Magmatic cycles and formation of the upper oceanic crust at spreading centers: Geochemical study of a continuous extrusive section in the Oman ophiolite

[1] The stratigraphy of the well-preserved Solund-Stavfjord Ophiolite Complex in the West Norwegian Caledonides documents the volcanic evolution of a spreading center in a Late Ordovician back-arc basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the 470-800 m thick extrusive sequence, and are organized stratigraphically in a cyclic manner. Cyclic units vary in thickness from 5 m to 225 m and are typically composed of basal sheet flows or lava flows with large pillows that are succeeded by flows with progressively smaller pillows and volcanic breccias. Thick, independent breccia units also occur in the stratigraphy. In sheet-flow dominated parts of the sequence the cyclic units are thicker (average 85 m) than in pillow-dominated parts (average 20 m). Detailed logging of closely spaced profiles ( 1 km or less apart) shows that the proportions of sheet flows, pillow lavas and volcanic breccias varies laterally. Along an axial segment of less than 10 km, the volcanic products change from predominantly sheet flows, reflecting robust volcanic centers, to pillow lavas to volcanic breccias. Sheet-flow dominated volcanic centers seem to be spaced at intervals of 25-30 km, and we tentatively interpret their regularity as an expression of volcanic segmentation at an intermediate-to fast spreading center. Observations of modern ocean crust suggest that sheet flows dominate at fast spreading ridges, while pillow lavas dominate at slow-spreading ridges. Volcanic breccias are apparently rare in both of these environments. These features contrast with the stratigraphy of the Solund-Stavfjord Ophiolite Complex, where the proportion of different volcanic products varies laterally and volcanic breccias are common. We emphasize the importance of detailed studies of the volcanic stratigraphy of ophiolites, as complements to those of in-situ oceanic crust, in order to provide a more complete picture of volcanic evolution of oceanic crust in different spreading regimes.

Section: Architecture Of the Volcanic Sequence Of The Solund‐stavfjormentioning

confidence: 99%

Volcanic evolution of oceanic crust in a Late Ordovician back‐arc basin: The Solund‐Stavfjord Ophiolite Complex, West Norway

Furnes

Hellevang

et al. 2003

“…The Oman ophiolite is well known for the complexity of its lava sequence, and particularly for the presence of V2 and V3 lavas with ''subduction related'' major and trace element characteristics, over the Geotimes lavas [e.g., Alabaster et al, 1982;Einaudi et al, 2003;Ernewein et al, 1988;Pearce, 1980;Umino et al, 1990]. We used the oxybarometer based on the Fe-Mg olivine-spinel exchange [Ballhaus et al, 1990] to calculate fO 2 .…”

Section: Oxygen Fugacity Conditions During Meltingmentioning

confidence: 99%

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

Monnier

Girardeau

Mée

et al. 2006

[1] 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% (F max ) 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 . 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# (% F max ) 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.

“…(a) REE patterns and calculated liquid compositions in equilibrium with clinopyroxene (thin black lines). Included is an average primitive MORB (blue thick line) from Kelemen et al [2004] and the range of REE compositions in Oman extrusive rocks of V1 type (green shaded field [from Pallister and Knight , 1981; Alabaster et al , 1982; Ernewein et al , 1988; Einaudi et al , 2003; Godard et al , 2003]) and of V2 type (red shaded field [from Godard et al , 2003]). (b) Trace element patterns.…”

Section: Resultsmentioning

confidence: 99%

Petrogenesis of crustal wehrlites in the Oman ophiolite: Experiments and natural rocks

Koepke

Schoenborn

Oelze

et al. 2009

[1] In the Wadi Haymiliyah of the Oman ophiolite (Haylayn block), discordant wehrlite bodies ranging in size from tens to hundreds of meters intrude the lower crust at different levels. We combined investigations on natural wehrlites from the Wadi Haymiliyah section with an experimental study on the phase relations in a wehrlitic system in order to constrain the petrogenesis of the crustal wehrlites of the Oman ophiolite. Secondary ion mass spectrometry analyses of clinopyroxenes from different wehrlite bodies imply that the clinopyroxenes were crystallized from tholeiitic, mid-ocean ridge (MORB)-type melts. The presence of primary magmatic amphiboles in some wehrlites suggests a formation under hydrous conditions. Significantly enhanced 87 Sr/ 86 Sr isotope ratios of separates from these amphiboles imply that the source of the corresponding magmatic fluids was either seawater or subduction zone-related. The experiments revealed that under wet conditions at relatively low temperatures, a MORB magma has the potential to produce wehrlite in the ocean crust by accumulation of early olivine and clinopyroxene. These show typically high Mg# which is a consequence of the oxidizing effect of the prevailing high aH 2 O. First plagioclases crystallizing after clinopyroxene under wet conditions are high in An content, in contrast to the corresponding dry system. Trace element compositions of clinopyroxenes of those wehrlites from the Moho transition zone are too depleted in HREE to be in equilibrium with present-day MORB, implying a genetic relation to the V2 lavas of the Oman ophiolite, which are interpreted to be the result of fluidenhanced melting of previously depleted mantle. We present a model on the petrogenesis of the crustal wehrlites in an upper mantle wedge above an initial, shallow subduction zone at the beginning of the intraoceanic thrusting.