HR: 11:35h
AN: V52B-06    [Abstracts]
TI: Timescale for Incremental Construction of the Silurian Vinalhaven Intrusive Complex, Coastal Maine, USA
AU: * Hawkins, D P
EM: hawkins@denison.edu
AF: Dept. of Geology and Geography, Denison University, Granville, OH 43023 United States
AU: Wiebe, R A
EM: bob.wiebe@fandm.edu
AF: Dept. of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604 United States
AB: Subvolcanic plutons, such as the Vinalhaven intrusive complex, preserve field evidence for incremental growth from ephemeral chambers due to crystallization, replenishment, rejuvenation and accumulation. In the Vinalhaven intrusion, field relations indicate that the lower portion of the intrusion, which is characterized by a layered section of gabbro-diorite and cg granite, is stratigraphically older than the upper portion of the intrusion which is characterized by homogeneous, cg granite with a wide variety of schlieren structures (Wiebe and Hawkins, this volume). To evaluate the distribution of time within this stratigraphic framework, we determined high-precision U-Pb ages (ID-TIMS) on single crystals and single crystal fragments of zircon from four samples in the intrusion. Two samples of cg grained, one collected from the layered section near the base of the intrusion and the other along the western margin of the intrusion along strike from the layered section, yield concordant U-Pb crystallization ages of 421.5 $\pm$ 0.4 Ma and 421.3 $\pm$ 0.4 Ma, respectively. A third sample of cg granite collected near the top of the intrusion yields a preliminary concordant U-Pb crystallization age of 419.8 $\pm$ 0.7Ma. A fourth sample, collected from the fg granite in the core of the intrusion, a body we interpret as a silicic replenishment emplaced during the middle portion of the growth history, yields a concordant crystallization age of 420.3 $\pm$ 0.4 Ma. These four concordant U-Pb crystallization ages are consistent with the field relationships and indicate that, despite its moderate size (about 80 sq km of exposure) and shallow level of emplacement, the Vinalhaven intrusion was constructed over a nominal interval of 1.7 m.y. Inheritance patterns in the zircon populations of these samples may provide additional insights into the construction of the intrusion. Inherited zircon is difficult to avoid in the sample of fg granite; numerous grains and grain fragments yield discordant U-Pb dates with Pb-Pb dates ranging from 1465 to 460 Ma. These ages extend well beyond the ages of the exposed country rocks, and the inherited zircon crystals were probably derived from the crust beneath the intrusion. In contrast, inherited zircon is less common in the three samples of cg granite and the dates they yield are no older than 427 Ma, well within the age range of the exposed country rocks. Moreover, two zircon crystals from the youngest granite overlap in age (weighted mean 421.3 Ma) with the samples from the lower portion of intrusion. Although these inheritance patterns may reflect sampling bias, we think it is more likely that they reflect magmatic processes during pluton growth. If so, we suggest two implications. First, we see field evidence that fg silicic dikes (silicic replenishments) disrupt crystal mush as they intrude. Thus, a recently replenished chamber contains zircon crystals remobilized from older granite mush within the intrusion. If such grains erupted, they would yield unrealistically long residence times. Second, if fresh inputs of magma that replenish a chamber carry inherited zircon, then granite mush accumulating below that chamber could preserve a `layer' of cg granite that is relatively enriched in inherited zircon. Such layers could be used as stratigraphic markers of successive silicic replenishment events in the solidified pluton.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting