DOCUMENTING THE TIMESCALE OF PLUTON CONSTRUCTION: U-PB GEOCHRONOLOGY OF THE SUBVOLCANIC VINALHAVEN INTRUSION AND ASSOCIATED VOLCANIC ROCKS

The timescale of pluton construction and the relationship between granite plutons and rhyolitic volcanic complexes remain elusive problems that limit our understanding of crustal magmatism. High-precision geochronology of plutons associated with related volcanic rocks, such as the Silurian Vinalhaven Intrusive Complex (VHIC) on the coast of Maine, provide an ideal opportunity to constrain the timescale of pluton construction and establish the temporal relationship needed to examine the connection between subvolcanic rocks and associated volcanic rocks.

The VHIC is an excellent locality for such a study because the spatially-associated volcanic rocks, including flow-banded rhyolite domes mantled by block-and-ash-flow breccia deposits, almost certainly erupted from the intrusion. However, high-precision CA-TIMS U-Pb zircon ages of five granite samples from the intrusion indicate that the exposed level of the intrusion was constructed over at least 0.7 m.y. (419.9 ± 0.1 Ma to 419.2 ± 0.1 Ma), and two rhyolite tuff samples from the volcanic section indicate that the preserved volcanic rocks are 1.3 m.y. older than the intrusion. We believe this gap in time reflects both the poor preservation of the volcanic sequence and the level of exposure of the intrusion, a conclusion supported by a recent microgravity study of the VHIC, rather than a lack of consanguinity.

Two additional lines of evidence support this conclusion. First, granite samples from the intrusion yielded magmatic zircon crystals, with U-Pb ages that span the gap in time between the VHIC and the volcanic rocks. Given field evidence for rejuvenation of crystal mush during magma replenishment, these older zircon crystals represent antecrysts. Second, the Vinalhaven diabase (VHD), which occurs beneath the oldest block-and-ash flow breccias, contains ubiquitous quartz xenocrysts and rare xenoliths of granite. Together with whole rock major and trace element compositions, these observations suggest that the VHD formed by bulk mixing of gabbro and granite early in the history of the VHIC. Zircon xenocrysts in the VHD are similar in habit, size, color, and zoning characteristics to zircons from the VHIC granite, were almost certainly derived from the VHIC, and must be 421.2 Ma or older. Thus, we believe the pluton was constructed over about 2 m.y.