Dating transcurrent terrane accretion: An example from the Meguma and Avalon Composite Terranes in the northern Appalachians

Abstract

The polygenetic Minas Geofracture separates the Meguma and Avalon Composite Terranes of the Maritime Appalachians. Dynamically recrystallized muscovite and biotite occur within D3 shear zones which developed during dextral movement along the Geofracture. These record 40Ar/39Ar plateau ages between circa 360 and 370 Ma. Comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low‐pressure, contact metamorphic aureoles. These are similar to U‐Pb ages previously reported for igneous monazite, therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels. The D3 fabrics were superimposed on earlier (S1) structures which formed during initial amalgamation of the Meguma and Avalon Composite Terranes. Whole‐rock slate/phyllite samples collected outside high‐temperature portions of the plutonic contact aureoles yield 40Ar/39Ar plateau ages which correspond to groupings of circa 400–395 Ma, 390–385 Ma, and 380–375 Ma. These are interpreted to date diachronous formation of the S1 fabrics and concomitant low‐grade regional metamorphism. In the circa 25 Ma between initial amalgamation and granitic plutonism, it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening. This led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to D3 dextral shearing.