Papuan Ultramafic Belt Research Articles

Ultra-Refractory Peridotites of Phanerozoic Mantle Origin: the Papua New Guinea Ophiolite Mantle Tectonites

AbstractHarzburgites and dunites forming the base of the Late Cretaceous–Paleocene Papuan Ultramafic Belt (PUB) and Marum ophiolites of Papua New Guinea (PNG) are among the most refractory mantle peridotites on Earth. We present a new integrated dataset of major element, bulk plus mineral trace element and Re–Os isotopic analyses aimed at better understanding the genesis of these peridotites. The PUB harzburgites contain olivine (Fo92–93), low-Al enstatite (less than or equal to 0.5 wt. % Al2O3 and CaO), and Cr-rich spinel (Cr# = 0.90–0.95). The Marum harzburgites are less refractory with olivine (Fo91.9–92.7), enstatite (~0.5–1.0 wt. % Al2O3 and CaO), minor clinopyroxene (diopside), and spinel (Cr# = 0.71–0.77). These major element characteristics reflect equivalent or greater levels of melt depletion than that experienced by Archean cratonic peridotites.Whereas bulk-rock heavy rare earth element (HREE) abundances mirror the refractory character indicated by the mineral chemistry and major elements, large-ion lithophile elements indicate a more complex melting and metasomatic history. In situ olivine and orthopyroxene REE measurements show that harzburgites and dunites have experienced distinct melt-rock interaction processes, with dunite channels/lenses, specifically, showing higher abundances of HREE in olivine. Distinctive severe inter-element fraction of platinum group elements and Re result in complex patterns that we refer to as ‘M-shaped’. These fractionated highly siderophile element (HSE) patterns likely reflect the dissolution of HSE-rich phases in highly depleted peridotites by interaction with subduction-related melts/fluids, possibly high-temperature boninites.Osmium isotope compositions of the PNG peridotites are variable (187Os/188Os = 0.1204 to 0.1611), but fall within the range of peridotites derived from Phanerozoic oceanic mantle, providing no support for ancient melt depletion, despite their refractory character. This provides further evidence that highly depleted peridotites can be produced in the modern Earth, in subduction zone environments. The complex geochemistry indicates a multi-stage process for the formation of the PNG mantle peridotites in a modern geodynamic environment. The first stage involves partial melting at low-pressure (<2 GPa) and high-temperature (~1250°C–1350°C) to form low-K, low-Ti tholeiitic magmas that formed the overlying cumulate peridotite–gabbro and basalt (PUB only) sequences of the ophiolites. This is inferred to have occurred in a fore-arc setting at the initiation of subduction. Later stages involved fluxing of the residual harzburgites with hydrous fluids and melts to form replacive dunites and enstatite dykes and interaction of the residual peridotites in the overlying mantle wedge with high-temperature hydrous melts from the subducting slab to generate the extremely refractory harzburgites. This latter stage can be linked to the eruption of low-Ca boninites at Cape Vogel, and other arc-related volcanics, in a nascent oceanic island arc. Both ophiolites were emplaced shortly after when the embryonic oceanic island arc collided with the Australian continent.

The Louisiade ophiolite: A missing link in the western Pacific

AbstractRecent dredging of a 100 km long ridge along the northernmost part of the Louisiade Plateau (LP) recovered serpentinized peridotites, MORB (mid‐ocean‐ridge basalt) and volcaniclastic breccia–conglomerates. Clinopyroxene, Cr‐spinel and bulk rocks show that the serpentinites are harzburgites to dunites, whereas hornblende phenocrysts from volcaniclastic rocks reflect hydrous, andesitic volcanism. The association of MORB, depleted mantle rocks and fingerprints of hydrous arc magmatism is typical of supra‐subduction zone ocean lithosphere formed above a nascent subduction zone. Seismic and high‐resolution bathymetry data reveal structures consistent with an extensive east–west elongated ophiolite estimated to have obducted onto the LP between 53 and 80 Ma. This represents a major eastwards continuation of the Papuan Ultramafic Belt and forms a crucial link with ophiolites farther south in New Caledonia, providing support for major subduction initiation events in marginal basins along the northern and eastern margins of Australia and Zealandia in the Palaeocene–Eocene.

