Magnetic overprinting in the Thanet gabbro complex, Ontario

Buchan, Kenneth L.

doi:10.1139/e78-148

Cited by 29 publications

(11 citation statements)

References 24 publications

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“…HB A ), 30^35 ‡ south of the equator, to still younger B poles from CO, HB and the Thanet Gabbro (TH) of the Elsevir Terrane [10], which lie 20^25 ‡ north of the equator. The A to B link is necessitated by the observation that B is a thermal overprint of A, on the basis of adjacent but non-overlapping A and B blocking temperature ranges, a de¢ning property of partial TRMs (pTRMs).…”

Section: Geology and Paleomagnetic Backgroundmentioning

confidence: 99%

Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

Dunlop

2002

Earth and Planetary Science Letters

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We report geomagnetic field intensities for the A (V1000 Ma) and B (V850 Ma) natural remanent magnetizations of the Cordova Gabbro of southern Ontario. We interpret A and B to be secondary thermal overprints produced during regional uplift and cooling following the Grenvillian Orogeny. Multivectorial A+B paleointensity data were analyzed using pairs of Arai diagrams for the separated A and B vectors. From 18 reliable A results and 15 reliable B results, we find paleofield values of 18.1 þ 2.1 WT and 7.1 þ 1.5 WT, respectively. The corresponding virtual axial dipole moments (VADMs) are (3.12 þ 0.36) and (1.82 þ 0.38)U10 22 A m 2 , about 70% and 40% of the average Phanerozoic dipole moment based on data from 0.3^300 Ma. The Cordova data and other Precambrian paleointensities are generally within a range of approximately 0.5^1.5 times the Phanerozoic mean VADM. Three sets of paleofield values as a function of paleolatitude, including the Cordova data, support a dipole configuration for the Earth's field in 251 50 Ma time windows around 2700 Ma, 1100 Ma, and 850 Ma. ß

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Section: Geology and Paleomagnetic Backgroundmentioning

confidence: 99%

Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

Dunlop

2002

Earth and Planetary Science Letters

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“…Berger, York & Dunlop (1979) estimate the ages of HA and HC at 980 f 10 and G 820Ma respectively by incremental 40Ar: 39Ar heating experiments. In the Thanet Gabbro (Buchan 1978) components ThA and ThB yield poles corresponding to HA and HB respectively although the blocking temperature spectra here suggest that the sequence is ThB +ThA; to conform to the simple loop path of Fig. 5 these latter data must either apply to the return part of the loop or ThB must have a later (CRM) origin.…”

Section: Fennoscandianmentioning

confidence: 57%

Proterozoic palaeomagne tism and single continent plate tectonics

Piper

1983

Geophysical Journal International

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Summary. Proterozoic (2700–570 Ma) palaeomagnetic poles from the major shields conform to a single apparent polar wander path on a contiguous reconstruction which brings into alignment, both with each other and with the long axis of the primary continental crust, tectonic and magmatic features ranging from late Archaean (≤ 2900 Ma) to late Proterozoic (1100 Ma) age. This discovery implies that the continental lithosphere behaved on a megatectonic scale as a single coherent plate during this era, subject to internal deformation but not breaking up in a brittle fashion until late Precambrian times. Peripheral parts of the Supercontinent were redistributed over a short interval at ca. 1100 Ma but the continental crust was not dismembered and widely dispersed until 570–530 Ma. This latter episode correlates with the Proterozoic‐Phanerozoic transition and defines the onset of multi‐continent Plate Tectonics. The Proterozoic apw record comprises nine large loops with periods ranging from 270 to 110Ma and averaging 200Ma. Between 2150 and 1000 Ma the loops had a similar signature and their resultant effect was to return the continental lithosphere repeatedly to a stable position with its centre at the geographic pole, probably by the Goldreich‐Toomre mechanism. This periodicity correlates with theoretical and experimental studies of mantle convection predicting avalanche‐type behaviour due to build up of instability in the surface boundary layer with cycles of 50–200 Ma. The consolidation of the continental lithosphere defines several phases in the history of mantle convection: (1) between 3500 and 2800 Ma the sialic scum resulting from differentiation of a thickened and subducting lithosphere was accumulated into the primary body by a large scale mantle convection system; (2) this operated simultaneously with a subcontinental‐lithosphere small‐scale (50–100 km) convection system showing a transition from bimodal to cylindrical roll mode represented by the later greenstone belts (2900–2200 Ma) and straight belts (ca. 2800–2400 Ma); (3) a transition to larger‐scale episodic subcontinental convection is represented by the Proterozoic mobile belts (2200–1100 Ma); (4) a modification of the major suboceanic lithosphere convection is suggested by reorganization of the peripheral shields at 1100 Ma, and (5) a subdivision of this major system resulted in continental dispersal during Lower Cambrian times.

