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We present new 40Ar/39Ar data for two of the Tore-Madeira Rise (TMR) volcanic seamounts. A sample from Tore East seamount on the northern part of the TMR yielded an ultra-precise age of 80.50 ± 0.1...

International audience; A new high-resolution bathymetric map combined with a regional Digital Elevation Model (DEM) analysis reveal the modalities of occurrence and emplacement of post-spreading magmatism along the NNE-SSW oriented, 1000 km long Tore-Madeira Rise (TMR) as well as its relationship with the activity of major fault systems including the Estremadura Fault System (ESF) and the Azores-Gibraltar Fracture Zone (AGFZ). Morphological and structural analysis of the bathymetric map were performed to map volcanic features such as eruptive cones, vents and fissures together with faults along the TMR. The new bathymetric map shows that the main NNW-SSE seamount alignment is formed by three structurally distinct volcanic massifs, the Tore, the Josephine and the Southern Volcanic Groups. The majority of the volcanoes of each group emplaced within or along specific portion of pre-existing faults (ESF and AGFZ) including splay fault, releasing bend, fault tips and interaction zones between different segments. Magmas were channelled into sub-vertical pre-existing lithospheric faults that acted as preferential pathways for the vertical magma ascent. Migration and final eruption of magma are controlled by the local stress variation induced by complex fault geometries, change in plate kinematics as well as strong shear zone anisotropy as suggested by the emplacement within localised areas of transtension. We conclude that post-spreading magma emplacement in the southern part of the Iberia margin was related to the development of a transtensional plate boundary between the Iberian and African Plate during the Late Cretaceous. More generally, our findings emphasize that the distribution of volcanism as the expression of the interaction between shallow plate tectonic and mantle processes should be included in plate kinematic reconstruction. This study also demonstrates that the accurate mapping of oceanic seafloor is pivotal to better understand tectono-magmatic evolution of volcanic seamount chains and geological processes in oceanic domains.

Spinel-bearing peridotite and pyroxenite xenoliths dredged from the Dragon Seamount (southern Tore–Madeira Rise, West Iberia and Morocco margin) give an insight into the composition of the underlying lithosphere. These xenoliths are devoid of evidence of strong host lava–peridotite interaction and re-equilibration or late impregnation in the plagioclase facies. The spinels and pyroxenes from the Dragon peridotites have compositions distinct from those of both lherzolites and harzburgites from the Iberia margin and the Mid-Atlantic Ridge. They display a highly depleted composition, in particular, high Cr-number, up to 0.63 in the spinels, consistent with a melting degree between 12 and 19%. Because of the strong chemical similarities between the Tore–Madeira Rise, Newfoundland peridotites, and peridotites from supra-subduction zones, we propose that the Dragon peridotites formed in a similar context. The pyroxenites display a cumulate texture and are probably a high-temperature–high-pressure cumulate formed by fractional crystallization from a melt. The Tore–Madeira Rise peridotites may represent a former mantle wedge in an oceanic arc, later included into the continental lithosphere and finally tectonically disseminated within the lithosphere during the rifting of the Newfoundland–Iberia continental lithosphere. As a consequence, rifting processes may produce heterogeneities in the oceanic lithosphere and influence isotopic compositions of ocean island basalt-type lavas during plume–lithosphere interactions, as inferred for the southern Tore–Madeira Rise. Supplementary material: Further information on the analytical results is available at http://www.geolsoc.org.uk/SUP18512.

Large massifs of alkaline rocks are exposed along ∼250 km of the Atlantic coast of Portugal. Their origin is still poorly understood, including the precise timing of their emplacement and their rel ...

The Tore–Madeira Rise is a seamount chain located 300 km off the Portugal and Morocco coasts attributed to hotspot activity. U–Pb ages of lavas from the northern and central Tore–Madeira Rise range between 103 and 80.5 Ma whereas 40 Ar/ 39 Ar ages from the central and southern Tore–Madeira Rise yield ages ranging from 94.5 to 0.5 Ma. We performed new 40 Ar/ 39 Ar measurements to better understand the geodynamic history of the Tore–Madeira Rise. Plagioclase ages from the Bikini Bottom and Torillon seamounts suggest ages of >90 Ma and ≥60 Ma, respectively. Amphiboles from the Seine seamount yield an age of 24.0 ± 0.8 Ma. Biotites from lavas of the Ashton seamount give ages of 97.4 ± 1.1 Ma and 97.8 ± 1.1 Ma. The geochronological database available on the Tore–Madeira Rise has been filtered on statistical criteria to eliminate unreliable ages. The resulting database reveals three pulses of alkaline magmatism on the Tore–Madeira Rise at 103–80.5 Ma, at c . 68 Ma and between 30 Ma and the present. The magmatism was continuous from 103 Ma until c . 68 Ma and from c . 30 Ma until the present on the Tore–Madeira Rise, the surrounding seamounts and the Portugal coast. We suggest that the space–time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the Iberian plate. Supplementary material: A detailed Ar measurements dataset is available at http://www.geolsoc.org.uk/SUP18359.

It has been difficult to relate the segmentation of mid-ocean ridges to processes occurring in the Earth's 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 Earth's 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.

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 various chemical features, allowing to define 3 types of websterites and 2 groups of gabbros. They have mostly BAB characteristics (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 40K/40Ar 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.