Control of serpentinisation rate by reaction-induced cracking

Serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water. The solid volume increase during serpentinisation can lead to stress build-up and trigger cracking, which ease fluid penetration into the rock. The quantitative effect of this reaction-induced cracking mechanism on reactive surface generation is poorly constrained, thus hampering our ability to predict serpentinisation rate in geological environments. Here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid-ocean ridges. We develop a micromechanical model to quantify the propagation of serpentinisation-induced cracks in olivine. The maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses. The micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting, and thus bulk reaction rate. Our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation. We also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the Marum ophiolite and the Papuan ultramafic belt (Papua New Guinea). The natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40%. These results are in agreement with our model predictions, suggesting that reaction-induced cracking controls the serpentinisation rate. We use our model to estimate that, at mid-ocean ridges, serpentinisation occurs up to 12 km depth and reaction-induced cracking reduces the characteristic time of serpentinisation by one order of magnitude, down to values comprised between 10 and 1000 yr. The increase of effective pressure with depth also prevents cracking, which positions the peak in serpentinisation rate at shallower depths, 4 km above previous predictions.

Magmatic Enclaves and Andesitic Lavas from Mt. Lamington, Papua New Guinea: Implications for Recycling of Earlier-fractionated Minerals through Magma Recharge

Mt. Lamington is a composite, dome-forming volcano in Papua New Guinea, sitting on the Papuan Ultramafic Belt (PUB) ophiolite. The 1951 eruption produced andesitic dome lavas with numerous basaltic–andesitic enclaves and a few PUB ultramafic xenoliths. To understand the nature of the 1951 eruption, and to assess the effect of assimilating ophiolitic crust in modifying the geochemistry of arc magmas, we carried out petrological, mineralogical and geochemical studies on andesitic lavas as well as magmatic enclaves and ultramafic inclusions. The mineralogy of the enclaves is dominated by amphibole and plagioclase, similar to the andesitic lava hosts. The textures of the enclaves vary from fine-grained diktytaxitic to coarser-grained plutonic textured. We interpret this variation to result from variable cooling rates in the enclave-forming magma body when it invades the overlying andesite. The diktytaxitic enclaves contain variable proportions of host-derived amph + plag antecrysts and xenocrysts of ol + sp ± cpx ± amph with disequilibrium textures, indicating interaction with host lava and assimilation of foreign materials, respectively. A previous study argued that the olivine xenocrysts with chromian spinel inclusions are derived from the PUB, and thus that the PUB contaminated the Mt. Lamington magmas. We demonstrate that this is highly unlikely on the basis of morphological and compositional discrepancies between PUB ol + sp, sampled in nodules, and the xenocrysts. Mass balance indicates that the high whole-rock Ni contents of enclaves and andesitic hosts can be explained by olivine incorporation and do not require any PUB involvement. The olivines are considered to represent crystal mush fractionated from precursor(s) of andesitic and/or pre-1951 shoshonitic lavas. Their presence in enclaves represents recycling of earlier-fractionated components through magma recharge. We argue that this recycling is an important and underestimated process in shaping arc magmas.

Suckling Dome and the Australian–Woodlark plate boundary in eastern Papua: the geology of the Keveri and Ada'u Valleys

