Protracted magma assembly and evolution of the Toba Caldera Complex, Indonesia

Abstract:

The 2500 km² Toba Caldera Complex of northern Sumatra overlies a seismically anisotropic middle crust, interpreted as containing an active magmatic sill complex [1]. Maximum crustal shear wave attenuation occurs at depths (5-7 km) that match those of mineral equilibration for the 74 ka Youngest Toba Tuff (YTT), an extraordinary 2800 km³ of densely to non-welded ignimbrite and co-ignimbrite ash-fall. The distribution of rhyolite and dacite domes erupted over ~15 kyr in the wake of the YTT resembles that at Laguna del Maule, Chile, and has an uncertain relation to caldera ring fractures. A continuum of U-Th and/or U-Pb crystallization ages, spanning in some cases >50% of final zircon and allanite growth, show that accessory phase nucleation and growth in the YTT was likely episodic over protracted time intervals of tens to hundreds of kyr, and from diverse magmatic conditions [2]. A similar crystallization fingerprint is also found in zircons from post-YTT rhyolites and dacites, showing that there was significant residual magma in the crust following eruption of the YTT [this study; 3]. The compositional variations defined by the heterogeneous YTT magma are mirrored by variations in YTT zircon trace element ratios (e.g., Zr/Hf, Eu/Eu*, Nd/Yb). Titanium concentrations in zircon (1-10 ppm) generally vary as expected with interelement ratios, suggesting a large (~200°C) temperature interval for zircon crystallization. In contrast, the trace element characteristics of biotite and matrix glass are specific to individual samples, which indicate that compositionally discrete domains persisted within the YTT for at least the duration of biotite growth. The contrasting crystal records of the YTT zircons, as well as evidence for intermittent crystal isolation and remobilization associated with magma recharge, is favored at the cool and wet eutectoid conditions that characterize at least half of the YTT, wherein heat fluxes could dissolve major phases like biotite but have only a minor effect on larger zircon crystals. Repeated magma recharge may have contributed to compositional zoning in the YTT but regular perturbations to the magma reservoir over >400 ka did not lead to eruption until 74 ka ago.

[1] Jaxybulatov, K., et. al. (2014), Science; [2] Reid and Vazquez (2017), G-cubed; [3] Mucek, A.E., et al. (2017), Nature Comm.