scholarly journals Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

2013 ◽  
Vol 29 ◽  
pp. 1-24 ◽  
Author(s):  
Brian G.J. Upton
Keyword(s):  
Mafic Magma ◽  
Dyke Swarm ◽  
Small Scale ◽  
Alkaline Magmatism ◽  
East African ◽  
The North ◽  
Intrusive Rocks ◽  
Alkali Granites ◽  
African Rift ◽  
Very High

The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c . 1163 Ma Younger giant dyke complex (YGDC) mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokken) is considered to be coeval and presumably comagmatic with the YGDC. At the unconformity between the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustal assimilation. A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik). Three post-extensional complexes (Tugtutôq, Ilímaussaq and late Igdlerfigssalik) along the former rift mark the end of magmatism at c . 1140 Ma. The latter two complexes have oblate plans reflecting ductile, fault-related strain. The Tugtutôq complex comprises quartz syenites and alkali granites. The Ilímaussaq complex mainly consists of nepheline syenite crystallised from highly reduced, Fe-rich phonolitic peralkaline (agpaitic) magma, and resulted in rocks with very high incompatible element concentrations. Abundant anorthositic xenoliths in the mafic and intermediate intrusions point to a large anorthosite protolith at depth which is considered of critical importance in the petrogenesis of the salic rocks. Small intrusions of aillikite and carbonatite may represent remobilised mantle metasomites. The petrological similarity between Older and Younger Gardar suites implies strong lithospheric control of their petrogenesis. The parental magmas are inferred to have been derived from restitic Ketilidian lithospheric mantle, metasomatised by melts from subducting Ketilidian oceanic crust and by small-scale melt fractions associated with Gardar rifting. There are numerous analogies between the southern Gardar rift and the Palaeogene East African rift.

scholarly journals Small-scale thermal upwellings under the northern East African Rift from S travel time tomography

2016 ◽  
Vol 121 (10) ◽  
pp. 7395-7408 ◽  
Author(s):  
Chiara Civiero ◽  
Saskia Goes ◽  
James O. S. Hammond ◽  
Stewart Fishwick ◽  
Abdulhakim Ahmed ◽  
...  
Keyword(s):  
Travel Time ◽  
East African Rift ◽  
Small Scale ◽  
East African ◽  
Travel Time Tomography ◽  
African Rift

Geochemical and petrophysical characterization of mantle metasomatism beneath the North Tanzanian Divergence, East African rift.

2021 ◽  
Author(s):  
Adeline Clutier ◽  
Fleurice Parat ◽  
Michel Gregoire ◽  
Benoit Gibert ◽  
Stéphanie Gautier ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Lithospheric Mantle ◽  
East African Rift ◽  
Mantle Xenoliths ◽  
P Wave ◽  
S Wave ◽  
East African ◽  
The North ◽  
Alkaline Magmas ◽  
African Rift

<p>The North Tanzanian Divergence (NTD) is the prolongation of the eastern branch of the East African Rift and is a place of intense volcanism. Numerous volcanoes erupted deep subalkaline to highly alkaline magmas, including the particular active natrocarbonatite Oldoinyo Lengai. On the North-South axis (Natron to Manyara basins), three highly alkaline volcanoes, Pello Hills, Lashaine and Labait, erupted melilite magmas that originated from low degree of partial melting of asthenospheric mantle (depth > 120 km). The particularity of these volcanoes is that they sampled numerous mantle xenoliths during ascent. This represents a unique opportunity to study the composition and the rheology of lithospheric mantle. Mantle xenoliths are deep garnet-bearing peridotites (120 km depth), amphibole and phlogopite peridotites and phlogopitites. They contain abundant hydrous minerals as isolated crystals or veins that attest to an important metasomatism beneath the NTD. Previous geochemical and petrological studies have highlighted interactions of alkaline magmas and the thick cratonic lithosphere as metasomatic agent. However, the presence and composition of magmas, the degree of metasomatism, and the role of metasomatism on mantle rheology below the NTD is still debated.</p><p>To characterize these previous parameters, in this study we performed geochemical and petrophysical analyses on metasomatized, fertile and refractory mantle xenoliths from Labait (on-craton volcano) and Pello Hills (in-rift volcano). Using mineral compositions and thermobarometer calibrations, we estimated the depth of mantle xenoliths between 40 and 140 km (14 to 47 kbar) and temperatures from 930 to 1340°C. EBSD analysis on thin sections indicate that peridotites and amphibole/phlogopite-bearing mantle xenoliths display a moderate to strong deformation induced crystal preferred orientation. In contrast, weak mineral orientations have been observed in phlogopite-amphibole-clinopyroxene-bearing veins. Calculation of seismic properties using MTEX program show that peridotites are seismically anisotropic, up to 12.4% for P-wave velocity (Vp) and 6.8% for S-wave velocity (Vs). The Vp and Vs in hydrous veins are lower than in peridotites (Vp: 7.5-7.9 and 8.3-9.6 km/s; Vs: 4.4-4.6 and 5.0-5.3 km/s respectively) and therefore the Vp and Vs velocities decrease with the increasing proportion of metasomatic minerals. We estimate that a peridotite with 20 vol.% metasomatic vein has a velocity decrease of 3.5% for Vp and 2.9% for Vs, compared to a fertile peridotite.</p><p>These geochemical and petrophysical approaches are important to understand P- and S-wave propagation in the lithospheric mantle beneath the NTD and more specifically in metasomatized lithospheric mantle. The new in situ data and models from mantle xenoliths will be compared to tomographic acquisition and discussed in term of temperature, presence of melt or metasomatism processes. Both petrophysical and geophysical data will help to precisely determine the structure and rheology of the lithospheric mantle, which may control the propagation of the rift at early stage rifting between the Tanzanian craton and the mobile Proterozoic belts.</p>

Seismic wave attenuation in the lithosphere of the North Tanzanian divergence zone (East African rift system)

2017 ◽  
Vol 58 (2) ◽  
pp. 253-265 ◽  
Author(s):  
A.A. Dobrynina ◽  
J. Albaric ◽  
A. Deschamps ◽  
J. Perrot ◽  
R.W. Ferdinand ◽  
...  
Keyword(s):  
Seismic Wave ◽  
Wave Attenuation ◽  
East African Rift ◽  
Rift System ◽  
East African Rift System ◽  
East African ◽  
Seismic Wave Attenuation ◽  
The North ◽  
African Rift

Geology of Nubia and Surrounding Regions

2021 ◽  
pp. 47-62
Author(s):  
James A. Harrell
Keyword(s):  
Nile River ◽  
East African ◽  
Basement Complex ◽  
The North ◽  
Central Sudan ◽  
Tectonically Active ◽  
Eolian Sediments ◽  
Rift Zones ◽  
Sandstone Formation ◽  
African Rift

Nubia occupies a low, interior plain extending from southern Egypt to central Sudan. It is surrounded by high plateaus and mountains on all sides except the north. Across this plain flows the Nile River. The bedrock geology is dominated by metamorphic and igneous rocks of the mainly Precambrian to Paleozoic Basement Complex, and sedimentary rocks of the Cretaceous Nubian Sandstone Formation. Cataracts and rapids exist wherever the Nile River flows over Basement Complex rocks. The bedrock is blanketed in many areas by unconsolidated alluvial, lacustrine, and especially eolian sediments of mostly Quaternary age. The tectonically active Red Sea and East African rift zones are responsible for Cenozoic and still ongoing volcanism and seismicity in the Nubian region with additional tectonism resulting from uplift along the Nubian Swell in southern Egypt and northern Sudan.

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