Roberts Lake Syncline mafic lavas (NE Superior craton): A proposed extension of the Cape Smith belt

Abstract

The Superior craton is bordered on its northern and eastern sides by the approximately coeval Cape Smith fold belt and the Labrador Trough, two major Paleoproterozoic volcano-sedimentary belts that record broadly similar tectonic evolution but which show some distinctive characteristics and have notably different mineral resource endowments. The Roberts Lake Syncline, situated directly between the Cape Smith and Labrador Trough, hosts two Paleoproterozoic mafic volcanic sequences and has uncertain tectonic affinities to its neighboring fold belts. It was previously linked to Labrador Trough magmatism but re-interpretation of the regional geology suggests a closer resemblance to the successions of the Cape Smith belt. We present new lithogeochemistry that shows that the Roberts Lake Syncline comprises two distinct magmatic events rather than the single event previously interpreted. The lower and more voluminous suite of the Roberts Lake Syncline lavas comprises multiply-saturated basalts characterized by normalized trace element patterns similar to E-type mid-ocean ridge basalts (E-MORB). The upper suite of the Roberts Lake Syncline stratigraphy is composed of volcanic rocks with depleted, N-MORB-like trace element signatures rocks spanning a range from komatiites and picrites with near-primary compositions to high-Mg basalts with flat normalized trace element patterns. Both suites were apparently undersaturated with sulfide liquid in their mantle source. The E-MORB magmas were affected by a sulphide-saturation event, leading to widespread and variable degrees of depletion of chalcophile elements, despite showing no significant crustal contamination. The N-MORB magmas also underwent saturation with sulfide melt, generating minor occurrences of magmatic sulfide mineralization hosted by ultramafic intrusions within the older volcanic suite and small but significant depletions in chalcophile elements in the erupted picritic basalts. The geochemistry of the lower E-MORB suite is similar to that of the 2170 Ma Biscotasing dike swarm that occurs in and near the Labrador Trough and the 1998 Ma Minto-Povungnituk large igneous province (LIP) of the Cape Smith belt, whereas the upper N-MORB-like suite in the Roberts Lake Syncline has compositions resembling those of the 1880 Ma Circum-Superior LIP. Our detailed geochemical comparison suggests a close resemblance to the chemical stratigraphy observed within the Cape Smith belt, leading us to consider the Roberts Lake Syncline as the southeasternmost portion of the Ungava Orogen rather than the northernmost part of the Labrador Trough. These insights contribute to a revised LIP and plume centre map for this NE portion of the Superior craton and help to constrain the large-scale tectonic architecture of the margins of the Superior Province.