10 Precambrian Assembly of a Continent from the Northeast to the Mid-Continent to the Southwest: Modern Approaches to Study Ancient Crust (Posters)

Sunday, 19 March 2017
8:00 AM - 12:00 PM
Grand Ballroom, 17th Floor

A PRELIMINARY RECORD OF HIGH GRADE METAMORPHISM FROM CHEMICAL ZONING PRESERVED IN GARNET FROM PELITIC SCHISTS OF THE NORTHERN TAOS RANGE, NEW MEXICO

Paper number: 10-2
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Exposures of supracrustal rocks from the northern Taos Range of north-central New Mexico record upper amphibolite to granulite facies regional metamorphism. During this period of metamorphism, significant garnet growth occurred within a pelitic schist with the assemblage garnet + biotite + quartz + muscovite + plagioclase + sillimanite ± [cordierite] + Fe–Ti oxides + accessory minerals. Cordierite was present in some garnet-bearing domains though it is completely pseudomorphed by an aggregate of sillimanite + quartz ± biotite ± oxides ± phyllosilicates. Electron Microprobe mapping of major elements in garnet provides clues to the metamorphic history of this crustal section. Analysis shows zoning with an increase of Fe/(Fe+Mg) in a 200 micron-thick rim of a garnet from sample TR25. Mapping of this garnet shows Ca content highest in the center with irregular elongate patches of lower calcium content in the core. An abrupt decrease is observed near the rim, followed by a thin, discontinuous higher Ca rim. Mn shows a broad core to rim decrease. The higher Ca rim may represent minor garnet growth during partial melting reactions involving biotite. Another schist, TR19, from a higher elevation outcrop contains the assemblage garnet + biotite + quartz + plagioclase + sillimanite + muscovite + tourmaline + Fe–Ti oxides + accessory minerals. Analysis of two garnet porphyroblasts in sample TR19 show zoning with an increase of Fe/(Fe+Mg) in a 300 micron-thick rim of the larger garnet porphyroblast. Ca zoning is more uniform compared to sample TR25. Mn mapping of TR19 is similar to TR25 in that content is highest in the core with a broad decrease to the rim. Garnet zoning in both rocks show no evidence for polymetamorphic overgrowths, nor cation diffusion related to resorption along garnet rims. Preliminary zoning and petrologic analysis results are interpreted to be consistent regional metamorphism during a single tectonic cycle of associated with the ~1.45-1.35 Ga Picuris Orogeny.

Geological Society of America Abstracts with Programs. Vol. 49, No. 2

doi: 10.1130/abs/2017NE-291219

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