40Ar/39Ar ages from the Botwood – Mount Peyton region, Newfoundland: possible paleomagnetic implications

Abstract

Hornblende and biotite from the freshest available sample of the gabbroic (early) phase of the Mount Peyton batholith have both yielded 40Ar/39Ar apparent age plateaus. An age of 420 ± 8 Ma is suggested for this phase. This means that there is a resolvable age difference between the mafic and the acidic rocks of the batholith (Rb/Sr age of the latter is 390 ± 15 Ma). Biotite and hornblende from a gabbroic inclusion and biotite from the latter's granitic matrix have all yielded very disturbed age spectra and variable total gas ages. Previously published paleomagnetic data suggest that these inclusions were remagnetized at the time of granite intrusion. The present radiometric data give some support to this interpretation, but do not clearly indicate the timing of any such event. A whole-rock sample from the (Lower Silurian?) Botwood Group rhyolite has also yielded a disturbed age spectrum; its total gas age is 385 ± 13 Ma. The Botwood pole is distinct from the granite pole, so either remagnetization did not occur in this case or it occurred at a later time, a time not clearly revealed in the present radiometric data.On the basis of previously published magnetic data and the now improved age control on the Mount Peyton complex, it appears that the 400 Ma paleopole for Newfoundland may differ substantially from other North American poles.