The Ronda Peridotite, south of Andalusia (Spain), was imaged by AVIRIS in 1991 and partially sampled by us in the field with a GER 3700 spectrometer in 1997 in order to get experience in processing hyperpectral images of planetary surfaces with probes such as ISM Phobos (1989), OMEGA Mars Express (2003) and VIMS Cassini (2004). The high spectral resolution of the images (224 channels from 400 to 2455 nm) is necessary to conduct geological analysis with remote petrological determinations of rock types. On Earth, it is also necessary to determine species of vegetation because of their strong influence in mapping lithology, even in dry areas like the Ronda peridotite. The Ronda AVIRIS image was first processed to infer geological features using photo-interpretation of colour composite images extracted from 150 useful channels compared to geological maps and checked on the field during the campaign of July 97. This allows us to distinguish easily the peridotite massif from its surrounding rocks and its own serpentine zoning. Since this work followed the work of Chabrillat et al. [2000] we chose to explore the AVIRIS data with other techniques. We chose to remove the contribution of the atmosphere with spectra collected in the field on a white target at various altitudes and to remove the main vegetation with spectra of the most characteristic vegetation of the peridotite. In both cases we first estimated the amount of atmosphere and vegetation with band ratios and remove them with two similar empiric corrections of the reflectance. From the spectroscopy data, after removal of the atmosphere and some vegetation signal, we were able to clearly distinguish the crustal rocks from the mantle ones, as well as compositional variations due to pyroxene and mostly serpentine abundance within the peridotites. Hyperspectral infrared spectrometry will provide good geological mapping of the main rocks on planetary surfaces, if images can also be calibrated with in situ field measurements which will not miss any unexpected component. However, some ambiguities remain between certain types of rock which have close mineralogical composition (e.g. harzburgite compared to lherzolite) or which have resulting spectra very similar to each other (plagioclase and lizardite in peridotites). Some other ambiguities between spectra are also introduced by techniques of analysis based on relative reflectance. By not taking into account absolute intensity of the reflectance, because of roughness and topographic shading effects, small mineral variations are not always visible.

The Wuqbah massif (central Oman Ophiolite) comprises a well preserved

Several mechanisms have been proposed to account for the rotation of the nearly north-south abyssal hill fabric formed on the East Pacific Rise north of the Easter Microplate to the nearly east-west trends in the northern microplate interior. Proposed mechanisms include rigid microplate rotation, transform fault – parallel shear, and bookshelf faulting during the transfer of lithosphere from the Nazca Plate to the microplate. We used a submersible magnetometer on a NAUTILE dive program to measure the magnetic vector rotation of a pillow basalt and dike spur near Pito Deep, the present location of the tip of the propagating rift system that created the microplate. Our results, although too limited to draw strong conclusions from, suggest clockwise rotations of the seafloor magnetic vectors inconsistent with the transform-parallel shear model, and larger than can be explained solely by rigid microplate rotation.

The Wuqbah peridotites (Wuqbah massif, central Oman Ophiolite) constitute the mantle part of a complete ophiolitic sequence and their field deformation geometry is thought to reflect mantle dynamics in a fossil overlapping ridge settings (Girardeau et al., 2002). These peridotites comprise dominantly residual harzburgites and dunites. Nearly 70% of the harzburgites are clinopyroxene-free, and the rest contains less than 1%. The mineral chemistry of olivine, pyroxenes and spinel, and whole rock major and rare-earth element data, indicate that the Wuqbah peridotites are all strongly refractory and that they record a major percolation event, marked by strong enrichments in incompatible elements. At the massif scale, the Central Zone contains rocks with the most refractory features (20% melt extraction), as expected in an area of mantle upwelling. In the overlapping ridge senario, it corresponds to the overlap zone whose formation is discussed.

15·433–15·634; Sr/Sr = 0·70330–0·70476; Nd/ Petrographic and field data indicate the existence of four main rock Nd = 0·512539–0·512916) suggest involvement of an entypes within the allochthonous Cabo Ortegal ultramafic units: (1) riched component in their mantle source, which may be related to harzburgites; (2) dunites; (3) massive, occasionally garnet-bearing, the subduction of terrigenous sediments (i.e. EMI). The new data pyroxenites; (4) less abundant mafic rocks with variable amounts obtained confirm that ultramafic units of Cabo Ortegal experienced of garnet-rich pyroxenite. The major and trace element compositions a complex tectonothermal history similar to that of other units of of the analysed ultramafic rocks define well-delimited fields in binary the same area and allow us to distinguish at least two different variation diagrams. Normalized trace element patterns, however, events. Sm–Nd whole-rock–clinopyroxene ages suggest formation of exhibit large ion lithophile element (LILE) and light rare earth the ultramafic units at >500 Ma, an age similar to that of element (LREE) enrichment that do not correlate with the main formation of the protoliths of associated HP/HT units. Internal rock types distinguished. NiO contents and fo-number of olivine in Sm–Nd isochrons (Cpx–Grt–whole rock) from two pyroxenites the harzburgites match those of the mantle olivine array, whereas a indicate ages of >390 Ma for garnet crystallization in these rocks, fractional crystallization trend is observed from dunites to pyroxenites. which is consistent with previous U–Pb dating of HP/HT Spinel and olivine in the harzburgites have residual characteristics recrystallization in Cabo Ortegal. Pyroxenite–dunite-rich ultramafic comparable with those of abyssal peridotites or peridotites from arc massifs such as Cabo Ortegal might have originated within the settings, whereas in most of the dunites and pyroxenites the range lithospheric mantle above a subduction zone, variably modified by of fo-number and Cr/(Cr + Al) ratio suggests crystallization fluid or melt infiltration from the subducted oceanic crust. from primitive subduction-related magmas. Whole-rock major and trace element and Pb–Sr–Nd isotope data suggest that regionalscale massive pyroxenites from Cabo Ortegal originated from relatively homogeneous parental melts. Fractional crystallization processes

Since the Mesozoic, the north–south oblique convergence between the Australian and Pacific plates produced the obduction of pieces of oceanic lithospheres now incorporated into the New Guinea Island. The largest one corresponds to Central Ophiolite Belt (COB), located in the central part of Irian Jaya, that displays well-preserved peridotites, gabbros, dolerites and basalts. The ultramafic rocks, characterized by the abundance of harzburgites, show residual characteristics implying residues of a 20 to 25 % melting. The associated basalts are MORB-like, but display significant Nb and Ta depletions suggesting a subduction zone environment for their origin. These data suggest that COB was formed in a back-arc environment, along the Australian continental margin rather than within a large oceanic domain.

Along the Galicia and Gorringe banks and in the Iberia Abyssal Plain of the North Atlantic, unroofed sub-continental mantle fills the gap between ‘true’ oceanic crust and the continental crust margin. These lithospheric peridotites are intruded by gabbros and dolerites, and locally covered by basalts. Primary magmatic zircons extracted from gabbros and meta-gabbros of the two banks were dated by the U–Pb chronometer, and initial hafnium isotope signatures (ϵHfi) were determined on the same grains. For Mt. Gettysburg at Gorringe, gabbro emplacement ages of 137.5±0.5 (2σ) Ma and 135.7±0.8 Ma are obtained, and corresponding ϵHfi lie at +20.5±0.3 (2σ) and +19.5±0.4, substantiating magma formation from severely LILE-depleted mantle domains. Gabbro zircons from Mt. Ormonde at Gorringe yield a much younger age of 77.1±0.4 Ma and the Hf isotopes document an intermediately LILE-depleted mantle source having a ϵHfi of +7.6±0.4. Given its age and Hf signature, emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the Iberian Continent in Upper Cretaceous time. Concerning the Galicia section, zircons from a meta-gabbro yield an emplacement age of 121.7±0.4 Ma and a ϵHfi of +14.0±0.2, and a ϵHfi of +14.6±0.2 is obtained for zircons from a previously dated meta-gabbro of identical age. These results indicate magma extraction from mantle reservoirs that are slightly less LILE-depleted than those sampled by the about 20 Myr older Gorringe gabbros. The data demonstrate that magmatism occurring prior to complete separation of Europe from America was essentially of asthenospheric origin. Both the 138–135 Ma ages for the Gorringe gabbros and 122 Ma ages for the Galicia gabbros are at least 5 Myr older than the oldest sediments on Gorringe, and the break-up unconformity at the Galicia Bank, respectively. Magma source signatures of the syn-rift gabbros are in agreement with values expected for differently depleted Cretaceous MORB-type mantle reservoirs, and the age-difference for magmatism between the Gorringe and Galicia banks suggests a rate of 4.4±0.3 cm/yr for northward progression of continental rifting. Based on the new results, a structural model for Iberia–America rifting is discussed putting forward the idea that magma emplacement produces a level of weakness and decollement between the rifting crust and its underlying lithospheric mantle.

The Cyclops massif (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge basalts (N-MORB) and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT)-high pressure (HP) mafic rocks metamorphosed during the Miocene. Mineral chemistry and bulk rock rare-earth element (REE) abundances of the peridotites are characteristic of highly residual mantle rocks. The high Cr# [Cr#=100*Cr/(Cr+Al)] of spinel (up to 60) and very low heavy rare-earth element (HREE) concentrations of peridotites (< 0.1 time the chondritic values) are in agreement with residues of 25 to 35% melting as expected for peridotites from supra-subduction zone environments. Ti-enrichments in spinels and secondary clinopyroxenes (up to 1%, and 0.5%, respectively) are likely a consequence of reaction between mantle-derived melts and the host peridotites. High light rare-earth element (LREE) concentrations reaching up to chondritic values and high field strength element (HFSE) anomalies suggest that the initial composition of the residual peridotites has been previously modified by the passage of boninitic melt(s). The associated basalts and related cumulate rocks display major and trace element contents with Nb-negative anomalies typical of back-arc magmas. New40K/40Ar isotopic ages obtained from the back-arc basin basalts (BABB - 29 Ma) and boninites (43 Ma) combined with the geochemical signatures of the rocks studied here, indicate that the Cyclops Mountains may have formed in a single suprasubduction environment. This implies southward plunging subduction of the Australian oceanic lithosphere beneath the northern part of the Australian margin. The ultramafic rocks and related lavas (boninites) likely formed during the Eocene in a forearc environment, before their southward obduction onto the island arc crustal welt during the early Miocene. The Pliocene back-thrusting event has led to the slicing of the backarc basin series onto the arc and fore-arc sequences.

New petrological data on magmatic rocks obtained from the Iberia Abyssal Plain and from the Gorringe Bank, combined with those already known on the Galicia Bank, allow to better constrain the formation of the West Iberian Margin. These three zones were sampled along East-West transects of the ocean-continent transition, immediately West or at the foot of the last tilted continental block of the margin.

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.

A Nautile submersible investigation of Pito Seamount documents vigorous hydrothermal venting at 23° 19.65'S, 111° 38.41'W and at a depth of 2270 m. The data indicate the volcano is young and recently active, as predicted from analyses of SeaMARC II side-scan and swath bathymetry, and geophysical data. Pito Seamount lies near Pito Deep (5980 m), which marks the tip of the northwestward propagating East rift of the Easter microplate. Bathymetry surrounding Pito Seamount consists of a series of ridges and valleys with relief up to ∼ 4 km. The 4-km submersible-transect to the summit of Pito Seamount crossed areas of very glassy basalt with little or no sediment cover, suggesting the lava flows are very young. Most of the lava samples from Pito Seamount are depleted normal MORB (mid-ocean ridge basalt). Lava samples associated with active and dead hydrothermal vents consist of phyric and aphyric transitional and enriched MORB. Sulfides consist primarily of sphalerite and pyrite, with traces of chalcopyrite. The active hydrothermal chimney on Pito Seamount has a small, undiversified biological community similar to northern East Pacific Rise vent sites (alvinellid worms, bythograeid crabs and bythitid fishes) and western Pacific back-arc basin sites (alvinocaridid shrimps). No vestimentiferan worms were observed. Previous geophysical data, and new geochemical data and visual observations, suggest that the vigorous black smoker is a result of deep, extensive crosscutting faults formed by extensive tectonic thinning of Pito Deep, and a very robust magmatic supply being supplied from upwelling asthenosphere. Although no biological or vent fluid samples were obtained, geological and biological observations, such as the large number of inactive chimneys, old hydrothermal deposits, and starfish, as well as the occurrence of dead mollusks (gastropod and mussels), suggest a recent waning of hydrothermal activity near the summit. The speculative interpretation that Pito Seamount is acting as a focal point for the formation of a new seafloor spreading axis trending northwest (310°) from the seamount summit towards Pito Deep is supported by the new data and observations reported here. These include the similar geochemistry of young lava samples obtained from Pito Seamount and from a small volcanic mound within Pito Deep and the strong SeaMARC II side-scan backscatter amplitudes along most of the ∼ 50 km rift zone connecting the summit of Pito Seamount to Pito Deep.

Submersible investigations along the East Rift segments, the Pito Deep and the Terevaka transform fault of the Easter microplate eastern boundary, and on a thrust-fault area of the Nazca Plate collected a variety of basalts and dolerites. The volcanics consist essentially of depleted (N-MORB), transitional (T-MORB) and enriched (E-MORB) basalts with low (0.01−0.1, 0.25, > 1.2–2) K/Ti and(La/Sm)N ratios, respectively. The Fe-Ti-rich ferrobasalt encountered among the N-MORBs are found on the Pito Deep Central volcano, on the Terevaka intra-transform ridge, on the ancient ( 61) are found on topographic highs (2000–2300 m) and lower values (Mg# < 56) at the extremities of the East Rift segments (2500–5600 m depths). The deepest area (5600 m) along the East Rift is located at 23 °S and coincides with a Central volcano constructed on the floor of the Pito Deep. Three major compositional variabilities of the volcanics are observed along the East Rift segments studied: (1) the 26 °S East Rift segment where the volcanics have intermediate Na8 (2.5–2.8%) and Fe8 (8.5–11%) contents; (2) the 23 °S East Rift segment (comprising Pito seamount and Pito Deep Central volcano) which shows the highest (2.9–3.4%) values of Na8 and a low (8–9%) Fe8 content; and (3) the 25 °S (at 24 °50′–26 °10′S) and the 24 °S (at 24 °10′–25 °S) East Rift segments where most of the volcanics have low to intermediate Na8 (2.6–2.0%) and a high range of Fe8 (9–13%) contents. When modeling mantle melting conditions, we observed a relative increase in the extent of partial melting and decreasing melting pressure. These localized trends are in agreement with a 3-D type diapiric upwelling in the sense postulated by Niu and Batiza (1993). Diapiric mantle upwelling and melting localized underneath the 26, 25 and 23 °S (Pito seamount and Central volcano) East Rift segments are responsable for the differences observed in the volcanics. The extent of partial melting varies from 14 to 19% in the lithosphere between 18 and 40 km deep as inferred from the calculated initial (Po=16kbar) and final melting (Pf=7kbar) pressures along the various East Rift segments. The lowest range of partial melting (14–16%) is confined to the volcanics from 23 °S East Rift segment including the Pito seamount and the Central volcano. The Thrust-fault area, and the Terevaka intra-transform show comparable mantle melting regimes to the 25 and 26 °S East Rift segments. The older lithosphere of the EMP interior is believed to have been the site of high partial melting (17–20%) confined to the deeper melting area (29–50 km). This increase in melting with increasing pressure is similar to the conditions encountered underneath the South East Pacific Rise (13–20 °S).

UPb and RbSr dating of various minerals from the mafic-ultramafic Cabo Ortegal complex reveal the occurrence of high-grade metamorphism and mantle melting between 406 and 383 Ma. This event reflects deep subduction of oceanic lithosphere underneath a continental plate margin, including melting of detrital material and carbonates. Magmatism lead to the injection of locally garnet-bearing pyroxenite layers into the ultramafic rocks. It also caused the emplacement of carbonate-rich garnet-clinopyroxene rocks and pegmatites into peridotites and mafic granulites. The upper time limit is events is defined by the crystallization of 406 ± 4 (2 σ) Ma zircon in carbonate-rich garnet pyroxenites, and the lower limit is given by 383 ± 1 Ma old rutile from a garnet pyroxenite, and by 383 ± Ma titanite that crystallized in a carbonate-rich layer. All other zircon, monazite and titanite analyses lie between these two age limits defining an average age of 388 ± 1 Ma. These small age differences observed between the different minerals substantiate that the UPb chronometer in zircon, monazite and titanite behaved as a closed system in the garnet-clinopyroxene stability field (∼800°C and 1.35–1.65 GPa). The new geochronological data neither confirm the occurrence of a 480 or 420 Ma high-grade metamorphic event, as suggested earlier. In consequence, the Cabo Ortegal complex most likely was produced by a single subduction event (406–383 Ma), which was immediately followed by obduction, and incorporation of the complex into the orogenic belt of NW Iberia. Zircon dating of a plagiogranite in the Ophiolitic Series that tectonically underlies the complex substantiates basaltic crust formation at 472 ± 3 Ma, in association with melting of Precambrian detrital material. This Ordovician magmatic event probably occurred coevally with formation of the protoliths from which the mafic granulites and eclogites of the Cabo Ortegal complex were produced by subduction ∼70 m.y. later. On the other hand, emplacement of the exceptionally well-developed pyroxenite layers in the Cabo Ortegal complex appears to be exclusively a result of peridotite re-melting during earliest Devonian subduction at 406–383 Ma.

This paper presents detailed petrologic and geochemical analysis of mafic and ultramafic rocks from the Taitao Peninsula, South Chile, and contributes to improve the knowledge of the Taitao ophiolite. These data allow us to identify three different units on the Taitao Peninsula, which include an ophiolitic body (peridotites, gabbros and dykes) and two Pliocene to Pleistocene volcanic-sedimentary units which accumulated as subduction of the Chile ridge occurred. The petrologic, mineralogical and geochemical data strongly suggest an oceanic origin of the peridotites and the gabbros, possibly at an active spreading center; harzburgites and CPX-bearing harzburgites can be considered as fragments of obducted mantle, whereas werhlites could represent a different magma source, enriched in K, Ti, Na and water, that impregnated the peridotites in a later stage. The two volcanic-sedimentary units, termed the Main Volcanic Unit (MVU) and the Chile Margin Unit (CMU) are geochemically unconnected with the ophiolite. They range in composition from basaltic to dacitic and can be subdivided into three groups: (i) a group with MORB affinities (ii) a group with calc-alkaline affinities (iii) a group with intermediate characteristics. MVU exhibits greenschist facies mineral assemblages whereas CMU shows assemblages ranging from zeolite facies to very low temperature conditions. We suggest that these different metamorphic overprints are a consequence of the collision of two different ridge segments with the South-America margin in front of the Taitao Peninsula.

The ultramafic sequence and associated chromitites of the Nan-Uttaradit ophiolite in the northeastern part of Thailand have been studied in the field and by applying petrography and geochemistry to whole rock samples and minerals. The ultramafic rocks comprise irregulary shaped bodies of dunite, harzburgite, orthopyroxene-rich lherzolite and orthopyroxene-rich harzburgite, clinopyroxene-rich dunite and intrusive clinopyroxenite-websterite bodies. Three types of chromitite were distinguished. Type I chromitite lenses and type II layers which are hosted in orthopyroxenite in the northern part and in dunite in the central part of the ophiolite. Type III chromitite forms lenses or layers in clinopyroxenites in the central and southern parts of the belt. According to the modal and chemical composition the peridotites and orthopyroxenites are strongly refractory. They originated during different stages of interaction between percolating melts and peridotite. The chromitites of types I and II, which are very rich in Cr (up to 68 wt.% Cr203), crystallized from a boninitic parental magma under highly reducing conditions in the northern part and moderate oxygen fugacities (FMQ) in the central part of the ophiolite. The chromitite of type III which are characterized by the highest Fe 3+ (Fe 3+ + Cr + Al) -ratios, and hosted in intrusive clinopyroxenite-websterite-rocks, cumulated from a CaO-rich transitional boninitic melt under fO2 conditions around FMQ.

