S Meffre

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental ribbons, including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These microcontinental ribbons were isolated from Australia and from each other during a protracted extension episode from ca 120 to 52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major change in Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spreading centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty-Entrecasteaux arc. Continued subduction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 70-60 million years old western volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorly exposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty-Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist and eclogite facies metamorphic assemblages. Following locking of this subduction zone at 38-34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above which formed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatu and Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first the South Fiji Basin (31-25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong Java Plateau, a large igneous province, with the Solomons section of the Vitiaz arc resulted in a reversal of subduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South Fiji Basin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc and produced the Lau Basin-Havre Trough. This southwest Pacific style of crustal growth above a rolling-back slab is applied to the 600-220 Ma tectonic development of the Tasman Fold Belt System in southeastern Australia, and explains key aspects of the geological evolution of eastern Australia. In particular, collision between a plume-triggered 600 Ma volcanic passive margin and a 510-515 Ma boninitic forearc of an intra-oceanic arc had the same relative orientation and geological effects as that which produced New Caledonia. A new subduction system formed probably at least several hundred kilometres east of the collision zone and produced the Macquarie arc, in which the oldest lavas were erupted ca 480 Ma. Continued slab rollback induced regional extension and the growth of narrow linear troughs in the Macquarie arc, which persisted until terminal deformation of this fold belt in the late-Middle to Late Devonian. A similar pattern of tectonic development generated the New England Fold Belt between the Late Devonian and Late Triassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward to locations in New Zealand, and on the Lord Howe Rise and Norfolk-New Caledonia Rise, during the post- 120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustal accretion like the southwest Pacific since 120 Ma, facing the open Pacific Ocean, we question whether the eastern (Australia-Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined to Laurentia.

A North American provenance for Neoproterozoic to Cambrian sandstones in Tasmania

Abstract Tasmania forms an enigmatic province within the Neoproterozoic to Cambrian history of Australia. It lies at the boundary between Australia and North America in most Rodinia reconstructions but no reliable lithostratigraphic correlations have been reported with either mainland Australia or North America. We used detrital zircon age spectra, measured by LAM-ICP-MS, of Neoproterozoic and Cambrian sandstones in Tasmania to search for evidence of correlations with these two continental blocks during the time slice critical to Rodinia breakup. The Tasmanian sandstones are dominated by 1600–1900 Ma and 1200–1500 Ma age zircons. There is little evidence for Grenville (∼1100 Ma) and Ross (∼550 Ma) Orogen sources in these sandstones, in contrast to detrital zircon age spectra of similar age rocks in South Australia. The detrital zircon age spectra of Tasmanian sandstones are different from age spectra reported from British Columbia. They are very similar to age spectra reported from Cambrian sandstones of Nevada, supporting Rodinia reconstructions that place southwestern USA near to Tasmania in the Neoproterozoic.

Re-evaluation of contact relationships between Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen

Some published tectonic reconstructions of the eastern Lachlan Orogen in New South Wales have shown Ordovician volcanic and volcaniclastic rocks of the Macquarie Arc conformably overlying or interfingering with a coeval Ordovician quartz-rich turbidite sequence. Re-examination of key contacts between the volcanic and quartz-rich successions has found no evidence to support this interpretation, and suggests that the two packages are separate tectonostratigraphic terranes. The contacts between these two coeval successions are generally marked by major faults containing mylonites, cataclasites and, at some locations, fragments of mid-ocean ridge-type pillow basalt and chert. The quartz-rich turbidites are generally highly deformed and of higher metamorphic grade than the adjacent volcanics. At Oberon and Mudgee, contacts are faulted but there are no mylonites or significant differences in metamorphic grade. At Palmers Oaky and Black Springs, Silurian quartz-rich sandstones overlying the Ordovician volcanics have been mistakenly assigned to the Ordovician in previous studies. Throughout the Lachlan Orogen, there is no mixing of framework grains. Quartz-rich turbidite successions are dominated by quartz with lesser feldspar and rare tourmaline, zircon and monazite derived from recycled continental sources. In contrast, the volcaniclastic sandstones contain feldspar, clinopyroxene and lithic fragments derived from subduction-related clinopyroxene-phyric basalt and plagioclase-phyric andesite. Detrital-zircon populations also differ, with separate U/Pb age populations and almost no overlap. Comparison of the Ordovician sequences of the Lachlan Orogen with modern turbidites from continental- and arc-related sedimentary basins suggests that complete separation of sedimentary sources is only possible if the sandstones were deposited hundreds of kilometres apart, in separate tectonic environments. The two sequences were juxtaposed along major faults in the Late Ordovician or Early Silurian, probably when the Macquarie Arc collided with a thick Ordovician sedimentary wedge located on the Gondwanan continental margin.

Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia

The Central Chain ophiolites in New Caledonia are fragments of a supra-subduction zone (SSZ) ophiolite, now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick Middle Triassic to Upper Jurassic volcaniclastic sequence. Most of the fragments were formed by a single tholeiitic magmatic episode, but one of these, the Koh ophiolite, was formed by two tholeiitic magmatic episodes separated by boninites. The first event in the Koh ophiolite formed cumulate gabbros, dolerites, plagiogranites, and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements (LREE) and abnormally low TiO2 (0.5% at Mg#=60). Shortly after their eruption, these tholeiitic lavas were overlain by a high-Ca boninitic unit with a basal section of boninite pillows, flows, and breccias and an upper section of boninitic dacites and tuffs. The last magmatic phase involved eruption of evolved tholeiitic basalts, as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite. This second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts (BABB) erupted during the early rifting history of modern back arc basins. The boninitic volcanics belong to a high-Ca series with slightly lower SiO2, Al2O3, and TiO2 compared to those from modern island arc systems, and they lack the positive Zr spike relative to adjacent rare earth elements (REE) in normalised element variation patterns. These boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40–60% extension of the initial basin crust and eruption of the upper tholeiites. The dominance of BABB-like tholeiites throughout the Central Chain ophiolites in New Caledonia, the restricted occurrence of boninites, and the stratigraphy and chemistry of the Koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation. We suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites, during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre. The occurrence of a thick blanket of calc-alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the Koh boninites were not associated with the initiation of subduction. A close modern analogy for the Koh ophiolite exists on the Hunter Ridge protoisland arc between southernmost Vanuatu (New Hebrides island arc) and the Fijian islands; there, high-Ca boninites lacking positive Zr spikes occur together with low-Ti tholeiites and more typical BABB tholeiites where the southern spreading centre of the North Fiji Basin is propagating into the protoarc crust of the Hunter Ridge.

Middle and Late Ordovician magmatic evolution of the Macquarie Arc, Lachlan Orogen, New South Wales

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of ∼9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive Nd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee – Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee – Narromine Volcanic Belt have ages that cluster in the 470 – 460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calc-alkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley – Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee – Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley – Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian – Devonian opening of the Hill End Trough. Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian – Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee – Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment, occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Volcanics in the Junee – Narromine Volcanic Belt, and Alaskan-type zoned ultramafic intrusions of the Fifield complexes farther west. The latter were emplaced through deformed Ordovician turbidites of the Girilambone Group, and their radiogenic isotope signatures show significant crustal involvement.

