Chemical stratigraphy and petrogenesis of the early Proterozoic Amisk Lake volcanic sequence, Flin Flon-Snow Lake greenstone belt, Canada

The Amisk Lake area contains an 8-5 km thick sequence of subaerial and subaqueous flows and volcaniclastic rocks, including shallow water turbidites. The lower stratigraphy is dominated by transitional and tholeiitic basalt and basaltic andesite flows and volcaniclastic rocks. The transitional rocks are similar in many respects to modern boninites [concave-up rare earth element (REE) patterns, high mg-number, Cr and Ni, and low Ti/V] and the parental magmas were produced by partial melting of a refractory harzburgitic mantle, previously metasomatized by H₂O±large ion lithophile element (LILE)-rich fluids from subducting Early Proterozoic oceanic crust. Mixing with an ascending tholeiitic partial melt or mid-ocean ridge basalt (MORB)-Source diapir produced the variable light REE (LREE) enrichment in these rocks. Tholeiitic and olivine-phyric picritic rocks range from LREE-depleted to moderately LREE-enriched and are transitional from T-type MORB to E-type MORB, with mild addition of a slab-derived fluid. The lower part of the Amisk stratigraphy we interpret to have formed as a large volcano or volcanic complex within a backarc basin that was characterized by rapid volcanism and subsidence. The intercalation of subaerial and subaqueous lawas indicates focused volcanism consistent with a tholeiitic seamount or ocean island. The upper stratigraphy is more silicic and is dominated by calc-alkalic basaltic to andesitic volcanic rocks with trace element and LREE patterns similar to high-K calc-alkalic and shoshonitic rocks from modern island arcs, including strong negative Nb anomalies and LILE enrichment. These rocks represent the transition to island arc magmatism in the Amisk Lake sequence.