Felsic magmatism in the Southern Mariana arc: Petrogenetic comparison between Zealandia Bank and East Diamante

Felsic magmas have been recognized to be a significant component in intra-oceanic arc systems (IOAS), but their origin in this setting is still a matter of debate. Zealandia Bank and East Diamante are two submarine volcanoes in the Southern Marina arc that have erupted felsic magmas. Previous geologic and petrologic studies of the Diamante cross-arc volcanic chain suggest that the felsic magmas of East Diamante may be caused by the remobilization of tonalitic middle crust (Stern et al., 2010). The petrogenetic comparison between Zealandia and East Diamante volcanoes conducted in this study provides further constraints on the genesis of felsic magma in an IOAS. ROV Hyper-Dolphin dives during cruise NT09-08, on R/V Natsushima, were carried out in the Zealandia area and on the Diamante cross-arc volcanic chain. Zealandia Bank, which is built on the volcanic front, has two neighboring edifices, North and West Zealandia, on the rear-arc side. The western half of Zealandia Bank is dominated by felsic lavas, with mafic lavas having been recovered from a ridge extending to the south of the main edifice and a parasitic cone. Magnesian olivine basalts have been recovered from West Zealandia. The Diamante cross-arc volcanic chain consists of three major edifices, a caldera with felsic resurgent domes on the volcanic front (East Diamante), and two basaltic cones on the rear-arc side (West and Central Diamante). There are silica gaps at around 60 % SiO2 in both the Zealandia Bank and East Diamante magmas. Felsic lavas from East Diamante are generally porphyritic, with total phenocryst abundances up to 41 % and the assemblage consisting of quartz, plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides. Felsic lavas from Zealandia Bank, on the other hand, generally contain less phenocrysts, with an assemblage of 1-3 % plagioclase, 0.1-0.3 % clinopyroxene, <0.1 % orthopyroxene and Fe-Ti oxides, and no quartz. Microprobe results show felsic lavas from Zealandia Bank have Ca-rich plagioclase and Mg-rich pyroxenes relative to East Diamante felsic lavas. Equilibrium temperatures of 900-1000°C are obtained from the felsic lavas of Zealandia Bank, whereas 800°C is estimated for those of East Diamante. Felsic lavas in Zealandia Bank exhibit lower Th/Nb and higher Ba/Th than East Diamante. 87Sr/86Sr and 143Nd/144Nd in mafic to felsic samples from Zealandia Bank are similar to those from East Diamante, while 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are higher. Our petrographic evidence from Zealandia Bank, such as the silica gap, higher temperature, lower abundance of phenocrysts and lack of quartz in the phneocryst assemblage, suggests crustal melting here, similar to what has been inferred at Sumisu volcano, Izu arc (Shukuno et al., 2006). If so, this indicates that the felsic magmas from Zealandia Bank may be produced by remelting of tonalitic middle crust under higher temperatures and at higher degrees of melting than at East Diamante. The differences in trace element and isotope compositions between Zealandia Bank and East Diamante may be caused by an along-arc variation in a component added from the subducting plate during crustal formation.