Christophe Monnier

Back-arc basin origin for the East Sulawesi ophiolite (eastern Indonesia)

The East Sulawesi ophiolite is one of the three largest ophiolites in the world. It displays all the components of a typical sequence, from residual mantle peridotites to cumulate gabbros, sheeted dolerites, and lavas of normal mid-oceanic-ridge basalt (MORB) composition. Trace element data on the lavas and dolerites, and particularly their depletion in Nb compared to neighboring incompatible elements, suggest a subduction-zone environment for their origin. The chemical similarity between the East Sulawesi ophiolite lavas and those from the Eocene Celebes Sea back-arc basin crust together with their identical age strongly suggest a back-arc tectonic environment for this ophiolite, which represents a fragment of the Eurasian plate obducted onto the East Sulawesi basement of Australian origin.

Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge.

It has been difficult to relate the segmentation of mid-ocean ridges to processes occurring in the Earths underlying mantle, as the mantle is rarely sampled directly and chemical variations observed in lavas at the surface are heavily influenced by details of their production as melt extracted from the mantle. Our understanding of such mantle processes has therefore relied on the analysis of pieces of fossil oceanic lithosphere now exposed at the Earths surface, known as ophiolites. Here we present the phase chemistry and whole-rock major- and trace-element contents of 174 samples of the mantle collected along over 400 km of the Oman Sultanate ophiolite. We show that, when analysed along the fossil ridge, variations of elemental ratios sensitive to the melting process define a three-dimensional geometry of mantle upwellings, which can be related to the segmentation observed in modern mid-ocean ridge environments.

Cenozoic Plate interaction of the Australia and Philippine Sea Plates: ''hit-and-run'' tectonics

Abstract Recent studies in northwest New Guinea have shown the presence of at least two marginal basins of different age, both of which formed in back-arc settings. The older basin opened between the Middle Jurassic and Early Cretaceous, a remnant of which is now preserved as the New Guinea Ophiolite. Its obduction started at 40 Ma and it was finally emplaced on the Australian margin at ∼30 Ma. The younger basin was active during the Oligocene to Middle Miocene and was obducted in the Early Pliocene. Studies of the western edge of the Philippine Sea also reveal an important deformation of the Philippine arc in the Oligocene, which hitherto has remained unexplained. Using information from these systems, paleomagnetic results, kinematic reconstructions and geochemistry of the supra-subduction ophiolite, we present a plate model to explain the regions Eo–Oligocene development. We suggest that an extensive portion of oceanic crust extended the Australian Plate a considerable distance north of the Australian Craton. As Australia began its steady 7–8 cm/year northward drift in the Early Eocene, this lithosphere was subducted. Thus, the portion of the Philippine Sea Plate carrying the Taiwan–Philippine Arc to its present site may have actually been in contact with the ophiolite now in New Guinea and obduction led to deformation of the Philippine Sea Plate itself. Neogene Plate kinematics transported the deformed belt in contact with the Sunda block in the Late Miocene and Pliocene. This interpretation has implications for the origin for the Philippine Sea Plate and the potential incorporation of continental fragments against its boundaries.

Along‐ridge petrological segmentation of the mantle in the Oman ophiolite

Oman ophiolite mantle has been sampled over a distance of about 400 km, all along the paleo-ridge axis. Primary phases have been analyzed in 174 peridotites (mainly harzburgites) and major and trace element contents measured in 90 and 156 samples, respectively. Most samples display depleted characteristics with very low incompatible element bulk rocks and very low HREE contents. On the basis of the spinel Cr# and in agreement with Yb concentrations in the bulk rocks, an average of 16.5% (Fmax) of melt extraction is estimated. These rocks show light REE enrichments marked by high LREE/MREE ratios that well correlate with the extent of melting. The light REE were possibly gained during the latest stage of melting in an open-system melting model, or through interaction with influxed fluid after melting. Chemical data have been processed Fourier Transforms to study the along-ridge variations, which gives results similar to those obtained using the seven point running average (Le Mee et al., 2004). When plotted along ridge, spinel Cr# display variations with two types of wavelengths, defining four 50–100 km long segments (70 km in average) and numerous 10–20 km shorter ones making undulations within the longer ones. All segments have a center marked by high values of spinel Cr# (≈ Fmax) and edges with the lowest values. The large, 50–100 km segments (70 km in average) may correspond to large asthenospheric mantle upwellings between major deep mantle discontinuities, while the smaller ones possibly relate more superficial mantle instabilities similar to the structural diapirs of Nicolas et al. (1988a). We consider that the variation in degree of melting in the short-scale instabilities relates fluid/melt flux melting variations. By comparison with mid-oceanic ridge models, the long Oman segments can correspond to second-order segments and the smallest to third- to fourth-order ones. Our data on the geometry of the melting zones will constrain models of the dynamics of the mantle beneath ridges. They provide a new perspective for further characterization of the segments in the Oman ophiolite.

Dynamics and age of formation of the Seram-Ambon ophiolites (Central Indonesia)

The Seram-Ambon ophiolitic series comprise peridotites, websterites, gabbros and lavas. Petro-geochemical data show that the peridotites are weakly depleted rocks, except for the rare Cpx-free harzburgites. They underwent a sub-solidus metamorphic re-equilibration in the plagioclase field. The associated websterites and gabbros display va- rious chemical features, allowing to define 3 types of websterites and 2 groups of gabbros. They have mostly BAB cha- racteristics (presence of negative anomalies in Nb, Zr, Ti and Y), except the group 2 gabbros which have N-MORB features and the type 3 websterites which bear adakitic affinities. Lavas also display a variety of compositions, including high-Mg IAT and Mg-rich BABB with sub-alkaline affinities. Both IAT and BABB display high Th/Nb ratios which support an origin close to a continental crust environment. Our 20 to 15 Ma 40 K/ 40 Ar ages calculated for the BABB and 15-9 Ma for the IAT show that the basin and arc formed in a very short span of time, before their obduction 9-7 Ma ago (Linthout et al., 1997). Considering the paleogeographic situation in the Miocene (Haile, 1979 ; Haile, 1981) and our data, we propose that the Seram-Ambon ophiolites formed during the early Miocene in a small, short-lived (10 Ma), transtensive basin bordered on its east by an active margin and on its western part by a passive continental margin over which it was later obducted towards the SW direction.

Kinematics of mantle flow beneath a fossil Overlapping Spreading Center: The Wuqbah massif case, Oman ophiolite

The Wuqbah massif (central Oman Ophiolite) comprises a well preserved <2 km thick crustal unit over a 6–8 km thick mantle sequence. The mantle comprises harzburgites and few dunites cut by various types of dykes. With the exception of basal and late fault zones, peridotites display granoblastic and porphyroclastic textures, acquired by high-temperature plastic deformation. They locally suffered important recrystallization as shown by neoblastic shapes and arrangements. Macroscopically, the peridotites have well-defined foliations and lineations, whose features allow the massif to divide into three structural domains. It comprises a N110° trending central zone that suffered an anticlockwise rotation, with an upwelling zone in its southwestern part, bound between two NS areas sheared in right-lateral conditions. This peculiar geometry is thought to reflect an overlapping spreading center (OSC) where the east and west bordering zones would define the trace, in the mantle, of two ridge segment terminations and the oblique N110° central zone the axial part of the overlap area. It is proposed that the northward migration of the southwest branch and its concomitant EW spreading caused the deformation and rotation of the overlap zone. In our OSC model, flow in the mantle would be continuous from one spreading center branch to the other, being curved in the overlap area to accommodate the offset. The mantle flow would therefore be more and more decoupled with the brittle crust when approaching the overlap zone, where extensional tectonics can occur. We consider that some mantle upwelling is possible due to its specific location, in between two ridge segments and over the main mantle flow. The distance between the two ridge axes is ∼15 km, which means that this paleo-OSC can be ranked in the class of intermediate OSCs, illustrating a third-order ridge segmentation. At the scale of the ophiolite, the Wuqbah OSC is located northwest of a major domain marked by the presence of a NW-SE trending propagating ridge responsible for the formation of a new oceanic crust. It is suggested that the Wuqbah OSC formed earlier and underlines the trace of a ridge that had later migrated to the East.

Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo, Indonesia)

Abstract The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with minor metavolcanic rocks. The ultramafic rocks include dominant lherzolites and pyroxenites with rather scarce harzburgites and dunites. Spinel peridotite, mineral chemistry data and bulk rock Rare Earth Element (REE) abundances show that most rocks underwent a low degree of partial melting. However, a few samples display significant depletions in Light REE (LREE), which are interpreted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase facies, as seen from phase chemistry data. These peridotites are locally crosscut by dikelets containing high-temperature K-and Cr-rich amphiboles. Lavas closely associated with the Meratus peridotites have REE compositions ranging from the ones typical of enriched MORB (E-MORB) to normal MORB (N-MORB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle that locally suffered a low degree of fractional melting during the last stages of a continental rifting phase, in agreement with the presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enriched subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere which might have produced N-MORB volcanic rocks. Back-arc basin basalts (BABB) now associated with E-MORB and N-MORB have also been found in the metamorphic soles of the peridotites. These rocks would have formed in a back-arc basin now accreted to the eastern margin of Eurasia. The latter are partly covered by calc-alkaline magmatism (Alino Formation). The ophiolitic series was later crosscut by calc-alkaline melts (Manunggul Formation). The Meratus ophiolitic series hence displays a dual origin. They witness 1) a continental episode mostly seen within the peridotites. and 2) later subductionrelated events marked by the emplacement of calc-alkaline magmas.

Premiers résultats des plongées du Nautile sur le banc de Gorringe (Ouest Portugal)

Abstract The oceanic crust exposed on the Gorringe Bank (SW Portugal) presents a laccolith-like body of gabbros, 500 m thick by 50 km long, within mantle peridotites. It also shows rare tholeiitic dikes and pillow-lavas resting locally directly over the peridotites. Gabbros, that crystallized in a closed system, subsequently underwent strong deformation in highto low-temperature conditions, in a west to east extensional flat shear zone system. This massif would likely be formed during the early stages of oceanic spreading, at the end of continental rifting. This is in agreement with kinematic reconstructions for the North Atlantic Ocean.