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Precambrian Crustal Evolution of the Rehoboth Province, Southern Africa
No full textThis thesis uses U-Pb, Sm-Nd Lu-Hf, Ar-Ar and Oxygen isotope data on well-documented rock
samples, to investigate crustal evolution of the Precambrian Rehoboth Province of Southern Africa. This
province is defined by its smooth magnetic character, reflecting deep magnetic basement in contrast to
the adjoining mobile belts and the Kaapvaal Craton. Most of the area is covered by sedimentary
sequences and the Kalahari sands.
A first indication of old crust in the Rehoboth Province was provided by granitoid and mafic cobbles
from the ~300 Ma Dwyka glacial diamictite. The granitoid cobbles have Archaean ages between 3100 to
2500 Ma and a Palaeoproterozoic group between 2050 and 2020 Ma. The mafic cobbles yield ages of
1123–1111 Ma. The likely provenance of the Dwyka cobbles is the basement of the Rehoboth Province
or the Kalahari Line east of Rietfontein. A source within the Kaapvaal Craton is excluded by the absence
of typical Kaapvaal cover rocks in the diamictite cobbles. Previous theories of crustal growth or
collision between the Rehoboth Province and the Kaapvaal Craton at either 1800 or 1200 Ma are not
supported by these data.
The Rehoboth Basement Inlier (RBI) is a tectonic terrane at the northern margin of the Rehoboth
Province, thrust up in the ~600 Ma Damara foreland. U–Pb, Lu–Hf and Oxygen isotope data for zircon
from metasedimentary and magmatic rocks provide new insights on the crustal evolution of the
Rehoboth Province. A small group of 3.41 to 2.45 Ga U-Pb zircon ages found in the metasediments of the
RBI strengthens the concept of an Archaean foundation to the Rehoboth Province. A group of
Palaeoproterozoic ages ranging from 2.2 to 1.92 Ga have not been identified in outcropping magmatic
rocks. The Lu-Hf isotope character of these zircons requires mixing between Archaean crustal source
rocks and juvenile mantle material. This again points to the presence of an Archaean nucleus within the
Rehoboth Province. The 2.05 Ga event, seen in both Dwyka cobbles and detrital zircons, corresponds in
age to the Bushveld event and thermal peaks in the Kaapvaal Craton and Limpopo Belt. It suggests that
the Rehoboth Province was attached to the Kaapvaal Craton before 2.05 Ga, but after 2.45 Ga.
A large age peak at 1.87 Ga corresponds in age to the largely metabasaltic 1870 ±5Ma Elim
Formation, now the oldest dated unit in the RBI. However, the Lu-Hf isotope data of the detrital zircons
from metasediments shows a source with distinctly older crustal residence and it is likely that the
magmatic event at 1.87 Ga was widespread over the Rehoboth Province.
The detrital zircon ages that correspond to the younger Palaeoproterozoic (1.83–1.72 Ga) magmatic
ages of the RBI show similar Lu-Hf character and the source for these zircons was thus mainly provided
by rocks related to the same subduction phase. The Palaeoproterozoic magmatic rocks of the RBI reveal
a complex arc-related tectonic history which probably represents an Andean subduction setting.
The Kaaien Terrane in South Africa is part of the complex suture zone between the Kaapvaal
Craton, Kheis Province, Rehoboth Province and the Namaqua–Natal Province related to a ~1200 Ma
collision event. New metamorphic pressure-temperature (PT)-calculations combined with
geochronology for unusual garbenschiefer rocks of the Groblershoop Formation reveal an unique burial
and uplift history. A segment of the Kaaien Terrane reached depths around 40 km. This caused peak
metamorphism at 1164 Ma (Lu-Hf on garnet), followed by rapid exhumation to hornblende and white
mica Ar-Ar closure temperatures by ~1140 Ma, thought to be controlled by a change in the tectonic
regime. This is the highest pressure found thus far in the Namaqua-Natal Province, most others being
less than 5kb. Other parts of the Kaaien Terrane remained at the surface during this period.
