40Ar/39Ar Thermochronology and Metamorphism of Amphibole from the Humboldt Bay High Strain Zone, Wabigoon Subprovince, Lake Nipigon, Ontario

Abstract

The Humboldt Bay High Strain Zone (HBHSZ) is a shear zone in the Onaman-Tashota Belt, a granite-greenstone belt of the Wabigoon Subprovince of the Archean Superior Province. The HBHSZ represents a boundary within the Onaman-Tashota Belt, separating domains with different tectonic histories. In order to constrain the age of metamorphism and deformation on this boundary, an 40Ar/39Ar study utilizing step-heating techniques on 10 samples of hornblende from the HBHSZ is presented. Important relationships seen at both macroscopic and microscopic scales include (1) rocks of the shear zone have undergone polymetamorphism, (2) an earlier phase of actinolitic amphibole (greenschist facies) is replaced in areas by a later phase of hornblendic amphibole (amphibolite facies), (3) peak metamorphism on the HBHSZ is amphibolite facies, and accompanied peak deformation on the shear zone, (4) zones of high strain are inferred to correlate with rocks with a larger amount of hornblende, and (5) zones of lower strain are inferred to correlate with rocks with a larger amount of porphyroclastic actinolite, surrounded by significant amounts of hornblende. 40Ar/39Ar spectra from these porphyroclastic samples show disturbed spectra, with step ages ranging from 2598 - 2712 Ma. Samples showing the highest degree of strain and recrystallization yield flat spectra that plateau at an approximate age of 2667 Ma (within error). This age is taken to represent the age of the last major deformation along the HBHSZ. The data suggest that this late deformation along the HBHSZ is syn-plutonic with respect to the adjacent North Wind Pluton. The age of the North Wind Pluton is thus constrained with a lower limit of 2667 Ma. The relative timing, similar kinematics, and proximity of the HBHSZ to the Wabigoon - Quetico subprovince boundary raises the question of whether peak deformation on the shear zone is related to the subprovince collision, or to a previous event related to the accretion of the eastern Wabigoon. Further work may enable the correlation of the HBHSZ tectonic boundary, with boundaries in the western Wabigoon.

Keywords: Humboldt Bay High Strain Zone, Wabigoon Subprovince, 40Ar/39Ar, polymetamorphism, amphibole, Archean, deformation. Pages: 148 Supervisor: Nicholas Culshaw