Trace element and Sm–Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake Greenstone belt, Superior Province: evidence for Archean plume–arc interaction

Abstract

The metavolcanic Lumby Lake belt comprises mafic tholeiites intercalated with thin felsic pyroclastic units. Al-undepleted komatiites are present towards the top of the stratigraphy. Identification of Al-depleted pyroclastic komatiites associated with chemical and siliciclastic sedimentary rocks indicates the upper portion of the 3 Ga Lumby Lake stratigraphy is directly comparable to a stratigraphic sequence developed on a paleoregolith in the nearby Steep Rock greenstone belt. The lower portion of the Lumby Lake sequence therefore represents a rarely preserved association of komatiite–tholeiite and calc alkaline volcanism developed prior to rifting episodes identified in ∼3 Ga terranes of the northern Superior Province. Al-undepleted komatiites are characterised by elevated MgO (18–24 wt.%) and Ni (600–1500 ppm) contents, in conjunction with variable LREE depletion (La/Sm n =0.5–0.8). Intercalated spinifex textured komatiitic basalts possess lower MgO (10–11 wt.%) and Ni (150–180 ppm) and flat to weakly enriched LREE (La/Sm n =0.9–1.1). Pyroclastic Al-depleted komatiites (Al 2O 3/TiO 2=4–5) are strongly LREE enriched (La/Yb n =6.7–10.5) with variable HFSE anomalies. Compositionally uniform tholeiites with variably depleted to enriched LREE (La/Sm n =0.8–1.2) and minor HFSE anomalies dominate the stratigraphy of the belt. A distinct subset of tholeiites, occurring towards the centre of the belt, is characterised by low Al 2O 3/TiO 2 ratios, LREE enrichment and the absence of HFSE anomalies. Minor intermediate (SiO 2=53–64 wt.%) volcanic rocks with pronounced REE fractionation (La/Yb n =0.8–1.1), high Al 2O 3/TiO 2 and Zr/Y ratios also occur throughout the belt. Two distinct subtypes of felsic pyroclastic rocks are recognised intercalated sporadically throughout the stratigraphy. Both types display pronounced LREE enrichment (La/Sm n =3.9–6.1) but Type 1 has strongly fractionated HREE patterns (Gd/Yb n =1.5–4.6) whereas, Type 2 HREE patterns are generally flat (Gd/Yb n =1.7–1.9). Intrusive counterparts to both subtypes have been identified within the Marmion Lake batholith to the south of the greenstone belt. The felsic subtypes may be derived from similar parental magmas by variable degrees of hornblende involvement during fractionation processes. Sm/Nd isotope systematics from a range of rock compositions have yielded ε Nd values of +2 to +5, typical of the range for Archean volcanic and intrusive rocks. Komatiite–tholeiite associations within Archean terranes are interpreted as the result of plume related magmatism likely in a geodynamic setting comparable to Phanerozoic oceanic plateaux. In contrast intermediate and felsic rocks are typical of calc alkaline suites generally attributed to Archean subduction related environments. The coeval eruption of the two magma suites in the Lumby Lake belt is best interpreted as the result of the sporadic subduction of plume-modified oceanic spreading centres over 10s of m.y. and the eventual impingement of a mantle plume upon an active subduction zone. This process can also account for the young age of basement material relative to overlying rift sequences in 3 Ga terranes of the Superior Province.