THERMAL HISTORY OF THE ACADIAN OROGEN FROM DETRITAL MONAZITE STEPPED HEATING AND (U-Th)/(He-Pb) DOUBLE DATING

The Devonian–Mississippian Acadian orogeny was part of a series of major Paleozoic mountain building phases along the margin of Laurentia that had major impacts on the continent’s geosphere, hydrosphere, biosphere, and atmosphere. However, the thermal and exhumation history of the Acadian orogen is obscured by post-Acadian tectonic overprinting, uplift and erosion, and burial reheating. In the Acadian foreland basin, post-Acadian burial heating has reset most low-temperature thermochronometers, precluding our ability to assess orogen exhumation histories. Because monazite (U-Th)/He typically has higher closure temperatures (~250–300°C) than zircon and apatite and may preserve a thermal history record of the Acadian orogen, we collected new detrital monazite (U-Th)/(He-Pb) double dates from the Pocono Formation and combined these data with zircon and apatite (U-Th)/He thermal modeling to reconstruct pre-to-post burial thermal histories. Monazite ⁴He diffusion parameters can vary and therefore affect the calculated closure temperature and geologic interpretation, so we performed full incremental stepped heating experiments on six monazites and abbreviated stepped heating experiments on 18 monazites to directly determine diffusion parameters. As monazite diffusion parameters often vary with composition, we performed TREE analysis and EDS elemental mapping on each monazite.

Unimodal Th-Pb ages (425–375 Ma) and uniform monazite–(Ce) elemental compositions indicate that Pocono Fm. monazites were likely derived from a single Acadian source and have a shared thermal history. Some diffusion stepped heating experiments yielded linear results; others showed more complex relationships between ⁴He diffusivity and temperature, such as increases in activation energy with increasing temperature and instances of multi-domain diffusion behavior that coincide with Th zonation. The full monazite He diffusion results yielded a range of effective closure temperatures (~100–200°C and ~260–300°C), indicating that the Pocono Fm. monazites vary in terms of their diffusion parameters and behavior despite deriving from the same general source rock and having relatively uniform compositions. The diffusion parameters determined from the stepped heated monazites enable robust thermal modeling and reconstruction of thermal histories of the Acadian orogen.