The Zealandia Volcanic Complex: Further evidence of a lower crustal “hot zone” beneath the Mariana Intra‐oceanic Arc,Western Pacific

This paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO₂; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.