Ages and origins of metamorphic fabrics and the tectonics of southeastern New England

This study examines metamorphic fabrics in rocks from the two low-grade (Merrimack and Avalon) terranes exposed in eastern Massachusetts. Polymetamorphic argillites are the principal rock types in the Merrimack terrane, where later generations of chlorite-zone cleavages regionally overprint higher grade cleavages. Truncation of deformed older minerals by younger strain free minerals; preferential precipitation of muscovite and Fe-Ti oxides at the expense of quartz, chlorite and plagioclase; and the increase in aspect ratios of quartz and plagioclase strongly suggest that pressure solution is the dominant mechanism involved in the formation of all cleavage types in these argillites. Electron microprobe analyses of muscovite, chlorite, and plagioclase reveal chemical heterogeneities at the intragranular scale, demonstrating that pressure solution operated in closed, fluid-limited systems, where local mineral assemblages internally buffer the compositions of small fluid reservoirs. Chemical equilibrium is, thus, limited to domains of microns between adjacent cleavage-forming minerals. 40Ar/39Ar dating of muscovite in relict high-grade cleavages yielded Late Devonian (Acadian) cooling ages; in contrast, later muscovite generations that define overprinting chlorite-grade fabrics preserve Late Paleozoic (Alleghanian) growth ages. The Merrimack terrane and other cover rocks in eastern Massachusetts are underlain by the Avalon terrane, which is comprised of variably metamorphosed igneous and sedimentary rocks. Compilation of geochronologic data together with microtextural study of Avalon terrane meta-igneous rocks reveal field gradients in 40Ar/ 39Ar cooling ages and deformation intensity. In northeastern Massachusetts, Silurian to Devonian 40Ar/39Ar magmatic cooling ages were obtained in weakly recrystallized diorites and granites that preserve primary igneous textures. In southeastern Massachusetts, orthogneisses are known to have Neo-Proterozoic to Devonian magmatic crystallization ages, but new Permian to Triassic40Ar/39Ar cooling ages from these rocks become younger with increasing deformation intensity and loss of primary igneous fabrics toward the terrane-bounding Bloody Bluff fault, indicating thermal resetting caused by Alleghanian amphibolite facies metamorphism as Putnam-Nashoba rocks thrust eastward over Avalon. Truncation of north-south trending cooling age isopleths by the Bloody Bluff fault implies a transition to normal fault motion in the Mesozoic.