Geochemical and U–Pb zircon geochronological constraints on the development of a Late Archean greenstone belt at Birch Lake, Superior Province, Canada

Abstract

This paper presents new high-precision U–Pb zircon ages that place constraints on the timing of magmatic activity (volcanic and plutonic), deformation and gold mineralization within the Late Archean Birch–Confederation Lakes greenstone belt within the Uchi subprovince of the Superior Province. Geochemical data, including Sm–Nd isotopic analyses place additional constraints on the nature of the magmatic activity. Three volcanic rocks from the Birch Lake area have similar ages ( 2742.2 +1.8 −1.7 , 2739.1±2.8 and 2741.7 +2.3 −2.2 Ma) and together with previously published data suggest that a major proportion of the volcanic rocks of the greenstone belt were erupted in a narrow time interval from 2742 to 2735 Ma. Older volcanic rocks recognized to the south of the Birch Lake area are either absent or of limited extent. Plutonic rocks include the pre- to syn-tectonic Horseshoe Island stock (2725.0±2.5 Ma), the syn-tectonic Mainprize Lake batholith ( 2724.3 +1.8 −1.5 Ma) and the late- to post-tectonic Shabumeni Lake stock ( 2710.8 +1.9 −1.3 Ma) and Okanse Lake pluton ( 2700.8 +1.9 −1.4 Ma). These ages and structural relationships indicate that the development of a (D 2) regional penetrative fabric, at least in part, post-dates the emplacement of the Horseshoe Island stock. This is consistent with the interpretation that D 2 deformation developed synchronously with, and perhaps in response to, the emplacement of the Mainprize Lake batholith. An earlier (D 1) penetrative deformational event as well as possible large scale subhorizontal structures (D 0) developed subsequent to the deposition of the youngest volcanic rocks and prior to D 2. Plutons are younger than and unrelated to volcanism. In contrast to volcanic rocks, plutons show no indication of having interacted with older felsic crustal rocks and are interpreted to be derived from partial melting of metasomatized mantle and subducted tholeiitic basalt. The Okanse Lake pluton has affinities with the sanukitoid suite and is closely comparable in age to similar plutons over a wide area. It is proposed that a 2700 Ma sanukitoid magmatic event may reflect melting of previously metasomatized mantle in response to thermal re-equilibration associated with the transition from an earlier subduction dominated regime to a later, post-collisional stage culminating in the stabilization of the northern Superior Province.