HIGH-NA DACITE FROM THE JEAN-CHARCOT-TROUGH-(VANUATU), SOUTHWEST PACIFIC

Rifting that produced the Jean Charcot Trough (JCT) in the Vanuatu bac k-arc region commenced 2 3 Ma, leading to SiO2-bimodal magmatism in th e northern half of the trough. Dacite with high Na2O (> 6.0%) and low K2O (< 1.0%) was recovered from the northern termination of the JCT, a ccompanied by thioleiitic N-MORB-type back-arc basin basalt similar to that erupted at the central spreading axis of the North Fiji Basin (N FB). Incompatible element ratios are very similar in the dacite and as sociated basalt, and also in the NFB back-arc basin basalts (Zr/Rb > 1 0, Nb/Rb > 0.5, Y/Rb > 2.0, Nb/La > 0.5, and Ba/Zr < 1.5), but are qui te different to lavas from the active Vanuatu volcanic arc. The dacite s are compositionally close to oceanic plagiogranite produced during f ractional crystallization of an N-MORB basaltic magma, and mass balanc e calculations show that they may be similarly derived from the associ ated basalts. We show that JCT basalts are derived from a mantle sourc e compositionally very close to that beneath the central spreading axi s of the NFB, with minimal contamination by a subduction component. We stward injection of diapiritic source mantle similar to that of the NF B back-arc basin basalts may have caused rifting of the JCT. During in cipient back-arc rifting, high Na/K felsic magmas may have evolved fro m parental low-K basalts via side-wall crystallization within a large lower crustal magma chamber.