Peridotite Ridge Research Articles

Extensional tectonics in the Gorringe Bank rocks, Eastern Atlantic ocean: evidence of an oceanic ultra‐slow mantellic accreting centre

The Gorringe Bank corresponds to an upper mantle peridotite ridge enclosing a 500‐m thick/ 50‐km long laccolith‐like body of gabbro, locally cut and poorly covered by tholeiitic rocks. Strain and kinematic analysis of orientated gabbros and peridotites sampled during the GORRINGE diving cruise (1996) provides new kinematic constraints on extensional high‐temperature deformation recorded at deep levels during stretching, near an accreting centre axis of a mantle‐dominant oceanic lithosphere. It is argued that the Gorringe Bank lithosphere formed at an oceanic ultra‐slow, N010°–020°‐trending accreting centre, mostly by passive tectonic denudation of the mantle, without any synchronous large magmatism. This peculiar lithosphere may be representative of the Iberia oceanic domain located between the continent and the J anomaly ridge, which likely marks the beginning of true spreading at an oceanic spreading ridge.

Cooling history of granulite samples from the ocean–continent transition of the Galicia margin: implications for rifting

Zircon and apatite fission track ages were obtained on two granulite samples that were recovered from the sea floor in the ocean–continent transition area of the Galicia margin (North Atlantic) using the French submersible Nautile. Zircon ages indicate that the rocks cooled through about 250°C in Carboniferous to Early Permian time (307 ± 42 Ma and 287±35 Ma). Hence, the granulites do not represent the prerift lower crust but were in an upper crustal position long before rifting started. Apatites yielded Early Cretaceous ages (126 ± 6.7 Myr and 129 ± 13.4 Myr), indicating cooling through 90 ± 30°C coeval with the main rifting phase that preceded continental breakup. We assume that the granulite samples originate from a tectonic breccia cropping out near one of the sample locations. This breccia formed along a synrift detachment accommodating continental breakup and final exhumation of the Galicia margin’s peridotite ridge.

The ocean‐continent boundary off the western continental margin of Iberia: Crustal structure west of Galicia Bank

A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin‐parallel peridotite ridge (PR). Immediately west of the PR, oceanic crust is only 2.5–3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a ∼60‐km‐wide lens‐shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust. When superimposed on a reflection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal levels, eventually coinciding with the top of the serpentinized peridotite lens (original crust‐mantle boundary). These observations render almost impossible the seafloor exposure of the PR by S acting as a top‐to‐the‐west detachment fault. Numerical models of melting and borehole subsidence information constrain our rifting model. The easternmost continental crust experienced a total stretching factor of 4.3 (most likely in two stages); it probably occurred over ∼25 m.y., with the highest rate of stretching at the beginning of the main earlier rift phase (Valanginian; 141–135 Ma). The 3 (4.7) km thick continental crust (depending on whether serpentinized peridotite is assigned to crust or mantle), which may include melt products, requires stretching factors of more than 11 (7) and a rift duration of more than 25 (13) m.y. The thin oceanic crust immediately west of the PR is explained by conductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after breakup.

Models of the development of the West Iberia rifted continental margin at 40°30′N deduced from surface and deep‐tow magnetic anomalies