The Owen Stanley Fault Zone (OSFZ) is the low-angle thrust boundary between the Australian and Woodlark plates. The eastern extension of the OSFZ links with the Woodlark Basin spreading centre. Recent tectonic models of eastern Papua depict the OSFZ boundary passing through the Mt Suckling district, with the Keveri Fault a key component. Gravity data clearly show that the OSFZ and the Papuan Ultramafic Belt (PUB) pass north of Mt Suckling. Tectonised mafic and ultramafic rocks of the Mt Suckling district, previously referred to the PUB, are reassigned to the Awariobo Range Complex (new name). Extensive pillow basalts previously referred to the middle Eocene part of the Kutu Volcanics at the top of the PUB sequence are, in the map area, reassigned on lithological and biostratigraphic grounds to the late Oligocene–middle Miocene Wavera Volcanics. The detailed work reported here indicates that the Keveri Fault is unrelated to the OSFZ with no evidence for thrusting along the structure. The area's tectonic history has been dominated by large vertical displacements along the Keveri Fault. The commencement of late Miocene buoyant uplift of the Suckling Dome (new name), related to granite intrusion into thick crust of the eastern Papua region, marks the inception of the Keveri Fault and coincides with the initiation of Woodlark rifting. The fault facilitated much of the rapid vertical movement of the dome, with an estimated 8000 m of uplift (2.5 m/103 a) since the late Miocene. Movement on the Keveri Fault is notably different from structures flanking other metamorphic core complexes in eastern Papua. There is no field evidence for the development of a low-angle, south-dipping detachment fault along the southern margin of the Suckling Dome. The Suckling Dome is the westernmost of the eastern Papua domes, localised within a broad extensional zone that continues to propagate westward along the OSFZ plate boundary.

The chemical characterization and tectonic significance of ophiolite terrains in southeastern Papua New Guinea

AbstractThe evolution of the southeastern Gondwana margin in the Papua New Guinea segment is manifested by ophiolites signifying plate collision, volcanic arcs marking subduction, and metamorphic core complexes that have developed during extensional events and exhumation associated with sea floor spreading in the Woodlark Basin to the east. In detail the Papuan Ultramafic Belt marks a well‐documented collision between continental crust and a subduction system. However, to the southeast, an extensive sequence of basaltic rocks known as the Milne Terrain is more problematic. Geochemical data indicate that these upper Cretaceous and Eocene rocks have MORB‐type affinities, and their most likely tectonic association is with the opening of the Coral Sea Basin. Milne Terrain rocks represent the lower plate in the obduction system along which the Papuan Ultramafic Belt was emplaced, and thus they are the structural equivalent of the continental crust which was separated from the Australian continental block by the opening of the Coral Sea. Spectacular uplift (>4 km) of the oceanic basaltic crust of the Milne Terrain may be due to the underlying presence of underplated material associated with a Late Miocene‐Pliocene episode of subduction immediately prior to the encroachment of the Woodlark spreading center into the Papuan area.

Hydrothermal Ni: Doriri Creek, Papua New Guinea

The Doriri Creek (DC) Ni–Pd–Pt prospect was discovered in 1966 in the Papuan Ultramafic Belt (PUB) in PNG. The DC was interpreted as a hydrothermal Ni accumulation. The DC is located in the southern proximity of Mt Suckling (~180km SE of Port Moresby), where local intrusive rocks are intermediate to acid dykes and small stocks, within the tec tonized contact zone of the Australian and Woodlark Plates. The active volcanoes of Mount Victory and Waiowa indicate recent thermal activity in the area.The Doriri Creek prospect is the result of episodic hydrothermal fluid flow running through the Doriri prospect, that resulted in Ni concentration of up to 1.55wt.%, formed by alteration of an ultramafic unit of peridotites/pyroxenites within a Mg-rich gabbronorite envelope. Ni was concentrated in chlorite and serpentine group minerals in addition to Fe oxides, with a minor amount in pentlandite in locally sulfidic samples. Ore mineralogy is also associated with a high phosphorous content as apatite, that concentrates LREE (light rare earth elements). Palladium concentrations are up to 0.37ppm. Platinum is present in concentrations up to 0.06ppm within the ore.The alteration halo associated with Doriri Creek mineralization is ~100m in width. Primary mineralogy comprises pyroxene, olivine and plagioclase, which have been altered extensively to amphibole and chlorite–serpentine group minerals. This halo is characterized by enrichments of U, K and W over background values.Local magnetite concentration is up to ~35% of whole rock, which is very pronounced in the sulfide rich area of the system. The top part of the DC system is overprinted by tropical weathering at metric scale, which displays LREE enrichment and positive Ce anomalies.The Papuan Ultramafic Belt is described as a highly prospective ground for hydrothermal Ni systems based on its availability of Ni, active thermal flow engines, and the geologic regional context dominated by mafic rock suites and the presence of carbonate/siliciclastic units.

