Age constraints and characteristics of subaqueous volcanic construction, the Archean Hunter Mine Group, Abitibi greenstone belt

Abstract

The Archean Hunter Mine Group (HMG) is a 6–7 km-thick south-facing volcanic sequence at the southern limit of the Northern Volcanic Zone, Abitibi greenstone belt. The HMG is composed of (1) felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits; (2) pyroclastic fire-fountain products and high- to low-concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron-formation; (3) mafic pillowed and brecciated flows; (4) a prominent felsic dyke swarm; and (5) abundant mafic intrusions. The stratigraphy is complex, as inferred by detailed volcano-sedimentary facies mapping. A 5–7 km-wide, N-trending felsic dyke swarm, coupled with abundant synvolcanic and synsedimentary faults, chaotic breccias and mineralization suggest a central volcanic depression, which is consistent with a caldera structure. Voluminous effusive activity and explosive fire-fountaining eruptions contributed to caldera development. Rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills. The lithofacies and overlying komatiites flows are consistent with a deep-water marine setting. U-Pb age determinations help constrain the HMG caldera formation to ≈6 million years. The sampled quartz–feldspar–phyric dykes yielded U-Pb zircon ages of 2728.3+4/−3.4 and 2728.9±0.8 Ma and together with a previously published dyke age of 2729.6±1.4 Ma from the same dyke swarm, a case is made that the swarm evolved over 1 million years (2728.6–2729.6 Ma). A lobate flow and flow breccia revealed similar ages of 2724.6+4.6/−1.7 and 2727.6+4.2/−2.0 Ma, whereas the up to 1 km-thick, gabbro-diorite Roquemaure sill precedes both felsic dykes and flows, as indicated by the 2731.8+2.2/−2.0 Ma age. The 2706.4+5.7/−4.3 Ma feldspar–quartz–phyric dykes occurred late in the history of the HMG and were not associated with subaqueous volcanic construction. The 207Pb/ 206Pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the HMG complex. Collectively, these age determinations and inheritance, as well as stratigraphy, support a tripartite division into a 2734–2730 Ma lower formational stage, a 2732 Ma middle formational stage and a 2730–2728 Ma upper formational stage. The study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice.