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