Evolution and tectonic significance of intermediate to felsic plutonism associated with the Hemlo greenstone belt, Superior Province, Canada

Abstract

Plutonic rocks from the Hemlo area in the southern Superior Province span a period from ca. 2720 to 2677 Ma. Early TTG plutons intruded into lower basaltic volcanic sequences and were probably derived from melting of basaltic crust at upper mantle depths. They predate major regional deformation and span ages from 2719 to 2697 Ma. Felsic to intermediate volcanism (2697–2688 Ma) partially overlaps TTG plutonism but is mostly slightly younger and lacking plutonic equivalents. Regional deformation and metamorphism of the greenstone belt and TTG plutons progressed rapidly following deposition of the youngest supracrustal rocks. Syn- to late-tectonic plutons emplaced between 2684 and 2677 Ma have a distinctive dual primitive-evolved character that is interpreted to reflect their derivation by direct melting of long-term depleted mantle that was metasomatically enriched in a wide range of LILE and HFSE <50 Ma prior to melting. The earliest of these plutons contain abundant zircon inheritance indicating crustal interaction whereas the latest plutons show no significant zircon inheritance. This secular evolutionary change in the nature of intermediate to felsic plutonism at Hemlo and its relationship to regional deformation and metamorphism parallels that occurring more regionally within the Superior Province and is interpreted to be the consequence of a transition in the nature of large-scale tectonic processes. Intermediate to felsic magmatism within a protracted period of volcanism and TTG plutonism originated primarily by melting of subducted basaltic crust, probably in an oceanic arc-related environment. The termination of this style of magmatic activity and initiation of regional deformation and metamorphism at ca. 2688 Ma is interpreted to be a consequence of collisional orogeny. The petrogenetic characteristics of the latest period of magmatism at 2684–2677 Ma requires that the melting regime migrate into sub-arc lithospheric mantle that was metasomatized during the earlier magmatic stage. Slab breakoff following terrane accretion is suggested as one possible mechanism for the abrupt termination of TTG magmatism and ensuing rapid and progressive introduction of large amounts of heat into sub-arc mantle and basal portions of the crust. This pattern of development is seen in many Superior Province greenstone belts but is older in the north. Therefore, it is probably an inevitable consequence of the tectonic processes that led to accretion of the Superior Province.