Tectono-metamorphic evolution of western Maine, Northern Appalachians, USA

Section A: Multiply-deformed schists in Western Maine were multiply metamorphosed under lower-amphibolite-facies conditions during the Acadian orogeny. Most of the many deformations were partitioned on a regional scale into zones dominated by coaxial bulk shortening and surrounded by anastomosing zones of non-coaxial shortening and shearing. Protection provided by plutons emplaced at different times during this history resulted in the matrix foliation orientation varying with respect to pluton proximity as well as to the regional-scale structures. Garnet and staurolite porphyroblasts grew episodically many times as deformation was partitioned and repartitioned during successive tectonic events. A succession of five FIAs trending ESE-WNW, N-S, E-W, ENE-WSW and NE-SW were measured from both garnet and staurolite porphyroblasts. Thus at least five periods of growth are inferred for these phases, in contrast with only two periods of growth as was previously believed. Pitch measurements of inclusion-trail orientation in staurolite porphyroblasts revealed that the foliations preserved as inclusion trails have similar trends with FIAs and are steeply dipping. Well-defined inclusion trails revealed complex porphyroblast-matrix relationships resulting from deformation and porphyroblast growth both being partitioned very heterogeneously across the region. The deformation and metamorphism were totally synchronous, and the partitioning of the deformation at the scale of a porphyroblast always controlled the location of its growth. Section B: Emplacement of the Mooselookmeguntic pluton, located in the western Maine region of the Northern Appalachians, was thought to have occurred towards the end of the Acadian deformation at around 370 Ma. Foliation inflection/intersection axes within porphyroblasts (FIAs) from its aureole reveal at least five periods of garnet and staurolite growth. EPMA dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of FIA development reveals a succession of apparrent ages from 410 Ma to 345 Ma. This succession indicates that deformation and metamorphism outlasted the Acadian orogeny and continued into Alleghanian, the thermal structure of the orogen progressively evolved to enable pluton emplacement, and it continued to develop afterwards with magmatic fluids still forming at depth. Section C: Microstructural measurements of FIAs (foliation intersection/inflection axes preserved in porphyroblasts) in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado front range and 5 in the Palaeozoic of western Maine. Monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1), 1720.4±6.8 Ma (FIA 2), and 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8.8 Ma (FIA 3), 372.1±5.5 Ma (FIA 4), and 352.7±4.2 Ma (FIA 5) in Maine, confirming the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Thermodynamic modeling in the NCMnKFMASH system reveals that this episodic growth of staurolite occurred over a similar bulk compositional range and PT path. Multiple phases of growth by one reaction in the same and adjacent rocks in both regions strongly suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that each stage of growth in both areas occurred during deformation and indicate that the local partitioning of deformation at the scale of a porphyroblast was the controlling factor on whether or not the reaction took place. Section D: Tectonic activity took place virtually continuously from before 420 Ma to less than 345 Ma in the vicinity of the Mooselookmeguntic Pluton. Field observations, petrological studies, foliation intersection/inflection axis (FIA) measurements, monazite dating of FIAs, and phase diagram modeling reveal at least five periods of growth of garnet and staurolite porphyroblasts over most of this period. At least one period of andalusite growth followed, before decompression into the cordierite field led eventually to retrogression. Peak metamorphic conditions of ~4 kbars and 600 °C were attained after the five periods of garnet and staurolite growth in the sillimanite field, although, locally, some fibrolite formed very early. The syn-kinematic Mooselookmeguntic pluton was progressively constructed at approximately the same structural level during successive deformations and over a long period of time rather than being the result of one discrete event as it has been previously proposed