U-Pb tantalite, Re-Os molybdenite, and 40Ar/39Ar muscovite dating of the Brazil Lake pegmatite, Nova Scotia: a possible shear-zone related origin for an LCT-type pegmatite

Authors

  • Daniel J. Kontak Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, NS B3J 2T9
  • Robert A. Creaser Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3
  • Larry M. Heaman Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3
  • Douglas A. Archibald Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6

DOI:

https://doi.org/10.4138/655

Abstract

The Brazil Lake pegmatite of southwestern Nova Scotia is a rare example of Li-Cs-Ta (LCT) -type pegmatite in the Meguma terrane of Nova Scotia. This spodumene-rich pegmatite is hosted by metasedimentary and metavolcanic rocks of the Silurian White Rock Formation, which records both Acadian and Alleghanian deformation. In order to constrain the petrogenesis and thermal history of the pegmatite, samples of tantalite, molybdenite, and muscovite were collected for U-Pb, Re-Os, and 40Ar/39Ar dating, respectively. A single, large, euhedral tantalite crystal from an albite-quartz-spodumene assemblage yielded concordant U-Pb ages between 365 and 395 Ma (all ± 1– 2 Ma). In contrast, molybdenite from quartzite adjacent to the pegmatite gave a Re-Os age of 353 ± 2.5 Ma. Finally, a coarse muscovite grain from a quartz-feldspar-muscovite intergrowth surrounding a blocky K-feldspar megacryst was dated with the 40Ar/39Ar spot laser technique and yielded a maximum age of 347.6 Ma for a core analysis and minimum age of 318.4 Ma for a rim analysis. These data are interpreted to indicate that the time of pegmatite crystallization occurred at 395 Ma with the younger U-Pb tantalite ages (i.e., youngest ~ 365 Ma), the 353 Ma Re-Os molybdenite age, and the oldest 40Ar/39Ar age (i.e., 347 Ma) recording hydrothermal activity related to later magmatic events in this part of the Meguma terrane (e.g., 357 Ma Wedgeport Pluton). The muscovite ages record resetting and cooling related to the ca. 357 Ma magmatic event and later overprinting by Alleghanian deformation at ca. 320 Ma in this area. The older age for the pegmatite does not correlate with any known magmatic event in the area and therefore, its origin may have been related to Acadian metamorphism. RÉSUMÉ La pegmatite du lac Brazil dans le Sud-Ouest de la Nouvelle-Écosse constitue la seule pegmatite de type Li-Cs-Ta (LCT) reconnue dans le terrane de Meguma en Nouvelle-Écosse. Cette pegmatite riche en spodumène est incluse dans des roches métasédimentaires et métavolcaniques de la Formation silurienne de White Rock, marquée à la fois par les déformations acadienne et alléghanienne. Pour limiter la pétrogenèse et le passé thermique de la pegmatite, on a prélevé des échantillons de tantalite, de molybdénite et de muscovite en vue de datations U-Pb, Re-Os et 40Ar/39Ar, respectivement. Un simple gros cristal automorphe de tantalite provenant d'un assemblage d'albite-quartz-spodumène a présenté des âges U-Pb concordants se situant entre 365 et 395 Ma (tous ± 1-2 Ma). En revanche, la molybdénite du quartzite adjacent à la pegmatite a accusé un âge Re-Os de 353 ± 2,5 Ma. On a finalement daté un gros grain du muscovite provenant d'une intercroissance de quartz-feldspath-muscovite entourant un phénocristal en bloc de K-feldspath au moyen de la technique laser du spot de détection 40Ar/39Ar, et on a obtenu un âge maximal de 347,6 Ma d'une analyse de carotte et un âge minimal de 318,4 Ma d'une analyse de bordure. Ces données sont interprétées comme une indication que la cristallisation de la pegmatite est survenue vers 395 Ma; la datation U-Pb de la tantalite plus récente (la plus récente étant d'environ 365 Ma), la datation Re-Os de la molybdénite de 353 Ma et la datation 40Ar/39Ar la plus ancienne (c.-à-d. 347 Ma) témoignent d'une activité hydrothermale apparentée à des phénomènes magmatiques ultérieurs dans cette partie du terrane de Meguma (p. ex. pluton de 357 Ma de Wedgeport). Les datations de la muscovite signalent une remise en place et un refroidissement reliés au magmatisme d'environ 357 Ma et une surimpression ultérieure par la déformation alléghanienne dans ce secteur vers 320 Ma. L'âge plus ancien de la pegmatite ne correspond à aucun phénomène magmatique connu dans ce secteur; son origine pourrait par conséquent être apparentée au métamorphisme acadien.

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Published

2005-01-01

How to Cite

Kontak, D. J., Creaser, R. A., Heaman, L. M., & Archibald, D. A. (2005). U-Pb tantalite, Re-Os molybdenite, and 40Ar/39Ar muscovite dating of the Brazil Lake pegmatite, Nova Scotia: a possible shear-zone related origin for an LCT-type pegmatite. Atlantic Geoscience, 41(1). https://doi.org/10.4138/655

Issue

Vol. 41 No. 1 (2005)

Section

Articles

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