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Monazite crystallization ages have been measured in situ using SIMS and EMP analysis of samples from the Bronson Hill anticlinorium in central New England. In west-central New Hampshire, each major tectonic unit (nappe) displays a distinctive P-T path and metamorphic history that requires significant post-metamorphic faulting to place them in their current juxtaposition, and monazite ages were determined to constrain the timing of metamorphism and nappe assembly. Monazite ages from the low- pressure, high-temperature Fall Mountain nappe range from c. 455 to 355 Ma, and Y zoning indicates that these ages comprise three to four distinct age domains, similar to that found in the overlying Chesham Pond nappe. The underlying Skitchewaug nappe contains monazite ages that range from c. 417 to 307 Ma. 40 Ar/ 39 Ar ages indicate rapid cooling of the Chesham Pond and Fall Mountain nappes after 350 Ma, which is believed to represent the time of emplacement of the high-level Chesham Pond and Fall Mountain nappes onto rocks of the underlying Skitchewaug nappe. Garnet zone rocks from western New Hampshire contain monazite that display a range of ages (c. 430-340 Ma). Both the metamorphic style and monazite ages suggest that the low-grade belt in western New Hampshire is continuous with the Vermont sequence to the west. Rocks of the Big Staurolite nappe in western New Hampshire contain monazite that crystallized between c. 370 and 290 Ma and the same unit along strike in northern New Hampshire and central Connecticut records ages of c. 257-300 Ma. Conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks. These monazite ages confirm that the metamorphism observed in the Big Staurolite nappe occurred significantly later than that in the units structurally above and below. These data support the hypothesis that the Big Staurolite nappe represents a major tectonic boundary, along which rocks of the New Hampshire metamorphic series were juxtaposed against rocks of the Vermont series during the Alleghanian.

An exposure near Gassetts, Vermont, contains lithologies varying from staurolite-kyanite grade aluminous schists with paragonitic muscovite to potassic gneiss with phengitic muscovite. Single-crystal laser fusion 40 Ar/ 39 Ar ages for paragonitic and phengitic muscovite yield similar distributions with ranges between 366 ± 4 and 326 ± 4 Ma. Intracrystalline ages vary from ca. 394 ± 4 to 330 ± 4 Ma. Thus, we find that the intracrystalline (core-rim) age distribution of relatively large, single crystals essentially encompasses the range of ages obtained through total fusion of smaller crystals, consistent with models for development of diffusion profiles and 40 Ar-closure during cooling with a diffusion dimension controlled by the physical grain size. However, some of the larger crystals studied, particularly those with prominent microscopic defects (features readily evident such as internal grain boundaries and twin planes), yield relatively young ages and lack significant core-rim age discordance. Furthermore, the overall distribution of single-crystal ages in the two samples is bimodal, and we suggest that this age distribution reflects metamorphic deformation and recrystallization event(s) superimposed on early generation muscovite. Thus, the mean age of muscovite in these samples (typical of K/Ar and 40 Ar/ 39 Ar incremental heating analysis of bulk mineral separates) has little relationship to any single, hypothetical closure temperature. In view of the similar results we obtain for muscovite of contrasting composition, the net effects of variations in grain size, deformational character, and growth history are interpreted to be more important in forming the observed variations in age than are the chemical substitutions in these samples.

Mountain, Skitchewaug and Big Staurolite nappes. Reactivation of A new, detailed tectonic model is presented for the Acadian orogenic this fabric during thrusting is recorded in some rocks of the Big belt of central New England (Vermont and New Hampshire) that Staurolite nappe by rotated garnets that grew during near-isothermal accounts for a wide range of petrological and structural observations. loading. Only the sillimanite isograd crosses the Fall Mountain– Three belts are considered: the Eastern Vermont, Merrimack, and Skitchewaug nappe boundary. Metamorphic breaks across the Skitintervening Bronson Hill belts. Specific observations in eastern chewaug–Big Staurolite nappe boundary, at the base of the Big Vermont that are accounted for in the model include the following. Staurolite nappe, and at the margin of the Keene and Alstead domes P–T paths are clockwise with maximum pressures near the Athens, require post-metamorphic thrusting when P–T conditions were in Chester, and Strafford domes of 8–11 kbar, but with maximum the greenschist facies. These observations can be explained by a pressures decreasing to 3–5 kbar at the boundary with the Bronson relatively simple model involving in-sequence thrusting from east to Hill belt. Differential exhumation of the Vermont domes relative to west commencing in central New Hampshire at 400–410 Ma. the rocks in easternmost Vermont is required by the recorded differences Preservation of the low-grade belt along the Vermont–New Hampin maximum pressure (5–6 kbar; 15–20 km) and the present-day shire border requires that crustal thickening in Vermont was not geographical separation (7–10 km). Specific observations in New caused by emplacement of New Hampshire nappes onto eastern Hampshire that are explained include the following. P–T paths in Vermont and that the nappes of western New Hampshire had time the Merrimack belt are counter-clockwise with maximum pressures to cool before final juxtaposition against the low-grade belt. Cooling of 4–5 kbar and are related to high regional heat flow and heat ages constrain this final juxtaposition to have occurred in the transfer by early Acadian plutons. P–T paths in the Bronson Hill Carboniferous, suggesting that the Acadian was a prolonged event belt are intimately associated with structural position. An early spanning as much as 100 Myr. contact metamorphism is evidenced in the Skitchewaug and Fall Mountain nappes near contacts with the early Acadian Bethlehem gneiss (>400–410 Ma). Peak metamorphic temperature rises upwards in the nappe sequence (an inverted metamorphic sequence)

