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

Abstract

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 H 2O bearing fluid phase. Quartz from the tonalites contains early secondary and secondary liquid-dominated inclusions (88–99 wt.% H 2O), with an average salinity of 18 wt.% (calculated as NaCl eq). Combined gas and ion chromatography shows that the major ions in the fluid are Cl −, Ca 2+, Na + with smaller amounts of K +, Mg 2+, Sr 2+, Br − and NO 3 −. The dominant non-H 2O volatile species is N 2 (0.5–10%), and small amounts of CO 2, CH 4 and other hydrocarbons are also present. The cation 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 Ca 2+ in exchange, explaining the variable Na +/Ca 2+ 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. It is 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.