Origin and Evolution of the Fatu Kapa Magmatic System (North-Western Lau Back-arc Basin): Insight on the Genesis of High-Silica Lavas

Abstract The South Eastern Futuna Volcanic Zone (SEFVZ; 50 km × 80 km) is located in the NW of Lau back-arc basin at northern boundary and YAO microplates. Fatu Kapa an area (∼15 10 diffuse volcanic activity center SEFVZ intersection two axes with fracture zone. At this place, volcanic, tectonic, hydrothermal activities are enhanced. most differentiated series whole region found area. Based on major trace element data, four groups rocks distinguished main form a well-defined transitional suite from basalts to andesitic dacitic lavas that display enrichments Zr, Cl heavy rare earth elements. mafic group extracted Indian-type depleted mantle mixed component isotopic characteristics similar nearby Samoa plume. There no evidence subduction input. Petrological modelling shows melts (Group 1A) evolve towards trachyandesites through fractional crystallization coupled assimilation % Cl-rich Th-poor melt generated by low-degree (≤2 %) partial melting hydrothermally altered basaltic crust. Rocks Groups 1B(a) 1B(b) were then mixing Group 1A obtained wallrock shallow magma chambers. Extremely high concentrations can be explained interstitial K- brines pore water before these rocks. This good agreement unusual extension indicates that, north basin, microplate boundaries privileged zones generate complex felsic magmatism within areas widespread alteration. Our study confirms importance processes generating high-silica environments decoupled subduction.