The ocean‐continent transition (OCT) on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading. The nature of the OCT off western Iberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by Leg 149 of the Ocean Drilling Program. Here we concentrate on the interpretation of a new digital magnetic anomaly chart for the Iberia Abyssal Plain between 39.5° and 42.2°N. The most striking anomaly is a trough which exists immediately east of, and locally parallel to, anomaly J and is closely coincident with a basement peridotite ridge. We conclude from modeling surface magnetic anomalies and a deep‐tow magnetometer profile that seafloor spreading began about 129.9 m.y. ago (Barremian) at a rate of 10.0 mm/yr, consistent with drilling and seismostratigraphic results from the southern Iberia Abyssal Plain. A second feature is the existence of linear low‐amplitude isochron‐parallel magnetic anomalies east of the peridotite‐ridge trough and a clear change in the trend of these anomalies east of about 11°15′W. It is possible to model these anomalies by a series of 7‐ to 30‐km‐wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust. The west‐to‐east anticlockwise change in trend at 11°15′W suggests either a major change in stress regime and/or in the type of magnetic anomaly source. We speculate that the region between the peridotite ridge and 11°15′W represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading. We explain the change in trend at 11°15′W by a model featuring a rift which propagated through continental crust from south to north. We consider three scenarios for the formation of the transitional zone crust; first, continental crust is surrounded and impregnated by intrusive and extrusive material, formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate, second, such crust is surrounded by unroofed upper mantle material, or third, the zone is the product of ultraslow seafloor spreading. Present evidence tends to support the first scenario. In this scenario, the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust, fill the space caused by additional extension, and form the transitional crust, already described, over a broad zone at least several tens of kilometers wide. At breakup, upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization. In a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading, initially producing thin oceanic crust because of a restricted melt supply. The width of continuous technically extended continental crust in the southern Iberia Abyssal Plain is less than was previously supposed.

Gabbro and related rock emplacement beneath rifting continental crust: U [sbnd]Pb 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 determined 39Ar 40Ar 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.

Na, Al IV and Al VI in clinopyroxenes of subcontinental and suboceanic ridge peridotites: A clue to different melting processes in the mantle?

The concentrations of Na, Al IV and Al VI in clinopyroxene from plagioclase-free, spinel-lherzolites and harzburgites recovered from the mid-Atlantic ridge are compared with data from subcontinental and pre-oceanic, plagioclase-free, spinel-peridotites. The concentration of Al 2O 3 in clinopyroxenes varies within the same range for both suboceanic and subcontinental peridotite groups, from highly aluminous to Al-poor. However, most clinopyroxenes in suboceanic peridotites have Na contents (0.02–0.6 wt% Na 2O) and Al VI/Al IV ratios (0.3–1) lower than Na contents (0.7 to > 2 wt% Na 2O) and Al VI/Al IV ratios ( > 1) of subcontinental peridotite clinopyroxenes. These differences appear to be unrelated to both the degree of melting undergone by the two peridotite groups, and to their post-melting evolution (e.g., late metasomatism and magma/peridotite interactions). The clinopyroxene compositional variations within the oceanic peridotite group result from variable degrees of melting due to adiabatic mantle upwelling below mid-ocean ridges, starting in the garnet stability field. In contrast, the range of clinopyroxene compositions observed in subcontinental peridotites cannot be explained only by melting during similar adiabatic decompression. The data suggest that the subcontinental upper mantle was enriched in a jadeite component before partial melting, and that change in pressure and/or temperature was not the major cause of melting. Partial melting in subcontinental upper mantle could have been brought about by the introduction of water-rich fluids in the hot ultramafics.

Mantle Lherzolites from Troodos Ophiolites: Mineralogy and Ion Probe Geochemistry of Clinopyroxenes

Introduction. Peridotites of the Troodos ophiolite complex are represented by highly depleted harzburgite (e.g. Gass, 1990; Greenbaum, 1977). Recently we found spinel lherzolites between Amiandos mine and Pashi Livadia as a rock body at least 3 km in largest dimension, apparently the same rock type previously described as high-Al harzburgite (Duncan & Green, 1987). The spinel lberzolites are characterized by slightly depleted mineral composition similar to Mid Oceanic Ridge peridotites (MORP). Petrography and mineral composition of peridotites. Troodos peridotites are tectonites with protogranular or porphyroclastic textures typical of residual mantle peridotites (Mercier & Nicolas, 1975). Spinel Lherzolite. 70-75% olivine (O1) (Fo 90.5-91), 15-20% orthopyroxene (Opx) (Mg# 90-91), 5-7% clinopyroxene (Cpx) (Table 1) and 1-2% Cr-A1 spinel (Sp) (Cr/(Cr + AI) = 0.28-0.45). Cpx occurs as deformed irregular grains (1.5-2.5 mm) with Opx exsolution lamella. Cpx-bearing harzburgites (2-3% Cpx) are closely associated with spinel lherzolite and have transitional mineralogy. Vernadsky Institute of Geochemistry, RAS, Kosigin St., 19, Moscow 117975, Russia. Dept. Volcanology and Petrology, GEOMAR Research Center, Wischhofstr. 1-3, D-2300 Kiel, Germany