The thinned continental crust of the west Galicia margin is bound by a belt of serpentinized peridotites (‘peridotite ridge’) lying about 300 km off the coast in the North Atlantic ocean. From this ridge, a gabbro and a chlorite rock were studied in an attempt to substantiate rift-related subcontinental magmatism, occurring prior to sea-floor spreading. U-Pb dating of 13 different zircon fractions yields a precise age of 122.1 ± 0.3 Ma (2σ) for the emplacement of the chlorite rock protolith, from which more than 50% of Si and alkali-calc-alkali elements were lost during greenschist facies tectonometamorphism. Sr and Nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in LILE, relative to a chondritic reservoir. No evidence for the presence of continental material in the magma source regions can be observed. From the new zircon age of 122.1 ± 0.3 Ma, and earlier determined39Ar40Ar age of 122.0 ± 0.6 Ma for amphibole from the same locality, it can be documented that magma formation, solidification and unroofing of the mantle rocks occurred during a short period of time of about 3.4 Ma, which means that the peridotite ridge detached from the continent and rose to the surface immediately after, or even coevally with mantle melting.

Textural and petrological data are presented that record the formation of plagioclase-bearing wehrlites and peridotites at slow-spreading and fast-spreading ridges that are comparable to the wehrlites and peridotites formed in ophiolitic complexes. Evidence is provided for locally pervasive magma percolation in the oceanic residual upper mantle. The samples studied come from the East Pacific Rise (EPR) (Hess Deep) and the Mid-Atlantic Ridge (MAR) (DSDP Site 334). At both sites, parts of the peridotites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine and subidiomorphic spinel crystals. Petrofabric data for olivine suggest a magmatic origin for the Hess Deep wehrlite. Its phase chemistry is comparable to intrusive plagioclase-wehrlites and lherzolite from ophiolites that are thought to have crystallized from a crystal mush. It differs drastically from the associated Hess Deep diopside-bearing harzburgites and dunites, which are quite similar to oceanic and ophiolitic residual peridotites. The EPR wehrlite is therefore considered as an intrusive rock. The Leg 37 plagioclase-peridotites display a weak lattice fabric of olivive which may have resulted from high-temperature plastic flow; these rocks have a phase chemistry that is typical of ophiolitic and oceanic residual rocks. These rocks are hence interpreted as residual dunite or harzburgite which has been pervasively impregnated by a melt from which the clinopyroxene and plagioclase crystallized. These peridotites could have an intrusive origin or may have formed in situ at the crust-mantle transition in association with intrusions of large gabbro bodies.

[1] An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies by a direct comparison at high spectral resolution between field and airborne spectra despite strong variations in outcropping conditions such as (1) lighting, (2) surface roughness geometry, (3) blocks coated with red/brown patina and exfoliation products, or (4) deep hydrothermal weathering. On the basis of spectral signatures, we are able to distinguish three end-members of olivine-orthopyroxene bearing assemblages in the mantle sequence: (1) harzburgites, (2) dunites, and (3) a harzburgite with interstitial carbonate. Because plagioclase is spectrally featureless in the wavelength range studied it cannot be detected. In the crustal sequence, we therefore identified four end-members with variable abundance of clinopyroxene: (1) massive gabbros, (2) amphibolized (upper) gabbros associated with intrusive dykes, (3) wehrlite with high serpentine content, and (4) gabbronorite (a lithology not previously recognized in the studied area). With the exception of wehrlite, spectra of olivine-rich end-members display characteristic Mg-OH narrow absorption features caused by their high serpentine content. We take advantage of this observation to split the data into two subsets, corresponding to the mantle and crustal sequences, respectively. Pixels of an image often correspond to heterogeneous areas in the field and a direct comparison between airborne and in situ spectra is not straightforward. However, comparing spectra of pixels associated with the most homogeneous areas in the field with the spectra acquired in situ at the same location, we found a systematic change both in mean intensity and overall spectral shape. Dividing each spectrum by its low-pass trend removes the effects caused by surface light scattering associated with each scale of analysis and results in an exceptional match between field and airborne spectra. However, the albedo information is lost and as a consequence, rock types only characterized by albedo change cannot be discriminated. A spectrum of a mixture of powdered minerals is usually seen as a linear combination of mineral spectra proportional to their abundance. However, this is no longer the case when minerals occur in complex arrangements in rock types. We thus develop a synthetic spectral library of all possible combinations of rock types covering the surface area of a pixel and use a simple distance calculation to identify the best match between each pixel and modeled spectra. This procedure allows the determination of the fractional cover of each rock type in a given pixel and to establish maps for each spectral end-member. The final product is a geological map, derived from the combination of end-member fractional cover maps, and is broadly consistent with the existing geological maps. Beyond this general agreement which demonstrates the potential of this new approach for geological mapping, imaging spectrometry allows (1) to map in detail the outline of the Moho north of Maqsad and (2) to identify a new crustal sequence enriched in silica south of Muqzah, revealing the presence of orthopyroxene, the nature and distribution of which are of relevance to the petrological and tectonic understanding of the Oman ophiolite evolution.

The age and origin of alkaline rocks emplaced into the sediments of the rift-related continental Lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between Iberia and Newfoundland. The U–Pb titanite ages are 146.5 ± 1.6 (2σSTERR), 145.3 ± 1.4 and 142.3 ± 1.0 Ma, and initial Pb isotopic ratios of feldspars lie at 18.418–18.978 for 206Pb/204Pb, at 15.594–15.925 for 207Pb/204Pb and at 37.105–39.216 for 208Pb/204Pb. Initial 87Sr/86Sr ratios measured in the same feldspar fractions lie at 0.705409–0.706462. This episode of magmatic activity lasting for at least 4.2 ± 2.6 Myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface. Such zones are preferentially re-activated Palaeozoic faults of the Iberian plate. The isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental Iberian mantle.

