GSA Field Forum 2005
Rethinking the Assembly and Evolution of Plutons: Field Tests and Perspectives

The leaders of this stimulating forum intended to provoke discussion about how multiple dike injections may assemble plutons, analogous to multiple injections of basalt form mid-ocean ridges. Ideally, we would have visited the McDoogle pluton southwest at the beginning of the trip, where the published evidence is quite strong in support of the process. Regrettably, that visit, as well as a sequential approach to the hypothesis, was logistically precluded by inaccessibility, limits of time, and convenient order of march.

With those caveats, we began the trip in one of the largest plutonic complexes in the Sierra Nevada, the Tuolumne intrusive suite (TIS) in Yosemite National Park, with the most cryptic field relations of all observed during the forum. Most of the participants came away with a good understanding of the ages and field relations among the larger units of the TIS, but I believe few of us were able to accept the multiple dike hypothesis to account for assembly of the entire TIS from the observed field relations in the May Lake area.

The Lamarck granodiorite about 75 km southeast of Yosemite is at the northeast end of the McDoogle pluton and, presumably, the two plutons may have had some common basis of assembly. Although we spent two days in the Lamarck pluton, I didn’t hear anyone saying that they bought the multiple injection hypothesis based on anything they saw there. I believe most all of us are reserving judgment about how widespread the multiple dike process is for assembling plutons, until we can search our own plutons for features alleged to be characteristic of the process.

The final day of the field forum was spent in discussion sessions about what information is necessary to advance the understanding of pluton assembly and emplacement. The conclusions were the most disappointing part of the forum, as interesting and informative as the discussions were, because nothing really new materialized. To the same old calls for more chemistry and geochronology – both very necessary and desirable – were nebulous calls for increased three-dimensional understanding through geophysics, and for looking at the larger scales of rock textures on the scale of kitchen countertops. Some participants thought numerical modeling would somehow yield deeper insights, but others were dubious, believing that granitic plutons are in the field, not inside computers.

In my own experience, not all plutons can be explained by incremental dike injections, and not all can be explained by diapirism. I have mapped plutons that most certainly were fed by a main feeder dike; I have mapped plutons that just as certainly intruded as diapirs. I believe, like H. H. Read said 60 years ago, that there are “granites and granites”. I think the key to understanding their genesis, assembly, and emplacement depends on crustal depth of their exposure. Thus, a granitic pluton exposed at a shallow level of the crust will manifest itself differently than one eroded to mid-crustal or deep crustal depths. J. J. Sederholm found this to be true over 80 years ago when he made a traverse across Scandinavia from deep crust exposures in Finland, across the mid-crust in Sweden, to the (relatively) shallow crust in Norway. Based on his interpretation of the transitions Sederholm saw, Eugene Wegmann wrote a classic paper about diapirism (Über Diapirismus), which is generally neglected by students today, because it is not in English or downloadable off the web. I commend it to your attention, even though it was written 75 years ago, and I'll send you a translation if you can’t read it in German.

Arthur Gibbs Sylvester, 9 November 2005