How are silicic volcanic and plutonic systems related? Part 2: Insights from phase-equilibria, thermodynamic modelling and textural evidence

Petford, Nick

Abstract

Thermodynamic modelling shows that, although high-silica rhyolitic melts can form through fractionation of
low-silica rhyolitic magmas the complementary cumulates do not have compositions similar to natural plutonic
granites. Rather than being granitic, the predicted cumulates would be monzonitic to granodioritic. Thus, it is
unlikely that the majority of monzogranitic to syenogranitic batholiths formed in this way and although many
high-silica rhyolites may be cogenetic with plutonic rocks, they seem to be magmatically decoupled from most
accompanying plutonic masses. We suggest that mush heating may not be the major cause of apparent resorption
textures in phenocrysts and antecrysts in rhyolitic magmas. A significant cause is simple magma ascent under
near-isothermal conditions, and embayments in quartz seem to be growth rather than resorption features. Thus,
the presence of ‘resorption’ textures should not be regarded as firm evidence for mush heating and remobilisation.
Although some glomerocrysts may have been harvested from mush environments, the modelled melt
temperatures, compositions and the near-liquidus mineral assemblages are generally incompatible with such an
origin. Many petrogenetic puzzles surrounding silicic magma systems stem from an assumption that there is a
close magmatic connection between silicic volcanic rocks and granitic plutons, and because of a model that
assumes the existence of large, shallow magma reservoirs in which fractionation and crustal assimilation occur.
Models predicated on the concept of mush rheological lock-up, mush reactivation and melt extraction from
mushes to form eruptible rhyolitic liquids should be re-evaluated. In general, silicic plutonic rocks are neither
compositional equivalents nor cumulate complements of silicic volcanic rocks.

Citation

Petford, N. (2022). How are silicic volcanic and plutonic systems related? Part 2: Insights from phase-equilibria, thermodynamic modelling and textural evidence. Earth-Science Reviews, 235, Article 104250. https://doi.org/10.1016/j.earscirev.2022.104250

• Journal Article Type: Review
• Acceptance Date: Nov 7, 2022
• Online Publication Date: Nov 16, 2022
• Publication Date: 2022-12
• Deposit Date: Oct 28, 2024
• Journal: Earth-Science Reviews
• Print ISSN: 0012-8252
• Electronic ISSN: 1872-6828
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 235
• Article Number: 104250
• DOI: https://doi.org/10.1016/j.earscirev.2022.104250
• Keywords: Volcanic Plutonic Granite Rhyolite Mush Differentiation Remobilisation
• Public URL: https://durham-repository.worktribe.com/output/2990591
• Publisher URL: https://www.sciencedirect.com/science/article/abs/pii/S0012825222003348