Geochemistry of mid-Cretaceous Pacific crust being subducted along the Tonga–Kermadec Trench: Implications for the generation of arc lavas

Abstract

The Pacific Plate is currently being subducted at a rapid rate beneath the Indo-Australian Plate along the Tonga–Kermadec Trench in the southwestern Pacific. It has long been assumed that the lithosphere being subducted is relatively old and homogeneous in composition. Basaltic lavas dredged from the upper crust of the incoming lithosphere along the length of the trench are mid- to late-Cretaceous in age. Although the samples are mainly N-MORB, they range from tholeiitic to alkalic basalts. Concentrations of incompatible trace elements show a high degree of variability (e.g., Ba=8 to 270 ppm, Rb=0.4 to 39 ppm, Sr=60 to 625 ppm, and Nd=2.3 to 25 ppm). Neodymium, Sr and Pb isotopic data also show wide ranges (εNd(T)=5.7–11.2; 87Sr/86Sri=0.70224–0.70311; 206Pb/204Pbi=17.84–20.22). More importantly, the basaltic crust being subducted displays a latitudinal compositional variation that is similar to that shown by the Tonga–Kermadec arc lavas. Previous studies have proposed that the variably depleted sub-arc mantle, which was preconditioned through a backarc melt extraction or displacement process, is mainly responsible for the latitudinal variation in the Tonga–Kermadec arc lavas. However, our new results suggest a greater role of the lithospheric input into the source of arc lavas. The three end-member possibilities linking the latitudinal variation of the lithospheric input to the source of arc lava output are: (1) the mantle wedge beneath the volcanic arc on the west side of the trench, the main source of Tonga–Kermadec arc lavas, is a western extension of the Cretaceous Pacific upper mantle east of the trench; (2) the altered oceanic crust melts and the resultant slab melt modifies the mantle source of arc lavas; and (3) fluids dehydrated from the altered oceanic crust effectively transfer the compositional signature of the subducted slab into the mantle source of arc lavas.