Abstracts
Abstract
As part of a major study of the Avalon zone in eastern Newfoundland, the paleomagnetism of the Finn Hill ignlmbrite sequence located in the Colliers-Harbour Main area is described. Detailed alternating field and thermal experiments performed on this Hadrynian ash-flow tuff sequence indicate that it is characterized by two significantly different mean directions of magnetization; I) A(S-l28, I=+25, 095 =7.1°, K-L7I, N-4 sites) in-situ and (D-225, D + 59, a95=l3°, K=48) after tilt correction, 2) B (D-69, I=+52, N= 2 sites) in-sltu and (D=303, I=+40) after tilt correction.
The A component is believed to be secondary and the paleopole corresponding to the in-situ direction of magnetization is I4°S, l°W. The B tilt corrected component (D-303, I=+40) is interpreted as a pre- or syn-folding direction of magnetization corresponding with the time of formation of the ignimbrite sequence or slightly later; its corresponding paleopole position Is 39°S, 29°E. It is worth noting that the paleopole (14°S, l°W) of the secondary component found in the present study is not significantly different from the paleopole (5°S, 8°W) obtained in the Cloud Mountain basalt overlying Grenville basement of northwestern Newfoundland, the age of which is close to latest Precambrian (=620 Ma), this suggests that in tate Precambrian time, the eastern and western Newfoundland blocks were not very distant from each other or else they were far apart but at the same paleolatitude. The secondary component is Interpreted as representing the magnetic imprint of the Avalonian (Cadomlan) orogenic event. This last interpretation is reasonable since nearby Cambrian units (to the west) are located unconformably above the Precambrian units and are almost flat lying. Consequently, the chances of this secondary component being of Acadian age are relatively small. The new data set of this reasearch fills a definite gap in the paleomagnetic record of the Avalon zone.
RESUME
La description du paleomagnetlsme de la sequence d'ignimbrite de Finn Hill, situee dans la region de Colliers - Harbour Main, s'lnsere dans le cadre d'une etude d'envergure de la zone Avalon de Test de Terre-Neuve. Cette sequence hadrynienne de tuf volcanique a ete soumise a des experiences poussees de champ alternatif et d'anlyse thermique. Les resultats indiquent que la sequence est caracterisee par deux sens moyens de I'aimantatlon qui different de facon significative: I) A(S=128, I-+25, ags =7.1", K=171, N-4 sites) in-situ et (D>225, D+59, 0^5-13', K«48) apres correction de 1'incllnaison, 2) B (D-69, I-+52, N»2 sites) In-situ et (D=303, I=+40) apres correction de l'lnclinaison.
La composante A serait secondaire et le paleopOle correspondent au sens in-situ de I'aimantation est de I4°S, l°0. On interprete la composante d'inclinaison B corrigee (D=303, I=+40) com me le resultat d'une aimantation qui a precede ou qui etait con tern poraine au plissement, et correspondent done au temps d'emplacement de la sequence d'ignimbrite, ou survenant peu apres; la position du palepOle correspondent est de 39°S, 29°E. 11 est interessant de noter que le palepOle (14°S, 1°0) de la composante secondaire, releve dans la presente etude, ne differe pas de facon significative du palepOle {5°S, 8°0) obtenu a partir du basalte de Cloud Mountain qui recouvre le socle grenvillien dans le nord-ouest de Terre-Neuve et dont l'Sge est proche du Precambrien tardif (620 ^Ma). Ceci suggere que tard durant le Precambrien, les blocs est et ouest de Terre-Neuve n'etaient pas tres distants, ou bien qu'ils etaient tres eloigners l'un de 1'autre mais situes a la mfime paleolatitude. On interprete la composante secondaire comme une emprelnte magnetique representant l'orogenese avalonienne (Cadomien). Cette derniere interpretation est raisonnable puisque les unites cambriennes voisines (a 1'ouest) reposent subhorizontalement et en discordance sur les unites precambriennes. Consequemment, il est peu probable que cette composante secondaire soit d'Sge Acadien. Ces nouvelles donnees comblent une lacune importante dans I'histoire palfio-magnetlque de la zone Avalon.
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