Slab segmentation, anomalous arc volcanism, and giant porphyry copper deposits in Indonesia

Most Holocene volcanoes in Indonesia display geochemical and geophysical properties characteristic of formation via mantle wedge metasomatism. However, some volcanoes (Toba, Ungaran, Muriah, Rinjani, Tambora, Sangeang Api, and Batu Tara) are anomalous, showing alkaline geochemical affinities, anomalously violent recent eruptions, and/or positioning above sections of slab generally considered too deep to induce mantle melting via slab dehydration. These anomalous volcanoes are spatially and temporally associated with giant Pliocene-recent porphyry Cu deposits (in thin continental or oceanic crust), contrary to global occurrences of porphyry Cu deposits that are typically restricted to thick (>45 km crustal thickness) continental arcs. Using geophysical data to model slab geometry and geochemical data to evaluate source compositions and melting mechanisms, we demonstrate that anomalous Indonesian volcanoes and giant porphyry Cu deposits are linked to slab segmentation due to slab tearing. In the slab beneath Toba, the mechanically weak Investigator Fracture Zone, which separates a more buoyant Cenozoic segment of the Indo-Australian plate from older, denser, and more steeply dipping Jurassic-Cretaceous lithosphere to the east, likely created a sub-vertical slab tear feeding volcanism via poloidal mantle flow. The alkaline volcanoes Muriah, Ungaran, Tambora, and Sangeang Api (as well as the sub -alkaline volcano Rinjani) are positioned along edges of slab segments defined by negative seismic velocity anomalies at depths of similar to 185-400 km, which we interpret as elongate slab windows formed during localised choking of subduction by anomalously buoyant oceanic lithosphere. Although no negative velocity anomalies are associated with Batu Tara, we speculate that this alkaline volcano is located above a small-scale slab tear, potentially associated with the subducted Scott Plateau. We propose that poloidal asthenospheric flow along slab segment boundaries has triggered deep, low-degree partial melting with varying contributions of slab melt, sediment, and fluid. Products of this melting regime are sub-alkaline and alkaline lavas with trace element patterns enriched in incompatible elements relative to the main arc volcanoes. Based on spatial, temporal, and geochemical associations of giant Pliocene-recent porphyry Cu deposits with proposed slab tears (and an absence of evidence for slab melting in the formation of these deposits), we suggest that slab tears may have generated oxidised magmas by facilitating melting in the garnet stability field, emulating the petrogenetic effects of thick continental crust and slab melting. We further hypothesise that slab tearing locally enhanced magma flux, leading to extremely violent Quaternary eruptions at Toba, Rinjani, and Tambora.