IN SITU RB-SR MICA GEOCHRONOLOGY OF WESTERN MAINE PLUTONS

Western Maine hosts plutons with petrographically diverse characteristics of both I- and S-type granites with U-Pb zircon (crystallization) ages ranging between 452 and 320 Ma. Here, we present in situ Rb-Sr dates of biotite and muscovite within 16 samples from six plutons to investigate the cooling history of these rocks. In situ Rb-Sr dates were determined by LA-ICP-MS/MS at the University of Maine MAGIC Lab using an ESL NWR193UC excimer laser coupled to an Agilent 8900 ICP-MS/MS. Biotite dates from 14 samples range between 336-238 Ma, which average ~125 Ma younger than their U-Pb ages. Muscovite dates from 11 samples range from 403-251 Ma, which average ~70 Ma younger than their U-Pb ages. Comparatively, muscovite dates are consistently older than biotite by ~55 Ma. Out of six plutons, Phillips Pluton was the only sample in which Rb-Sr muscovite dates overlapped with U-Pb zircon ages, indicating that Rb-Sr muscovite dates record the crystallization ages. However, there is also evidence for partial resetting of the muscovite from the Phillips pluton. The significant difference in timing between U-Pb zircon dates, Rb-Sr muscovite dates, and Rb-Sr biotite dates suggest either a very protracted cooling history across western Maine, or a potentially complex resetting history. The latter is more likely; however, a higher sampling density and additional trace element analyses would help elucidate between possible fluid-mediated or thermal events. In either case, our results show that each of the geochronological systems, particularly Rb-Sr muscovite and biotite, behaves differently in terms of closure to diffusion of radiogenic Sr.