerozoic. Although now seen as more protracted, it is still one of the most striking and critical events in the history of the biosphere. Almost all paleogeographic reconstructions for the early Cambrian feature a previously isolated Laurentia, the core of ancestral North America. Yet geological evidence from five continents, integrated here for the first time, indicates that the present-day "southern cone" of Laurentia was still attached to the newly amalgamated supercontinent of Gondwanaland into Cambrian times. Laurentia was then isolated by the development of a major deep oceanic connection between the opening Iapetus Ocean basin and the already well-developed paleo-Pacific. As the marine transgression advanced, major changes in ocean chemistry occurred, upwelling generated phosphorite deposits, and the number of fossilized metazoan phyla "exploded" with morphologic disparity between Laurentia and Gondwanaland already established. The development of this deep oceanic gateway, and of an ocean floor–consuming and arc-generating subduction zone along virtually the entire margin of Gondwanaland shortly thereafter, need to be taken into account in consideration of the global environmental and biotic changes associated with the Neoproterozoic-Phanerozoic transition.

Evidence of Rifting From Cryogenian NeoProterozoic Laurentia

Petrogenesis of Gunbarrel magmatic rocks: Homogeneous continental tholeiites associated with extension and rifting of Neoproterozoic Laurentia

Authors:

Sandeman et al

Abstract:

The ca. 780 Ma Gunbarrel Igneous Event of northwest Laurentia consists of spatially discrete suites of sills, dykes and lavas distributed over a vast area extending from Wyoming in the south to the Wopmay Orogen and the Mackenzie Mountains of Northwest Canada. Thick (≤ 100 m) sills and rare dykes in Wopmay orogen and thinner (≤ 30 m) sills, dykes and rare lavas in the Mackenzie Mountains are moderately evolved, augite + oligoclase-labradorite + ilmenite-magnetite gabbros and amygdaloidal basalts. Systematic petrochemical differences between units reveals that each is likely derived from subtly distinct parental magmas collectively exhibiting mutually consistent element variations. The dataset is remarkably homogenous, in particular, the incompatible trace elements and the Sm-Nd isotopes. All rocks preserve petrochemical evidence of an enriched MORB-like mantle source, but a small lithospheric component in the primary magmas resulted in elevated LILE, minor negative HFSE anomalies and sub-depleted mantle but supra-bulk earth ɛNd values. The lithospheric component was slightly older, modestly fractionated, Sr-depleted, garnet-free (pyroxenitic?) lower crust or, similar material that was previously recycled into the lithospheric mantle. Mineral chemical data for plagioclase and clinopyroxene in chill margin samples from a Hottah sheet in Wopmay orogen, indicates rapid and repeated turbulent mixing of geochemically and thermally similar magmas. These were staged from large, lowermost crust(?) magma chambers centered over an asthenospheric thermochemical anomaly thought to lie to the west of present-day North America. These magmas were then rapidly emplaced across western Laurentia. The complex mineral chemical features along with the homogeneity of the rocks imply turbulent, rapid mixing of numerous similar magmas in one, or many large chambers

New Paleomagnetic Data From Southern Sweden Has Implications for Tonian Neoproterozoic Laurentia, Baltica Geography

A palaeomagnetic and 40Ar/39Ar study of mafic dykes in southern Sweden: A new Early Neoproterozoic key-pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops

Authors:

Elming et al

Abstract:

We present the results of palaeomagnetic and 40Ar/39Ar studies of the Proterozoic mafic dykes in the Norrköping and Falun areas of the southern Sweden. The primary remanence of two 939 ± 3 Ma dykes is supported by the rigorous baked contact test. The remanence direction of two other dykes, one of which was previously U-Pb dated at 946 ± 1 Ma is close to the reverse direction of 939 Ma dykes. Using these results together with previously published 935 ± 5 Ma palaeomagnetic data from the Göteborg-Slussen mafic dykes and some dykes from the Falun area we calculated the mean 946 -935 Ma palaeopole for Baltica (0.9°S, 240.7°E, A95 = 6.7), which can be qualified as the key pole. Using this pole together with other date we conclude that the Grenville and Sveconorwegian loops of Laurentian and Baltican Apparent Polar Wander Paths are temporary displaced by 100-150 m.y. We propose new palaeogeographic reconstructions of Baltica and Laurentia at ca. 940 Ma and ca. 850 Ma. We also present two new Mesoproterozoic non-key poles from 1410 Ma and 1595 Ma dykes.

