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

摘要

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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar 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~(40)Ar/~(39)Ar age (i.e., 347 Ma) recording hydrothermal activity related to later magma tic events in this part of the Meguma terrane (e.g., 357 Ma Wedge port Pluton). The muscovite ages record resetting and cooling related to the ca. 357 Ma magma tic 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 magma tic event in the area and therefore, its origin may have been related to Acadian metamorphism.