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“…They "see through the 1000 Ma Grenville event. The only other study that may indicate an older pole was conducted on the Thanet gabbro of the Hastings Lowlands of southeastern Ontario (Buchan 1978). The three poles (Th(AI), Th(A2), Th(B)) derived from that study are plotted in Fig.…”

Section: Park An1mentioning

confidence: 99%

“…Two areas, the Hastings Lowlands of southeastern Ontario, and the northeastern part of the Grenville Province in Labrador, have significantly lower grades of metamorphism, and thus provide better opportunities for recovering primary magnetizations (Dunlop et al 1980). In the Hastings Lowlands, paleomagnetic studies of the Thanet gabbros have offered equivocal evidence of the presence of primary magnetization (Buchan 1978;Dunlop et al 1980). Previous studies in Labrador on units along the Grenville Front (Fahrig and Larochelle 1972;Fahrig et al 1974) have also yielded somewhat equivocal results, possibly due to inadequate analysis.…”

Section: Introductionmentioning

confidence: 99%

Paleomagnetic history of the Mealy dykes of Labrador, Canada

Park¹,

Emslie²

1983

Can. J. Earth Sci.

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Paleomagnetic analysis of the Mealy diabase dykes of Labrador reveals magnetizations that predate the Grenville event at about 1000 Ma. These dykes intrude the Mealy Mountains anorthositic complex in the Grenville Structural Province. They are well south of the Grenville Front Tectonic Zone, but were apparently never subjected to temperatures as high as 500°C during their post-consolidation history.Four distinct magnetic components were uncovered by thermal and alternating field treatments and a fifth remained unresolved. The major magnetic mineral present, titanomagnetite, is thought to record two magnetic directions acquired during cooling from magmatic temperatures. These are B ( D = 305", 1 = -76"; N = 18 sites; K = 12; ag5 = 1 lo) and A ( D = 095", 1 = $52'; N = 20 sites; K = 46; (u95 = 5"). Component B has much within-site dispersion. The other two components, C ( D = 274", I = -47"; N = 10 sites; K = 15; = 13') and D ( D = 292', 1 = -74"; K = 5; ag5 = 31°), probably reside in magnetite and pyrrhotite, respectively. Component C , antiparallel to A, was probably acquired at about the same time as A. We suggest that C and A represent the first stable magnetizations retained by the dykes following an extensive period of cooling and re-equilibration of the magnetic minerals. Components B and D , which agree in direction, represent a later stage of cooling.Component B has a pole at 14g0E, 34"s (Sp = 18", Sm = 19") in agreement with regional metamorphic poles from the Grenville; A, however, has a pole at 173OW, 23% (Sp = 5", Sm = 7"), which apparently "sees through" the peak in Grenville activity. The A site poles have a linear distribution along the Keweenawan Track and probably relate to an age between 1000 and 1150 Ma.L'ktude pal6omagnCtique des dykes de diabase de Mealy au Labrador indiquent des aimantations qui antidatent l'kvtnement du Grenville B environ 1000 Ma. Ces dykes @nttrent dans le complexe anorthositique des montagnes Mealy de la province structurale du Grenville. 11s sont localisks bien au sud de la zone tectonique du front du Grenville, mais selon toute apparence ils n'ont pas kt6 soumis B des temphatures pouvant dkpasser 500°C depuis leur consolidation.Quatre composantes magnktiques distinctes sont dkduites des traitements thermiques et de champ altematif et une cinquitme demeure non-rksolue. Le principal mineral magnktique present, la titanomagnktite, semble porter deux directions magnktiques acquises lors du refroidissement magmatique. Elles sont B ( D = 305", I = -76"; N = 18 sites: K = 12; a95 = 1 lo) et A ( D = 095", I = +52"; N = 20 sites; K = 46; ag5 = 5"). La composante B est bien A l'intkrieur des sites de dispersion. Les deux autres composantes C ( D = 274", 1 = -47"; N = 10 sites; K = 15; ass = 13") et D ( D = 292', 1 = -74"; K = 5; a95 = 31") sont probablement portkes respectivement par la magnktite et la pyrrhotite. La composante C , antiparalble B A, fut probablement acquise plus ou moins simultankment A A. Nous suggkrons que C et A repdsentent les premieres aimantations stables r...

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Magnetic overprinting in the Thanet gabbro complex, Ontario

Cited by 29 publications

References 24 publications

Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

Proterozoic palaeomagne tism and single continent plate tectonics

Paleomagnetic history of the Mealy dykes of Labrador, Canada

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