Arc-continent collisional orogenesis in the SW Pacific and the nature, source and correlation of emplaced ophiolitic nappe components

Southwest Pacific ophiolitic nappes emplaced upon the former margin of eastern Australia provide a record of cyclical episodes of arc-continent collisional orogenesis that occurred throughout the Paleogene. Cenozoic nappes in Papua-New Guinea, New Caledonia and Northland, New Zealand consist of two key components: (1) a basal and apparently older Late Cretaceous–Eocene, metamorphosed, normal to weakly-enriched MORB-like basaltic ocean floor terrane; and (2) an uppermost supra-subduction zone (SSZ) ophiolite that represents basement of the associated magmatic forearc. A detailed comparison of geochemical characteristics of unequivocal SSZ basalts with those considered as representative of the basaltic ocean floor terranes illustrates that formation of the majority of the latter is consistent with melting of a source transitional to normal (N-) and enriched (E-) MORB. These slight REE-enriched or transitional (T-) MORB have generally been considered as the result of seafloor spreading in a back arc basin (BAB) or basins to the east of Australia in the Late Cretaceous-Paleocene (~85–55 Ma). The enrichment recorded by the BAB basalts is likely a result of upwelling plumes associated with initial fragmentation of eastern Gondwana that left their signature on the mantle source consequently tapped by the BAB basalts. A subset of slightly more enriched basalts may owe their enrichment to source contamination (minor assimilation of thinned Gondwana-derived continental crust). Associated LREE-depleted N-MOR basalts previously considered as part of the basaltic ocean floor terranes on the other hand, are similar to SSZ basalts of the Northland and Papuan Ultramafic Belt ophiolites. Refinement of existing tectonic models is hindered by ambiguities in the stratigraphic relationships of the underlying basaltic ocean floor terranes with the overlying SSZ ophiolites, a paucity of age data and uncertainties in the tectonomagmatic affinities of the underlying basalts. Nonetheless, the geochemical characteristics displayed by LREE-depleted basalts of the basaltic ocean floor terranes suggest that these may instead represent younger, magmatic forearc basement components subsequently overthrust by lower sections of a genetically related ophiolite. Whether these and for that matter the majority of basalts of the basaltic ocean floor terranes, indeed represent complementary forearc crust to the SSZ ophiolite as opposed to a slice of Late Cretaceous-Eocene BAB floor backthrust beneath the SSZ ophiolite upon emplacement, will have to await the acquisition of corresponding radiometric age data.

New SW Pacific tectonic model: Cyclical intraoceanic magmatic arc construction and near‐coeval emplacement along the Australia‐Pacific margin in the Cenozoic

Various reconstructions of the SW Pacific for the Late Cretaceous and Cenozoic suggest that northeast dipping subduction began in the South Loyalty Basin (SLB) at 55–50 Ma and that subsequent closure of the SLB resulted in the diachronous emplacement of Cretaceous‐Paleocene ophiolitic nappes onto the Norfolk Ridge in New Caledonia at 40–34 Ma and in Northland, New Zealand, around 24–21 Ma. A fundamental problem with these models is that they do not account for the fact that NE dipping subduction had already been established offshore Papua New Guinea by at least 65–60 Ma which resulted in the emplacement of the Papuan Ultramafic Belt (PUB) ophiolite at 59–58 Ma. A second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the Loyalty arc and Three Kings Ridge. Finally, reconstructions of the Northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the Northland allochthon is Late Cretaceous‐Paleocene, when in fact it is Oligocene. A new model is presented whereby the PUB, New Caledonia, and Northland ophiolites formed and were emplaced in a cyclical fashion above an extensive NE dipping Cenozoic intraoceanic arc system which diachronously propagated (N‐S) along the entire eastern margin of the Australian Plate. These “infant arc” ophiolites represent fragments of suprasubduction zone lithosphere (SSZL) generated in the earliest stages of magmatic arc formation that were emplaced shortly after (<20 m.y.) as a result of forearc‐Australian Plate collision. Subduction inception was the result of subsidence of older MORB‐like lithosphere generated within an extensive “back arc basin” to the east of the Norfolk Ridge during the earliest stages of SLB formation above a southwest dipping Pacific Plate. During emplacement of each ophiolite, a crustal fragment of the older lithosphere was scraped off the NE dipping slab and subsequently back‐thrust beneath each ophiolite during its emplacement.

