GEOCHRONOLOGIC CONSTRAINTS ON THE ORIGIN OF THE ANDOVER GRANITE IN THE CONTEXT OF THE TECTONIC HISTORY OF THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

The Andover Granite is a complex multi-phase aluminous to peraluminous granitic suite that dominates the northern Nashoba terrane in eastern Massachusetts. The suite is composed of both foliated and unfoliated two-mica granites as well as unfoliated biotite granite, muscovite granite, and granodiorite phases. The youngest unfoliated phase was dated at 412±2 Ma (U-Pb ID-TIMS monazite; Hepburn et al., 1995) while the foliated two-mica phase was previously dated at ~450 Ma (whole rock Rb-Sr; Zartman & Naylor, 1984; Hanford, 1965). New preliminary U-Pb CA-TIMS zircon ages on two samples from the foliated two-mica phase are ~420 and ~419 Ma, bracketing the crystallization of the entire Andover suite between ~420 and ~412 Ma. Whole rock geochemical analyses of the newly dated samples indicate they have peraluminous compositions.

We interpret the Andover suite as having originated in the Late Silurian during subduction of the oceanic crust between the approaching Avalonian micro-continent and the trailing margin of Ganderia, on which the Nashoba terrane is currently located. At this time, significant heat was brought into the crust as a result of crustal thickening and the intrusion of intermediate composition calc-alkaline magmas such as the Sharpners Pond Diorite (430±5 Ma; U-Pb zircon, Zartman & Naylor, 1984). This heat influx caused upper amphibolite facies metamorphism (~423 Ma: U-Pb EMP monazite; Stroud et al., 2009; 425 ± 3 Ma, U-Pb ID-TIMS monazite, Hepburn et al., 1995) and likely triggered anatectic melting of the Proterozoic to early Paleozoic basement rocks of the terrane, providing source input for the Andover granitic suite. This scenario fits well with the regional tectonic model of van Staal et al. (2009) for the northern Appalachians in the Late Silurian as it shows the precursor of the docking of Avalonia to the eastern margin of composite Laurentia, similar to the early stages of the Acadian orogeny shown in their model.