The interplay of crystal fractionation, magma recharge, assimilation and mixing in Archean volcanic systems revealed in the geochemical diversity of the Deloro assemblage, Abitibi Greenstone Belt

Abstract

Volcanic rocks from greenstone belts represent major records of Archean geologic history. One such belt, the Abitibi Greenstone Belt, which forms the eastern part of the Abitibi-Wawa Subprovince (one of several terranes in the Superior Province, Canada) is notable for its relatively high proportion of supracrustal rocks and rich mineral endowment. Greenstone belt volcanic rocks do, however, present a challenge for geochemical modelling due to the variable extent to which their primary signatures can be overprinted by hydrothermal alteration and metamorphism. Nevertheless, the use of immobile elements for data exploration, and focussing on the least-altered samples for petrogenetic modelling, can overcome these challenges. Furthermore, modern modelling tools, like the Magma Chamber Simulator, offer unprecedented opportunities to constrain the origin and evolution of volcanic rocks, such as the Deloro Assemblage in the Abitibi Greenstone Belt, and thereby gain insights into Archean petrogenesis. The 2734–2724 Ma Deloro Assemblage is widespread, well sampled, geochemically diverse, and of economic interest due to a rich endowment of volcanogenic massive sulphide deposits. A large geochemical compilation and petrogenetic modelling using the Magma Chamber Simulator demonstrates that the major element diversity can be explained through a combination of processes, using a tholeiitic basaltic parental magma at approximately 3 kbar and a wallrock of average SE Superior Province TTG as a contaminant. This modelling highlights the roles not just of fractional crystallisation and assimilation, but also the previously unrecognised role of magma recharge in contributing to the geochemical diversity of the Abitibi volcanic systems. Additionally, the modelling provides constraints on what ranges of oxygen fugacities and parental magma water contents are suitable. The trace element evolutions of these models clarify the different petrogenetic pathways that give rise to the Deloro Assemblage rocks. We show that the volcanic rocks can be organized into nine geochemical groups, mostly based on a Th/Yb versus Yb/Ti diagram. Furthermore, trace element modelling illustrates the utility of this diagram for differentiating (a) between open- versus closed-system behaviour, and (b) between different types of open-system processes. The most primitive group, Tholeiitic basalts, has trace element patterns that suggest melting of primitive mantle at a range of depths. The Tholeiitic basalts, Transitional intermediate group, Tholeiitic dacites and Tholeiitic rhyolites record progressive evolution via fractional crystallisation and magma recharge. In contrast, coupling assimilation to fractional crystallisation accounts for the progression from Tholeiitic basalts to High-Th basalts and further onwards to the Calc-alkaline andesites, Calc-alkaline dacites and Calc-alkaline rhyolites. The high Th/Yb, low Yb/Ti character of the HREE-depleted intermediate group cannot be explained through AFC; rather these samples are part of a simple mixing trend between the tholeiitic basaltic parent magma and the TTG composition. In light of the geochemical groups and their petrogenesis, a general association between VMS deposits and extensive fractionation (particularly Tholeiitic rhyolites) can be observed. In contrast, the HREE-depleted intermediate group is infertile.