Partial melting of a refractory subducted slab in a Paleoproterozoic island arc: Implications for global chemical cycles

This study focuses on the rare occurrence of high-Fe, low-Si tholeiitic basaltic rocks of a Paleoproterozoic (1.9 Ga) backarc-arc succession (Amisk Lake, Flin Flon Greenstone Belt, Canada) and their implications for subarc mantle processes and the global elemental budget of Ta, Nb, and Ti. Volcanism at Amisk Lake is characterized by interbedded tholeiitic and boninite-like backarc basalts, overlain by intermediate to felsic calc-alkalic are volcanic rocks. The change from dominantly backarc to arc volcanism is marked by picritic flows overlain by the high-Fe tholeiites. The high-Fe tholeiites are characteristically high in Fe₂O₃(T) (15-21 wt%), low in SiO₂ (39-45 wt%), and slightly light rare earth element enriched ([La/Sm](n) = 1.3-1.93). They are unusually high in Ta, Nb, and Ti (normally depleted in arc-related rocks), and low in Zr, Hf, and Y, relative to normal-type mid-ocean ridge basalt. The petrogenesis of these rocks requires high degrees of partial melting of a dehydrated subducting slab, at low pressure. The high-Fe tholeiites represent either direct partial melts of the slab, or partial melts of the overlying mantle wedge, which has been metasomatized by this slab melt. These rocks provide evidence for the retention of Ta, Nb, and Ti in subducting slabs, and the subsequent isolation of these elements in the deep mantle, thus explaining the lack of complementarity of Ta, Nb, and Ti between the continental crust and depleted mantle. Conditions of higher heat flow and rapid subduction of young oceanic crust during the Paleoproterozoic could have resulted in partial melting of the subducted slab at shallow depths.