New Insights Into the Petrogenesis of Voluminous Crustal-Signature Silicic Volcanic Rocks of the Toba Eruptions (Indonesia)

The formation of large-volume silicic magmas in arc settings is fundamental for understanding trans-crustal magmatic systems related to subduction zones. Here, we present an integrated study of the four Quaternary Toba eruptions and pre-caldera Haranggaol Andesite on Sumatra, Indonesia. This rock suite has significantly enriched Sr-Nd isotopes (⁸⁷Sr/⁸⁶Sr = 0.71220–0.71517, ε_Nd = −8.9 to −10.6) compared with other volcanic rocks on Sunda-Banda arc, but is similar to the post-caldera (<74 ka) Sipisupisu basalts near the Toba Caldera. Thermodynamic modeling using Magma Chamber Simulator has revealed that the Toba silicic rocks can be produced by a two-stage assimilation and fractional crystallization of mantle-derived basaltic melts with compositions similar to the Sipisupisu basalts in the lower and upper crustal magma reservoirs. Binary modeling of Sr-Nd isotopes suggests that the rocks near the Toba Caldera can be produced by mixing of 5%–10% of the subducting Nicobar Fan sediments (⁸⁷Sr/⁸⁶Sr = 0.73493, ε_Nd = −14 on average) with depleted MORB mantle (DMM). Indeed, decompressional partial melting modeling of bulk mixtures of DMM with 7% subducting sediments using pMELTS indicates that the melts generated can have geochemical compositions similar to the Sipisupisu basalts. We therefore argue that hybridization of the subducting sediments with the mantle wedge could be an alternative scenario responsible for the enriched isotopic characteristics of the rocks near Toba. Prolonged fractionation of mantle-derived enriched/depleted basaltic melts, accompanied by crustal assimilation and crystal-melt segregation, could be common processes in generating large-volume silicic magmas on continental arcs.