INTRUSIVE RELATIONS AND ISOTOPIC AGES OF DEVONIAN GRANITES IN SOUTHERN AND CENTRAL VERMONT: EVIDENCE FOR A PROLONGED ACADIAN OROGENY AND PARTITIONING OF COMPRESSIONAL STRAIN

Granite and trondhjemite of the Devonian New Hampshire Plutonic Suite form NE- and NW-trending swarms of dikes and small discordant stocks that intrude Acadian (F1, S1) deformed rocks at a wide range of crustal levels, from subgarnet and garnet grade, along the western margin of the Connecticut Valley-Gaspe trough in the NW to staurolite-kyanite grade (Chester-Athens and Guilford domes) in the SE.

Ages of zircon from granites from four settings (NW to SE) have been obtained on the SHRIMP: (1) biotite granodiorite intruding the Gile Mountain and Waits River Formations at East Barre, 368±4 Ma (²⁰⁶Pb/²³⁸U); (2) trondhjemite dike intruding the Hoosac Formation, Plymouth, 365±6 Ma (²⁰⁶Pb/²³⁸U); (3) biotite granite in the Chester dome near Gassetts, 392±6 Ma (²⁰⁶Pb/²³⁸U); and (4) granite at Black Mountain, Guilford dome 366±4 Ma (composite ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb) and monazite (TIMS ²⁰⁷Pb/²³⁵U) age of 366±1 Ma.

Granites at Bethel and East Barre bracket Acadian F2 (S2), whereas similar dikes that intrude NE across the Athens dome are syn-Acadian F4 (S4) (Ratcliffe and Armstrong, 2001: USGS I Map 2636). 392 Ma nonfoliated dikes in the core of the Chester dome cross cut predome Acadian S1, record only vertical shortening, and predate published hornblende Ar/Ar cooling ages of 379 to 368 Ma and muscovite that has core to rim laser Ar/Ar ages of ~390 to 330 Ma. 367 Ma foliated dikes on the flanks of the major domes were folded by tightening of the same folds that they intrude. At many localities garnet grew in cleavages that affect the younger granites, indicating garnet growth as late or later than 366 Ma. At Plymouth, dikes locally are thrust faulted and generally crenulated by Acadian S2.

From these data we conclude that granite intrusion and unevenly distributed Acadian deformation started before 392 Ma and peaked about 365 Ma; in the Chester dome cooling through muscovite Ar retention temperatures was as late as 330 Ma. Igneous and metamorphic activity may have been episodic. F2(S2) structures may have formed diachronously, from SE to NW as protracted late Acadian strain accumulated from the Chester-Athens domes southeastward and coeval strain was partitioned into narrow shear zones and broad flexures to the west