Mantle source heterogeneity in a Neoproterozoic back-arc basin: Geochemical and thermodynamic modeling of the volcanic section of Wadi Ghadir ophiolite, Egypt

More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively. The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated. The Kelseaa type magmas were derived at shallow depth from a fluid-enriched, relatively depleted, mantle source,but some have a component derived from mantle with residual garnet. They are suggested to have formed in a back-arc environment. The more than 200 narrow dykes are olivine tholeiites (some picritic), alkali basalts, trachybasalts, basanites and a few phonotephrites. The magmas evolved by olivine and olivine + clinopyroxene fractionation. They have trace element characteristics which can be described mainly by mixing of two components: one is a typical OIB-magma (La/Nb < 1, Zr/Nb = 4, Sr/Nd = 16) and rather shallowly derived from spinel peridotite; the other is enriched in Sr and has La/Nb = 1.0 - 1.5, Zr/Nb = 9, Sr/Nd = 30 and was derived at greater depth, probably from a pyroxenitic source. Both sources were probably recycled material in a mantle plume. A few of these dykes are much more enriched in incompatible elements and were derived from garnet peridotite by a small degree of partial melting. The Kaas and Listed dykes (20-40 m) and related dykes are evolved trachybasalts to basaltic trachyandesites. They are most likely related to the Blekinge Dalarne Dolerite Group. The few NW-trending dykes are quartz tholeiites, which were generated by large degrees of rather shallow melting of an enriched mantle source more enriched than the source of the older Bornholm dykes. The source of the NW-trending dykes was probably a very hot mantle plume.

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Mantle source heterogeneity and magmatic evolution at Carlsberg Ridge (3.7°N): constrains from elemental and isotopic (Sr, Nd, Pb) data

Marine Geophysical Research ◽

10.1007/s11001-016-9292-1 ◽

2016 ◽

Vol 38 (1-2) ◽

pp. 47-60 ◽

Cited By ~ 4

Author(s):

Ling Chen ◽

Limei Tang ◽

Xing Yu ◽

Yanhui Dong

Keyword(s):

Mantle Source ◽

Magmatic Evolution ◽

Carlsberg Ridge ◽

Source Heterogeneity

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Sulfide mantle source heterogeneity recorded in basaltic lavas from the Azores

Geochimica et Cosmochimica Acta ◽

10.1016/j.gca.2019.10.012 ◽

2020 ◽

Vol 268 ◽

pp. 422-445 ◽

Cited By ~ 4

Author(s):

Christopher L. Waters ◽

James M.D. Day ◽

Shizuko Watanabe ◽

Kaan Sayit ◽

Vittorio Zanon ◽

...

Keyword(s):

Mantle Source ◽

Source Heterogeneity ◽

Basaltic Lavas

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Structure of oceanic crust in back-arc basins modulated by mantle source heterogeneity

Geology ◽

10.1130/g48407.1 ◽

2020 ◽

Author(s):

Ingo Grevemeyer ◽

Shuichi Kodaira ◽

Gou Fujie ◽

Narumi Takahashi

Keyword(s):

Oceanic Crust ◽

Mantle Source ◽

Seismic Data ◽

Lower Crust ◽

Subduction Zones ◽

P Wave ◽

Spreading Ridge ◽

Volcanic Arc ◽

Depleted Mantle ◽

Back Arc

Subduction zones may develop submarine spreading centers that occur on the overriding plate behind the volcanic arc. In these back-arc settings, the subducting slab controls the pattern of mantle advection and may entrain hydrous melts from the volcanic arc or slab into the melting region of the spreading ridge. We recorded seismic data across the Western Mariana Ridge (WMR, northwestern Pacific Ocean), a remnant island arc with back-arc basins on either side. Its margins and both basins show distinctly different crustal structure. Crust to the west of the WMR, in the Parece Vela Basin, is 4–5 km thick, and the lower crust indicates seismic P-wave velocities of 6.5–6.8 km/s. To the east of the WMR, in the Mariana Trough Basin, the crust is ~7 km thick, and the lower crust supports seismic velocities of 7.2–7.4 km/s. This structural diversity is corroborated by seismic data from other back-arc basins, arguing that a chemically diverse and heterogeneous mantle, which may differ from a normal mid-ocean-ridge–type mantle source, controls the amount of melting in back-arc basins. Mantle heterogeneity might not be solely controlled by entrainment of hydrous melt, but also by cold or depleted mantle invading the back-arc while a subduction zone reconfigures. Crust formed in back-arc basins may therefore differ in thickness and velocity structure from normal oceanic crust.

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