Cooling history of the northern Ford Ranges, Marie Byrd Land, West Antarctica

Abstract

Thermochronologic data from the Fosdick, Phillips and Chester mountains of Marie Byrd Land, West Antarctica, have been obtained through U‐Pb analysis of monazite, 40Ar/39Ar analysis of hornblende, muscovite, biotite and K‐feldspar, and apatite fission track methods. These data were collected to test the hypothesis that high‐grade metamorphic rocks in the Fosdick Mountains occupy the footwall of a Cordilleran‐style metamorphic core complex, exhumed during the breakup of this sector of Gondwana in early Late Cretaceous time. High‐grade metamorphism of rocks exposed in the Fosdick Mountains was followed by rapid cooling starting at ∼105 Ma, during the transition from convergence to extension in the adjacent continental margin of Gondwana. Monazite, hornblende, muscovite, biotite, and K‐feldspar from the Fosdick Mountains record rapid cooling (70±30°C/m.y.) from peak metamorphic conditions of 725°–780°C at 4.3–5.6 kbar to below 165°C between 105 and 94 Ma. Subsequent slow cooling was apparently punctuated by a short period of accelerated cooling through the apatite partial annealing zone (∼110°–60°C) between ∼80 and 75 Ma. Cooling rates decreased to an average of ∼l°C/m.y. after 70 Ma. Cooling ages become progressively older to the south; metamorphic grade decreases in concert with the increasing cooling ages. The southernmost samples, from the Chester Mountains, probably cooled to below K‐feldspar closure temperature (∼165°C) before inferred reheating associated with metamorphism in the Fosdick Mountains. North of the Fosdick Mountains, Devonian Ford granodiorite in the Phillips Mountains was below K‐feldspar closure temperature by early Cretaceous time. Byrd Coast granite intrusions in the eastern Phillips Mountains and east of the Fosdick and Chester mountains were emplaced between 100 and 105 Ma, and these plutons cooled very rapidly (>100°C/m.y.) to below biotite closure temperature, consistent with their epizonal character. The relationship of these granitoids to metamorphic rocks in the Fosdick Mountains is uncertain. We hypothesized the following sequence of events during the transition from convergence to extension along the Pacific margin of Gondwana. Voluminous intrusion into the lower and middle crust led to increased heat flow and high‐tem‐perature, low‐to moderate‐pressure metamorphism, forming the Fosdick metamorphic complex (FMC) exposed in the Fosdick Mountains. Decrease in strength due to intrusion and partial melting resulted in large‐scale flow, probably driven by extension‐related differential stresses. This deformation ended before the onset of rapid cooling of the FMC at ∼105 Ma. Cooling rates determined for the FMC can be modeled by decreasing the heat flux into the crust and exhuming the complex at a rate of 1.5 mm/yr. The decrease in cooling rate between closure of K‐feldspar 40Ar/39Ar at ∼94 Ma and cooling into the apatite fission track partial annealing zone by ∼80 Ma is interpreted to indicate that exhumation was at least a two‐stage process. Our observations indicate that the Fosdick and Chester mountains are part of a coherent block that was tilted ∼20° to the south during the exhumation of the FMC, probably by movement along an east trending, north dipping, normal fault between the Fosdick and Phillips mountains. The Fosdick Mountains are not a Cordilleran‐style metamorphic core complex, but the FMC provides a record of middle‐crustal processes related to the rifting of New Zealand from Gondwana in the Late Cretaceous.