U-PB CONSTRAINTS ON MAGMATISM RELATED TO THE VINALHAVEN INTRUSION, COASTAL VOLCANIC BELT, MAINE

The two main units within the Vinalhaven (VH) pluton are intruded by basaltic and granitic dikes and by a small pluton of fg granite, which record the input of mafic and silicic magma into a silicic magma chamber. Field relationships in the cg granite and the layered gabbro-diorite-granite unit preserve evidence for the accumulation of granitic mush on the floor of the silicic chamber and indicate which way was up while the system was active. The roof of the pluton includes a folded sequence of volcanic and fossiliferous sedimentary rocks that preserves the early magmatic history. High-precision U-Pb zircon geochronology of volcanic rocks in the roof and plutonic rocks from within the intrusion constrains the age of the crustal column beneath the pluton, the duration of magmatism, and perhaps the timescale over which the silicic magma chamber was active.

Inherited cores in zircon crystals from tuff, cg granite and fg granite range in age from 435 to 1465 Ma. Metamorphic rocks of early Cambrian age are exposed throughout the Coastal Volcanic Belt, but the pattern of inheritance clearly indicates that the crust beneath the VH intrusion is more heterogeneous than rocks exposed at the surface.

Magmatism in the area persisted for at least 6 m.y. The oldest magmatic rocks, rhyolite tuffs interbedded with sedimentary rocks, erupted at 426 ± 1 Ma. These rocks are overlain by a thick package of rhyolite tuffs that are compositionally similar to the adjacent VH granite and overlap in age. A tuff collected near the top of the volcanic section yields an age of 420 ± 2 Ma, whereas zircons from two granite samples collected at different stratigraphic levels in the pluton yield an age of 420.3 ± 0.5 Ma (n=8; MSWD=0.16). The upper portion of the volcanic section appears to have erupted from the VH magma chamber and magmatism in the area spanned the interval of 426 to 420 Ma.

Even though all eight U-Pb analyses from the two samples of cg granite produce a precise average age, nominal Pb-Pb dates from the stratigraphically higher sample are systematically younger by about 0.7 m.y. Although our preliminary U-Pb data are insufficient to draw firm conclusions, we suggest that the VH magma chamber was active for hundreds of thousands of years, during which time it periodically erupted high-silica rhyolite and was periodically replenished by both mafic and silicic magma.