THE LASER’S EDGE: PUSHING DETRITAL ZIRCON GEOCHRONOLOGY TO THE LIMIT TO STUDY COMPLEX ROCKS IN THE PELHAM DOME, NORTHERN APPALACHIANS

Detrital zircon geochronology in rocks with complex tectono-thermal histories is challenging. Metamorphism affects the U-Pb systematics of detrital zircon through recrystallization and Pb loss, resulting in dates that are too young. Also problematic is intrusion of granitoids followed by intense deformation because that adds post-depositional zircon that can be difficult to distinguish from the detrital zircon based on composition and age. Studies that combine high spatial resolution (SIMS or LA-ICPMS) with high age precision (CA-TIMS) are often necessary to ensure accurate detrital ages.

The Pelham dome in central MA is one of 21 domes in the Ordovician Bronson Hill composite arc (BHCA) and the only that has Neoproterozoic rocks of the Dry Hill Gneiss (DHG) in its core. The DHG was interpreted as metavolcanic based, in part, on its intercalation with the metasedimentary Poplar Mountain Gneiss (PMG), which was also considered Neoproterozoic. Our goal was to determine if Paleozoic dates (~330, 390, 420, and 470 Ma) obtained from 18 grains (7%) from one PMG lithology are accurate detrital ages or too young. Creation of 30-40 mm thick zircon wafers polished on both sides allowed for 3-8 LA-ICPMS dates per grain and two CA-TIMS dates per grain from fragments that were cut out with a laser. Within-grain age agreement suggests all dates are accurate; there is no mixing with ubiquitous ~290 Ma rims that are clearly metamorphic. Compositions of the ~330-470 Ma grains show they did not grow during metamorphism or pegmatite crystallization, but instead are likely from arc magmas. To test whether these grains were added to the PMG as inherited cores in zircon from pegmatite that was sheared into the PMG, a pegmatite-rich rock was analyzed. It has 330 Ma zircon, but lacks 390-470 Ma grains, making an inherited origin for them unlikely. We conclude that parts of the PMG were deposited after 390 Ma, possibly after 330 Ma, thereby providing evidence for a Carboniferous extensional or transtensional basin on a Gondwanan-derived accreted terrane and Alleghenian underthrusting of it under the BHCA.