DISTINGUISHING PLUTON EMPLACEMENT MECHANISMS

Proposed mechanisms by which wall rocks accommodate magma intrusion involve mainly brittle cracking (stoping; injection of dikes, sills or laccoliths) or mainly ductile flow (diapirism; ballooning). Practically all such mechanisms have been debated for more than a century. The debate has lasted this long largely because typical field observations tend to be inconclusive: wall-rock xenoliths and discordant wall-rock contacts may reflect either stoping or sequential injection of dikes and/or sills, and an ovoid intrusion surrounded by concentrically foliated wall rocks can result from diapirism, ballooning, or laccolith emplacement. Other lines of evidence and reasoning thus are needed. Recent field and geophysical studies have demonstrated that many plutons are roughly tabular in overall form, broadly either laccolithic or dike-like. This result tends to favor construction by injection of magma into dilatant cracks. Detailed mapping and U/Pb geochronology indicate that some, and perhaps most, large granitic intrusions are far more composite than is superficially apparent and grew incrementally over millions of years (e.g., Coleman et al., this meeting). This result severely limits the applicability of mechanisms such as stoping and diapirism that require the bulk of a pluton to be fluid magma at a given time. Further, quantitative aspects of rock fragmentation and of observed xenolith suites cast serious doubt on stoping as a significant mechanism of magma ascent (Glazner and Bartley, this meeting) and suggest that most xenoliths are isolated by successive injection of intrusive sheets. Field relations of many plutons previously interpreted in terms of stoping are equally well or better explained by this mechanism. Simple quantitative estimates of wall-rock strain magnitudes and patterns required for diapirism, ballooning, and laccolith emplacement indicate that strains required for diapir emplacement probably are too large and the strains required by ballooning are too areally extensive; laccolith emplacement appears to be the most viable explanation of ovoid plutons surrounded by concentric foliation. Present theoretical and observational considerations combine to favor the predominance of crack opening to construct plutons by incremental addition of dike and/or sill-like bodies.