THERMOCHRONOLOGIC CONSTRAINTS FOR METAMORPHIC CORE COMPLEX FORMATION ON NORMANBY ISLAND, SOUTHEASTERN PAPUA NEW GUINEA

The D'Entrecasteaux Islands (DI) are active metamorphic core complexes (mccs) located just west of a westward propagating seafloor spreading center within the Woodlark Basin, SE Papua New Guinea. Normanby Island, the easternmost of the DI (and closest to the ridge tip), is a newly documented mcc. The island is composed of two lithologically distinct lower plate domains; retrogressed epidote blueschists of the Prevost range in the east and high grade felsic gneiss in the northwest. These are separated by a central basin of upper plate ultramafics, granodiorite, and volcanics. ⁴⁰Ar/³⁹Ar step heat analyses were conducted on mineral separates from these three regions in order to constrain the timing of mcc formation. These results, when combined with structural analyses, document heterogeneity in timing and style of extensional deformation prior to the onset of seafloor spreading. In the Prevost range, ⁴⁰Ar/³⁹Ar step heat experiments on white mica separates from quartz-mica schists, calc-silicates, and retrogressed epidote blueschists yield complex age spectra with individual apparent ages ranging from ~7 – 3 Ma. Results are interpreted to reflect incomplete thermal resetting of older white mica ages, heterogeneity in non-radiogenic ⁴⁰Ar/³⁶Ar ratio, or the presence of multiple mica growth stages during exhumation, which brought these rocks from 400 – 500°C through the brittle-ductile transition by 3 Ma. Plagioclase analyses from Prevost range yield saddle shaped spectra with older apparent ages in some steps (100s of Ma), but also containing minimum apparent ages from 5 - 3 Ma, likely reflecting thermal overprinting of protolith ages during mcc formation. In contrast, ⁴⁰Ar/³⁹Ar analyses on white mica and K-feldspar from the high grade felsic gneiss on NW Normanby document rapid cooling during exhumation from ~2.5 – 1.8 Ma. Exhumation on NW Normanby coincided with mcc exhumation and cooling of felsic gneiss on Fergusson and Goodenough Islands to the west, and was concordant with ~2 Ma granodiorite intrusion on Goodenough, Fergusson, and central Normanby Islands. Results from Normanby Island document temporally and structurally distinct phases of mcc formation, and provide an analogue for progressive mcc development prior to the propagation of a developing seafloor spreading center.