Mafic to felsic magmatism and crustal recycling in the Obonga Lake greenstone belt, western Superior Province: evidence from geochemistry, Nd isotopes and U–Pb geochronology

Abstract

The central Wabigoon Subprovince of the western Superior Province contains Mesoarchean granitoid and supracrustal rocks (3.01–2.83 Ga) and 2.78–2.69 Ga granitoid plutons and supracrustal sequences. It is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement. The Obonga Lake greenstone belt contains two distinct assemblages: (1) a <2.724 Ga northern assemblage consisting of sedimentary and mafic volcanic rocks; and (2) a ca. 2.73 Ga southern assemblage dominated by gabbroic, mafic and felsic volcanic rocks. Mafic to ultramafic plutonic rocks separating the northern and southern assemblages have been dated at 2733±7 Ma, similar in age to the volcanic rocks to the south. A 2690±1 Ma gabbro–anorthosite pluton intrudes part of the northern assemblage. At the southern margin of the belt a granitoid phase has been dated at 2930 Ma. This may represent an inherited component, but suggests the presence of Mesoarchean basement to at least part of the greenstone belt. The northern assemblage mafic rocks are enriched in light REE and HFSE and depleted in heavy REE. Despite being HFSE-enriched, mantle normalised abundances show trends of La>Ta>Nb>Th. They have ε Nd values of +0.7 to +2.4. A dacitic unit in the northern assemblage has high La/Yb, high Sr/Y, low Nb, Y and heavy REE and is interpreted to represent a mantle-modified slab melt similar to adakites in Cenozoic arcs. The associated enriched basalts may represent melts from the mantle wedge modified by slab melt (adakitic) metasomatism, or they may represent an enriched (OIB-like) asthenospheric source. Rocks of the southern assemblage mostly show Th and light REE enrichment and negative Nb and Ta anomalies. Low ε Nd values (down to −0.9) can be modelled through contamination of a mafic liquid by 3.3–3.2 Ga sialic crust, although rocks of that age have not been observed. Suggested Mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years. The southern assemblage volcanism may have occurred in a continental arc to continental back-arc system where crustal recycling played an important role.