Age of the metamorphic sole of the Papuan Ultramafic Belt ophiolite, Papua New Guinea

The Papuan Ultramafic Belt (PUB) ophiolite is former oceanic crust and upper mantle emplaced onto continental crust in Papua New Guinea (PNG) in a zone of general convergence between the Pacific and Australian plates. The metamorphic sole beneath the ophiolite is best exposed in the Musa–Kumusi divide and comprises a 40- to 300-m-thick body of granulite and amphibolite facies rocks. Geochronological studies on the metamorphic sole, using amphiboles from the granulites and amphibolites, yield measured K–Ar ages ranging from 65.0±0.7 to 57.2±0.6 Ma and average 40Ar– 39Ar direct total fusion ages ranging from 67.0±0.7 to 59.5±0.2 Ma. Five of the six 40Ar– 39Ar plateau ages, derived from age spectra, lie between 58.6±0.2 and 57.8±0.2 Ma, with an overall mean age of 58.3±0.4 Ma. The large spread in measured K–Ar and 40Ar– 39Ar total fusion ages is thought to be caused by the presence of variable amounts of excess argon. The mean plateau age for five samples of 58.3±0.4 Ma is interpreted to mark the time of cooling of the metamorphic sole following peak metamorphism. We suggest that the development of the metamorphic sole and emplacement of the PUB ophiolite onto the PNG crust occurred in a relatively short time interval in the Paleocene.

Forearc ophiolites: a view from the western Pacific

Abstract Many of the world’s largest ophiolite masses are now being interpreted as remnants of oceanic forearcs stranded on continental margins in the course of arc-continent collision. This interpretation implies a former association between the ophiolite and a volcanic arc at a distance of 100–200 km. The New Caledonia region of the SW Pacific is one of the few areas where there is good evidence for the presence of such an arc. On New Caledonia itself, the Grand Massif du Sud ophiolite has been thrust over the Norfolk Ridge continental fragment, while the coral islands of the Loyalty Group, c. 100 km to the NE, cap large sub-sea edifices with volcanic morphology. New Caledonia can be correlated geologically with New Guinea, despite the considerable width of open ocean that now separates the two. In central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges, the similarities include the relationship between New Caledonia and the Loyalty Islands. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, which is backed to the north and east by the oceanic Solomon Sea. An associated volcanic arc can be recognized in this area only by assuming a complicated history of collision, post-collision arc-forearc separation and sea-floor spreading, followed by renewed contraction and a very recent and continuing second collision. The case for such a sequence of events can be made on a number of grounds. If the processes responsible are general in their nature, they could explain the apparent absence of arc-volcanic belts in association with many other supposedly forearc ophiolites.

Geophysical contributions to the COASTPLAN project, Lae area, Papua New Guinea, 1998

The Lae Coastal Hazards study is one of three pilot projects currently sponsored by CCOP as part of the overall COASTPLAN programme. The aim of the study is to assess the vulnerability of the coastal region at the head ofthe Huon Gulf, and particularly the Lae urban area, to natural disaster. Detailed investigations of possible risk made in early 1998 by the COASTPLAN team were supported by a gravity survey designed to clarify the factors controlling uplift and subsidence in this tectonically active region. The total of93 new gravity stations established in the Leron-Lae-Finschafen area confirmed the presence of a major foreland basin gravity low in the northern Markham Valley and the probability of an extension of the Papuan Ultramafic Belt beneath the Huon Peninsula. Correlations between gravity patterns and local and regional seismicity suggest that some potential long term and short term sources of geological hazards may have been overlooked.

