Protracted late- to post-Ottawan cooling and exhumation of the mid crust in a Grenvillian inlier, central Appalachians, USA – New data and a new hypothesis

Abstract

New 40Ar/39Ar hornblende and biotite data from granulite-facies gneisses across the Grenvillian New Jersey Highlands inlier in the central Appalachians permit estimates of the rates of regional cooling and exhumation following the Ottawan phase of the Grenvillian Orogeny. 40Ar/39Ar hornblende and biotite results yielded plateau ages of ca. 947–914 Ma and ca. 887–819 Ma, respectively, constraining post-Ottawan cooling from the peak metamorphic temperature of ∼770 °C to ∼300 °C at a slow rate of 1.9–3.5 °C/Myr. Following assumptions about the physical properties of these mid-crustal rocks, we calculate they were exhumed from a peak burial depth of ∼22 km, and passed through the 500 °C and 300 °C isotherms at depths of ∼14 km and ∼7.5 km, respectively, indicating a slow exhumation rate of 0.06–0.11 km/Myr.The dual signal of slow cooling and exhumation is interpreted to be due to an extended period of elevated mantle heat flow triggered by delamination or convective removal of the lower mantle lithosphere beneath the orogen. Together with lower crustal intrusion and (or) crustal underplating by asthenospheric magmas, this is interpreted to have resulted in extensional collapse of the overthickened crust, juxtaposition of crustal levels, and the formation of large metamorphic core complexes. Mafic intrusions and (or) underplating not only provided an important heat source in the lower crust, but also increased the average crustal density, thereby slowing the rates of uplift, erosion and exhumation. We interpret the dual signal as a cryptic signature of pervasive extensional orogenic collapse after the Ottawan phase of the Grenvillian Orogeny.