24 Future Directions in Appalachian Tectonics: Building on Recent Lithotectonic Syntheses I

Monday, 19 March 2012
10:15 AM - 12:20 PM
Capital Rooms 2 & 3

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

Paper number: 24-3
Program: Theme Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

Monazite overgrowths in sedimentary rocks can be used to date and characterize fluid influx/alteration events, and possibly, to unravel the timing of distal loading (i.e. tectonic) events. We report here the results of in-situ analysis by electron microprobe of Paleozoic authigenic and low-grade LREE and HREE phosphates (monazite and xenotime) occurring as overgrowths on detrital monazite and zircon, respectively. This study reveals the ability of low-grade sedimentary rocks to record several tectonically-driven fluid pulses, which are most likely associated to the burial and loading of sediments during Appalachian tectonism between ca. 480 and ca. 200 Ma.

Samples are from the Cambrian Potsdam Formation, deposited uncomfortably on Proterozoic (Grenville) basement in New York, and slightly metamorphosed in the lower greenschist facies. In addition to U-Th-Pb dating, this study 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 subhedral. Monazite and xenotime overgrowths have low Th and U, typical of 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 reveal four to five major overgrowth events between ca 500 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) and possibly the rifting of Laurentia during Pangea break-up. We suggest that fluid migration, driven by orogenic loading, was responsible for dissolution of detrital monazite and zircon. Authigenic reprecipitation of monazite and xenotime results from changes in fluid composition, redox conditions, or temperature. These results document the power of in-situ analysis and dating to resolve complex fluid-related growth histories of REE-phosphates in sedimentary rocks.