Grenville Diabase Dyke Swarm: Extent, Age and Comparative Geochemistry
Quartz diabase dykes of the Grenville swarm extend from west of Sudbury, Ontario, eastward almost as far as Montreal, Quebec, a distance of 650 km. The dykes are vertical and trend east-west, parallel to faults related to the Ottawa graben of the St. Lawrence rift system, within a swath about 200 km wide in central Ontario and southwestern Quebec. In the vicinity of Lake Temiskaming, northwest-trending dykes (Lac Pommeroy) are considered to be a splay of the Grenville swarm, as are southwest-trending dykes (Rideau) in the Frontenac region of eastern Ontario; southwest-trending diabase dykes in the Adirondack region of New York State may be similarly related. This westerly fanning suggests a source region to the east. Grenville dykes have been dated by U-Pb methods at ca. 590 Ma at several localities, including two reported herein. Paleomagnetic study of Grenville dykes is important for constraining paleopole position during a period of apparently rapid polar wander and/or plate motion in the late Neoproterozoic (Ediacaran). However, Grenville dykes can be confused in the field with other diabase dykes of similar trend and appearance. For example, some dykes of the 1.24 Ga Sudbury swarm have a similar trend to Grenville dykes at the western end of the Grenville swarm, and the two are not everywhere correctly distinguished on maps. In the Frontenac region of eastern Ontario, the Rideau dykes have the same trend as Brockville dykes (undated), but different from the north- to northwest-trending Kingston dykes, herein dated at 1160 Ma (U-Pb baddeleyite). In adjacent southernmost Quebec, however, poorly exposed diabase near Buckingham is superficially similar to Grenville diabase but has a preliminary U-Pb zircon age of 1165 Ma. Although careful petrographic study can help to distinguish diabase of these different swarms, whole-rock geochemistry provides unequivocal identification (and is cheaper and less time-consuming to obtain than geochronology). In summary, Sudbury and Grenville diabase are olivine- and quartz-normative respectively; Sudbury diabase is characterized by being more enriched in Fe, K and high field strength minor and trace elements (particularly Ba) than Grenville diabase, which in turn is markedly enriched in Cu. Lac Pommeroy and Rideau diabase have identical chemical signatures to Grenville diabase. Brockville diabase, quartz-normative like Grenville diabase, in addition to being amygdaloidal and carrying plagioclase xenocrysts, is comparatively enriched in Ti, Zr, and P (but not Ba), and impoverished in Cu. The 1165-Ma diabase near Buckingham is also quartz normative, but is highly enriched in alkalis, Ba, Sr, P and impoverished in Cu and REE; it is entirely different to the coeval Kingston diabase, which is olivine-normative and comparatively poor in Ba, Sr, P.
U-Pb Zircon Dating and Paleomagnetism of Proterozoic Mafic Dykes in the Adirondacks at Rand Hill, NY
We studied several of the east- west trending mafic dykes with chilled margins that cut the Grenville Province in the Adirondacks of New York State near Rand Hill. It has been suggested that these dykes are Neoproterozoic in age and originated from an Iapetan mantle plume that also fed the ca. 590 Ma Grenville dyke swarm. According to Isachsen et al. (1988), 17 of the Rand Hill dykes carry a magnetite-borne remanence with an easterly declination and about a 64 degree inclination on average. Baked contact tests that we attempted for three Rand Hill tholeiitic dykes that carry a similar remanence proved inconclusive. For one of these three tholeiitic dykes, U-Pb dating of liberated and in situ zircons, using laser ablation ICP-MS, supports a Mesoproterozoic emplacement age of 1036 +/-3 Ma with a Neoproterozoic lead loss event at about 640 Ma. The tight range of zircon Th/U ratios (0.40 to 0.49) and the interstitial locations of most of the dated in situ zircons strongly support Mesoproterozoic emplacement for this tholeiitic dyke. In contrast, for the one alkalic Rand Hill dyke studied, liberated and in situ zircons provide strong evidence for a Neoproterozoic emplacement age of 624 +/-2 Ma. However, this alkalic dyke is dominated by a hematite-borne shallow southerly remanence that is likely a Kiaman overprint. Hence, our study suggests a complex intrusion and magnetization history for the east-west Adirondack mafic dykes. Some of the tholeiitic dykes may represent magmatism during the waning of the Mesoproterozoic Grenville Orogeny, whereas some of the alkalic dykes may represent early OIB-like magmatism from a Neoproterozoic Iapetan mantle plume.
