FROM SUBDUCTION TO RIFTING: THE LATE CRETACEOUS–CENOZOIC TECTONIC EVOLUTION OF EASTERN PAPUA NEW GUINEA

Late Miocene–Pliocene high-pressure (HP) and ultrahigh-pressure (UHP) rocks occur in the lower plates of metamorphic core complexes (mcc) in the Woodlark Rift of Papua New Guinea. They form topographic highs, and occur structurally beneath basalts and ultramafic rocks that are inferred to represent remnants of an obducted ophiolite. Petrologic characteristics of these rapidly (>cm a^-1) exhumed HP–UHP rocks share similarities with other UHP terranes globally including peak metamorphic conditions of >2.6 Ga and 600–760°C and coesite preserved in mafic enclaves hosted by metasedimentary and granitoid gneisses. However, along strike variations in the transient Australia (AUS)–Woodlark (WDK) plate boundary zone evolution have yielded important temporal and spatial differences now preserved in the geologic, petrologic, and isotopic record that have yet to be demonstrated elsewhere. Presently rifting of a subduction complex transitions along strike into the Woodlark Basin seafloor spreading system. Since ~6 Ma the spreading center rift tip has propagated westward resulting in the separation of the once contiguous Woodlark Rise (northern rifted margin) and Louisiade Archipelago (southern rifted margin).

The Late Cretaceous–Cenozoic evolution of the AUS-WDK plate boundary zone involved northeastward subduction of the Australian passive continental margin beneath an island arc built on oceanic lithosphere. Collision led to metamorphism of Jurassic–Cretaceous sediments and basalts (i.e., Owen Stanley metamorphic rocks), and southward obduction of oceanic crust and mantle to form the 12–16 km-thick ophiolite of the Papuan Ultramafic Belt (PUB). Based on foraminifera in sediments found within the basalts previous studies estimated the PUB crystallization age to be Maastrichtian (71 to 65 Ma; Late Cretaceous). Late Paleocene K-Ar and ⁴⁰Ar/³⁹Ar amphibole ages (58.3 ± 0.4 Ma) have been interpreted to mark the time of cooling of the metamorphic sole following peak metamorphism. The PUB is regionally extensive and is well exposed west of 149°E. However, its continuation to the south and east remains controversial with possible correlative mafic and ultramafic rocks exposed throughout the Woodlark Rift where, in general, they occur in the upper plates of mccs. Dolerite recovered from the Moresby Seamount yielded a zircon U-Pb age of 66.4 ± 1.5 Ma, indicating that crustal extension immediately west of the active sea floor spreading tip is being accommodated in part by normal faulting within latest Cretaceous to early Paleocene oceanic crust.

Within the mcc lower plate rocks, a Lu-Hf garnet-whole rock isochron age (65 ± 6 Ma) from a mafic boudin in mylonitic gneiss within the shear zone carapace bounding the Goodenough Island mcc is interpreted to record garnet growth during prograde metamorphism. This age is concordant with the inferred Late Cretaceous age of the PUB and suggests that zircon crystallization within dolerite, obduction of young (hot) oceanic lithosphere and the onset of subduction zone metamorphism in eastern Papua New Guinea occurred in rapid succession. The subsequent Paleogene–Early Miocene prograde history of metamorphic rocks in the Woodlark Rift has yet to be revealed. However the conditions and timing of peak metamorphism varied from east to west. In the Misima mcc on the southern rifted margin Mid–Late Miocene greenschist to amphibolite facies metamorphism and subsequent exhumation from depths >15 km occurred prior to seafloor spreading in the Woodlark Basin. West of the active seafloor spreading rift tip, and synchronous with seafloor spreading in the Woodlark Basin, blueschists and greenschists formed in the Pliocene and were subsequently exhumed in the Prevost Range mcc of Normanby Island. Late Miocene eclogite facies metamorphic rocks formed and were exhumed from mantle depths and are now found in the lower plates of the D’Entrecasteaux Islands mccs. Outstanding questions regarding the prograde metamorphic history include: What caused the apparent time lag between the onset of collision and HP-UHP metamorphism? Were HP–UHP rocks stored at mantle depths before rapid exhumation occurred? Or are multiple subduction events recorded by the HP­–UHP rocks in this plate boundary zone known for complex microplate interactions?

Work in progress aims to determine the age of prehnite-pumpellyite rocks in the Louisiade Archipelago on the southern rifted margin, and the spatial and temporal evolution of volcanism within the rift. Of particular interest are xenocrysts and xenoliths in Pliocene–Recent volcanic rocks from the Woodlark Rise whose composition indicates possible derivation from 1) lower plate gneisses (e.g., garnet amphibolite and possibly eclogite), and 2) oceanic lithosphere (e.g., basalt, dunite and peridotite). These will provide clues regarding the composition of material at depth that the ascending magma has sampled, and help answer remaining questions regarding the relationship between subduction and magmatic processes in the Woodlark Rift.