Remnants of PaleoProterozoic Volcanic Arc Bonnetia Found in Yukon

The Wernecke igneous clasts in Yukon, Canada: fragments of the Paleoproterozoic volcanic arc terrane Bonnetia

Authors:

Alexander B. Nielsen, Derek J. Thorkelson, H. Daniel Gibson and Daniel D. Marshall

Abstract:

The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U-Pb titanite at 1599 Ma. Four U-Pb zircon ages on the Wernecke igneous clasts (1714-1706 Ma) demonstrate that the clasts are older than the Wernecke Supergroup (less than 1.64 Ga) and indicate that the clasts were not derived from dykes within the Wernecke Supergroup. Instead, the clasts were derived from an obducted terrane named Bonnetia. Geochemical characteristics of the Wernecke igneous clasts infer that Bonnetia formed as a volcanic arc with a component of within-plate magmatism. Neodymium mantle depletion ages of 2080-2760 Ma suggest that the arc was built on older continental crust. Consequently, Bonnetia may have been a volcanic arc, possibly built on a rifted fragment of Laurentia, on another continental fragment, or possibly on the leading edge of another continent. The subsequent event of breccia-formation may represent a hydrothermal response to obduction-caused tectonic loading of the crust. The characterization of Bonnetia as a volcanic arc complex that underwent obduction requires that northwestern Laurentia was flanked by an ocean basin in the late Paleoproterozoic

Orogeny During Katian Ordovician Up Ended Marine Ecosystems

New Ohio University research suggests that the rise of an early phase of the Appalachian Mountains and cooling oceans allowed invasive species to upset the North American ecosystem 450 million years ago.

The study, published recently in the journal PLOS ONE, took a closer look at a dramatic ecological shift captured in the fossil record during the Ordovician period. Ohio University scientists argue that major geological developments triggered evolutionary changes in the ancient seas, which were dominated by organisms such as brachiopods, corals, trilobites and crinoids.

During this period, North America was part of an ancient continent called Laurentia that sat near the equator and had a tropical climate. Shifting of the Earth's tectonic plates gave rise to the Taconic Mountains, which were forerunners of the Appalachian Mountains. The geological shift left a depression behind the mountain range, flooding the area with cool water from the surrounding deep ocean.

Scientists knew that there was a massive influx of invasive species into this ocean basin during this time period, but didn't know where the invaders came from or how they got a foothold in the ecosystem, said Alycia Stigall, an Ohio University associate professor of geological sciences who co-authored the paper with former Ohio University graduate student David Wright, now a doctoral student at Ohio State University.

"The rocks of this time record a major oceanographic shift, pulse of mountain building and a change in evolutionary dynamics coincident with each other," Stigall said. "We are interested in examining the interactions between these factors."

Using the fossils of 53 species of brachiopods that dominated the Laurentian ecosystem, Stigall and Wright created several phylogenies, or trees of reconstructed evolutionary relationships, to examine how individual speciation events occurred.

The invaders that proliferated during this time period were species within the groups of animals that inhabited Laurentia, Stigall explained. Within the brachiopods, corals and cephalopods, for example, some species are invasive and some are not.

As the geological changes slowly played out over the course of a million years, two patterns of survival emerged, the scientists report.

During the early stage of mountain building and ocean cooling, the native organisms became geographically divided, slowly evolving into different species suited for these niche habitats. This process, called vicariance, is the typical method by which new species originate on Earth, Stigall said.

As the geological changes progressed, however, species from other regions of the continent began to directly invade habitats, a process called dispersal. Although biodiversity may initially increase, this process decreases biodiversity in the long term, Stigall explained, because it allows a few aggressive species to populate many sites quickly, dominating those ecosystems.

Paper link.