Geochemistry and tectonic implications of Hawley formation meta-igneous units: Northwestern Massachusetts

In northwestern Massachusetts, the Hawley Formation is the eastern unit of the Rowe-Hawley Belt (RHB), a proposed Taconian (Ordovician) arc-trench gap sequence (Stanley and Ratcliffe, 1985). The Hawley Formation consists or three informal members: (1) the Warfield Mountain Amphibolite of volcanic origin, (2) the Legate Hill Brook Metadacite also of volcanic origin, and (3) conformably overlying Sanders Brook Black Slates. The Dell Metatrondhjemite intrudes the Hawley Formation; all units in the eastern half of the Hawley Formation are intruded by the Charlemont Mafic Intrusive Suite. Amphibolites from the Warfield Mountain Amphibolite Member of the Hawley Formation have IAT, MORB/BABB, and boninitic geochemical signatures. The Legate Hill Brook Metadacite and Dell Metatrondhjemite have arc or forearc geochemical signatures. The Charlemont Mafic Intrusive Suite amphibolites have a backarc basin geochemical signature. All map units are affected by a dominant, planar, north-northeast trending, steeply east-dipping Acadian (Siluro-Devonian) foliation. The best minimum age for the Hawley Formation is a 462 Ma U/Pb zircon age from a tonalitic intrusive into a correlative of the Hawley Formation in southern Vermont (Karabinos and others, 1996). The presence of boninitic amphibolite in the Hawley Formation suggests that the early mafic units had a forearc origin because "modern" boninites have only been found in situ in forearc settings. The geochemical complexity of the Hawley Formation is also similar to that observed in "modern" forearc environments. Forward modeling and the bimodality of the Hawley metaigneous units suggests that the trondhjemites were produced as partial melts; these melts may have been generated from mafic units such as Group I and II amphibolites. The forearc origin of the Hawley Formation may be explained by either of two basic models of boninite genesis: (A) initiation of subduction ("infant arc") model, (B) forearc rifting/spreading model. In either case, we propose that the trondhjemites were generated later as partial melts from the subducting oceanic slab or from the base of the arc crust. Finally, the enriched tholeiitic sills (group IV) may have resulted from an episode of Ordovician backarc extension. Potential correlatives to Hawley Formation geochemical units can be found in the adjacent Moretown and Collinsville (Shelburne Falls Dome) formations. Amphibolites from the easternmost Moretown Formation are geochemically similar to MORB and, although separated from the Hawley Formation by a major structural discontinuity (Zoar/ South Newfane Thrust), are very similar to Hawley Group II amphibolites. Trondhjemites and mafic sills from part of the Shelburne Falls Dome (Collinsville Formation) are very similar to the Hawley trondhjemites and boninitic and Group I amphibolites, respectively.