THE CASE AGAINST MAGMATIC STOPING

Magmatic stoping is widely called upon to explain discordant pluton contacts, but several lines of evidence indicate that stoping is not a significant process in the emplacement of most plutons. First, xenoliths rarely make up even a few percent of most plutons; this sparseness, even on well-exposed pluton floors, argues that few xenoliths make it into plutons. Second, piles of rock fragments have sufficiently high porosities (~50%) that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much, if not all, of the available magma, producing enormous magmatic breccias that are not observed. Third, although Stokes settling calculations show that large (meter-scale or larger) xenoliths could sink rapidly in magma, natural fragmentation processes produce a fractal distribution of particle sizes, with small particles far more abundant than large ones. The absence of small xenoliths at pluton contacts thus argues against stoping. Fourth, at many plutons geochemical data clearly rule out significant assimilation of local wall rocks. Finally, there is growing evidence that many large plutons never were large molten bodies of magma, but grew incrementally. Stoping cannot occur in such systems. Where xenoliths are locally abundant in plutons, they probably represent isolation of wall-rock bodies between successive intrusive increments rather than wall-rock fragments that were engulfed by magma. Although stoping may occur during the emplacement of some plutons, it is unlikely to be a volumetrically significant process.