Constraints from ⁴⁰Ar/³⁹Ar geochronology on the tectonothermal history of the Kapuskasing uplift in the Canadian Superior Province

Abstract

The Kapuskasing uplift (KU) in the Superior Province of the Canadian Shield has been interpreted as an oblique cross section through the Archean mid-crust. However, the time of juxtaposition of the granulites of the KU against the lower grade rocks of the Abitibi greenstone belt (AGB) along the Ivanhoe Lake fault zone is problematic. To constrain the postmetamorphic tectonothermal history of the KU, we have conducted 57 40Ar/39Ar step-heating analyses on mineral and rock samples collected in a transect across the southern KU and adjacent AGB. The age spectra record a complex thermal history. Amphiboles from the AGB in the footwall of the Ivanhoe Lake fault zone have ca. 2.66 Ga dates, similar to closure ages for amphiboles from farther east in the AGB. Amphibole dates of 2.46–2.52 Ga from the deepest structural levels of the KU place an upper limit on the time of major uplift of the granulites and their juxtaposition with the AGB. Biotite and muscovite dates from the transect cluster into three age groups. The presence in the deepest structural levels of the KU of biotite with 2.40–2.45 Ga dates indicates that significant uplift (15–20 km or more) of the granulites had occurred by this time. Micas with dates in the 2.25–2.30 Ga range are close to fault zones; these dates may indicate a ca. 2.30 Ga episode of fault reactivation. Feldspar, fault-related whole rocks, and some micas record events post 2.1 Ga. These correspond to the emplacement of mafic and lamprophyric dykes and fault reactivation.

Creation of a continent recorded in zircon zoning

“…Growth of lower crustal zircon at 2.51 Ga has also been detected in kimberlite xenoliths in the Abitibi subprovince and attributed to contact metamorphism of the lower crust due to subcrustal mafi c intraplating during dike swarm emplacement (Moser and Heaman, 1997). The minor growth of ~600 °C zircon tips thus refl ects the thermal perturbation of the deep crust caused by the igneous intraplate prior to fi nal ≥2.43 Ga cooling (Hanes et al, 1994). Stage IV rifting signals the onset of behavior of the Superior lithosphere as a conventional, rigid continental plate.…”

Magnetic properties of rocks of the Kapuskasing uplift (Ontario, Canada) and origin of long-wavelength magnetic anomalies

“…The primary period of thrust uplift predated intrusion of the 2454 Ma Matachewan dike swarm (West & Ernst 1991). This estimate is confirmed by 40 Ar/ 39 Ar amphibole ages of 2460–2520 Ma from the deepest levels of the KSZ, immediately adjacent to the ILCZ, contrasting with 2660 Ma ages in the adjacent footwall rocks of the Abitibi belt (Hanes et al 1994). However, uplift must have continued for at least another 400 Ma, through the time of intrusion of the 2040 Ma Kapuskasing dikes (Percival & West1994).…”

A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

“…The identified K-addition to feldspar most likely occurred during the protracted uplift of the KSZ. Proterozoic Ar-Ar and Rb-Sr mica and feldspar ages have been reported and were attributed to hydrothermal activity both syn-and post-uplift [24,26,34]. Furthermore, clouding of feldspar in dykes that intruded the KSZ is closely associated with a post-2.45 Ga magnetic reversal [35].…”

“…Zircon from samples collected from the same outcrops as the samples listed above have ages ranging from 3200-2640 Ma [25] and indicate that partial melts were extracted from at least some of the granulites. The KSZ experienced differential uplift of >20 km during a protracted history [26].…”