2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 130-8
Presentation Time: 3:40 PM-3:55 PM


ALLAZ, Julien1, SELLECK, Bruce W.2, WILLIAMS, Michael L.1, and JERCINOVIC, Michael J.1, (1) Dept. of Geosciences, University of Massachusetts, Amherst, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003, jallaz@geo.umass.edu, (2) Department of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346

A number of recent studies suggest that REE-phosphates are stable over a large range of pressure and temperature, and that REE-bearing accessory phases can be dissolved and later re-precipitated as a function of fluid composition and metamorphic conditions. Such conditions include the growth of authigenic monazite during diagenesis and low-grade metamorphism.

We report here the results of an in-situ dating by electron microprobe of Paleozoic authigenic and low-grade monazite and xenotime overgrowths on detrital monazite and zircon, respectively. Samples are from the Cambrian Potsdam Formation, deposited uncomfortably on Proterozoic (Grenville) basement in New York. This study also focused on the textural and chemical relationship of these REE-bearing accessory phases. Detrital monazite and zircon are rounded and commonly fractured; new REE-phosphate overgrowths are commonly subeuhedral. Data were acquired on the CAMECA SX-100 “Ultrachron” microprobe. Monazite and xenotime overgrowths have low Th and U, typical of burial diagenetic conditions. A clear enrichment of LREE and a depletion of HREE (relative to relict Grenville cores) are observed in the monazite overgrowths. EPMA dates have relatively large errors, due to low Th, U and Pb content, but document the power of the technique to resolve complex fluid-related growth history of REE-phosphate in sedimentary rocks.

Studied samples reveal four to five major overgrowth events between 510-490 Ma (deposition time) and ca. 200 Ma in both monazite and xenotime. These events broadly correlate with the major orogenic events recorded in the Appalachians to the east (Taconic, Salinic, Acadian, Neo-Acadian and Alleghanian). We suggest fluid migration, driven by orogenic loading, to be responsible for dissolution of detrital monazite and zircon. Authigenic reprecipitation of monazite and xenotime then results from changes in fluid composition, redox conditions, or temperature.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting
Session No. 130
Monazite: The Ultimate Geologic Record
Minneapolis Convention Center: Room 102BC
1:30 PM-5:30 PM, Monday, 10 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 330

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