On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes

TLDR

Rhyolitic melts accumulate in silicic crystal mushes in the upper crust, often preserved as granodioritic batholiths, and after 40–50 % crystallization intermediate magmas contain high‑SiO2 interstitial melt akin to crystal‑poor rhyolites erupted in mature arc and continental settings. The study investigates whether system‑wide extraction of high‑SiO2 interstitial melt from crystal mushes is feasible, offering an explanation for compositional gaps in ignimbrites, evolved bodies at batholith roofs, zircon age ranges, and low‑velocity zones beneath silicic calderas. Segregation is proposed to occur through hindered settling, micro‑settling, and compaction once convection stalls at the rheological locking point of ≥50 % crystals, with rates constrained by end‑member settling velocities and compaction rates. Estimated timescales for producing >500 km³ of crystal‑poor rhyolite are 10⁴–10⁵ years, compatible with mush residence times, and the model integrates silicic magmatism by linking plutonic and volcanic evolution up to upper‑crust storage, where granitoids represent remnants of rhyolitic eruptions.

Abstract

The largest accumulations of rhyolitic melt in the upper crust occur in voluminous silicic crystal mushes, which sometimes erupt as unzoned, crystal-rich ignimbrites, but are most frequently preserved as granodioritic batholiths. After approximately 40–50% crystallization, magmas of intermediate composition (andesite–dacite) typically contain high-SiO2 interstitial melt, similar to crystal-poor rhyolites commonly erupted in mature arc and continental settings. This paper analyzes the feasibility of system-wide extraction of this melt from the mush, a mechanism that can rationalize a number of observations in both the plutonic and volcanic record, such as: (1) abrupt compositional gaps in ignimbrites; (2) the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths; (3) the observed range of ages (up to 200–300 ka) recorded by zircons in silicic magmas; (4) extensive zones of low P-wave velocity in the shallow crust under active silicic calderas. We argue that crystal–melt segregation occurs by a combination of several processes (hindered settling, micro-settling, compaction) once convection is hampered as the rheological locking point of the crystal–melt mixture (≥50 vol. % crystals) is attained. We constrain segregation rates by using hindered settling velocities and compaction rates as end-members. Time scales estimated for the formation of >500 km3 of crystal-poor rhyolite range from 104 to 105 years, within the estimated residence times of mushes in the upper crust (>105 years, largely based on U/Th and U/Pb dating). This model provides an integrated picture of silicic magmatism, linking the evolution of plutonic and volcanic systems until storage in the upper crust, where granitoids become the leftovers from rhyolitic eruptions.