ODP drills the West Iberia rifted margin

Leg 149 of the Ocean Drilling Program drilled a transect of basement holes across the ocean‐continent transition (OCT) of the rifted continental margin off the west coast of Portugal. The principal objective was to sample a series of basement highs beneath the Iberia Abyssal Plain, which were thought to span the transition from the oldest oceanic crust to a peridotite ridge separating oceanic from extended continental crusts to a broad region of highly‐extended continental crust.Drilling demonstrated that the late rift‐stage exposures of peridotite at the seafloor are more laterally extensive than previously thought; peridotite was found on two highs 19 km apart. From the region expected to be extended continental crust, we cored 57 m of metagabbro basement whose continental or oceanic affinities are not yet clear. This rock may be Hercynian in age, and therefore part of the continental crust that was rifted to form this margin, or it may have been emplaced, uplifted, and exposed during rifting some 125 m.y.a. At our most landward site, Tithonian sediments, deposited about 20 m.y. before the onset of seafloor spreading on this segment of margin, are interpreted to overlie extended continental crust. The results of the leg will shed new light on the mechanisms by which upper mantle and perhaps lower crustal rocks are exposed at the seafloor during the late stages of continental breakup.

A Cold Suboceanic Mantle Belt at the Earth's Equator

An exceptionally low degree of melting of the upper mantle in the equatorial part of the mid-Atlantic Ridge is indicated by the chemical composition of mantle-derived mid-ocean ridge peridotites and basalts. These data imply that mantle temperatures below the equatorial Atlantic are at least approximately 150 degrees C cooler than those below the normal mid-Atlantic Ridge, suggesting that isotherms are depressed and the mantle is downwelling in the equatorial Atlantic. An equatorial minimum of the zero-age crustal elevation of the East Pacific Rise suggests a similar situation in the Pacific. If so, an oceanic upper mantle cold equatorial belt separates hotter mantle regimes and perhaps distinct chemical and isotopic domains in the Northern and Southern hemispheres. Gravity data suggest the presence of high density material in the oceanic equatorial upper mantle, which is consistent with its inferred low temperature and undepleted composition. The equatorial distribution of cold, dense upper mantle may be ultimately an effect of the Earth's rotation.

Oceanic lithosphere at the edge of a cenozoic active continental margin (northwestern slope of Galicia Bank, Spain)

A basaltic seafloor was surveyed and sampled by the French submersible Nautile on the northwestern slope of the Galicia Bank (Spain); the morphology and petrology of the volcanic rocks indicate an oceanic crust, although rather enriched in incompatible elements. The radiometric age of the basalts (about 100 Ma) suggests that they were emplaced a short time after the onset of seafloor spreading in that part of the North Atlantic. This oceanic seafloor remained attached to the overriding Iberian plate when the north and northwest passive Galicia margin was turned into an active margin in the Paleocene-Eocene. Mesozoic basalts now form the inner wall of the fossil northern Galicia trench. Mesozoic structures of the margin, including the northern continuation of the peridotite ridge, drilled to the west of Galicia, were slightly deformed by Cenozoic tectonism, although some inversion and shortening did occur locally. We consider this remnant basaltic seafloor to have been joined to the Galicia basement since the Mesozoic. Consequently, it cannot be interpreted as a wedge of oceanic lithosphere accreted to the Iberian plate as a result of Cenozoic subduction. Indeed, the initial oceanic lithosphere and the ocean-continent boundary are preserved at the edge of the former active continental margin. This setting can help to understand the processes of ophiolite emplacement in the case of further collision.

Kinematics of peridotite emplacement during North Atlantic continental rifting, Galicia, northwestern Spain

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 (> 10 2 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.

