HR: 16:30h
AN: V44C-03    [Abstracts]
TI: Longevity of the Silurian Vinalhaven Intrusive Complex, Maine, USA
AU: * Hawkins, D P
EM: hawkins@denison.edu
AF: Denison University, Department of Geosciences, Granville, OH 43023, United States
AU: Wiebe, B A
EM: bob.wiebe@fandm.edu
AF: Franklin & Marshall College, Department of Earth & Environment, Lancaster, PA 17604, United States
AB: Many silicic intrusions preserve apparent stratigraphic sections of interlayered silicic, hybrid and mafic rocks. These sections provide a potential temporal framework that can be used to constrain the longevity of plutonic systems and delineate detailed histories of pluton construction. Field relations indicate that the Silurian (419 Ma) Vinalhaven intrusive complex (VIC) has a stratigraphic record of magma chamber evolution and pluton growth by crystal accumulation on magma chamber floors that was interrupted by episodes of replenishment, rejuvenation and eruptions (Wiebe and Hawkins, this volume). The system preserves evidence for a wide range of magma chamber processes including, multiple replenishments of compositionally varied magmas, mingling and mixing of those magmas, and mafic replenishments that rejuvenated granitic crystal mush (Wiebe et al., 2004) and perhaps triggered roof collapse associated with caldera formation (Hawkins and Wiebe, 2004). The purposes of this study are to test the stratigraphic model interpreted from field relationships in the VIC and to constrain the timescale of pluton construction via high-precision ID-TIMS U-Pb zircon geochronology. We collected four samples of cg granite from different stratigraphic levels in the intrusion and one sample of fg granite from a large-volume intrusion interpreted as a silicic replenishment late in the system's history. The U-Pb ages were determined from the weighted mean of the 206Pb/238U dates of 5 to 9 concordant analyses of either whole single crystals of zircon or individual crystal terminations (tips) sliced from whole zircon crystals. Each grain or grain fragment was either air-abraded or annealed and chemically abraded prior to dissolution. Ages have uncertainties of 110 – 70 k.y. at 419 Ma and are statistically robust. The three cg granite samples from the layered granite-hybrid-gabbro unit near the base of the intrusion yield ages that are consistent with the interpreted stratigraphy and span about 300 k.y. In that time interval, more than 100 individual mafic intrusions were trapped within the resident magma chamber(s) and silicic magma repeatedly replenished the system to construct several kilometers of layered section. A fourth cg granite sample collected in the upper portion of the intrusion is indistinguishable in age from the youngest sample collected in the layered section. This indicates that about 300 k.y. after the oldest granite sample formed, the intrusion had a thick wall/roof of crystallized granite, at least locally. The sample of fg granite is about 700 k.y. younger than the granite at the margin of the intrusion at the base of the layered unit. Therefore the intrusion, as exposed today, was constructed in at least 700 k.y.
DE: 1036 Magma chamber processes (3618)
DE: 1115 Radioisotope geochronology
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting