THE LUDLOW BATHOLITH: GEOCHRONOLOGY AND PETROLOGY OF A LIKELY COGENETIC INTRUSIVE SUITE
The major felsic units in the Bristol Mountains exhibit concentric zoning and decrease in age from rim to core. Three new zircon U-Pb radiometric ages show ages of 77.4±1.0 Ma at the rim of the pluton, and ages of 75.6±0.9 Ma and 75.8±1.0 Ma in samples closer to the core. These results are consistent with preexisting U-Pb zircon age data from Hess, 2017. From rim to core the four main units are equigranular hornblende biotite granodiorite with columnar biotite, equigranular biotite granodiorite with flaky biotite, porphyritic biotite granodiorite with flaky biotite, and porphyritic biotite granodiorite with glomeroporphyritic biotite. Mafic enclaves are predominantly present on the southern rim of the pluton and more scarcely distributed in eastern areas. The western side of the field area is dominated by mylonitic and meta-plutonic rocks, while the eastern side consists of several scattered patches of gabbro, which range in size within the outcrops of granodiorite. From the data presented thus far, it can be concluded that the Bristol Mountain units are cogenetically related.