Provenance of intra-Rodinian basin-fills: The lower Dalradian Supergroup, Scotland
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Dalradian
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Laurentia
Rodinia
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Baltica
Laurentia
Rodinia
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Pangaea
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Abstract Late Ediacaran opening of the Iapetus Ocean is typically considered to reflect separation of Baltica and Laurentia during final breakup of the Rodinia supercontinent, with subsequent closure during the Caledonian Orogeny. However, evidence of the pre‐opening juxtaposition of Baltica and Laurentia is limited to purportedly similar apparent polar wander paths and correlation of Rodinia‐forming orogenic events. We show that a range of existing data do not unequivocally support correlation of these orogens, and that geologic and palaeomagnetic data instead favour separation of Baltica and Laurentia as early as 1.1–1.2 Ga. Furthermore, new detrital zircon U–Pb age and Ar–Ar thermochronological data from Norway point towards an active western Baltican margin throughout most of the Neoproterozoic and early Palaeozoic. These findings are inconsistent with the majority of palaeogeographic reconstructions that place Baltica near the core of the Rodinia supercontinent.
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Abstract The core of the Rodinia supercontinent has long been considered to have consisted of three cratons – Baltica, Laurentia and Amazonia – amalgamated along the late Mesoproterozoic Sveconorwegian, Grenville and Sunsas orogens. In recent years, however, it has become increasingly clear that the metamorphic and magmatic evolution of the Sveconorwegian orogen is inconsistent with a collisional model. Although geological data alone do not rule out proximity to Rodinia, palaeomagnetic data indicate significant latitudinal separation of Baltica and Laurentia during supercontinent assembly. In this contribution, we briefly review two recently proposed and mutually exclusive tectonic models for the Sveconorwegian orogeny and present a compilation of previously published and new chemical and isotopic data. A lack of crustal thickening throughout much of the orogen and few if any changes in lower-crustal sources and melting conditions between 1.3 and 0.9 Ga suggest that the western part of the Sveconorwegian orogeny represents a change from a dominantly extensional to a compressional back-arc regime, but without a significant change in overall tectonic setting. This orogenic evolution is incompatible with amalgamation into Rodinia and suggests that Baltica may have been isolated until the Silurian Caledonian orogeny.
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A total of 4344 magmatic U-Pb ages in the range 2300 to 800 Ma have been compiled from the Great Proterozoic Accretionary Orogen along the margin of the Columbia / Nuna supercontinent and from the subsequent Grenvillian collisional orogens forming the core of Rodinia. The age data are derived from Laurentia (North America and Greenland, n = 1212), Baltica (NE Europe, n = 1922), Amazonia (central South America, n = 625), Kalahari (southern Africa and Dronning Maud Land in East Antarctica, n = 386), and western Australia (n = 199). Laurentia, Baltica, and Amazonia (and possibly other cratons) most likely formed a ca. 10 000-km-long external active continental margin of Columbia from its assembly at ca. 1800 Ma until its dispersal at ca. 1260 Ma, after which all cratons studied were involved in the Rodinia-forming Grenvillian orogeny. However, the magmatic record is not smooth and even but highly irregular, with marked peaks and troughs, both for individual cratons and the combined data set. Magmatic peaks typically range in duration from a few tens of million years up to around hundred million years, with intervening troughs of comparable length. Some magmatic peaks are observed on multiple cratons, either by coincidence or because of paleogeographic proximity and common tectonic setting, while others are not. The best overall correlation, 0.617, is observed between Baltica and Amazonia, consistent with (but not definitive proof of) their being close neighbours in a SAMBA-like configuration at least in Columbia, and perhaps having shared the same peri-Columbian subduction system for a considerable time. Correlation factors between Laurentia and Baltica, or Laurentia and Amazonia, are below 0.14. Comparison between the Grenville Province in northeastern Laurentia and the Sveconorwegian Province in southwestern Fennoscandia (Baltica) shows some striking similarities, especially in the Mesoproterozoic, but also exhibits differences in the timing of events, especially during the final Grenville-Sveconorwegian collision, when the Sveconorwegian evolution seems to lag behind by some tens of million years. Between the other cratons, the evolution before and during the final Grenvillian collision is also largely diachronous. After 900 Ma, magmatic activity had ceased in all areas investigated, attesting to the position of most of them within the stable interior of Rodinia.
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Meso– to Neoproterozoic geological and paleomagnetic data support a direct connection between Baltica and Laurentia in both Nuna and Rodinia supercontinents, however in different relative configurations. Previous paleomagnetic data limit the time of break-up of Nuna core configuration and ca. 90° rotation of Baltica relative to Laurentia between 1.27 Ga and 0.99 Ga. Despite the well documented relative motion of continents, the tectonic mode during the Meso– to Neoproterozoic has been questioned and the operation of single lid tectonics at 1.6–1.0 Ga during the Nuna-Rodinia supercontinent cycles has been proposed. In this study, new paleomagnetic and whole rock 40Ar-39Ar geochronological data from a basic dyke in Stugun central Sweden are combined with coeval data to produce a 1.22 Ga (1.244–1.200 Ga) moderate-quality paleomagnetic pole. This is done to better estimate the break-up time of the core of Nuna and explore the plate tectonics at the Mesoproterozoic. The new pole fills part of the 1.247–1.140 Ga gap in the paleomagnetic record of Baltica. By comparing apparent polar wander paths (APWPs) and calculated drift velocities, a break-up of Baltica and Laurentia at 1.12–1.04 Ga is suggested. Plate velocities calculated for Laurentia, Baltica and Siberia for the time of the Nuna supercycle are similar and low to moderately high corresponding with the present-day tectonic speeds. Furthermore, the obtained velocity peaks may be related with onset of the break-up of the Nuna supercontinent, the break-up of the direct Baltica–Laurentia connection in Nuna and nascent Rodinia. We suggest that the velocity peaks and large oscillating shifts in late Mesoproterozoic pole positions for Laurentia and Baltica result from a combination of relative plate motion and inertial interchange true polar wander (IITPW) events. Both IITPW events and relative plate motions argue for an operation of plate tectonics in the Meso– to Neoproterozoic.
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Abstract The Grenville, Sveconorwegian, and Sunsas orogens are typically inferred to reflect collision between Laurentia, Baltica, and Amazonia at ca. 1.0 Ga, forming a central portion of the Rodinia supercontinent. This triple‐junction configuration is often nearly identical in otherwise diverse Rodinia reconstructions. However, available geological data suggest that although the Grenville and Sveconorwegian provinces shared a similar tectonic evolution from pre‐1.8 to ca. 1.5 Ga, they record distinctly different tectonic histories leading up to, during, and possibly following Grenville–Sveconorwegian orogenesis. Moreover, palaeomagnetic data suggest the two continents were separated at peak orogenesis, further invalidating any direct correlation. A number of possible interpretations are permissible with available geological and palaeomagnetic data, of which a “classic” triple‐junction configuration appears least likely. In contrast to the commonly inferred intertwined Proterozoic evolution of Baltica and Laurentia, the possibility remains that they were unrelated for a billion years between 1.5 and 0.45 Ga.
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Passive margin
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