COMBINED ALLANITE-MONAZITE PETROCHRONOLOGY ON POLYMETAMORPHIC MIGMATITES FROM THE CENTRAL APPALACHIANS

The Smith River Allochthon (SRA) of the central Appalachians preserves a complex polymetamorphic record of orogenic processes, including several phases of intense heat flux associated with magmatic intrusion. Here, we combine monazite and allanite U-Pb geochronology and trace element chemistry with thermodynamic modeling to constrain part of this high-temperature polymetamorphic history. We combine these new P-T-t constraints with previously collected monazite ages from Bob Tracy to present a comprehensive P-T-t tectonic evolution of the SRA.

Monazite in SRA pelitic migmatites is found as inclusions in garnet crystal cores and in high-Ca garnet rims. It also occurs in the matrix, commonly surrounded by double coronae of apatite and allanite. Monazite U-Pb dates from five investigated samples and in all textural locations range between >500 Ma and ~350 Ma. Based on the compositions and textural setting of these crystals, the data are interpreted to represent at least five distinct age populations: 1) early Cambrian ages, previously interpreted to represent a cryptic thermal event unique to the SRA 2) ~475 Ma Taconic regional metamorphism 3) ~460-445 Ma crystallization coeval with heating from multiple felsic intrusions (SRA tonalites at 460 Ma and the Leatherwood granite at 445 Ma), 4) ~430 Ma high-temperature contact metamorphism and anatexis, and 5) ~350 Ma recrystallisation during fluid infiltration, coeval with initial allanite growth.

Allanite occurs as both a double corona with apatite surrounding monazite, and as individual grains in heavily metasomatized domains. U-Pb dates ranging between ~350 and ~280 Ma are interpreted to represent crystallization during both neo-Acadian and Alleghanian deformation. The initial allanite and apatite forming reaction (~350 Ma) is texturally linked to the reaction plagioclase + biotite = garnet at upper amphibolite facies P-T conditions. Young monazite age domains adjacent to allanite are depleted in HREE and show a trend of decreasing Eu/Eu*, consistent with this interpretation.

This range of ages reveals a prolonged polymetamorphic history for the SRA, from initial docking along the Laurentian margin during the early Ordovician Taconic orogeny to thrusting and fluid infiltration in the Permian during the Alleghanian orogeny. This study also highlights the benefit of dating both allanite and monazite in polymetamorphic rocks, and the importance of detailed petrographic observations, always a signature of Bob Tracy’s work.