Gravity fields in eastern Halmahera and the Bonin Arc: Implications for ophiolite origin and emplacement

Classic ophiolites, as exemplified by the Troodos Massif in Cyprus and the Papuan Ultramafic Belt in eastern New Guinea, are large overthrust masses which are generally associated with large positive gravity anomalies. However, similar rocks occurring in extensive fragmented terranes which have also been described as ophiolitic do not produce large gravity effects. The eastern part of the island of Halmahera, in northeastern Indonesia, is an ophiolite of this latter type. On the two eastern arms of the island, a Mesozoic ophiolitic basement is overlain by, and imbricated with, Upper Cretaceous and Paleogene arc volcanic and sedimentary rocks. Bouguer gravity values are generally in the range +50 to +150 mGal and are characterised by steep local gradients indicative of shallow sources. The Bouguer gravity average suggests that the crust is at least 20 km thick, and it must be even thicker if a significant part of the anomalous gravity field is due to the presence of a cold and therefore dense, lithospheric slab within the asthenosphere, associated with the present‐day subduction beneath Halmahera. The absence of any exposures of continental basement rocks or of quartzose sediments in eastern Halmahera suggests that these ophiolites have not been overthrust onto continental crust and that the thickening occurred in an intraoceanic island arc. The Paleogene arc was evidently characterised by volcanism occurring over an unusually wide area. In this it resembles the Izu‐Bonin volcanic arc, which, like Halmahera, has been situated at the margin of the Philippine Sea Plate throughout its history. The gravity field of the Halmahera ophiolite is comparable with that of the Bonin volcanic arc, but there is no Halmahera parallel to the very high gravity fields recorded over the Bonin Islands forearc ridge. The equivalents of this part of the Paleogene arc may be represented by the ophiolitic complexes now distributed along the northern margin of the orogenic belt in New Guinea.

The Sadowa Igneous Complex, Eastern Papua New Guinea: Ophiolite or Not?

Milsom and Smith (1975) suggested the Sadowa Igneous Complex (SIC) could represent a fragment of oceanic crust that has been thrust into its present position. Pigram and Symond (1991) suggested another contemporaneous spreading center north of the Coral sea. The SIC floored that basin between the continental fragments of the Eastern, Papuan and proto-Owen Stanley terranes (Fig. 1). The late Cretaceous to Eocene SIC is mainly composed of gabbro, dolerite, basaltic conglomerate, breccia, lower (with pelagic clasts) and upper volcanic rocks (interbedded and overlained by volcaniclastic graywackes) and a few serpentinites, serpentinized dunites, clinopyroxenites and plagiogranites. The SIC is conformably overlain by pelagic sediments and is cut by reverse or normal faults. It was obducted onto the Owen Stanley metamorphics during the Late Eocene to Middle Oligocene to form the Eastern Papuan Composite Terran (EPCT) after the emplacement of the Papuan Ultramafic Belt (Pigram and Davies, 1987). Subsequently tectonism could be expected to have reversed the southward subduction to northward subduction from the Port Moresby trench (Hamilton, 1979). During the Miocene the EPCT collided with Eastern and Papuan Plateaus (Pigram and Davies, 1987). The Astrolabe agglomerate contains subduction

Structural setting, petrology and emplacement of serpentinites in the Koki Fault Zone, Port Moresby, Papua New Guinea

