Chemical Stratigraphy and Petrogenesis of the Early Proterozoic Amisk Lake Volcanic Sequence, Flin Flon-Snow Lake Greenstone Belt, Canada

Abstract

The Amisk Lake area contains an 8·5 km thick sequence of subaerial INTRODUCTION and subaqueous flows and volcaniclastic rocks, including shallow Many ancient and modern volcanoes and volcanic sucwater turbidites. The lower stratigraphy is dominated by transitional cessions are complex, characterized by rapid lateral and and tholeiitic basalt and basaltic andesite flows and volcaniclastic vertical changes in facies and composition. Petrogenetic rocks. The transitional rocks are similar in many respects to modern studies of volcanoes and volcanic successions are thus boninites [concave-up rare earth element (REE) patterns, high mginfluenced by a large number of variables, and are number, Cr and Ni, and low Ti/V] and the parental magmas strongly dependent on three-dimensional stratigraphic were produced by partial melting of a refractory harzburgitic mantle, control. Most previous studies of ancient volcanic belts previously metasomatized by H2O±large ion lithophile element have relied on chemical discrimination diagrams to de(LILE)-rich fluids from subducting Early Proterozoic oceanic crust. duce the tectonic setting of the volcanic successions, with Mixing with an ascending tholeiitic partial melt or mid-ocean ridge little emphasis on petrogenetic processes or stratigraphic basalt (MORB)-source diapir produced the variable light REE relationships. For example, on the basis of previous (LREE) enrichment in these rocks. Tholeiitic and olivine-phyric geochemical studies, the Early Proterozoic Amisk Lake picritic rocks range from LREE-depleted to moderately LREEsuccession has been interpreted as either an island-arc enriched and are transitional from T-type MORB to E-type volcano (Fox, 1976; Watters & Armstrong, 1985; Watters, MORB, with mild addition of a slab-derived fluid. The lower 1991) or a backarc volcanic sequence (Gaskarth & part of the Amisk stratigraphy we interpret to have formed as a Parslow, 1987). Volcanic rocks of the Flin Flon area, large volcano or volcanic complex within a backarc basin that was 20 km to the east, have been interpreted to have been characterized by rapid volcanism and subsidence. The intercalation erupted in a combination of arc and backarc enof subaerial and subaqueous lavas indicates focused volcanism vironments (Syme, 1990), or in an oceanic island arc consistent with a tholeiitic seamount or ocean island. The upper (Stauffer et al., 1975; Thom et al., 1990). stratigraphy is more silicic and is dominated by calc-alkalic basaltic More recently, models of Early Proterozoic volcanism to andesitic volcanic rocks with trace element and LREE patterns in the Flin Flon Belt have been refined by regional similar to high-K calc-alkalic and shoshonitic rocks from modern and detailed mapping and lithogeochemical studies (e.g. island arcs, including strong negative Nb anomalies and LILE Heaman et al., 1993; Reilly, 1993; Watters et al., 1994; enrichment. These rocks represent the transition to island arc Stern et al., 1995a, 1995b; Lucas et al., 1996). These magmatism in the Amisk Lake sequence. studies have shown that the Flin Flon Belt represents a collage of different tectono-stratigraphic domains which were assembled during arc accretion events at around