The western part of the Galicia passive continental margin (western edge of Spain) is bordered by serpentinized peridotites over a N-S distance of about 125 km. These peridotites were emplaced at the end of continental rifting and/or at the very beginning of oceanic accretion. They have been observed in situ and sampled in five sites with the Nautile submersible during the Galinaute cruise (1986). Some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge. Most of the escarpments covered by the Nautile on the peridotite ridge have yielded only ultramafic rocks, except at Dive 10 where the peridotites are overlain by a sheared chlonte-bearing schist, and at Dive 14 where the ultramafic rocks are covered by basalts. All the peridotites are plagioclase-bearing harzburgites and Iherzolites, locally cross-cut by rare plagioclase-rich veins and dioritic intrusives. Some pyroxenites and gabbros are associated with the peridotites at Dive 14. The plagioclase-bearing peridotites are similar in composition to those drilled during ODP Leg 103. Primary structures have not been found, except in the northern part of the studied zone where a few samples exhibit magmatic textures. Elsewhere, the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high-temperature fabric. At these localities, boudinage structures and ultramylonitic bands, referred to as shear bands, occur and the rocks have a disrupted mylonitic texture. A few dioritic dykes have also suffered this mylonitization. Shearing occurred in a rotational regime, at high and decreasing temperature (1000-850 °C) and under a high deviatoric stress (> 102 MPa) i.e. under lithospheric conditions. The structural pattern of the peridotite ridge, deduced from measurements on oriented samples, is complex. The trend of the mylonitic foliation appears nearly constant from south to north (N125-N105), but its dip increases (20–45°) northward. From south to north, the stretching lineation pitch varies from subvertical to subhorizontal. Stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area. To the north, it occurred in a northwesterly direction with a strike-slip component. Stretching in the chlorite-bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation. This study confirms the uplift of the Galicia peridotites during the continental rifting of the North Atlantic Ocean in Cretaceous times. During their ascent, the peridotites underwent some melting and high-temperature-low-stress ductile deformation, followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures. The geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress-strain field prevailing in the continental lithosphere during the rifting episode. It is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point. The kinematics of emplacement agree well with an E–W opening of the North Atlantic Ocean.

High-pressure, subduction-related metamorphism is recorded in two structural units of the Cabo Ortegal complex of Spain: (i) an upper thrust unit (Concepenido-La Capelada) composed of granulites, eclogites and metaperidotites and (ii) a structurally underlying unit (Chimparra-Banded) formed of gneisses with minor metabasic intercalations. In the upper unit, a metamorphic episode of Lower Ordovician age caused the formation of eclogites from basic rocks with N-MORB compositions, while high-pressure, garnet-clinopyroxene granulites were formed from basic to intermediate protoliths with calc-alkaline affinities. Garnet replaces spinel in ultramafics of the same unit and may have originated simultaneously. The PT -conditions were c. 800°C at 13.5 kbar in the granulites, whereas similar temperatures and higher pressures ( > 17 kbar) prevailed in the eclogites. The migmatitic Chimparra and Banded gneisses contain boudins of garnet and omphacite-bearing rocks, with or without plagioclase. The basic protoliths of these rocks are similar to LREE-enriched basalts with alkaline affinities. The Silurian metamorphism in this gneissic unit took place at c. 700°C or lower and at pressures around 15 kbar. This metamorphism could have occurred when a volcano-sedimentary complex, formed in a thinned-crust environment, was subducted beneath an overlying active margin/volcanic arc complex. An ophiolitic unit, made up of the N- and T-type MORB metabasites of the Candelaria and Purrido-Pena Escrita Formations, underlies the gneisses. Metamorphism in this unit took place during the Early-Middle Devonian, mainly giving rise to amphibolite-facies associations. Sporadic coronitic metagabbros/dolerite and garnet-clinopyroxene metabasites indicate that amphibolite-granulite transitional conditions ( > 700°C, 8 kbar) were attained locally. Retrograde amphibolitic assemblages in the upper units are more or less coeval with this episode, which suggests that it may have been related to the cessation of obduction and the stacking of the overlying high-grade units.

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.

We conducted a detailed field and laboratory study of internal segregation structures of two hand-size pillow lavas samples. They were dredged, respectively, on the Josephine seamount, Tore-Madeira Rise (TMR), and on a small quaternary volcanic edifice located on the continental edge of the trench close to the Chile Triple Junction (CTJ). Both pillows display a combination of four types of segregation structures (spherical vesicles, pipe vesicles, vesicle cylinders, and vesicle sheets) observed so far only within subaerial basalt flows typically 2–10 m thick. In particular, the samples offer a remarkable exposure of the transition between pipe vesicles and cylinders. We show that the vesicle sheets are not generated by the same mechanism in both occurrences; they do not seem to be connected to cylinders in the CTJ pillow as they are in the TMR pillow. The two pillows are geochemically distinct, the TMR being alkaline and the CTJ calc–alkaline. Two types of internal differentiation are proposed. The first one implies the extraction of the residual liquid from the host lava and transport towards the segregation structures, whereas the other one results from in situ crystallization within one given structure. In the latter case, glass composition is highly dependant on the nature of the neighbouring crystallizing minerals. The degree of crystallization required to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows (ca. 25% crystallization) than in vapor-differentiated basaltic lava flows (35% crystallization).

The high spectral resolution of Airborne Visible Infrared Imaging Spectrometer (AVIRIS; 224 channels from 400 to 2455 nm) and HyMap (127 channels between 437 and 2485 nm) images is necessary to conduct geological analysis with remote petrological determinations of rock types or soils, or to determine vegetation groups. When airborne images and field spectra are well adjusted between each other, and when the vegetation does not interfere in the analysis, the spectral shape analysis (SSA) method represents an easy treatment to reveal a large amount of geological information. The method presented in the paper takes into account both wells and peaks of spectra resulting from a combination of absorption features and continuum shapes. It was conducted on the Ronda peridotite, in the south-west of Spain, which was imaged by AVIRIS in 1991 and by HyMap in 2000, and which was partially sampled in the field using a GER 3700 spectrometer in 1997, 2000 and 2001. In this study, the AVIRIS and HyMap images are processe...

