Methods of deciphering complex thermo-chronological histories of migmatitic metapelites: Applications within the Adirondack Highlands, New York.

摘要

Detailed petrography was performed on metapelites from the southern Adirondacks. Textures and zoning patterns in garnet indicate dehydration melting of biotite. Zoning of grossular in garnet is interpreted as a prograde record of growth, whereas Fe/(Fe+Mg) and spessartine distributions show diffusive profiles established during cooling. A general pelitic MnNCKFMASH pseudosection predicts bell-shaped core to rim profiles of grossular during anatexis, similar to those observed in the rocks. Garnet-biotite thermometry and GASP barometry indicate peak temperatures of at least 790°C at about 7--9 kbar, similar to conditions determined for the central Adirondacks. Cooling rates determined from finite difference modeling of spessartine and Fe/(Fe+Mg) diffusional profiles indicate a multi-stage cooling history in which some period of rapid cooling (>200°C/Ma) is required.; Electron microprobe CHIME analyses were used to constrain the timing of monazite growth events within southern Adirondack metapelites. Monazite displays complex chemical variation, accommodated primarily by continuous huttonite and brabantite substitution. Distinct zones within monazite record at least 3 major periods of growth (1160 +/- 9, 1105 +/- 15, and 974 +/- 10 Ma). The{09}∼1100 Ma population is volumetrically the most significant, with gradational zoning patterns indicating growth from a melt. Increases of Th/U in monazite from ∼20--100 at around 1100 Ma are correlated with the onset of melting, followed by continuous monazite growth over a 100--200 m.y. interval. Late inclusion of 1020--970 Ma monazite within major phases is thought to correspond to heating, possibly during late reactivation of convergence.; Ti-in-quartz and Zr-in-rutile thermometry was applied to metapelites from the southern and western Adirondack Highlands. Within the southern Adirondack rocks, maximum Ti-in-quartz temperatures (∼810°C) are consistent with the results of garnet biotite thermometry from the same samples (≥790°C), and are interpreted as retrograde. Zr-in-rutile temperatures, reaching values of up to 850°C, are interpreted as peak. Ti-in-quartz and Zr-in-rutile temperatures from the western Adirondack sample are in close agreement. Temperature maxima on the order of 870°C are roughly consistent with previous temperatures estimated by Darling and others (2004).