HR: 09:15h
AN: V51E-06 INVITED     [Abstracts]
TI: Construction of the Vinalhaven Intrusive Complex, Maine, USA: the Plutonic Record of Evolving Magma Chambers Affected by Multiple Episodes of Replenishment, Rejuvenation, Crystal Accumulation and Eruption
AU: * Wiebe, R A
EM: bwiebe@fandm.edu
AF: Robert A. Wiebe, Department of Earth and Enviroment, Franklin and Marshall College, Lancaster, PA 17604 United States
AU: Hawkins, D P
EM: hawkins@denison.edu
AF: David P. Hawkins, Department of Geology and Geography, Denison University, Granville, OH 43023 United States
AB: Increasingly, the plutonic roots of volcanic systems can be shown to contain temporal records of events inferred from the study of volcanic rocks. The Vinalhaven intrusive complex preserves evidence for multiple episodes of silicic and mafic replenishments, rejuvenation of granite, and probable eruptive events over a nominal time-span of 1.7 Ma (Hawkins and Wiebe, this volume). The complex is about 12 km in diameter and consists mainly of cg granite, a thick section of arcuate, inward-dipping gabbro-diorite sheets in the southeastern half of the complex, and a circular core of fg granite. Field relations demonstrate that the base of the intrusion is along the southeastern margin of the complex, and the top is along the northwestern margin where it intrudes coeval volcanic rocks. Aphyric basaltic and granitic dikes fed this essentially bimodal intrusion. When basaltic dikes intersected a silicic chamber, basalt spread across a floor of silicic crystal mush to form gabbro-diorite sheets in granite. Several extensive layers of angular blocks of country rock occur within the mafic rocks. Granitic dikes and the fg granitic core of the complex have sharp to gradational contacts with cg granite, and, locally, both granites are intimately mixed and commingled. These relations indicate that new silicic injections mixed into partly crystallized resident magma. Several irregular bodies of porphyry (0.2 to 0.5 km in average dimension) intrude cg granite with sharp, gradational, or commingled contacts. The porphyry has 5 to 40% corroded phenocrysts, identical in composition to crystals in the granite, and a variably quenched matrix. Some of these bodies formed when late injections of basalt remelted largely solid portions of cg granite. New silicic input may have contributed to other porphyry bodies. The matrix probably quenched because of a sudden decrease in pressure, possibly due to eruption of magma from the chamber. The cg granite and inter-layered mafic rocks preserve a stratigraphic section that records magma chamber evolution during the early growth of the Vinalhaven intrusion. Near the base of this section, mafic sheets flowed across almost the entire width of intrusion. The large volume and comparable extent of country rock blocks at this level suggest a major collapse of the roof of a large tabular chamber, and this collapse probably records a major eruption of silicic magma from the chamber. At higher levels of this section, mafic sheets and country rock blocks gradually become restricted to the eastern third of the complex and are entirely absent in the upper half of the intrusion. The less extensive layers of country rock blocks may record smaller eruptions from more restricted chambers. The lower western and entire upper parts of the complex lack mafic rocks and consist of homogeneous, cg granite with a wide variety of schlieren structures that demonstrate granite formed largely by crystal accumulation on a chamber floor. Preliminary measurements of schlieren orientations suggest that the evolving chamber floors were highly irregular and consisted of basin forms with diameters that were much smaller than the width of the plutonic complex - perhaps as little as 1 to 2 km. Although the granite appears homogeneous and to lack any sharp internal contacts, it may have accumulated largely in small, semi-isolated chambers that waxed and waned due to crystallization, replenishment, rejuvenation and accumulation.
DE: 8434 Magma migration
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting