Holger Sommer

Holger Sommer
University of Namibia | UNAM · Department of Geology

Professor

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45
Publications
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675
Citations

Publications

Publications (45)
Article
Tonalite, trondhjemite, granodiorite (TTG) rocks in Viti Levu, Fiji Islands formed through hydrous partial melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek during late-Eocene time. This was followed by the formation of a quartz diorite unit by anatexis...
Article
This study reports igneous petrology, zircon ages and geochemistry for Palaeoproterozoic I-type granitic, granodioritic, Qtz-monzonitic, Qtz-monzodioritic and monzodioritic rocks from the Usagaran domain of southwestern Tanzania. These rocks can be subdivided into three groups according to their age, T-t evolution and geochemistry. The oldest group...
Article
This study investigated Neoproterozoic (Pan-African) eclogite- and high-pressure-granulite (E-HPG) facies rocks from the Mozambique belt of east-central Tanzania, collected close to the town of Ifakara and the adjacent Furua area from different tectonic settings, the Palaeoproterozoic Usagaran and the Neoproterozoic Mozambique belt. The studied roc...
Article
Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a ste...
Article
Full-text available
Recrystallized zircons extracted from intermediate granulites of the Letseng-la-Terae Diamond Mine in northeastern Lesotho provide evidence that the Tugela Terrane of the Mesoproterozoic high-grade Namaqua-Natal mobile belt may extend ~200 km farther west than previously thought. The rock-forming minerals in the investigated lower crustal granulite...
Article
The metamorphic rocks in the Neoproterozoic (Pan-African) Mozambique belt of southwestern Tanzania, around the town of Songea, can be subdivided into one- and two pyroxene bearing charnockitic gneisses, migmatitic granitoid gneisses and amphibolite-facies metapelites. Lower-grade amphibolite-facies rocks are rare and can be classified as sillimanit...
Article
The Roberts Victor kimberlite pipe-dike system is well known as type locality for intensively studied eclogitic xenoliths. Since more than 95% of the Roberts Victor xenoliths are rather of eclogitic than of peridotitic type, mineralogical data of lherzolitic peridotites from the mine is extremely rare. In fact, there is no detailed petrological stu...
Article
We present here a novel of high-resolution synchrotron based FTIR measurements, which gave evidence for strongly heterogeneous distribution of volatiles within natural nominally anhydrous minerals such as olivine and garnet from about 150 km depth in the Earth's mantle. It is long known that there can be a substantial amount of volatiles stored in...
Article
It has been suggested that much of the lithopheric mantle beneath the Colorado Plateau was hydrated by the dehydration of the Farallon plate when it was undergoing low angle subduction during the Laramide orogeny. If correct, low angle subduction could be a viable mechanism for weakening laterally extensive regions of continental lithosphere, allow...
Article
First evidence from the Jwangeng diamond mine in South Botswana reveals a possible mechanism of near-sonic speed diamond extraction. Our data support the formation of Weertman cracks as a transport mechanism for the diamond bearing kimberlitic-melt from the Earth's mantle to the surface. Weertman cracks are vertical fluid filled cracks, which can m...
Article
The mechanism of kimberlite eruptions plays a significant role in diamond exploration. The composition of the kimberlitic melt is a major physical aspect in understanding the eruption dynamics of kimberlites and therein diamond resorption due to changes in fO2. In the studied eclogites, water and CO2 is commonly dissolved in nominally water free mi...
Article
An important feature in diamond exploration is to understand the mechanism of the kimberlite eruption and dynamics of kimberlites in terms of the composition of the kimberlitic melt. Water is found as OH in nominally water free minerals (NAMS), which is caused due to mantle metasomatism. The water has a higher solubility in the kimberlitic melt and...
Article
Full-text available
The hypothesis that much of the lithosphere of the Archaean Tanzania Craton was hydrated, by the dehydration of a buoyant subduction 2Ga ago is presented in this study. Buoyant subduction is a potential mechanism for thermal erosion and metasomatism of extensive regions of the cold overlying continental lithosphere. This hypothesis could explain wh...
Article
In diamond exploration it is important to understand the mechanism of the kimberlite eruption and dynamics of kimberlites in terms of the composition of the kimberlitic melt. Water is found as OH in nominally water free minerals (NAMS) caused by mantle metasomatism. The water has a higher solubility in the kimberlitic melt and thus is removed from...
Article
Full-text available
Various melt inclusions in chrome spinel crystals of tempered samples from the lower chromitite stringer of the 'Merensky Reef' at Karee Mine in the western Bushveld Complex, were investigated to obtain an indication of the initial melt composition of the upper Critical Zone. The material was heated to 1300°C in a furnace under a graphite buffered...
Article
The mechanisms of kimberlite eruptions plays a significant role in diamond exploration. One major physical aspect in understanding the eruption dynamics of kimberlites is the composition of the kimberlitic melt. Water is commonly dissolved in nominally water free minerals (NAMS) as OH caused by mantle metasomatism. Due to the eruption of the kimber...
Article
