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

Abstract

Listen

Translate article

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.