Correlating Volcanic and Plutonic Perceptions of Silicic Magma Chamber Processes: Evidence from Coastal Maine Plutons
Bob Wiebe, Geological Sciences, Franklin and Marshall College, Lancaster, PA 17604
Don Snyder, Geological Sciences, University of Michigan, Ann Arbor, MI 48109
David Hawkins, Geology and Geography, Denison University, Granville, OH 43023
This second GSA Field Forum brought together volcanologists, experimentalists, field petrologists, structural geologists, and theoretical fluid dynamicists, specialists who think about silicic magmas from diverse vantage points. We felt that this forum was needed because those who work on the volcanic and plutonic silicic rocks generally make very different assumptions about the character of silicic magma, and the two kinds of records should be complementary. We hoped that these two communities would discuss to what degree inferences about silicic volcanism can be drawn from silicic plutons, and vice versa. Some key questions appeared to be: Are erupted products typical of the chambers they sample, or are they peculiar samples? How much of the observed compositional variation in silicic rocks is produced in upper crustal magma chambers? How should geochemical data be interpreted in light of the rich structures observed that appear to record floor accumulation in silicic chambers? What are the relative roles of fractionation and mixing?
The forum was convened in Ellsworth, Maine, September 1418, 2000. Field excursions concentrated on the evidence for silicic magma chamber processes in nearby coastal exposures of shallow granitic plutons and associated volcanic rocks. Parts of these plutons contain stratigraphic sections that record crystal accumulation and periodic injection of mafic magma and preserve spectacular magma mingling relationships. These layered sections appear to be the plutonic record of mafic replenishment events so widely inferred from the study of silicic eruptive systems. The forum explored the implications of these stratigraphic sequences for the mineralogical and geochemical evolution of silicic magma chambers and the possible impact of the magma chamber processes on the chemical and mineralogical evolution of silicic volcanic rocks and granitic plutons. Other features in these plutonic rocks that may be connected to volcanic phenomena included two kinds of composite (mafic-felsic) dikes, mafic enclaves, extensive zones of felsic enclaves near a pluton roof, and a major "shatter zone" along the margin of the Cadillac Mountain granite.
We spent the first two days on Mount Desert Island (Acadia National Park), examining the Cadillac Mountain intrusive complex and the spatially and temporally associated Cranberry Island volcanic series. For the last two days we examined a wide range of structures in the Gouldsboro, Corea, and Pleasant Bay intrusions, which lie farther northeast along the coast. Informal sessions before dinner focused on themes relevant to the day's field trip stopse.g., processes operating when mafic magma enters a silicic chamber, possible evidence in plutons for eruptive events. Sessions after dinner extended these discussions and included presentations relating to participants' work in other field areas.
The forum brought together scientists who might not otherwise converse at regular meetings, despite common themes that underlie their research interests. The need for such interchange was illustrated by the voluminous electronic correspondence among the participants after the meeting. Both the discussions at the forum and the subsequent correspondence reveal how all of us see these problems through the lens of our own experiences or field areas, causing us to have different sets of prejudices that we may consider implicitly understood. Informal communication, like this forum, can make these prejudices explicit in a way that formal publication cannot. Exposing and examining these predispositions must precede a general understanding of the problems.
We think that these predispositions create an impediment to arriving at a group consensus on some of the issueseach scientist places different weight on different kinds of evidence. Some are not willing to accept that a certain process occurred unless they see structures in the field that record it. Others may be willing to accept the same hypothesis because it explains geochemical evidence, even without seeing direct evidence in the rocks. Some require a specified physical mechanism for a process such as magma mixing, while others are willing to infer its occurrence on the basis of geochemical evidence, leaving the mechanism unspecified, certain that future work will reveal its identity. Some require that interpretations of plutonic systems also fit the observed volcanic ejecta, while others are satisfied to explain one without the other, uncertain that they are related. Despite these contrasting criteria for substantiation, we feel that many of the participants came away at least challenged to defend their ideas against a wider range of tests, and that other field settings might shed light on their own field areas.
If H.H. Read was correct when he said that the best geologist is the one who has seen the most rocks, then the experience of seeing the rocks in Maine and the slides and discussion of similar rocks elsewhere made us all better geologists.
Mary Beth Cheversia
Monica Lopez de Luchi