Timing and conditions of formation of the D'Entrecasteaux Islands, southeastern Papua New Guinea.

摘要

The Woodlark Basin of southeastern Papua New Guinea is a region of transition from rifting to seafloor spreading, where a seafloor spreading center has propagated from east to west from ∼6 Ma to present (Taylor et al., 1995; 1999). The D'Entrecasteaux Islands, Goodenough, Fergusson, and Normanby Islands, are metamorphic core complexes located within 100 km to the west of this spreading center tip, and have played an active role in extension, crustal thinning, and the exhumation of mid- to lower-crustal rocks. Metamorphic core complex lower plates have been exhumed from beneath predominantly low-angle top-to-the-north shear zones and northward-dipping detachment faults. Lithologies include retrogressed eclogites (Goodenough and Fergusson Islands) from > 70 km and blueschists (eastern Normanby Island) from ∼20 km depth. Combined in situ ion microprobe U-Pb zircon age analyses and trace and rare earth element chemistry from five variably retrogressed eclogites from Fergusson and Goodenough Islands document latest Miocene-Pliocene (∼8-2 Ma) eclogite formation. Age results require exhumation rates > 2.5 cm/yr. Temperature estimates for eclogite-facies metamorphism made from in situ ion microprobe analyses of [Ti] in zircon and electron microprobe analyses of [Zr] in rutile range from 611-870°C (Watson and Harrison, 2005; Watson et al., 2006). 40Ar/39Ar experiments on white mica and plagioclase from retrogressed blueschist and greenschist-facies rocks from the Prevost Range of eastern Normanby Island constrain Pliocene exhumation and cooling of the lower plate of this newly documented metamorphic core complex. A combination of 40Ar/39Ar and structural analyses (Little et al., 2006) lends support to the interpretation of a rolling hinge mechanism for the exhumation of the Prevost Range metamorphic core complex. 40Ar/39Ar analyses of mineral separates from andesites and granodiorites from central Normanby, and from felsic gneiss on northwestern Normanby Island document intrusion, volcanism, rapid exhumation and cooling from ∼2.2-1.8 Ma. The D'Entrecasteaux Island metamorphic core complexes formed as a result of rapid extension and crustal thinning ahead of a propagating seafloor spreading center, and are believed to play a vital role in the transition from extension to seafloor spreading within the Woodlark Basin.