U/PB AND RE/OS DATING OF THE SOUTH MOUNTAIN AND MUSQUODOBOIT BATHOLITHS, NOVA SCOTIA: EVIDENCE FOR PROTRACTED MAGMATIC-HYDROTHERMAL SYSTEMS

The large, peraluminous South Mountain (SMB) and Musquodoboit (MB) batholiths dominate the Meguma Terrane (MT) of southern Nova Scotia and are two of the most prominent intrusions in the Appalachians. Whereas these intrusions have been the focus of detailed mapping and petrological studies, some ambiguity remains regarding the duration of magmatic and hydrothermal activity despite previous U/Pb monazite (SMB), Rb/Sr (SMB) and Ar/Ar (SMB, MB) dating, in part because of partial resetting. Well constrained absolute ages are important in terms of the overall geologic and tectonic evolution of the MT and also the internal evolution of these metallogenetically (Sn, W, U, Cu, Zn, Ag) important intrusions. Herein we present results integrating new age dates (U/Pb zircon and monazite, Re/Os molybdenite) with previously published U/Pb, Rb/Sr and Ar/Ar ages to constrain the magmatic and hydrothermal evolution of these intrusions.

In this study the earliest phase of the SMB is represented by a sample of granodiorite from a drill hole near Dibgy (NSDNR-85-1), whereas a biotite monzogranite from the oldest phase of the MB was selected for dating. Two samples from a deep drill hole at the former tin mine at East Kemptville (EK, NSDNR-90-1) were selected to represent the most differentiated part of the SMB, these being a coarse leucomonzogranite (K-510) and an albitite (K-513). Coarse, disseminated molybdenite from a pegmatite was used as a proxy for mineralization at EK. The results follow: (1) SMB granodiorite zircon age is 380 ± 3 Ma; (2) MB monzogranite monazite age is 377 ± 1 Ma with two discordant zircons indicating a 385 Ma age; (3) coarse-grained leucomonzogranite from EK (K-510) has a discordant zircon age of 385 Ma, whereas the albitite (K-513) has a monazite age of 306 ± 3 Ma; (4) Re/Os age of 376 ± 3 Ma for molybdenite from EK.

Collectively these results combined with previous work suggests the following: (1) a minimum time of emplacement of the two batholiths at ca. 380 Ma, but older plutonic phases (to ca. 385 Ma) are possible; (2) protracted hydrothermal activity and cooling histories such that cooling to <350^oC occurred at ca. 365-370 Ma; (3) episodic hydrothermal activity to ca. 306 Ma. Together these warrant caution in using isolated samples to infer magmatic and hydrothermal histories for large intrusive bodies.