WHY DOES ALKALI-FELDSPAR COMMONLY GROW TO MEGACRYSTIC SIZES IN FELSIC MAGMAS WHEREAS COEXISTING QUARTZ AND PLAGIOCLASE DO NOT? INSIGHTS FROM THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

Populations of alkali-feldspar (af) crystals in felsic calc-alkaline rocks often include “megacrysts” more than 3 cm long. Coexisting quartz and plagioclase crystals seldom grow to comparable sizes, however, despite textural and experimental evidence that these phases begin to crystallize prior to af in dacitic and many rhyolitic magmas.

Af crystals in megacryst-bearing rocks are characterized by (1) “hump-shaped” crystal size distributions that are enriched in intermediate size fractions and (2) inverse correlations between crystal radius and roundness, both of which suggest that megacryst growth is the result of textural coarsening driven by repeated cycles of dissolution and crystallization. A numerical model of this process (Simakin and Bindeman, J Vol Geotherm Res, 2008) indicates that its efficiency increases with the mass fraction of a mineral dissolved during each cycle, since greater dissolution leads to more effective transfer of material from smaller to larger crystals.

Because plagioclase and quartz typically begin to crystallize before af, a heating event that dissolves equilibrium proportions of these minerals from a magma that is initially saturated with all three will inevitably dissolve a larger mass fraction of af than of either of the others. A dacitic magma modeled after the Whitney granite porphyry that initially contains 83 wt % crystals, for example, is calculated to undergo dissolution of 30 wt % of its total crystal mass when 50 kJ of heat is added per kg of magma. This total, however, includes 74 wt % of the af crystals initially present but only 43 wt % of the quartz and 13 wt % of the plagioclase. As a result, repeated dissolution-crystallization cycles are expected to coarsen af crystals more than those of either quartz or plagioclase.

This difference in the efficiency of coarsening may explain why af commonly grows to megacrystic sizes in felsic calc-alkaline magmas whereas coexisting quartz and plagioclase, which have experienced similar thermal histories, do not.