THE EXTREME ENDMEMBER OF OPEN SYSTEM BEHAVIOR: A ROLE FOR LOW TEMPERATURE LIQUIDS IN SILICIC IGNEOUS ROCK PETROGENESIS?

For 60 years, formation of silicic igneous rocks (granites and rhyolites) has been built on the foundation that the bulk composition of these rocks reflects a melt separated from a quartz-feldspar residue at the minimum melt point (MMP) at ~700°C (Tuttle and Bowen, 1958). However, old (TB58) and new work (Lundstrom, 2020, in review) both show that hydrous peralkaline liquids actually coexist with qtz and feldspar down to temperatures as low as 315°C. These liquids, with up to 40 wt% H₂O, will have viscosities ~10 orders of magnitude less than a silicic melt at the MMP. Their buoyancy relative to crustal rocks means that once formed in magmatic systems, the liquids will ascend by porous flow, react with minerals, potentially differentiating a mostly crystalline rock to a qtz-feldspar assemblage. Thus, granitic rocks may represent the crystalline residue after prolonged flow of this “low temperature liquid” and never reflect a prior stage as a rhyolitic composition melt. Furthermore, the advective flow of a liquid with 40wt% H₂O will control heat flow in magmatic systems.

I will present results of experiments showing the continuous change in liquid composition in equilibrium with qtz+feldspar from the MMP down to 315°C and the profoundly important and rapid exchange of Na-K between reacting liquid and alkali feldspar. Cases studies from Troodos ophiolite plagiogranites and Torre del Paine granites will illustrate mineralogical progressions showing that a flow-through differentiation process can viably explain differentiation from quartz diorites to granite. Observed mineral compositions in most granitoids provide evidence for equilibration at <=500°C. Anti-correlations of bulk rock Na₂O and K₂O in interior rocks of some convergent margin granitoids such as the Tuolumne Intrusion provide evidence for focusing of low temperature liquid into a central region, potentially explaining the normal compositional zonation of these bodies, w/ published thermal histories being consistent with this proposal. Finally, the delivery of H₂O via ascent of low temperature liquids through the magmatic arc crust to upper crustal magma chambers may represent the ultimate example of an open system process—eruptions driven not by magmatic water originally in the magma chamber but by arrival of pulses of low temperature liquid from below.