Formation of tonalite in island arcs by seawater-induced anatexis of mafic rocks; evidence from the Lyngen Magmatic Complex, North Norwegian Caledonides

The Lyngen Magmatic Complex (LMC) of North Norway, consists of a western suite of layered gabbros of normal-mid oceanic ridge basalt (N-MORB) affinity and an eastern suite of layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiitte) to boninitic affinity. The boundary between the suites is defined by a large-scale ductile shear zone, the Rypdalen shear zone (RSZ). In this shear zone anatectic tonalites were generated by partial melting of the gabbro in the presence of an H2O bearing fluid phase. from the tonalites contains early secondary and secondary liquid-dominated inclusions (88–99 wt.% H2O), with an average salinity of 18 wt.% (calculated as NaCleq). Combined gas and ion chromatography shows that the major ions in the fluid are Cl−, Ca2+, Na+ with smaller amounts of K+, Mg2+, Sr2+, Br− and NO3−. The dominant non-H2O volatile species is N2 (0.5–10%), and small amounts of CO2, CH4 and other hydrocarbons are also present. tion concentrations in the fluid are variable, due to element exchange during interaction of the fluids with the tonalites, amphibolites and metagabbros of the RSZ. The fluid contributed Na+ and K+ to the melt and gained Ca2+ in exchange, explaining the variable Na+/Ca2+ ratio of the fluid. The Br− and Cl− contents of the fluid inclusions plot on the same line as evaporating sea water, which strongly suggests a seawater origin for the fluid phase, and a seawater source fits well with other geochemical signatures and the tectonic setting of the LMC. suggested that seawater escaped from a subducting slab and was channelled along the Rypdalen shear zone. This caused anatexis of the gabbro, generating tonalitic melts at 0.5–0.9 GPa and 680–800°C.