A test of high-latitude Laurentia: paleomagnetism and 40Ar-39Ar geochronology of the 584 Ma Baie des Moutons syenite, Quebec
Laurentia's paleogeographic relations during the Precambrian-Cambrian transition are uncertain. Published paleomagnetic results from the Late Neoproterozoic (Ediacaran period) imply that Laurentia may have benn located at high southerly latitudes during 590-570 Ma, but conflict with other results from 615 Ma and from 565- 550 Ma which place Laurentia at low paleolatitudes. Paleomagnetic results from other continents over the same Ediacaran period are either nonexistent or equally ambiguous. To address this problem for Laurentia, we have obtained paleomagnetic and 40Ar-39Ar geochronologic results from the Baie des Moutons syenite, exposed in a failed Iapetan rift along the north shore of the Gulf of St. Lawrence in eastern Quebec. Three intrusive units of the syenite bear hornblende and biotite which provide overlapping 40Ar-39Ar plateau ages of 586-583 Ma and 590-578 Ma, respectively, confirming that the intrusion cooled rapidly upon emplacement. Of the 44 sites sampled in the Baie des Moutons syenite, eight yielded consistent characteristic remanent magnetization (ChRM) directions that were southeasterly and moderately steep (D=138, I=72.5; a95=11.5, k=24.2), retained by PSD magnetite. Four sites from late oxidized porphyry feldspar dykes gave shallow southerly directions (D=172, I=0; a95=12.6, k=53.8) that unblock by 450 degrees C. Notably for this collection, many sites fail to carry a detectable stable ancient remanence, often due to the presence of a viscous remanence along the present-day field direction carried by MD magnetite. AF cleaning and/or low-temperature demagnetization (LTD) methods were employed prior to thermal demagnetization, but LTD was found to "lock" in a portion of the VRM in specimens from some VRM-dominated sites, so these and other VRM-affected specimens have been excluded from the result. The sites bearing ChRM directions yield a preliminary paleopole at 23.9 N, 324 E (dp=18.1, dm=20.4) which is in between three 590-570 Ma paleopoles which place Laurentia at high paleolatitudes and other Laurentian paleopoles which put Laurentia at near-equatorial latitudes by Cambrian time.
Low-latitude Laurentia at 615 Ma: Paleomagnetism of the Long Range Dykes and Coeval Lighthouse Cove Formation, Northern Newfoundland and SE Labrador
Laurentia's apparent motion and paleogeographic relations during the Ediacaran Period (630-543 Ma) are uncertain and controversial. For early Ediacaran time Laurentia's only published paleomagnetic result, from the 615 Ma Long Range dykes of Labrador, is itself controversial being based on six dykes, three of which retain 'anomalous' directions. We have compiled existing published and unpublished data and greatly expanded upon the original work, sampling Long Range dykes and related flows of the Lighthouse Cove Formation in northern Newfoundland, Belle Isle, and southeastern Labrador to bring the combined total to over 40 sites. In the Long Range inlier of Newfoundland, 13 sites (including 11 newly sampled dykes) have a preliminary mean direction of D=141, I=39; a95=7.6, retained primarily by PSD magnetite and subordinately by hematite. For 12 sites in Labrador and 3 from Belle Isle, the preliminary mean direction is D=125, I=54; a95=6.6. Remaining sites had problems with remagnetization due to lightning strike or significant 'streaking' overlap of the characteristic remanence with a persistent steep viscous contribution along the present-day field direction. Some newly sampled dyke sites also retain 'anomalous' directions, in one case confirming the direction found in the same dyke by the previous study. In total, 28 sites provide a result that is consistent with the interpretation of the original study, that Laurentia lay at low paleolatitudes at 615 Ma. These new results also demonstrate, however, that the Newfoundland mean direction is 16 deg clockwise rotated with respect to the mainland Labrador mean direction, consistent with a 20 deg difference between the 020-trending dykes in Labrador and the 040-trending dykes in the Long Range inlier of Newfoundland. Comparison of the combined Long Range dykes result with that of the coeval Egersund dykes of Baltica shows that Baltica and Laurentia likely faced one another at low to mid paleolatitudes along their 'Caledonide' margins as the Iapetus Ocean began to open between them in the Late Neoproterozoic.
Paleomagnetic restudy of the late Ediacaran Sept Iles Mafic Suite
While many syntheses consider late Neoproterozoic paleomagnetic poles for Laurentia as either consistent with a high paleolatitude or else a low paleolatitude through Ediacaran time, it is also possible that both high- and low-paleolatitude results are robust, and Laurentia swung back-and-forthfive or six times in the celestial reference frame during Type II True Polar Wander (TPW). Such nonuniformitarian geodynamics is supported by apparent polar wander "swaths" for other major cratons in the late Neoproterozoic global paleomagnetic database. Intriguingly, overlapping of paleomagnetic "spinner" diagrams which assume that plate motions were dominated by multiple, fast TPW events essentially reconstructs the Gondwanaland supercontinent - completely independently of the seafloor magnetic anomaly record on which its paleogeography conventionally is based. Capture of any single TPW event "in motion" in the rock record has proven elusive. As a result, alternative explanations involving remarkable database bias or radically nonuniformitarian geomagnetic fields remain plausible. Previous paleomagnetic study of the ca. 564 Ma Sept Iles Mafic Suite (SIMS) of Laurentia's St. Lawrence rift arm has recovered both high- and low-paleolatitude magnetization components; if both components were proven robust, the SIMC may have crystallized and cooled during a mid-Ediacaran TPW event. Detailed rock magnetic screening of SIMS samples and hybrid demagnetization strategies indicate that the previous determination of the high-latitude magnetization component is likely compromised by recent viscous remagnetization of large single-domain and multi-domain magnetite grains. However, a dual-polarity, moderately high paleolatitude component is still retained by stable single domain crystals in a range of lithologies. Other sites also retain a dual-polarity, equatorial component, and some sites yield a magnetization which plots between these two end-members. It is not certain that the revised Sept Iles results, with site-mean magnetizations spread from ∼65° paleolatitude to the equator, must be interpreted as recording a TPW event. However, if a quick TPW event were recorded by a quickly-intruded, slow-cooling large igneous complex, it would presumably leave a paleomagnetic signature similar to that of Sept Iles complex. While the full thermochronology of SIMS is incompletely resolved, its mid-Ediacaran age is plausibly coincident with the otherwise difficult-to-explain, and also poorly-dated, "Shuram" disturbance to Earth's carbon cycle.