Is stoping a volumetrically significant pluton emplacement process?

Abstract

Magmatic stoping is widely called upon to explain discordant pluton contacts, but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons. First, xenoliths rarely make up more than 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 pluton-scale magmatic breccias that are not observed. Third, although settling calculations show that large (meter-scale or larger) xenoliths could sink rapidly in magma, natural fragmentation processes are likely to produce a fractal distribution of particle sizes, with small particles far more abundant than large ones. The absence of small xenoliths at pluton contacts, where magma cools most rapidly, thus argues against stoping. Fourth, although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common, geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks. Finally, there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally. Stoping cannot be volumetrically important in incrementally emplaced plutons, because blocks can only fall through the volume that is molten at any given time. Where xenoliths are abundant in plutons, they may commonly represent isolation of wall-rock bodies between successive intrusive increments rather than wall-rock fragments that were engulfed by magma. However, even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber. Although stoping undoubtedly occurs during the emplacement of some plutons, it is unlikely to be a volumetrically significant process.