The role of thermal migration and low-temperature melt in granitoid formation: Can granite form without rhyolitic melt?

Research output: Contribution to journalArticlepeer-review


The bulk composition of silicic igneous rocks hovers directly over the minimum melt located on the NaAlSi3O8-KAlSi3O8-SiO2 ternary. It is universally accepted that these rocks are igneous, reflecting the thermodynamic equilibrium between minerals and melt. However, there is a contradiction between the use of this phase diagram and current models of differentiation: fractional crystallization or partial melting (or any mechanical separation process) implies granites were melts extracted from a quartz-bearing residue.Here, I propose that a thermodynamically controlled process, wet thermal migration, provides a more consistent explanation for producing silicic rocks. This process, when coupled with slow incremental emplacement of sills, allows magmatic differentiation to take place without mechanical separation and produces minimum-melt compositions from input magmas not saturated in quartz. Examination of phase equilibria in the SiO2-Al2O3-Na2O-K2O-H2O system, as well as assessment of thermodynamic models, provides a template for understanding the wet thermal migration process (diffusion-based differentiation of crystal-mush in a temperature gradient). Phase equilibria in SiO2-Na2O-K2O-Al2O3 indicate a cotectic surface between quartz and alkali feldspar slopes down-temperature as melt peralkalinity increases. Experiments show quartz and two feldspars coexist with a single water-rich (>40 wt.% H2O) melt at 400°C and 0.1 GPa. Modelling suggests that development of water-rich melt at the hot end of a temperature gradient drives the process. Given the propensity of alkalis to rapidly diffuse down-temperature gradients and form these melts, small amounts (<5%) of interconnected melt form, leading to the differentiation of partially molten materials by wet thermal migration.Because the quartz-feldspar cotectic ranges from 650°C to 330°C, granites can reflect formation by a process that never involves the existence of rhyolitic melt. If this is correct, the process has implications for understanding the formation of plutons and batholiths and, therefore, the continental crust.

Original languageEnglish (US)
Pages (from-to)371-388
Number of pages18
JournalInternational Geology Review
Issue number3
StatePublished - Feb 17 2016


  • Granite
  • differentiation
  • phase equilibria
  • quartz

ASJC Scopus subject areas

  • Geology


Dive into the research topics of 'The role of thermal migration and low-temperature melt in granitoid formation: Can granite form without rhyolitic melt?'. Together they form a unique fingerprint.

Cite this