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.

The Xigaze ophiolite (29.2°N, 89.5°E), which outcrops in the Yarlung Zangbo suture zone, represents the remnants of an oceanic lithosphere formed in middle Cretaceous times between the Lhasa block to the north and the Indian plate to the south. In an attempt to define the paleo-orientation and latitude of the spreading center at which it has been created, a paleomagnetic study has been done on three sites in volcanics and overlying (or interbedded) radiolarites forming the upper part of the ophiolite sequence and also on seven sites in the Xigaze Group flysch which stratigraphically overlies the volcanics to the north. In each site, hand-blocks carefully oriented both with sun and magnetic compass have been sampled. The paleomagnetism data, combined with structural data on the ophiolite dolerite intrusives, allow a partial reconstruction of South Eurasia at the time of formation of the Xigaze ophiolite. The paleolatitude of accretion and deposition of the Xigaze ophiolite and overlying sediments is found to be 10–20°N. Both ophiolite and basin have encountered a 85 ± 20° anti-clockwise rotation. The corresponding ridge was close to the southern margin of the Lhasa block and was oriented N175 ± 25°.

The Xigaze ophiolite, (Tibet) displays unusual lithological, petrological, textural, and structural characteristics. There are no large masses of cumulate gabbros, but dolerite intrusives throughout the whole ophiolite sequence, some of which were intruded into already serpentinized peridotites, only minor residual harzburgites and dunites in dominantly Iherzolitic peridotites equilibrated at low temperatures and pressures, and relatively low-temperature deformation structures in the uppermost peridotites. These features suggest a very low heat flow at the spreading center where the Xigaze ophiolite was formed, in good agreement with a discontinuous and slowly-accreting spreading center origin. However, this ophiolite does not represent a typical mid-oceanic ridge ophiolite; rather it was formed in a small basin located at the southern margin of Eurasia, hence within a preexisting oceanic lithosphere. The opening of the nearly N-S Xigaze paleo-ridge resulted from the W-E drift of Africa relative to Eurasia from 180 to 110 Ma. The N-S emplacement onto the continent of the Xigaze ophiolite, formed 120 to 110 Ma ago, can be correlated to changes in direction of motion of the African and Indian plates: a primary intra-oceanic thrusting event probably occurring at 110 or 85 Ma and the final obduction near 50 Ma during the India-Eurasia collision.

The Xigaze ophiolite (Southern Tibet) exhibits an unusual ophiolite sequence characterized by: 1. (1) the limited thickness (less than 3 km) of its crustal unit which is mostly composed of dolerite intrusives, the gabbros being very scarce; 2. (2) the scarcity of harzburgites and dunites, the ultramafic rocks consisting mostly of Cr-diopside bearing harzburgites and Iherzolites. Although there are some textural and chemical variations from the basal Iherzolites to the upper harzburgite and dunites, no sharp discontinuity exists between the peridotite series of the Xigaze ophiolite, which all belong to the same peridotite suite. The peridotites have been equilibrated at relatively low temperatures and pressures (1155°C 0.5–0.6 GPa for the Cr-diopside bearing harzburgites and 1255°C 1.0–1.1 GPa for the stratigraphically 2 km deeper Iherzolites), indicative of a low thermal gradient at the Xigaze paleo-ridge. Accordingly, the peridotites have locally been deformed under low stress (0.2 GPa) conditions and at decreasing temperatures (from up to 1000°C to 900-800°C as evidenced by the activation of the (010) [100], (Ok1) [100] and (001) [100] olivine glide systems). The equilibrium temperatures and pressures estimated for the peridotites, the inferred shape of the melting zone beneath the Xigaze paleo-ridge, the low-temperature-low-stress conditions inferred for the deformation of the peridotites and the pattern of the mantle structure determined on the basis of structural and microstructural data, are in good agreement with a slow-spreading ridge origin for the Xigaze ophiolite.

The 1981 French–Chinese expedition to Tibet focused on the Lhasa block, extending earlier coverage 400 km north of the Tsangpo suture. The Lhasa block stood between 10 and 15° N latitude over most of the Upper Cretaceous and Eocene and, if Gondwanian in origin, had detached from Gondwana by early Permian. Seismic profiles reveal a complex Moho topography resulting both from multiple continental thrusting and large-scale strike-slip faulting. Subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated. This study emphasizes the role of smaller blocks in the accretion of the continental mosaic.

Large structures, lineations, foliations and sense of shear criteria are examined on the scale of the whole Ibero-Armorican Arc. Four sections (Galicia, Brittany-Vendee, Limousin and Eastern Massif Central) exemplify the major thrust sheets observed around the Arc. Stretching lineations are contemporaneous with the siluro-devonian metamorphism and are either transverse, oblique or parallel to the collision zone. A kinematic analysis shows that these lineations have resulted from a dominanuy transverse shear deformation which was followed by, or combined with, a longitudinal shear direction. On the scale of the entire Arc, this variation in the shear direction is interpreted as resulting from an early head on thrusting relative movement evolving to large scale movements parallel to the plate boundaries. Experiments with sand-silicone models support a model which generates the Arc by interaction between a transform sinistral direction, and a converning zone at a high angle to the transform direction.

During ODP Leg 103, serpentinized peridotite (clinopyroxene-spinel harzburgite) was cored within the basement approximatively at the boundary between the North Atlantic oceanic curst to the west, and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle derived peridotite on the seafloor occurred at the end of the period of rifting, roughly 110 Ma ago. Ductile shear zones observed in the cored peridotite are consistent with movements along a deep low-angle, normal fault rooted within the upper mantle and dipping eastward, beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, we use a non-uniform stretching model for the lithosphere, set up from the Wernicke's model (1985).

The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct “magmatic” diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioK f=(X Di opx )/(1 −X Di cpx ), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularX En cpx solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges.

The Xigaze ophiolite which outcrops along the Yarlung Zangbo river, southern Tibet, locally displays a complete ophiolitic sequence from marine sedimentary cover over basaltic volcanics to the north, to fresh Cr diopside-rich harzburgites to the south. In contrast with other ophiolites, the mafic part of the sequence is particularly thin. It is almost devoid of cumulate gabbros consisting of a diabase sill complex covered by lava-flows or pillow-lavas. The ultramafic unit is poor in residual harzburgites and dunites and consists dominantly of fresh Cr diopside-rich harzburgites. The origin of this peculiar ophiolite (of oceanic crust) is discussed.

