Journal of Petrology Advance Access originally published online on July 1, 2004
Journal of Petrology 2004 45(8):1565-1582; doi:10.1093/petrology/egh019
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Journal of Petrology 45(8) © Oxford University Press 2004; all rights reserved
On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes
DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF WASHINGTON, BOX 351310, SEATTLE, WA 98195-1310, USA
* Corresponding author. Present address: Section des Sciences de la Terre de l'Université de Genève, 13, rue des Maraîchers, 1211 Geneva 4, Switzerland. E-mail: olivier.bachmann{at}terre.unige.ch, bergantz{at}u.washington.edu
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.
KEY WORDS: crustal evolution; igneous processes; rhyolite; silicic magmatism; two-phase flow
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