INFLUENCES OF MELT COMPOSITION AND HEAT-LOSS RATE ON THE GROWTH OF ALKALI-FELDSPAR MEGACRYSTS IN FELSIC INTRUSIONS

Alkali-feldspar (Af) megacrysts commonly occur in the central parts of zoned granitic and granodioritic plutons and the inner members of nested intrusive suites. Despite subsolidus modification by exsolution and alkali transfer, megacrysts are igneous crystals that have been enlarged by textural coarsening. Previous studies suggest that megacrysts develop when a magma is (1) buffered near its Af saturation temperature for an extended period and (2) ‘thermally cycled’ as heat from nearby intrusions initially causes selective dissolution of small crystals and subsequent cooling leads to further growth of the larger surviving ones.

If we assume that the frequency of heat influxes due to intrusive events is similar throughout a pluton or intrusive suite’s history, then a magma’s potential to undergo coarsening will depend on how long it crystallizes along its Pl + Qz + Af ± fluid cotectic because the longer it does so the greater the chance that a randomly-timed heating event will trigger dissolution. How long magma will crystallize Af, in turn, depends on its orthoclase (Or) content and rate of heat loss. The effects of these two parameters were investigated by modeling (1) the crystallization histories of two magmas similar those that form the Or-poor equigranular and Or-rich megacrystic members of Sierran intrusive suites and (2) conductive cooling of these magmas emplaced into host rocks with different temperatures.

Calculations suggest that at the same rate of heat loss Or-rich magma will crystallize Af for about twice as long as an Or-poor one. Conductive cooling models indicate, on the other hand, that the inner, Or-rich members of these suites will lose heat 10 to 100 times more slowly than their outer, Or-poor members. Thus, although higher Or contents and lower rates of heat loss both contribute to the growth of megacrysts in the interiors of zoned plutons and intrusive suites, modeling suggests the rate of heat loss plays the dominant role.