The reaction muscovite+cordieritebiotite+Al2SiO5 +quartz+H2O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd +And± Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pressure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlockwise P–T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide conflicting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles, is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replacement of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction 1 and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) which satisfies most of the natural and experimental constraints. Contouring of the Fe–Mg divariant interval represented by reaction 1 allows for pressure estimation in garnet-absent andalusite+cordierite-bearing schists and hornfelses. The revised topology of reaction 1 allows for improved analysis of P–T paths from mineral assemblage sequences and microtextures in the same rocks.

Measurements of 60 single-grain, UV laser microprobe 40 Ar/ 39 Ar total gas ages for hornblende from metamorphic rocks of the Tobacco Root Mountains in southwest Montana yield a mean age of 1.71 ± 0.02 Ga. Measurements of 40 Ar/ 39 Ar step-heating plateau ages of three bulk hornblende samples from the Tobacco Root Mountains metamorphic rocks average 1.70 ± 0.02 Ga. We believe that these and the K/Ar or 40 Ar/ 39 Ar ages reported by previous workers are cooling ages from a 1.78 to 1.72 Ga, upper-amphibolite to granulite facies, regional metamorphism (Big Sky orogeny) that affected the northwestern portion of the Wyoming province, including the Tobacco Root Mountains and adjacent ranges. Based on the 40 Ar/ 39 Ar data, this 1.78–1.72 Ga metamorphism must have achieved temperatures greater than ~500 °C to reset the hornblende 40 Ar/ 39 Ar ages of samples from the Indian Creek Metamorphic Suite, which was previously metamorphosed at 2.45 Ga, and of the crosscutting metamorphosed mafi c dikes and sills (MMDS), which were intruded at 2.06 Ga. Biotite and hornblende from the Tobacco Root Mountains appear to give the same 40 Ar/ 39 Ar or K/Ar age (within uncertainty), indicating that the rocks cooled rapidly through the interval from 500 to 300 °C. This is consistent with a model of the Big Sky orogeny that includes late-stage tectonic denudation that leads to decompression and rapid cooling. A similar cooling history is suggested by our data for the Ruby Range. Three biotite samples from the Ruby Range yield 40 Ar/ 39 Ar step-heating plateau ages with a mean of 1.73 ± 0.02 Ga, identical to the best-estimate (near-plateau) age for a hornblende from the same rocks. Two samples of the orthoamphibole, gedrite, from the Tobacco Root Mountains were studied, but did not have enough K to yield a reliable 40 Ar/ 39 Ar age. Several biotite and three hornblende samples from the region yield 40 Ar/ 39 Ar dates signifi cantly younger than 1.7 Ga. We believe these samples were partially reset during contact metamorphism by Cretaceous (75 Ma) intrusive rocks. Hydrothermal alteration associated with ca. 1.4 Ga rifting led to growth of muscovite with that age in the Ruby Range, but this alteration was apparently not hot enough to reset biotite and hornblende ages there.

The island of Syros, Greece is part of the Attic-Cycladic blueschist belt, formed during Mesozoic Eurasia-Africa subduction. The rocks of Syros can be broadly divided into three tectono-stratigraphic units: (I) metamorphosed sedimentary and volcanic rocks (marble-schist sequence), (II) remnants of oceanic crust with fault-bounded packages of blueschist/eclogite-facies mafic rocks and serpentinite (mafic-ultramafic rocks) and (III) the Vari gneiss, which is a tectonic klippe. Low-temperature, high-pressure assemblages are found on several islands in the Cyclades. The best preserved of these rocks are on Syros and Sifnos islands. Mineral compositions and peak metamorphic assemblages are similar on both islands. Both islands are considered to share similar P-T histories with highest-pressure mineral assemblages reflecting conditions of at least 15 kbar and about 500°C.