Abstract A high-precision trace element study of ultramafic, mafic, and felsic rocks of the Kidd Creek Volcanic Complex reveals abundant compositional variety, which in turn indicates a previously unsuspected complexity of geodynamic processes. The stratigraphically lowest part of the Kidd Creek Volcanic Complex comprises mantle plume-related komatiites and komatiitic basalts. The komatiites exhibit light rare earth element (REE) depletion, near-chondritic Al 2 O 3 /TiO 2 ratios, and other features characteristic of Munro-type komatiites. A suite of low Ti tholeiitic basalt flows (TiO 2 typically <0.4 wt %; Al 2 O 3 = 11–22 wt %, MgO = 12–18 wt % in primitive examples), not previously documented in Archean terranes, is intercalated with the komatiites and rhyolitic sequences in the Kidd Creek Volcanic Complex. The low Ti tholei-ites are characterized by very high Al 2 O 3 /TiO 2 ratios (>50), high compatible element abundances (Ni = 300–500 ppm; Cr = 600–1,200 ppm), low incompatible element abundances, low Zr/Y ratios (1.3–2.0), and depleted light REE combined with fractionated heavy rare earth element (REE) ratios (La/Sm N = 0.4–0.7; Gd/Yb N = 0.4–0.5). The low Ti tholeiites and coexisting rhyolites are overlain by a zone of basalts with diverse trace element compositions, ranging from midocean ridge basalt (MORB)-like to arclike. Above the diverse zone, trace element compositions are more uniform over short stratigraphic intervals and two suites of arc-related basalts are identified. The lowermost suite consists of aphyric, primitive arc tholeiites with relatively high Ti contents (TiO 2 = 1.5–2.7 wt %), moderate light REE enrichment (La/Sm N = 1.2–1.5), unfrac-tionated heavy REE, Zr/Y ratios greater than primitive mantle (3.0–3.5), and small negative Nb anomalies (Nb/Nb* = 0.60–0.75). The upper portion of the Kidd Creek Volcanic Complex consists of weakly feldspar-phyric evolved arc tholeiites which are distinguished from the primitive arc basalts by greater LREE enrichment and Nb depletion (La/Sm N = 1.5–2.1; Nb/Nb* = 0.3–0.7) and systematic depletions of Zr and Hf (Zr/Zr* = 0.61; Hf/Hf* = 0.58). Petrographic and geochemical characteristics of the low Ti suite closely resemble post- Archean tholei-ites found in a restricted association with boninites and proto-arcs in extensional settings, involving high-temperature melting of depleted refractory mantle. Combined with the progressively more pronounced arc-type signatures in basalts upward in the Kidd Creek Volcanic Complex, the stratigraphic location of the low Ti tholeiites provides strong evidence that these Archean examples reflect 2.7 Ga arc initiation processes, similar to those now envisioned for Phanerozoic proto-arcs. Geochronological data and stratigraphic evidence indicate that plume activity did not cease with the onset of arc magmatism in the Kidd Creek Volcanic Complex. Instead, interaction of the plume with a trench system probably resulted in plate rearrangements and the local development of a new arc in the Kidd Creek Volcanic Complex. Accordingly, the Kidd Creek deposit is the product of an extensional sub-duction zone geodynamic setting.