EVIDENCE OF THE EARLY ACADIAN IN WESTERN MASSACHUSETTS FROM HIGH PRESSURE GARNET-KYANITE SCHISTS

Restitic schists from the Goshen Dome of western Massachusetts contain cm-scale garnet and multiple generations of kyanite and cordierite. Relict megacrystic (5cm) phosphatic garnets with prograde major-element zonation and anomalously ‘light’ O-isotopic ratios (d18O 2.0 core, to d18O 3.0 rim) are enveloped by this matrix and suggest subduction-related growth. Regional studies have reported dates consistent with the Taconic orogeny and a late Acadian overprint. The chronology of this previously unrecognized unit—particularly its prograde history—is therefore of great interest to regional tectonic interpretations.

This study presents laser ablation split stream monazite dates measured in situ from this unit. Monazite has not been found within the relict garnets, but is present within crd + ky pseudomorphs, as inclusions in cm-scale garnet, and in the matrix. Cores of monazite found in the pseudomorph domains yield the oldest ages (383 to 379 Ma), have steep HREE patterns and are depleted in Y. Overgrowths on these grains yield ages of 377 to 367 Ma, have shallower HREE patterns and are enriched in Y. These data suggest that the cores grew in the presence of a prior generation of garnet. These monazite cores may represent prograde growth, but we cannot rule out initial growth during retrograde metamorphism. The overgrowths likely formed during garnet breakdown, which we interpret as retrograde metamorphism.

The cores of monazite within matrix garnet yield a narrow age range of 381–379 Ma. Overgrowths on these cores—prior to their inclusion in late-stage garnet—yield overlapping ages of 380–378 Ma. Matrix monazites have cores as old as 378 Ma, with Y-enriched overgrowths that span 377­­­–361 Ma. These data demonstrate that 1) these rocks are related to the Acadian—and not the Taconic—orogeny, 2) if the monazite cores are prograde, the rocks were at peak conditions for only a few Ma, and 3) retrograde reactions occurring throughout exhumation produced a ca. 15 Myr record of monazite growth.