Three massifs in the Xigaze area (Southern Tibet, China), have been preserved from the intense deformation observed everywhere else in the Yarlung Zangbo Suture Zone. These massifs have similar lithologies and display a remarkable homogeneity in the attitude of most magmatic and tectonic structures such as lava flow planes, dolerite dikes and sills, and foliation planes in peridotites. This indicates that these ophiolites are only slightly dismembered, inasmuch as they locally display complete sequences from pillow lavas at the top to fresh Cr-diopside-bearing harzburgites at the base. Gabbroic rocks are rare and are present as screens between dolerite intrusives or in small-sized pockets at the limit between mafic and ultramafic rocks. One of the most important feature of this ophiolite is the occurrence of dolerite sills and dikes throughout the whole sequence indicating that this ophiolite results from to two successive magmatic events with (1) creation of the crustal part of the oceanic lithosphere with deposition of minor cumulates in small magma chambers over mantle residual rocks and (2) intrusion of dolerite sills and dikes throughout the whole sequence.

Leg 101 of the Ocean Drilling Program (ODP) was devoted discovering the tectonic and sedimentary evolution of the Atlantic Margin of the lberian Peninsula. A transect of five sites, with a total of 14 drill-holes was undertaken to the South of the Galicia Bank on the seaward edge of the margin. The data obtained revealed a complex history of subsidence and rifting preceding the initiation of sea floor spreading between Newfoundland and Iberia. The main findings include: 1) The Upper Jurassic-Lowermost Cretaceous shallow-water carbonate platform are the first Messozoic deposits at the margin. The «basement seismic reflector» is made-up of these carbonates. 2) The platform drowning, tilting of fault blocks and rapid subsidence preceded the spreading by as much as 25 million years. 3) A ridge of serpentiniced peridotites is located near the boundary between the oceanic and continental crusts. 4) The seismic reflector «S» does not, as widely believed represent a ductile-brinle boundary within the continental crust but is instead a reflector at the base of the synrift sediments.La campaña oceanográfica 103 del Ocean Drilling Program (ODP) ha estado dedicada a dilucidar la evolución tectónica y sedimentación del Margen Atlántico-Ibérico. Se realizaron un total de 14 sondeos., en cinco puntos de posicionamiento, sobre el extremo más profundo del margen; al S del Banco de Galicia. Los resultados obtenidos revelan que previamente al inicio de la expansión oceánica entre Terranova e Iberia ocurrió una historia compleja de distensión cortical, fracturación y subsidencia asociadas. Los resultados fundamentales son los siguientes: 1) Carbonatos de plataforma marina somera, de edad Jurásico superior-Cretáceo basal constituyen los primeros depósitos mesozóicos en ese ámbito del margen y dan lugar a un reflector sísmico considerado interiormente como basamento, 2) El hundimiento de la plataforma, fallamiento y basculamiento de los bloques ocurre desde 25 m.a. antes de iniciarse la acreción oceánica. 3) En el límite entre corteza oceánica-corteza continental se ubica una cresta constituida por peridotitas serpetinizadas. 4) El reflector sísmico «S», generalmente considerado como el límite dúctil-frágil en la corteza continental, corresponde realmente a la base de los depósitos sinrift.

Results are reported from the 1980 joint French–Chinese field expedition in Tibet. The area covered was from the High Himalaya in the south, to the region of Nagqu ∼250 km north of Yangbajain. Ophiolites in the Zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the Gangdise granodiorite belt in the late Mesozoic. The ophiolites were thrust to the south onto the Cretaceous melange and Triassic flysch but the age of this event is unclear. In the Tertiary, sediments of the Indian margin and the Xigaze basin were folded with steeply dipping cleavage, which indicates some north–south shortening of the crust. However, the Tertiary volcanic sequence on the Lhasa block and further north shows comparatively little folding and thrusting. In the Quaternary, deformation of Tibet was characterized by east–west extension.

The Xigaze ophiolite locally displays a 7-km-thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge. It consists from top to bottom of: 1. (1) pillow-lavas with δ18O between +9 and +16‰ and δD around −90‰ vs. SMOW; 2. (2) rare trondjhemites (+9.7‰ < δ18O < 13.4‰ and δD around −75‰); 3. (3) dolerite sills and dikes with δ18O between +7.0 and +9.5‰ and δD between −77 and −40‰; 4. (4) isotropic gabbros occurring as screens of the sills and dikes, and rare layered gabbros [these gabbros have δ18O between +4.8 and +11.0‰ and δD around −65‰ and bear high-δ18O albitized plagioclases (δ18O ≈ +10‰) which coexist with clinopyroxenes of mantle values (δ18O ≈ +5.5‰) or having values being slightly depleted (down to +3.0‰]; 5. (5) ultramafic rocks are either fresh or serpentinized; they have δ18O mainly around mantle values and δD's are between −250 and −100‰) for the mafic part of the ophiolite, the isotopic compositions correspond to hydrothermal interaction with seawater-derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in D. In particular, the gabbros reacted with seawater-derived fluids under amphibolite-facies conditions (as seen from their secondary mineralogy and their 18O-depleted pyroxenes) and subsequently were pervasively reacted under greenschist-zeolite-facies conditions during which the high-18O albitized plagioclase formed. This occurrence of amphibolite-facies conditions followed by greenschist-zeolite-facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower-temperature hydrothermal interaction if compared with other ophiolites. In contrast, the δD's of the serpentine (< −100‰) from the ultramafics are attributed to fluids of meteoric origin at high elevation. Nevertheless, the particular location of the serpentinized harzburgites just beneath the gabbros and the high-δD dolerite sills and dikes (−60‰ < δD < −40‰) enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction. The Xigaze ophiolite sequence displays very little 18O-depleted rocks (relative to mantle oxygen composition). They cannot balance the amount of 18O-enriched rocks. Consequently the result of the hydrothermal interaction between the Xigaze ophiolite sequence and the seawater is an 18O depletion of the seawater. However, the contribution of Xigaze-type “ridges” to the buffering of δ18O in Cretaceous seawater was probably small.