Laurentia at 571 Ma: Preliminary paleomagnetism and Ar-Ar age of the Ediacaran St Honore alkali intrusion, Quebec
The paleomagnetic record of Laurentia through the Ediacaran (~615 to 543 Ma) is pivotal to establishing the paleogeography of the opening of the Iapetus Ocean, but the handful of available paleomagnetic results are controversial. Some results appear to indicate that Laurentia resided at high southerly paleolatitudes from ca. 600 Ma to 570 Ma, whereas other interpretations hold that Laurentia resided at low paleolatitudes throughout the Ediacaran. We have sampled the mid-Ediacaran St. Honore alkali intrusion and related dykes in the Saguenay region of Quebec for a paleomagnetic and Ar-Ar geochonologic study to assess the 'high' and 'low' paleolatitude Laurentia interpretations. Ar-Ar geochronology of phlogopite separates from the carbonatite intrusion return plateau ages of 571.0 +/- 4.6 Ma, confirming and refining previous whole rock and biotite K-Ar ages. Preliminary paleomagnetic results from the intrusion and 12 carbonatite and lamprophyre dykes have a mean direction of D=122, I=69.8 degrees; a95=10, retained to high unblocking temperatures by PSD magnetite. Our preliminary result places St. Honore at 54 deg S at 571 Ma and implies that Laurentia lay at moderate paleolatitudes in the mid-Ediacaran. Notably, the paleopole location at 23 N, 322 E (dp=17, dm=15) is consistent with, and lies squarely in between the paleopoles which place Laurentia at high paleolatitudes and those which place Laurentia at low paleolatitudes. The preliminary St Honore result implies that at ca. 571 Ma Laurentia was in rapid transition from high to low paleolatitudes, or that Laurentian paleomagnetic results from the Ediacaran period may be even more complex than previously realized.
Paleomagnetism of the Adirondack Dyke Swarms: Filling Holes With Poles in the Ediacaran Apparent Polar Wander Path for Laurentia?
The Adirondack massif contains hundreds of dykes that postdate the Grenvillian basement complex but predate the onlapping Cambrian Potsdam Sandstone. Beyond this general age bracket, there are presently few if any more detailed age constraints on these dykes. The E-trending and putatively coeval Grenville dykes and the generally NE-trending Adirondack dykes form a radial fan that converge to a point near the Sutton mountain gravity and magnetic anomaly, inspiring the suggestion that both derive from a common Ediacaran mantle plume. Such an interpretation has seemed at odds, however, with large amounts of paleomagnetically- determined apparent polar wander (APW) for Laurentia, unless wholesale (not relative tectonic) motion of solid Earth is invoked. Yielding multiple seemingly reliable paleomagnetic remanences presented herein, the Adirondack dykes have the potential to fill in multiple gaps within the Ediacaran APW path for Laurentia. Immediately west of Dannemora, New York, samples from 18 of 29 dykes (1 site per dyke) were stable and reliably consistent within sites, resulting in 18 virtual geomagnetic poles (VGPs) including a full (i.e., hybrid magnetizations) positive baked contact test into the Lyon Mt. Granitic Gneiss. The distribution of those 18 VGPs is fairly scattered, but when subdivided into geographically consistent groups they define three paleomagnetic poles (mid-latitude, polar (in north), and polar (in south) with respect to present North America). These poles fill in gaps within the Laurentian APW path and may elucidate its anomalous, possibly global Ediacaran swath. Comparison of dyke characteristics (including trend, mineralogy, geochemistry, and geochronology) will test the validity of the VGP groupings, which currently depend only on the paleomagnetic data.
Ediacaran Paleogeography: Paleomagnetic Constraints and Alternative Models
Although the evolution of the Pangaea and Gondwana supercontinents is relatively well established, the exact configuration of the earlier supercontinent Rodinia and its predecessors are still widely debated due to our poor knowledge of Ediacaran palaeogeography. The Ediacaran to Cambrian Periods encompass rifting associated with the final breakup of Rodinia and widespread orogeny during the formation of Gondwana. Many high-quality palaeomagnetic poles have been used to construct Phanerozoic APWPs for the majority of continents, and there is general agreement about Phanerozoic tectonic history. In contrast, late Neoproterozoic palaeomagnetic data are scarce and controversial, and it is difficult at this stage to apply the traditional APWP method to arrive at a unique model for the latest Neoproterozoic - Cambrian global palaeogeography. Recent data do provide a good step forward, however. In the "Iapetan" realm of final Rodinia breakup, recent paleomagnetic data from Baltica verify the pre-Iapetian Laurentia-Baltica reconstruction and suggest that Baltica rifted off Laurentia-Amazonia around 600 Ma during the opening of the East Iapetus and Tornquist Sea, although details of this process are still debated. In most published scenarios, this was followed by separation of Amazonia and Laurentia and opening of Western Iapetus. At the same time, a complicated collision between several continental blocks on the other side of the globe caused closure of oceanic basins and the assembly of Gondwana, accompanied by major accretionary events along north Gondwanan (Cadomian orogeny), east Baltican (Timanian orogeny) and south Siberian (Baikalian orogeny) margins. At ~750 Ma Australia and Congo were in low latitudes and India was in middle latitudes. Australia is positioned at 755 Ma by the Mundine Well dykes palaeopole and by recent studies of the Neoproterozoic drill holes. From ~650 Ma to 550 Ma there is a swathe of palaeopoles, albeit with poor age constraints in many cases, that nevertheless forms a consistent pattern placing Australia in low latitudes throughout this time. The Malani Igneous Suite palaeopole determines the position of India at 770-750 Ma. In its reconstructed position relative to India, the Seychelles palaeopole, determined from 755-748 Ma granites, overlaps the Malani palaeopole. The position of Congo at ~ 750 Ma is constrained by the pole from Mbozi Complex and finds support from a recent study of the 765 Ma Luakela Volcanics. The Luakela and Mbozi results differ drastically from the Gagwe lavas pole, whose age has recently been reassessed as 795 ± 7 Ma. In India, the Harohalli dykes give a palaeopole with an imprecise age of 814 ± 34 Ma, but recent geochronological data suggest much older age for these dykes. Palaeomagnetism from the Bhander and Rewa Series is only broadly determined to be Mesoproterozoic or Early Cambrian. Recent paleontological, provenance and paleomagnetic data are still controversial. The recent Puga cap carbonate palaeopole from Amazonia indicates low latitude for Amazonia around 600 Ma. This pole is suspiciously close to the present-day pole, so it may represent a recent re-magnetisation. The post-orogenic Sinyai dolerite intrudes the East African orogen and provides a ~547 Ma pole for this part of proto-Gondwana. The number of reliable palaeomagnetic data is insufficient to produce a unique palaeogeographical model for the Gondwana assembly, but some alternative models can be demonstrated.
