MUSCOVITE AR-AR LASER PROBE AGES AND THERMAL MODELS COUPLED WITH LITHOSPHERIC MECHANICAL MODELLING

Provocative Rb-Sr and Ar-Ar biotite cooling ages in the 1.8 Ga crystalline basement of central North America distal to the 1.1 Ga Midcontinent rift system (MRS) delineate a discrete area that records <1.2 Ga ages, purportedly from a crustal bulge related to elastic responses during rapid lithospheric loading and flexure. Widespread Ar-Ar muscovite cooling ages, however, lack the MRS signal, and instead record older tectonothermal events. Laserprobe Ar-Ar thermochronometry on large single muscovite crystals from metamorphic units coupled with thermal diffusion modeling was conducted to better characterize the thermal effects imposed by rifting. Laser probe results yield grains that contain 50 to 500 m.y. age gradients that do not readily reveal distinct diffusion patterns. Samples proximal to the rift record Paleoproterozoic thermal histories related to known tectonic events. Muscovite closer to the bulge exhibits 1.4 and/or 1.1 Ga single spot ages on the edges of the crystals, which were then modelled as 1400 micron-radius spheres and cylinders subjected to ~400° and ~500°C thermal pulses. The age profiles from several grains show that the ages increase inwards away from the grain boundary at 1.4/1.1 Ga to 1.8 Ga. Modeling of the 40Ar* profiles yields the time of diffusion, which in this instance represents i) protracted residence at mid-crustal temperatures or ii) a short-lived thermal pulse associated with the MRS, in either case suggestive that muscovite was only partially reset. Numerical mechanical models of the lithosphere were used to validate the crustal bulge theory and to compare the predicted spatial pattern of crustal deflection with the distribution of cooling ages. We applied a visco-elastic Earth model to simulate both elastic flexure of the lithosphere and viscous flow of the underlying asthenosphere associated with volcanic loading. The geometry of the crustal load suggests that uplift would be largest opposite the aulacogen arm of the triple junction, but mechanical loading of the lithosphere and development of a crustal bulge would have little impact on resetting the isotopic systematics. We suggest the <1.2 Ga cooling ages reflect shallow intrusion reheating and isotopic resetting as a result of MRS magmatism, and not flexural uplift related erosion and cooling.