ALBERT

Notes: The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well).The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism.In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling.Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.

Notes: Acadian (Late Silurian to Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low-P, high-T (Buchan-type) metamorphism followed by thickening at high temperature (〉650d̀ C) and then by cooling to 100-200d̀ C below peak recorded temperatures before eventual unroofing. Mineralogical and textural evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, replacement of low-P cordierite-bearing assemblages by high-P garnet-bearing assemblages, and recrystallization of mylonites associated with late shear zones to form lower-T and higher-P assemblages. Peak conditions in the highest grade rocks were 685-780d̀ C and 5-6 kbar; the cooling path passed through 550d̀ C at about 6.5 kbar.The well-constrained P-T path documented from geological and mineralogical evidence for the CMT offers an unusual opportunity to examine characteristics of fluid inclusions that have experienced a long-lived metamorphic event spanning a broad range of P-T conditions. Fluid inclusion data from the CMT document a range of fluid compositions (CO2-rich, mixed CO2-N2-rich, N2-rich and H2O-rich) and densities during metamorphism. Densities of CO2 fluid inclusions range from 0.20 to 1.03 g cm-3. Medium-density CO2 fluid inclusions are contained in quartz inclusions within garnets in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650-700d̀ C at pressures below 5 kbar. Other CO2 fluid inclusions from matrix quartz yield isochores which pass through 700d̀ C and 5.2 kbar. The highest density inclusions associated with rocks containing the late high-P assemblages have isochores which pass below the estimated P-T conditions for recrystallization of the mylonite. Fluid inclusion evidence suggests an early low-P heating event, followed by thickening at high temperature, and then by nearly isobaric cooling to about 500d̀ C with later decompression. This interpretation is also consistent with previously published petrological models and supports an anticlockwise P-T path for the CMT of south-central Massachusetts.

Notes: Abstract Metre-scale amphibolite boudins in the Cheyenne Belt of south-eastern Wyoming are cut and deformed by shear zones which preserve a full strain transition across 7 cm, from relatively undeformed amphibolite with a relict igneous texture to mylonitic amphibolite with an L-S tectonic fabric. The strain transition is marked by the progressive rotation of amphibole + plagioclase aggregates into parallelism with the shear-zone boundary. An increase in strain magnitude is indicated by development of the tectonic fabric and progressive reduction of amphibole and plagioclase grain size as a result of cataclasis. Bulk chemistry of five samples across a single strain transition shows no significant or systematic variation in major element chemistry except for a minor loss of SiO2, which indicates that the shear zone was a system essentially closed to non-volatile components during metamorphism and deformation. Amphibolites throughout the shear zone consist of amphibole and plagioclase with only minor amounts of quartz, chlorite, epidote, titanite and ilmenite. Within the relatively undeformed amphibolite, amphibole and plagioclase have wide compositional ranges in single thin sections. Amphibole compositions vary from actinolitic hornblende to magnesio-hornblende with increases in Al, Fe, Na and K contents and decreases in Si and Mg that can be modelled as progress along tschermakite, edenite and FeMg-1 exchange vectors from tremolite. Plagioclase ranges from An60 in cores to An30 within grain-boundary domains. With increasing strain magnitude, local variation of amphibole composition decreases as amphibole becomes predominantly magnesio-hornblende. Plagioclase composition range also decreases, although grain-boundary domains still have higher albite content. These petrological data indicate that shear-zone metamorphism was controlled by the magnitude of strain during synmetamorphic deformation. SEM and microprobe imaging indicate that chemical reactions occurred by a dissolution and reprecipitation process during or after cataclastic deformation. This suggests that grain-boundary formation was an important process in the petrological evolution of the shear zone, possibly by providing zones for fluid ingress to facilitate metamorphic reactions. These results highlight the necessity for conducting detailed microstructural evaluation of rocks in order to interpret petrological, isotopic and geochronological data.

Notes: One-dimensional thermal (1DT) modelling of an Acadian (Devonian) tectonothermal regime in southern Vermont, USA, used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters. The area modelled lies within the Vermont Sequence of the Acadian orogen and includes: (i) a western domain containing garnet-grade pre-Silurian metasedimentary and metavolcanic rocks from the eastern flank of an Acadian composite dome structure (Rayponda–Sadawga Dome); and (ii) an eastern domain containing similar, but staurolite- or kyanite-grade, rocks from the western flank of a second dome structure (Athens Dome), approximately 10 km farther east. Using reasonable input parameters based on regional geological, petrological and geochronological constraints, the thermal modelling produced plausible P–T  paths, and temperature–time (T –t) and pressure–time (P–t) curves. Information extracted from P–T –t modelling includes values of maximum temperature and pressure on the P–T  paths, pressure at maximum temperature, predicted Ar closure ages for hornblende, muscovite and K-feldspar, and integrated exhumation and cooling rates for segments of the cooling history. The results from thermal modelling are consistent with independently obtained pressure, temperature and Ar cooling age data on regional metamorphism in southern Vermont. Modelling results provide some important bounding limits on the physical conditions during regional metamorphism, and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2.5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths. In addition, differences in peak metamorphic grade (garnet-grade versus staurolite-grade or kyanite-grade) and peak temperature for rocks initially loaded to similar crustal depths, differences in calculated exhumation rates, and differences in 40Ar/39Ar closure ages are likely to have been consequences of variations in the duration of isobaric heating (or ‘crustal residence periods’) and tectonic unroofing rates. Modelling results are consistent with a regional structural model that suggests west to east younging of specific Acadian deformational events, and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40Ar/39Ar closure during cooling. Modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward-thickening thrust-nappe pile rooted to the east (New Hampshire Sequence), as well as by diachronous structural processes within the lower plate rocks of the Vermont Sequence.