Jacques Girardeau

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.

Origin and evolution of the Paleozoic Cabo Ortegal ultramafic-mafic complex (NW Spain): UPb, RbSr and PbPb isotope data

Abstract 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.

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.

Gabbro and related rock emplacement beneath rifting continental crust: UPb geochronological and geochemical constraints for the Galicia passive margin (Spain)

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.

Plagioclase-wehrlites and peridotites on the East Pacific Rise (Hess Deep) and the Mid-Atlantic Ridge (DSDP Site 334): evidence for magma percolation in the oceanic upper mantle

Abstract 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.

138–121 Ma asthenospheric magmatism prior to continental break-up in the North Atlantic and geodynamic implications

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.

Petrology of the Easter microplate region in the South Pacific

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).

Petrology and geochemistry of the ophiolitic and volcanic suites of the Taitao Peninsula — Chile triple junction area

Abstract 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.

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.

Evidence of multi-phase Cretaceous to Quaternary alkaline magmatism on Tore-Madeira Rise and neighbouring seamounts from 40Ar/39Ar ages

Abstract: 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 40Ar/39Ar ages from the central and southern Tore–Madeira Rise yield ages ranging from 94.5 to 0.5 Ma. We performed new 40Ar/39Ar 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.