Mid Atlantic Ridge peridotites (from ODP Leg 109) geochemical compositions and conditions of partial melting of the upper mantle at slow spreading ridges

Mid Atlantic Ridge peridotites (from ODP Leg 109) geochemical compositions and conditions of partial melting of the upper mantle at slow spreading ridges

Diapiric Emplacement of the Ronda High-Temperature Ultramafic Intrusion, Southern Spain

Research Article| August 01, 1972 Diapiric Emplacement of the Ronda High-Temperature Ultramafic Intrusion, Southern Spain TIMOTHY P LOOMIS TIMOTHY P LOOMIS Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08540 PRESENT ADDRESS: DEPARTMENT OF GEOLOGY AND GEOPHYSICS, YALE UNIVERSITY, NEW HAVEN, CONNECTICUT 06520 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1972) 83 (8): 2475–2496. https://doi.org/10.1130/0016-7606(1972)83[2475:DEOTRH]2.0.CO;2 Article history received: 22 Jun 1971 rev-recd: 07 Jan 1972 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation TIMOTHY P LOOMIS; Diapiric Emplacement of the Ronda High-Temperature Ultramafic Intrusion, Southern Spain. GSA Bulletin 1972;; 83 (8): 2475–2496. doi: https://doi.org/10.1130/0016-7606(1972)83[2475:DEOTRH]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Ronda massif is an outcrop of generally unaltered peridotite covering approximately 300 km2 on the southern coast of Spain west of Malaga. An emplacement age between Triassic and Miocene is indicated from geological evidence.One margin of the intrusion was undisturbed by later tectonic movements, and the structural development along a traverse outward from this steep contact of the massif was examined in detail. The progressive increase in deformation toward the contact to form phyllite and two generations of gneiss, the spatial attitude of deformation structures and their tectonic interpretation, and the interaction of the deformation with contact metamorphic minerals all support the hypothesis that the structures observed are related to the emplacement of the massif. The structural information is combined with that from a petrologic study and gravity work to suggest that the massif penetrated vertically as a hot solid ridge.The peridotite ridge intruded the crust vertically, dragging up pelitic rock on the contacts from the lower crust, and displaced roof rock laterally away from the area above the massif as it penetrated the crust. The heat flow from the massif was modeled in an attempt to fit the observed gradient as deduced from the metamorphic aureole; this gradient can be matched only by taking into account the mass movement of material near the contact to bring up geothermal heat. An intrusion temperature of 1,000° to 1,200° C is considered to be most probable based on the metamorphic contact temperature and the petrogenesis of basic layers within the massif. Higher temperatures in the mantle are indicated.The massif was probably a diapiric intrusion with a rigid top pushed by a root extending through the lithosphere to the warmer astheno-sphere. Mechanical considerations suggest that the massif could have been driven part way through the crust by buoyant pressure in the mantle, as implied by the presence of crustal rock on the margins with a high-pressure history. A model of intrusion which would reduce the potential energy requirements of crustal thickening by the displaced crust allows the separation of adjacent crustal (lithospheric) blocks to open the volume of intrusion; plastic necking of the crust would occur near the hot massif.The ultramafic massif is considered to be part of two narrow ridges of high-density material along the margins of the Alboran Sea (continental graben) which were identified in a regional gravity survey. It is proposed that the ridges represent asthenospheric rock that welled up through extensional zones rapidly developed by rifting of lithospheric blocks. The geometry suggests a system in which the extensional zones radiate from a point near Gibraltar. These zones may be related to a counterclockwise rotation of Spain with respect to Africa about this point. Such a system would be expected if Spain acted as a roller block between Europe and Africa and is analagous to the opening of the Bay of Biscay.Continued extension and upwelling (as in other Mediterranean orogenic areas such as the Alps and Appenines) would have resulted in higher temperature diapirs, expulsion of the basic layers as basaltic magma, and a thinner crust to produce the ophiolite suite. In the extreme, a mid-ocean ridge system would be produced. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.