MAP PATTERNS OF ALLEGHANIAN METAMORPHISM MERGED WITH THERMOCHRONOLOGY IN THE BOSTON AVALON TERRANE REVEAL EARLY PALEOZOIC TECTONIC INHERITANCE, LATE PALEOZOIC THRUSTING, AND MESOZOIC EXTENSIONAL TECTONICS

The Boston Avalon terrane (BAt) of SE New England is host to many ⁴⁰Ar/³⁹Ar and older K-Ar ages of micas and amphiboles. The range of ages obtained reflect cooling of the host igneous rocks from the Ediacaran to Permian times, or of resetting of those ages by Alleghanian metamorphism. Unmetamorphosed sediments and volcanics from the eastern BAt show that these rocks have been close to sea level from Ediacaran times to the present.Amphibole cooling ages in the Dedham, Lexington, and Peabody intrusives in the NW BAt are all close to the zircon crystallization ages of Ediacaran, Silurian, and Devonian, respectively (Attenoukon, 2009), showing rapid cooling of these plutons and shallow emplaced in the Avalonian crust.

In contrast, all mica and amphibole ages in the western BAt are reset to Permian and early Triassic ages. The boundary between rocks containing minerals with reset and unreset ages coincides with the biotite and garnet isograds in the Narragansett Basin. Thus, the reset age boundary allows these isograds to be carried north of Providence to the Bloody Bluff fault zone in Mass. South of Providence the isograds turn to the SE north of Prudence Island. This pattern of metamorphism was caused by thrusting of the Putnam-Nashoba terrane (PNt) over the BAt (Wintsch and Sutter, 1986) and their curvature shows the shape of that tongue of the PNt over R.I. This curvature coincides with the NE limit of the New York Promontory suggesting that inheritance from the shape of the Laurentian margin established by the breakup of Rodinia was responsible for this metamorphic pattern.

The time of peak Alleghanian temperature can be inferred by the time of resetting of Devonian detrital muscovite ages to 280 Ma in the Narragansett Basin (Dallmeyer and Takasu 1992). This reset sample is just inside the biotite zone in Warwick, showing resetting at ~400°C, consistent with the typical closure temperature of muscovite. Exhumation and cooling in higher grade rocks began at this time, and all of the BAt rocks were cooled to lowest greenschist facies conditions by late Triassic times. This is also the time of thermal convergence of the PNt and the BAt, following the reactivation of the Lake Char fault as a normal fault (Goldstein 1989).