New Geochronologic and Paleomagnetic Results for the Grenville Dyke Swarm and Implications for the Ediacaran APWP for Laurentia
Grenville diabase dykes form a swarm that is concentrated along the Ottawa graben in the southern Canadian Shield. The swarm has been linked to the late Neoproterozoic opening of the Iapetus Ocean. U-Pb baddeleyite (+/- zircon) ages of 586+/-4, 589+3/-2 and 592+21/-3 Ma have previously been reported from three dyke sites. At the first site a stable magnetic remanence directed steeply down to the NW was determined, along with a positive baked contact test suggesting that the remanence is primary. The other two sites, although not as stably magnetized, also appear to carry this remanence. However, paleomagnetic directions obtained at many other Grenville dyke sites are significantly different, varying from steep down SE to shallow up SE, and occasionally steep up or intermediate up NW. These data could indicate that (a) there is a wide range in Grenville dyke ages, (b) true polar wander is responsible for different directions in dykes of only slightly different ages, (c) magnetic overprinting has occurred in dykes of the same age, or (d) some sites have been magnetized in anomalous directions during geomagnetic excursions or during the course of field reversals. Here we report a U-Pb baddeleyite age of 586+/-1 Ma for a dyke with an intermediate up NW direction, and a preliminary positive baked contact test for a dyke with a moderately steep down SE direction. As steep down NW and intermediate up NW directions are observed from different 586 Ma dykes, the results cannot easily be explained by either a large or small age difference among Grenville dykes. They suggest that magnetic overprinting has occurred in at least some sites and (or) that anomalous remanence directions have been acquired during geomagnetic excursions or field reversals. Which, if any, of the Grenville dyke remanences is primary is unclear because there appear to be positive baked contact tests for two distinct paleomagnetic directions. These results emphasize the complexity of Grenville dyke paleomagnetic data, and suggest that they are currently unsuitable for use in the construction of an Ediacaran apparent polar wander path (APWP). Like the Grenville dykes, multiple stable remanences have been reported from a number of other 615-565 Ma geological units (Long Range dykes, Catoctin volcanics and Sept Iles igneous complex) that have been used for the Ediacaran APWP for Laurentia. The age of these remanences is controversial, with some authors interpreting the data to indicate a low latitude for the continent throughout the interval, and others favouring a rapid change from high latitude to low latitude at ca. 565 Ma related to very rapid APW or true polar wander. The controversy arises for several reasons. For example, field tests to establish that remanences are primary are incomplete or open to more than one interpretation, and some magnetization directions are difficult to distinguish from that of the present earth's magnetic field. Given the complexity of the data, more detailed paleomagnetic and geochronologic work is required before paleopoles from these units, like those from the Grenville swarm, can be utilized to develop a robust APWP.
What Could Explain the nearly Coeval Shallow and Steep Inclinations of Ediacaran Paleomagnetic Results from Laurentia?
Paleomagnetic results obtained from rocks of Ediacaran age in several localities in Laurentia and Baltica persistently display co-existence of two magnetization components, one shallowly and the other steeply inclined. Both components pass criteria for a primary magnetization, so that it is not acceptable to reject one or the other half of the database. However, geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age. A straightforward interpretation of these results is therefore not evident, as it would imply that rocks acquired magnetizations in positions shifting rapidly from (or even switching back and force between) equatorial and near-polar latitudes. In a geographic reference frame, such large-scale (about 8000 km) and fast (up to 70 cm/year) migrations of a continent have long been rejected as dynamically implausible. Alternative explanations that are not rooted in plate tectonics involve either a very rapid migration of bulk lithosphere with respect to the rotation axis (TPW = True Polar Wander) or an unusual complexity of the magnetic field configuration. Theoretical arguments (Tsai and Stevenson, JGR, v. 112, issue B5, paper B05415, 2007) have been used to constrain the maximum velocities of TPW and its inertial interchange variety to less than about 30 cm/year; much less than those required to account for the paleomagnetic data. An unusual behavior of the geomagnetic field remains the only viable explanation for the Ediacaran data. For example, an alternation of the geomagnetic dipole axis between a co-axial and an equatorial alignment would have produced a set of shallow and steep paleomagnetic directions, similar to those observed. Theoretical arguments suggest that such switching from an axial to an equatorial dipole mode is possible at not-unreasonable conditions of outer core thickness and Rayleigh numbers (Aubert, J. and Wicht, J., EPSL, v. 221, p. 409-419, 2004). Given the global nature of the geomagnetic field, such a hypothetical equatorial dipole model can be tested if suitable rocks can be identified.