GEOCHRONOLOGICAL UPDATES ON UNDIFFERENTIATED GABBROS AND DIORITES IN SOUTHEASTERN NEW ENGLAND, USA

Complex relationships between granitoid rocks dominating Avalonia in SE New England and intermediate to mafic phases traditionally assigned to the Newburyport Quartz Diorite and Salem Gabbro-Diorite were well known before B.K. Emerson's 1916 Geologic Map of Massachusetts and Rhode Island. This map appeared with the caveat that "separate mapping is not feasible," and in 1983, compilers of the Bedrock Geologic Map of Massachusetts followed suit by rendering these assemblages as undifferentiated Neoproterozoic gabbro and diorite (Zdigb, Zdi). They also stressed geochronology as the path to further distinctions. Pioneering efforts of R.E. Zartman and R.S. Naylor already in progress at that time soon yielded an upper concordia intercept date of 450 ± 15 Ma based on combined zircon fractions from two Newburyport diorites intruding the Nashoba terrane near the New Hampshire border. Occurrences dated by J.C. Hepburn and colleagues fifteen later revealed a history of episodic Paleozoic alkalic magmatism among gabbro-diorites along the northwest margin of Avalonia.

Subsequent results obtained using refined U-Pb methods now provide increasingly precise and accurate age determinations from gabbro-diorites elsewhere in southeastern New England. Most recently, CA-TIMS analysis of 6 zircons from diorite at Rowley, MA establishes a weighted mean ²⁰⁶Pb/²³⁸U crystallization age younger than the reported 646 Ma K-Ar date of Zartman and Naylor, but ~ twenty million years older than 609-606 Ma U-Pb zircon dates from neighboring Dedham Granite north and west of Boston. Topsfield Granodiorite, long inferred from field relationships to be co-magmatic with the Rowley Diorite, yielded very little zircon, but CA-TIMS analysis of one of these is virtually identical to the Rowley result. The fault-bounded Rowley-Topsfield suite records the oldest known magmatism yet recognized in the Avalonian arc complex of southeastern New England, and prospective dates from similar-looking samples in more southerly locations may document a broader footprint. It is also clear that episodic mafic magmatism is not limited to Paleozoic alkalic events. Quartz diorite from the Zdi map unit south of Boston yielded a 589 ± 2 Ma upper intercept date consistent with its chilled contact against 599 ± 1 Ma Westwood Granite.