TEXTURAL ANALYSIS OF A LARGE POPULATION OF SHRIMP- AND TIMS-DATED MONAZITE IN WISSAHICKON FM. MIGMATITE FROM YORKLYN, DELAWARE

Monazite from the Wissahickon Fm. near Yorklyn, DE is used as an internal standard for geochronology (Aleinikoff, pers. comm., 2007) because it is abundant, compositionally uniform, and yields concordant TIMS ages (424.9 +/-0.4 Ma) that agree with SHRIMP ages (426 +/-3 Ma, Aleinikoff et al., GSA Bulletin, 2006). However, the metamorphic and microstructural context of this monazite has not been established. Here we report results of petrographic and SEM study of more than 200 monazite grains in 4 thin sections that were prepared from the same outcrop as was sampled by Aleinikoff et al., (2006).

Students in the fall 2010 Petrology class at WCU found evidence in the Wissahickon at Yorklyn for high-T metamorphism and migmatization. Cm- to decimeter-scale granitic leucosome contains perthitic microcline, plagioclase and quartz; micrometer-scale leucosome with Ba-rich ksp, pl and qtz occurs along mesosome grain boundaries. Melanosome is rich in gt and bt. No cordierite or orthopyroxene is observed; sillimanite is rarely preserved as aligned inclusions within pl. These observations suggest an origin by partial melting at conditions of T ~ 730-800 C and P ~ 0.6-0.9 GPa, followed by limited back-reaction to form bt + ms. Some leucosome contains bt fish parallel to mesosome foliation; other leucosome contains randomly-oriented bt. Microstructures suggest syn- to post-kinematic migmatization with respect to foliation formation, possibly due to ms-breakdown melting followed by less extensive bt-breakdown melting. Large, unoriented kyanite, present in leucosome of 1 sample, is post-migmatization, associated with ductilely-deformed qtz along grain boundaries and fills fractures in feldspars.

All thin sections in which monazite has been examined to date are cut perpendicular to strike of foliation. In this view, monazite ranges in size from 50 to 6000 sq. micrometers, with aspect ratios ranging from 1.15 to as great as 5.1 with an average of 2.0. It occurs as inclusions in bt, pl, qtz, and gt. Matrix grains, those most likely sampled for isotopic analysis, exhibit a strong preferred orientation parallel to foliation, suggesting syn-tectonic growth. Thus, the U-Pb age of monazite records the fabric-forming deformation that is approximately synchronous with max T in the Wissahickon Fm. at Yorklyn.