LINKING PARTIAL MELTING IN THE NASHOBA FORMATION TO THE ANDOVER GRANITE, EASTERN MASSACHUSETTS

The Nashoba terrane (NT) is a NW-dipping, shear zone-bounded, Cambrian-Ordovician, arc-backarc complex intruded by late Ordovician to earliest Carboniferous plutons. High-grade metamorphism in the NT caused the metasedimentary units of the Nashoba Formation (NF) to partially melt. The NE portion of the NT is dominated by the Andover Granite (AG). The contact between the NF and AG is poorly exposed. To investigate the relationship between the two units, U/Pb geochronology and whole-rock geochemistry were performed on the NF and compared to previous work on the AG.

Zircon grains from leucosome samples and slightly migmatitic gneiss from the NF were analyzed using secondary ion mass spectrometry techniques to determine their U/Pb isotopic ages and trace element abundances. Zircon cores were both oscillatory and sector zoned with multiple prominent overgrowths. Episodic zircon crystallization occurred from ~425 to ~385 Ma, indicating that migmatitic conditions lasted for ~40 million years. Four zircon populations were distinguished, based on grain morphologies, ages, Th/U ratios and REE patterns and have ages of ~420 Ma, ~415-412 Ma, ~407-405 Ma and ~395-393 Ma. Two pulses of AG have been dated at ~420 and ~412 Ma, indicating that early migmatization and granite crystallization were coeval.

Whole rock geochemical data were collected on NF leucosome, adjacent selvedges and previously interpreted “non-migmatitic” NF Bt+Sil+/-Gt gneiss. NF leucosome and pulses of the AG are peraluminous with similar major element oxide vs silica ratios, and comparable CIPW norm values that plot along the eutectic curve in Qtz-Ab-Or space. The geochemical data suggest that partial melting of the NF may have generated the AG, although an alternative AG source of comparable composition can’t be ruled out.

Selvedges and “non-migmatitic” gneiss have comparable chemistry and similar mineralogy. Primary muscovite is absent from both. The restitic composition of the NF “non-migmatitic” gneiss suggests that these rocks did undergo anatexis, and the leucocratic fraction exited the system. The geochemical and geochronological data suggests that anatexis of the NF possibly led to the formation of the AG, and/or perhaps plutons of similar age in the overlying Merrimack belt (e.g. Ayer, Chelmsford, and/or Fitchburg).