High-Na dacite from the Jean Charcot Trough (Vanuatu), Southwest Pacific

Rifting that produced the Jean Charcot Trough (JCT) in the Vanuatu back-arc region commenced 2–3 Ma, leading to SiO 2 -bimodal magmatism in the northern half of the trough. Dacite with high Na 2 O (>6.0%) and low K 2 O ( Zr Rb >10 , Nb Rb >0.5 , Y Rb >2.0 , Nb La >0.5 , and Ba Zr ), but are quite different to lavas from the active Vanuatu volcanic arc. The dacites are compositionally close to oceanic plagiogranite produced during fractional crystallization of an N-MORB basaltic magma, and mass balance calculations show that they may be similarly derived from the associated basalts. We show that JCT basalts are derived from a mantle source compositionally very close to that beneath the central spreading axis of the NFB, with minimal contamination by a subduction component. Westward injection of diapiritic source mantle similar to that of the NFB back-arc basin basalts may have caused rifting of the JCT. During incipient back-arc rifting, high Na/K felsic magmas may have evolved from parental low-K basalts via side-wall crystallization within a large lower crustal magma chamber.