NEW ZIRCON U/PB AGE CONSTRAINTS FOR THE AGAMENTICUS COMPLEX OF SOUTHEASTERN MAINE

The Agamenticus Complex, an intrusive igneous body located near York, Maine, consists of biotite granite (innermost unit), alkalic syenite, quartz syenite, and alkalic granite (outermost unit)¹. Previous geochronologic studies^2,3, using apatite fission track, Rb/Sr, and K/Ar analyses, produced ages ranging from 227 ± 3 Ma to 216 ± 4 Ma. The most recent work⁴ assigns it to the early Triassic at 233 ± 5 Ma, based on corrected^1,5 K/Ar measurements, making it the oldest member of the White Mountain Magma Series (WMMS). Given the problems with older dating methods and the potential for high precision measurements permitted by newer techniques, we reevaluate the geochronology of Agamenticus by presenting laser ablation ICP-MS measurements of zircon U/Pb content from each lithologic unit. In addition to providing new age constraints, our study motivates the investigation of other important questions. For example, given the timing of related volcanism, we may find ashes in the nearby Newark-Hartford basin record, a potential correlative marker for the astronomically calibrated time scale of the late Triassic⁶. However, our preliminary results suggest a late Permian age for the complex – important because its magnetizations could constrain the Kiaman Superchron as well as the plate position of North America. In this case, its ashes would likely be in the Honeycomb Point Formation of New Brunswick, Canada or the Ikakern Formation of Morocco (both assigned a late Permian age). Additionally, a high-resolution chronology of co-magmatic units allows us to test emplacement models for the complex: incremental assembly over a few million years or uniform cooling upon emplacement? Finally, if the complex is older than previously described, the geochronology of the entire WMMS may require reevaluation. In order to robustly evaluate and investigate these questions, future work includes refining our measurements with ultra-high precision analyses of zircon U/Pb content through CA-TIMS.

1. Hussey and Brooks (2014): Maine Geological Survey, Progress Map 14-1, scale 1:24000 2. Christopher GSA Bulletin 80.9 (1969): 1809-1814. 3. Foland et al. AJS 270.5 (1971): 321-330. 4. Foland & Faul. AJS 277.7 (1977): 888-904. 5. Dalrymple Geology 7.11 (1979): 558-560. 6. Kent & Olsen. JGR: Solid Earth (1978–2012) 104.B6 (1999): 12831-12841.