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Pelitic schists from contact aureoles surrounding mafic-ultramafic plutons in Westchester County, NY record a high- P (~0.8 GPa) high- T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km2) gabbroic body, consists of quartz-plagioclase-biotite-garnet-sillimanite-ilmenite-graphite-Zn-rich Al-spinel. Both K-feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk-rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620-640 °C and 0.5-0.6 GPa. In the aureole sample, WDS X-ray chemical maps show distinct Ca-enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet-biotite Mg-Fe exchange equilibria on inclusions and adjacent garnet host interior to the high-Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P-T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P-T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P-T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism ( c. 435 Ma). In the absence of contact metamorphism-induced recrystallization, it is likely that this regional-scale loading would remain cryptic or unrecorded. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The relatively new technique of "chemical" (non-isotopic) dating of monazite (REE phosphate) by electron microprobe (EMP) has been demonstrated to produce geochronologic results with precision comparable to U-Pb and Th-Pb isotopic techniques, but with advantages of superior spatial resolution and capability to date individual thermal and deformational episodes within a metamorphic continuum. Under appropriate circumstances, monazite partially re-crystallizes and undergoes partial age resetting, potentially providing a near-complete "tape recording" of the chronology of an orogenic event. Spatial resolution of less than 3 µm has an advantage over SHRIMP and ion microprobe analyses (minimum of about 15 The relatively new technique of "chemical" (non-isotopic) dating of monazite (REE phosphate) by electron microprobe (EMP) has been demonstrated to produce geochronologic results with precision comparable to U-Pb and Th-Pb isotopic techniques, but with advantages of superior spatial resolution and capability to date individual thermal and deformational episodes within a metamorphic continuum. Under appropriate circumstances, monazite partially re-crystallizes and undergoes partial age resetting, potentially providing a near-complete "tape recording" of the chronology of an orogenic event. Spatial resolution of less than 3 µm has an advantage over SHRIMP and ion microprobe analyses (minimum of about 15We have done much monazite dating in the Appalachians, but will focus in this presentation on two case studies from New England that illustrate the value of EMP dating: Acadian (broadly defined) to Alleghanian metamorphism from central Massachusetts and northeastern Connecticut, and Taconic, Acadian and Alleghanian metamorphism in western Connecticut and adjacent New York and Massachusetts. In both examples, patterns of ages from monazite EMP analysis indicate that the standard accepted tectonic models have significant problems, and may point the direction toward new models based on reinterpretation of structural and stratigraphic observations. We emphasize that these new dates are mainly of use in interpreting ambiguous field data and regional correlations. In central Massachusetts and adjacent areas (from the Bronson Hill arc eastward to the high-grade zones of the Sturbridge area), the conventional view has been of a principal Acadian tectonothermal event at ca. 400 Ma (Robinson's "classic Acadian") with a successor event at ca. 360 Ma (his "neo-Acadian"). Targeted monazite dating indicates highestgrade (near-granulite) metamorphism at ca. 435 Ma, with additional events at 400 Ma and 360 Ma. Abrupt age breaks across strike suggest major regional east-west shortening with cryptic major faults. To the west, approaching the Bronson Hill arc, a major belt of late recrystallization at 330 Ma indicates early onset of Alleghanian effects, and post-300 age resetting is seen as main-phase Alleghanian. In western Connecticut (west of the Mesozoic basin), the conventional model is of a Taconic fore-arc allochthonous on Laurentian margin (Grenville) with in situ post-Taconic sedimentary basins, all metamorphosed to kyanite-sillimanite grade in the Acadian, and with minor Alleghanian overprint to the south near Long Island Sound. Our dating shows a complex pattern of intermixed Taconic ages (ca. 520, 470-480 and 445-450 Ma) with multi-stage Acadian ages (> 420, 385-390 and 355-360 Ma), but some locations in older rocks within the supposed Acadian belt show no evidence of preceding Taconic metamorphism. Cameron's Line, a notable tectonic break highlighted by Rodgers on his 1985 Connecticut geologic map, has been interpreted as the western limit of imbricated Taconic thrust slices containing deep-water rocks.… [ABSTRACT FROM AUTHOR]

This article presents a study on the pressure-temperature-time (P-T-t) evolution of granulite facies metamorphism and partial melting in the Winding Stair Gap (WSG) located in the Central Blue Ridge province in North Carolina. The WSG features high-grade metamorphic rocks in the southern Appalachian orogen and is considered part of the thermal axis of the Paleozoic metamorphism. The petrographic and chemical data are presented, as well as thermobarometric estimates, and monazite uranium-thorium-lead (U-Th-Pb) dates for various samples taken from the WSG.

U(–Th)–Pb geochronology, geothermobarometric estimates and macro- and micro-structural analysis, quantify the pressure–temperature–time–deformation ( P– T– t– D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 °C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 °C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ∼650 ± 50 °C. P–T estimates record a decrease in pressure from ∼6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn- to post-solid-state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region. [ABSTRACT FROM AUTHOR]