CONSTRAINING THE ABSOLUTE AGE OF MULTIPLE FABRIC-FORMING EVENTS IN POLYMETAMORPHIC ARGILLITES, MERRIMACK TERRANE, MASSACHUSETTS

To determine the extent of Acadian and Alleghanian metamorphism in eastern Massachusetts, ⁴⁰Ar/³⁹Ar dating of whole rock and muscovite samples was applied to polymetamorphic argillites from the Worcester (WF) and Merrimack formations (MF) of the Merrimack terrane.  Two distinct metamorphic events are identified in the Merrimack terrane.  A higher grade metamorphic field gradient (M1), increases in grade to the west from chlorite through biotite, garnet, staurolite, and andalusite zones, crossing the MF and WF.  However, a second event (M2) constituting chlorite grade minerals in multiple fabrics regionally overprints rocks in all parts of the MF and WF.  The timing of the M1 event is constrained by the minimum ⁴⁰Ar/³⁹Ar cooling age of 370 Ma for muscovites from the andalusite zone.  This Late Devonian cooling age is supported by similar Late Devonian and Early Carboniferous age steps in age spectra of multiple muscovite populations in chlorite and biotite zone rocks.  Thus, the metamorphic field gradient is constrained to be older than Late Devonian, and is present even in low-grade rocks.  M1 is undoubtedly an Acadian event. 

The age of the M2 event is constrained by the age of overprinting muscovites in the muscovite-dominated cleavages (S_n+1).  An age spectrum from an andalusite zone sample, where almost all M1 muscovites have been recrystallized into S_n+1, yields an age minimum of 305 Ma, which gives the maximum age of this event.  However, the age minimum for a chlorite zone sample lacking an earlier cleavage is 275 Ma, which is probably a close estimate of the age of the M2 overprinting cleavage.  This age is supported by Permian age steps in age spectra of mixtures of M1 and M2 muscovites from other chlorite zone samples.  These results indicate that M2 is an Alleghanian event. 

We interpret all chlorite zone muscovites as preserving crystallization ages because they are much younger than the Late Silurian detrital muscovites in a chlorite-grade sandstone that were not reset by either M1 or M2.  Thus, the conspicuous field gradient in the Merrimack terrane was established in the Acadian orogeny, but the conspicuous cleavage in most outcrops was produced in the Alleghanian orogeny.