Implications of silurian granite genesis to the tectonic history of the nashoba terrane, eastern Massachusetts.

摘要

The Nashoba terrane is a highly metamorphosed and sheared Paleozoic tectonic block in eastern Massachusetts. The metamorphic rocks that compose the terrane are intruded by a series of diorites, tonalites, and granites. The Andover Granite is a complex multiphase granitic suite found in the northern part of the Nashoba terrane and is composed of both foliated and unfoliated granites as well as a granodiorite phase. The Sgr Group of granites is a series of unfoliated granites exposed along the Nashoba-Avalon terrane boundary. New crystallization ages for the foliated Andover Granite and the Sudbury Granite, southernmost body of the Sgr Group of granites, are presented. CA-TIMS U-Pb geochronology on zircons collected from these granites yielded 419.43 +/- 0.52 Ma and 419.65 +/- 0.51 Ma crystallization ages for the foliated Andover Granite and a 420.49 +/- 0.52 Ma crystallization age for the Sudbury Granite. Geochemical and petrographic analysis of these granites indicate that the foliated Andover Granite is a high-K calc-alkaline, peralmuminous, S-type, biotite + muscovite granite and the Sudbury granite is high-K calc-alkaline, metaluminous to slightly peraluminous, I-type, biotite granite. These two granites are interpreted to have formed from the anatexis of either Nashoba terrane metasedimentary rocks and/or its underlying basement just prior to the Acadian orogeny. It is proposed that when Silurian diorite/tonalite magmas intruded into the Nashoba terrane, the influx of magmatic heat was sufficient to trigger crustal melting and promote granite genesis. This petrogenetic scenario fits well with regional tectonic models showing the Silurio-Devonian convergence of Avalonia towards Ganderia (which formed the eastern side of composite Laurentia at the time) in the northern Appalachians. Prior to the collision of Avalonia to composite Laurentia, mafic and intermediate composition arc magmas intruded the eastern Ganderian margin. The large amount of heat that accompanied these intrusions is believed to have contributed to Acadian metamorphism and influenced the formation of granitic plutons along the margin. It is therefore proposed that the plutonic record of the Nashoba terrane shows that by the Late Silurian - Early Devonian, Avalonia was still outboard of Laurentia in the vicinity of southern New England.