New evidence from the Jwangeng diamond mine in South Botswana reveals a possible mechanism of near- sonic speed diamond extraction. Our data support the formation of Weertman cracks as a transport mechanism for the diamond bearing kimberlitic-melt from the Earth's mantle to the surface. Weertman cracks are two-dimensional liquid-filled cracks, whic...
Article
In this study we supply petrological evidence for a hydrous upwelling beneath the Colorado Plateau in the Western United States. We trace a hydrous upwelling by its signature of multi-stage water enrichment processes in garnet- and Cr-spinel bearing lherzolithes collected from the sample volcano "the Thumb". Multi-stage hydration is witnessed from...
Article
The Earth is a wet planet, where water is recycled from the mantle to the surface and back again. The mantle is generally too hot for the stability of hydrous minerals and too cold to store water within significant melt sheets. Therefore, the current paradigm in geosciences is that water resides and moves only through point defects in nominally anh...
Article
The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where...
Article
Full-text available
The Taita Hills-Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20-30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a...
Article
The Earth is a wet planet, where water is recycled from the mantle to the surface and back again. The mantle is generally too hot for the stability of hydrous minerals and too cold to store water within significant melt sheets. Therefore, the current paradigm in geosciences is that water resides within point defects in nominally anhydrous minerals...
Article
This study presents new zircon ages and Sm–Nd whole-rock isotopic compositions for high-grade gneisses from the Udzungwa Mountain area in the central part of the Mozambique belt, Tanzania. The study area comprises a succession of layered granulite-facies para- and orthogneisses, mostly retrograded to amphibolite-facies. The original intrusive conta...
Article
New SHRIMP zircon ages for high-grade rocks from the Pan-African Mozambique belt (MB) of central Tanzania document reworking of Archaean–Palaeoproterozoic crust during the formation of this Neoproterozoic collisional orogen. Several gneisses and granulites from the Great Ruaha river area yielded late Archaean emplacement ages of 2575–2680 Ma for th...
Article
Full-text available
We report SHRIMP zircon U-Pb ages for Post-Usagaran granitic- to granodioritic intrusives and a rhyolitic agglomerate from the Palaeoproterozoic terrain of southwestern Tanzania. This terrain consists of strongly deformed and metamorphosed rocks ascribed to the ca. 2 Ga Usagaran mobile belt, voluminous post-Usagaran granitoids, and minor supracrust...
Thesis
Full-text available
This doctoral thesis deals with the geological, petrological and geochronological evolution of East African basement rocks from the Neoproterozoic Mozambique Belt of central Tanzania. From 01/2001 to 6/2004 I worked on granulite facies rocks from the central part of Tanzania at the University Mainz, Germany. (together with A. Kröner, C. Hauzenberge...
Article
New data on the metamorphic petrology and zircon geochronology of high-grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan-African event. The metamorphic rocks are characterized by a clockwise P–T path, followed...
Article
Most of the geological and palaeogeographical models consider the Neoproterozoic supercontinent Gondwana (∼650-550 Ma) as the direct offspring of the disintegrated Mesoproterozoic supercontinent Rodinia (∼1300-750 Ma). One of the main classical sutures along which the dispersing Rodinia fragments were fused into a new supercontinent (Godwana) is id...
Article
Full-text available
In Central Dronning Maud Land, East Antarctica, rare metre-sized lenses of spinel peridotite are enclosed in high-grade metamorphic rocks. The rocks experienced a medium- P granulite-facies metamorphism at ∼575 Ma and a low- P amphibolite-facies overprint at ∼530 Ma. The latter is probably related to extensive granitoid magmatism between 530 and 50...
Article
The East African Orogen is an extensive Neoproterozoic (Pan-African) orogenic belt extending from Arabia to Mozambique and containing elements of both accretion and collision tectonics. The predominantly upper crustal northern part (Arabian-Nubian Shield, ANS) consists of Neoproterozoic juvenile arc assemblages that accreted onto the African contin...
Article
The Mozambique belt (MB) in East Africa is a mainly NNW-SSE trending mountain range that consists predominantly of high-grade metamorphic rocks. To the west it is bordered by the Usagaran/Ubendian mountain belt and the Archaean Tanzania craton, to the east by Neogene sediments. Pan-African granulite facies rocks have been reported from the (1) Tsav...
Article
Garnet corona textures in high-grade metamorphic rocks are well known. They form due to drastic increase in P-T conditions leading to instability of certain minerals and, consequently, to a change in the mineral paragenesis of a given bulk chemical composition. Such reactions often result in the formation of garnet layers, which separate an eventua...
Article
The Mozambique belt (MB) of East Africa contains large areas of granulite-facies rocks as well as migmatitic orthogneisses of Neoproterozoic, Palaeoproterozoic and Archaean age. The studied area is situated in central Tanzania along the Great Ruaha River where granitoid orthogneisses are interlayered with granulite-facies migmatitic metasediments....
Article
The 1.13 Ga Ilimaussaq intrusive complex, South Greenland, is composed of various types of alkali granite and silica-undersaturated alkaline to agpaitic nepheline syenites related to three subsequently intruded magma batches. Mineral chemistry indicates continuous fractionation trends within each rock type, but with distinct differences among them....

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