The possible mantle origin of serpentinites from the Koki Fault Zone (KFZ) is investigated by geochemical comparison with ultramafic rocks from the Papuan Ultramafic Belt (PUB), the Marum Ophiolite complex and serpentinite slivers from Oliguti, some 50 km south of Marum. The Cr spinels from the Koki serpentinites, but not those from the Oliguti serpentinites, show a partial overlap in composition with those of the PUB and also Marum Ophiolite cumulate and noncumulate (tectonite) members. Serpentinite whole rock analyses are refractory with high mg numbers (Mg/Mg + Fe 2+)% (87–92) and low TiO 2 (< 0.05 wt%) together with high Cr and Ni. They contain below-chondrite levels of REE with U-shaped patterns. The Koki serpentinites, but not those from Oliguti, are chemically comparable with cumulate members of an ultramafic ophiolite sequence. Structural mapping of the area has shown the presence of three main structural domains; an imbricate thrust stack, the KFZ and a possible passive roof duplex structure. These structures suggest that deformation occurred close to the front of a foreland thrust belt. It is speculated that serpentinite emplacement was achieved in two stages; an initial stage of diapiric upwelling consequent on hydration followed by a late stage of emplacement by thrusting within the KFZ.

Gravity measurements and terrane tectonics in the New Guinea region

Composite terranes consist of amalgamations of crustal blocks with differeing geological histories and crustal thicknesses which can usually be identified by conventional geological techniques. Times of amalgamation may be deduced from sedimentological and seismic stratigraphic studies in successor basins and palaeomagnetism can often provide some control on translational history. However, information on the geometries of terrane boundaries at depth is all too often lacking and cannot always be obtained by seismic reflection techniques. A considerable degree of control on this third dimension can be obtained from gravity measurements, especially where oceanic or ophiolitic terranes are in contact with continental blocks. The Papuan Ultramafic Belt of eastern New Guinea provides one excellent example of the ways in which terrane models can be evaluated by considering the gravity fields they would produce. Comparisons of the Bouguer anomaly patterns at various points along the Belt with those observed around geologically less well mapped ophiolites in other parts of New Guinea provide insights into structural settings which cannot be obtained from any of the other data sets available at present. The differences between types of ophiolitic terranes are clearly demonstrated. In some areas, as in the Weyland Terrane of western New Guinea, gravity results, instead of suggesting solutions to existing problems, raise new ones, complicating still further the task of formulating satisfactory structural models. Geological and geophysical studies must be closely integrated, and geologists and geophysicsts must structural closely together, if such problems are to be solved.

Late Cenozoic volcanism and extension in Eastern Papua

Summary The sea floor around eastern New Guinea is divided into a number of deep basins, separated by submarine ridges which are capped in places by islands of metamorphic, volcanic or coralline rock. The area has been subject to phases of extension since at least the Palaeocene, when the Coral Sea basin was formed. There have also been major compressive events, including the thrust emplacement of an ophiolite, the Papuan Ultramafic Belt, on to the Papuan Peninsula in the Oligocene. This peninsula, on the E of New Guinea, has been volcanically active from the Middle Miocene to the present day, and there have been eruptions on many of the surrounding islands and in the marine basins. Extension-related volcanic rocks include low-K tholeiites dredged from the floor of the Woodlark Basin and a peralkaline rhyolite association on islands near the E end of the peninsula. Volcanic rock types usually regarded as indicators of subduction, including andesites and high-K trachybasalts (shoshonites), are common. During the Neogene, two phases of extension with associated igneous activity can be interpreted; the first in the Middle and Late Miocene, and the second from the Middle Pliocene to the present day. Only in the first phase was the formation of marginal basins related to subduction. The later phase is seen as part of the response of the complexly fragmented Melanesian area to changes in relative motions of the surrounding major plates. Sea-floor spreading is currently occurring in the Woodlark Basin, and the post-Miocene calc-alkaline and shoshonitic rocks of the Papuan Peninsula and offshore islands reflect reactivation of subduction-modified mantle under this tensional regime, and not renewed subduction.

