Microprobe dating of monazite in relation to porphyroblast growth and thermodynamic modelling P-T paths for rocks affected by prolonged orogenesis

Pressure-Temperature-time-deformation paths were modelled using microstructural, petrologic, geochronologic data for multiply deformed Acadian metamorphic rocks currently exposed within and mantling the Chester and Athens Domes in southeastern Vermont, USA. Estimates of pressure, temperature and time were obtained for samples where the relative timing of distinct periods of garnet growth has been previously established through detailed microstructural analysis of foliation intersection axes (FIAs) of inclusion trails in garnet porphyroblast. Compositional mapping and in-situ microprobe analyses of monazite grains reveal that monazite growth was the result a series of reactions or growth events. Numerous monazite grains preserve complex compositional zoning patterns that cannot be easily explained by a single monazite growth event. Electron microprobe point analyses of U, Th, Pb and Y were used to determine the ages of monazite grains or distinct compositional domains within monazite grains. U-Th-Pb age distributions show monazite growth and subsequent metamorphism occurred over a period at least 80 m.y from 430 Ma to 350 Ma. A single sample was found to contain distinct populations when monazites where grouped by microstructural affinities. A single sample yielded ages of 424 ± 2.4 Ma, 405 ± 6.0 Ma, 386 ± 6.0 Ma, 366 ± 3.8 for monazite populations analysed in the cores, medians, and rims of garnet porphyroblasts and matrix respectively. Weighted averages calculated for monazite populations, which lie within the included/overgrown foliations for all of the samples clustered around 425 Ma, 405, Ma, 387 Ma, 377, Ma and 365 Ma and 350 Ma. These age distributions are taken to represent a best estimate of the timing of deformation and accompanying mineral growth for southeastern Vermont. P-T pseudosections were utilised to map mineral reactions for samples that contain garnet porphyroblasts with inclusion mineralogies that reveal partial mineral assemblages prior to garnet growth. Pseudosections were modelled in the system MnKFMASH for high-Al pelites and MnNCKFMASH for low-Al pelites and calcpelites. Thermobarometric calculations were compared with P-T pseudosections to construct P-T paths consistent with observed mineral reactions. P-T estimates for garnet cores are consistent with the position of garnet-in reactions on MnKFMASH pseudosections and are generally 25-50°C above the garnet-in reaction on the MnNCKFMASH pseudosections. P-T estimates for garnet cores reveal that garnet growth commenced at ª 525°C and moderate pressure (4-9 kbars) and proceeded along an up-pressure path reaching peak pressures between 13-14 kbars at 600-625°C. Monazite inclusions in garnet porphyroblasts bracket the timing of the onset of garnet growth between 425 Ma and 405 Ma. Peak pressure, which is estimated to have occurred at ª 600°C, was likely reached by 385 Ma. Peak temperatures of ª 650°C are recorded between 11-12 kbars, which was followed by decompression and cooling. Decompression occurred between 380 and 350 Ma with very little heating indicating that initial uplift and subsequently exhumation was rapid. Monazite ages from garnet rims and matrix mark the end deformation and mineral growth at 350 Ma. Previous tectonic models for Acadian orogenesis for the region have suggested that deformation and accompanying metamorphism was a two-stage process. Crustal thickening was thought to be the result of nappe style thrusting followed by regional scale doming. The integration of microstructural studies and in-situ monazite dating along with a detailed evaluation of the thermobarometric history reveals a more complex deformation and metamorphic history. The peak pressures and relatively low geothermal gradients associated with the early part of the P-T path are more consistent with subduction related tectonism than nappe style thrusting. Inclusion trails in garnet porphyroblasts also reveal that numerous near orthogonal foliations formed during garnet growth that cannot be easily explained by nappe and dome stage deformations. In the proposed model, foliation development and subsequent mineral growth occurs in response to crustal thickening related to continental subduction. This resulted in a greatly over-thickened crust that was unstable and underwent rapid uplift and exhumation with little heating. Monazite ages that have been linked directly with microstructures and mineral growth reveal that Acadian Orogenesis may have begun as early as the earliest Silurian and continued through to the Carboniferous.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

James Cook University

Year dc:date.issued

2003

Author dc:creator

• Welch, Peter W.

Language dc:language

en