DATING FLUID MIGRATION EVENTS THROUGH MICROPROBE DATING OF DETRITAL MONAZITE FROM THE POTSDAM FORMATION, NY
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.