Late Paleozoic tectonic evolution of the northern West Chinese Tianshan Belt

The northern West Chinese Tianshan is divided into three subunits: Carboniferous turbidite, ophiolitic mélange and Yili magmatic arc. Stratigraphical and petrological studies suggest that the turbidite and ophiolitic mélange form a subduction complex. The ophiolitic mélange that forms the North Tianshan suture was a result of intra-oceanic tectonism and subsequent redeposition and deformation during the subduction of the North Tianshan oceanic basin. The Yili arc-type granitoids are constained by single zircon U-Pb radiochronology between 361 and 309 Ma. The first-hand kinematic results on the deformed turbidite suggest that this suture zone was reworked by a Permian ductile dextral strike-slip fault. An evolutionary model of the study area allows three events to be distinguished: 1) Late Devonian to Carboniferous subduction of the oceanic basin below the Yili Block producing Yili magmatic rocks and subduction complex, 2) Late Carboniferous complete closure of this basin, 3) Permian right-lateral strike-slip faulting generating pull-apart basins and alkaline magmatism. A prominent reactivation during the Indo-Eurasia collision provoked the northward thrusting of the Paleozoic units upon the Cenozoic sediments of the Junggar Basin, consequently, hiding the bulk of this Late Paleozoic suture.

Benambran Orogeny in the Eastern Lachlan Orogen, Australia

The Benambran Orogeny reflects the accretion of the intra-oceanic Macquarie Arc to the Gondwana Plate. The ultimate cause was the northwards strike-slip transport of the allochthonous Bega Terrane along the eastern margin of Gondwana, into a forearc position outboard of the Macquarie Arc. Ensuing oblique compression in the outboard part of the Gondwana Plate drove the Macquarie Arc into and under its backarc Wagga Basin, represented by the Girilambone – Wagga Terrane, and led to a combination of thrusting and major strike-slip faulting within, inboard and outboard of the arc over 10 million years. In this time frame, the Benambran Orogeny in the Eastern Subprovince of the Lachlan Orogen in New South Wales consists of two phases of exhumation – deformation, at ca 443 Ma (late Bolindian to early Llandovery) and ca 430 Ma (late Llandovery), separated by a relaxation/extensional event. Both phases involved deformation and exhumation of the Macquarie Arc and coeval quartz-rich turbidites and black shales of the Adaminaby Superterrane. Closure of the former backarc basin was facilitated by limited east-dipping subduction that generated the short-lived Fifield arc. The second phase of the Benambran Orogeny also involved deformation and exhumation of overlying Llandovery strata (e.g. the Yalmy Group) and syn- to post-tectonic emplacement of granitoids. In both phases, deformation of Adaminaby Superterrane rocks was more intense than deformation of arc rocks.

The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende ( 40Ar/ 39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ∼56Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ∼0.5GPa at ∼800-950C. The thermal gradient inferred from the metamorphic assemblage (∼60°Ckm -1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ∼53Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ∼56Ma.

Cambrian metamorphic complexes in Tasmania: tectonic implications

Cambrian metamorphic complexes containing amphibolite‐ to eclogite‐grade rocks are present throughout western and northwestern Tasmania. These complexes contain mostly quartz‐albite‐biotite schists, garnet‐quartz‐albite‐biotite schists and mafic amphibolite lenses (up to 1 km long). The chemistry of these rocks is similar to unmetamorphosed, Late Neoproterozoic tholeiitic basalts and continental‐derived siliciclastics. A few rocks in the metamorphic complexes have compositions that are transitional between the amphibolites and the schists, representing metamorphosed volcaniclastic rocks formed by mixing between mafic and siliciclastic sources. The rocks in these complexes were probably located on the edge of a thin Late Neoproterozoic passive margin that was partially subducted during a Cambrian arc‐continent collision and uplifted during post‐collisional crustal re‐equilibration. The metamorphic condition, age and chemistry of both the schists and the amphibolites resemble those of metamorphic complexes in North Victoria Land in Antarctica. However, the structural setting of these complexes differs from those in Tasmania. Comparisons of the Tasmanian and North Victoria Land Cambrian structures and lithologies with those from more recent arc‐continent collisions worldwide show that both are compatible with a model involving east‐dipping subduction of a passive margin beneath an intraoceanic island arc. The differences between the two areas probably arise from differences in the geometry of the margins and the thickness of the passive‐margin sediments.