Emplacement of ophiolite in Papua New Guinea

Summary The major ophiolite complexes of Papua New Guinea lie on the northeastern margin of the Australian craton and are flanked, externally, by Palaeogene volcanic arcs. The ophiolites are segments of oceanic lithosphere which occupied the forearc zone prior to arc-continent collision. In the case of the easternmost ophiolite, the Papuan ultramafic belt, there is evidence that emplacement was preceded by development of a secondary rupture in the down-going plate, at some distance from the subduction zone, and was followed by extension, buoyant emergence, and reversed movement of the ophiolite. The westernmost ophiolite (April ultramafics) has more complex structure and probably developed as a series of thrust sheets in a subduction system; the thrust sheets were subjected to renewed deformation and thrusting by continued convergence after arc-continent collision.

The gravity field of the Marum ophiolite complex, Papua New Guinea

Summary The ophiolitic Marum complex crops out in northern New Guinea and to the south is in fault contact with sialic rocks of the continental core of the island. It is overlain to the north by thick sediments of the Ramu basin which give rise to a major gravity low. Gravity surveys over the complex and its margins have defined a Bouguer anomaly high offset towards the northern edge of the outcrops of basic rock. The anomaly is similar in form to, but much smaller than, the anomaly associated with the Papuan Ultramafic Belt to the east. Also, it is superimposed on a very much lower background field, so that positive Bouguer anomaly values occur only at the very peak of the high. A geological model of the area consisting of only three major units; continental crust, Neogene sediments and a heavily faulted island-arc frontal zone of mainly ultramafic rocks, is compatible with the gravity data. Other interpretations which are gravitationally acceptable are less easily accomodated to the geological constraints. A problem still exists in explaining the ‘thin-skin’ overthrust in the southeastern part of the complex.

Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome

The catastrophic explosive eruptions in January 1951 from Mount Lamington were followed by a year-long episode of activity which was thoroughly described by Taylor (1958). However, only preliminary major-element analyses and petrographic observations have previously been reported from this volcano. The present study was prompted by the description of anhydrite and numerous ultramafic inclusions in the lavas of Mount Lamington, together with reports of similar inclusions and anomalously high Ni and Cr contents in other volcanics of southeastern Papua. Mount Lamington, together with three other major andesitic centers — Mount Victory, Mount Trafalgar and Hydrographers Range — rest on a northeast-dipping ophiolite sheet, the Papuan Ultramafic Belt (PUB); there is no evidence for a Benioff-Wadati zone beneath this part of Papua. The lavas of Mount Lamington are porphyritic trachybasalt and high-Si, medium-to-high-K andesite. Phenocrysts are plagioclase, amphibole, and biotite, and less common minerals are magnetite, pyroxene, apatite, cristobalite, zircon and sulphide. Olivine and orthopyroxene grains and clusters are found as foreign fragments, and anhydrite is a minor constituent, particularly of pristine samples. Numerous inclusions of xenolithic harzburgite and amphibole-biotite-plagioclase-dominated, coarse-grained igneous inclusions are also present. The lava suite is characterized by enrichments in several incompatible elements relative to intermediate lavas from Bougainville Island and New Britain. Products of the 1951 eruption have Ni and Cr concentrations in the ranges 23–45 ppm and 54–111 ppm, respectively. There are dramatic compositional zonations in the constituent phases of both lavas and inclusions. For example, amphibole zones range from compositions compatible with crystallization from pristine intermediate magmas, to Mg and Si-rich and alkali-poor components. Spinels range from refractory, Al-poor chromite to magnetite in composition. Contamination of rising andesitic magmas by the PUB is demonstrated conclusively for Mount Lamington lavas, and overall interaction with crustal rocks in other andesitic provinces may be more generally prevalent than has previously been recognized. The high Ni and Cr contents of the lavas have resulted from partial digestion of PUB material, and are not the result of direct partial melting of an upper-mantle source; neither do the high values exclude derivation of the intermediate magmas by fractional crystallization of basalt below the ophiolite. Isotopic analysis indicates the source of the sulphur in anhydrite was of primary magmatic rather than sedimentary origin. Two possible sources for the sulphur are either remobilized sulphides in the PUB, or, more speculatively, low-pressure breakdown of sulphate meionite (scapolite), possibly present in the lower crust of southeastern Papua.