Tracking the exhumation of a Pliocene (U)HP terrane: U-Pb and trace-element constraints from zircon, D'Entrecasteaux Islands, Papua New Guinea

Abstract

Domal structures within the D'Entrecasteaux Islands of eastern Papua New Guinea expose ultrahigh-pressure (UHP) Pliocene (5.6-4.6 Ma) eclogites and evidence for partial melting. To better interpret the (U)HP exhumation history, U-Pb geochronology and trace-element abundances were determined in zircon from variably deformed host gneiss and crystallized melt (leucosomes, sills, dikes, and plutons) from the Goodenough and Normanby Domes by ID-TIMS (isotope-dilution thermal ionization mass spectrometry) and ICP-MS (inductively coupled plasma mass spectrometry), respectively, to constrain the timing of melt crystallization and deformation relative to UHP metamorphism. Zircons extracted from orthogneiss and deformed granodiorite sills of Normanby Dome, located ∼40 km southeast of the UHP eclogite, record HP metamorphism from 5.66 ± 0.02 to 5.04 ± 0.07 Ma, and melt crystallization at ∼4.1 Ma. Strongly deformed, layer-parallel leucosomes from Goodenough Dome, ∼20 km northwest of the UHP eclogite, began to crystallize by 3.85 ± 0.02 Ma. These dates indicate that melt crystallization began in the Goodenough and Normanby Domes within ∼0.75 m.y. of (U)HP metamorphism. The ID-TIMS dates from the orthogneiss and crystallized melt show that exhumation and cooling of the (U)HP rocks in the PNG terrane began first in the east, within Normanby Dome, then to the west, in the Goodenough Dome ∼1 m.y. later, and finally the middle dome rocks, exposed within the Mailolo Dome, cooled ∼2 m.y. after exhumation of Normanby Dome. All domes reveal synchronous crystallization of late, nondeformed melts, and final extension-driven exhumation by 1.82 ± 0.03 Ma.