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21.
On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes 总被引:22,自引:3,他引:22
The largest accumulations of rhyolitic melt in the upper crustoccur in voluminous silicic crystal mushes, which sometimeserupt as unzoned, crystal-rich ignimbrites, but are most frequentlypreserved as granodioritic batholiths. After approximately 4050%crystallization, magmas of intermediate composition (andesitedacite)typically contain high-SiO2 interstitial melt, similar to crystal-poorrhyolites commonly erupted in mature arc and continental settings.This paper analyzes the feasibility of system-wide extractionof this melt from the mush, a mechanism that can rationalizea number of observations in both the plutonic and volcanic record,such as: (1) abrupt compositional gaps in ignimbrites; (2) thepresence of chemically highly evolved bodies at the roof ofsubvolcanic batholiths; (3) the observed range of ages (up to200300 ka) recorded by zircons in silicic magmas; (4)extensive zones of low P-wave velocity in the shallow crustunder active silicic calderas. We argue that crystalmeltsegregation occurs by a combination of several processes (hinderedsettling, micro-settling, compaction) once convection is hamperedas the rheological locking point of the crystalmelt mixture( 相似文献