The Rehoboth Province is thus revealed as an ancient crustal block with Archaean foundations,
which may have been attached to the Kaapvaal Craton prior to 2.05 Ga. Major Palaeoproterozoic events
within the Rehoboth Province involved mantle additions mixed with reworked Archaean crust. Finally,
the Rehoboth Province played a major role during the evolution of the Mesoproterozoic Namaqua–
Natal Province, which led to the formation of the Kalahari Craton
Precambrian Crustal Evolution of the Rehoboth Province, Southern Africa
No full textThis thesis uses U-Pb, Sm-Nd Lu-Hf, Ar-Ar and Oxygen isotope data on well-documented rock samples, to investigate crustal evolution of the Precambrian Rehoboth Province of Southern Africa. This province is defined by its smooth magnetic character, reflecting deep magnetic basement in contrast to the adjoining mobile belts and the Kaapvaal Craton. Most of the area is covered by sedimentary sequences and the Kalahari sands. A first indication of old crust in the Rehoboth Province was provided by granitoid and mafic cobbles from the ~300 Ma Dwyka glacial diamictite. The granitoid cobbles have Archaean ages between 3100 to 2500 Ma and a Palaeoproterozoic group between 2050 and 2020 Ma. The mafic cobbles yield ages of 1123–1111 Ma. The likely provenance of the Dwyka cobbles is the basement of the Rehoboth Province or the Kalahari Line east of Rietfontein. A source within the Kaapvaal Craton is excluded by the absence of typical Kaapvaal cover rocks in the diamictite cobbles. Previous theories of crustal growth or collision between the Rehoboth Province and the Kaapvaal Craton at either 1800 or 1200 Ma are not supported by these data. The Rehoboth Basement Inlier (RBI) is a tectonic terrane at the northern margin of the Rehoboth Province, thrust up in the ~600 Ma Damara foreland. U–Pb, Lu–Hf and Oxygen isotope data for zircon from metasedimentary and magmatic rocks provide new insights on the crustal evolution of the Rehoboth Province. A small group of 3.41 to 2.45 Ga U-Pb zircon ages found in the metasediments of the RBI strengthens the concept of an Archaean foundation to the Rehoboth Province. A group of Palaeoproterozoic ages ranging from 2.2 to 1.92 Ga have not been identified in outcropping magmatic rocks. The Lu-Hf isotope character of these zircons requires mixing between Archaean crustal source rocks and juvenile mantle material. This again points to the presence of an Archaean nucleus within the Rehoboth Province. The 2.05 Ga event, seen in both Dwyka cobbles and detrital zircons, corresponds in age to the Bushveld event and thermal peaks in the Kaapvaal Craton and Limpopo Belt. It suggests that the Rehoboth Province was attached to the Kaapvaal Craton before 2.05 Ga, but after 2.45 Ga. A large age peak at 1.87 Ga corresponds in age to the largely metabasaltic 1870 ±5Ma Elim Formation, now the oldest dated unit in the RBI. However, the Lu-Hf isotope data of the detrital zircons from metasediments shows a source with distinctly older crustal residence and it is likely that the magmatic event at 1.87 Ga was widespread over the Rehoboth Province. The detrital zircon ages that correspond to the younger Palaeoproterozoic (1.83–1.72 Ga) magmatic ages of the RBI show similar Lu-Hf character and the source for these zircons was thus mainly provided by rocks related to the same subduction phase. The Palaeoproterozoic magmatic rocks of the RBI reveal a complex arc-related tectonic history which probably represents an Andean subduction setting. The Kaaien Terrane in South Africa is part of the complex suture zone between the Kaapvaal Craton, Kheis Province, Rehoboth Province and the Namaqua–Natal Province related to a ~1200 Ma collision event. New metamorphic pressure-temperature (PT)-calculations combined with geochronology for unusual garbenschiefer rocks of the Groblershoop Formation reveal an unique burial and uplift history. A segment of the Kaaien Terrane reached depths around 40 km. This caused peak metamorphism at 1164 Ma (Lu-Hf on garnet), followed by rapid exhumation to hornblende and white mica Ar-Ar closure temperatures by ~1140 Ma, thought to be controlled by a change in the tectonic regime. This is the highest pressure found thus far in the Namaqua-Natal Province, most others being less than 5kb. Other parts of the Kaaien Terrane remained at the surface during this period. The Rehoboth Province is thus revealed as an ancient crustal block with Archaean foundations, which may have been attached to the Kaapvaal Craton prior to 2.05 Ga. Major Palaeoproterozoic events within the Rehoboth Province involved mantle additions mixed with reworked Archaean crust. Finally, the Rehoboth Province played a major role during the evolution of the Mesoproterozoic Namaqua– Natal Province, which led to the formation of the Kalahari Craton
Intra-granitic pegmatites of the Las Chacras-Potrerillos batholith (Sierra de San Luis, Argentina): Geochemistry, dating, and a new model of magma evolution
No full textThe chemical composition of primary minerals and fine-grained border zones fromintra-granitic pegmatites have been analyzed to constrain the last stages of magmaticdifferentiation in the Devonian Las Chacras-Potrerillos batholith, central Argentina.Based on accessory phase assemblages and mineral chemistry, the pegmatites areassigned to the NYF (Nb - Y - F) family. They are emplaced in all six units of thebatholith and show an increased abundance, size and complexity in the granitic unitsof the southern region. Fractionation indicators in pegmatitic K-feldspar (e.g., K/Rb: 68- 301, Rb/Sr: 1- 47, Cs: 8 - 71 ppm, Ba: 66 - 800 ppm), white mica (e.g., K/Rb: 36 - 99,Rb/Sr: 40 - 826, Cs: 30 - 206 ppm, Ba 18 - 1889 ppm) and tourmaline (Fetot +Mgtot:2.6 - 4.0 apfu, Altot: 5.0 - 6.4 apfu) define a consistent pattern with the most evolvedcompositions in the southern pegmatites. This regional pattern of pegmatite complexityand mineral chemistry reflects compositional variations in the different parentalmagmas as well as variations in magmatic evolution and suggests higherconcentrations of fluids and fluxes in the southern area of the batholith. Although thesouthern granites are the host for the most complex pegmatites in the batholith, thetiming of emplacement was not constrained. A new zircon U-Pb age of 388.3 ± 2 Mafrom the southern granites constitute the oldest crystallization age of the batholith andresolves the previously debated emplacement sequence, which has importantimplications for the pegmatite evolution. The initial ascent and emplacement of thelower crustal derived southern granites heated the middle to upper crust and releasedfluids into the basement. The volatile-rich basement and the subsequent intrusion ofthe central biotite porphyritic granite, caused high volumes of partial melting andenhanced crustal contamination. Increased assimilation of volatile-rich metaclasticcrustal material into this subsequent central magma enabled the formation of highlyevolved melts and more complex pegmatites. We suggest that the intrusion of thebiotite porphyritic granite represent the parental magma for the complex southernpegmatites and caused the formation of a contact zone in the previously emplacedsouthern granites.Fil: Ribacki, Enrico. Universitat Potsdam. Mathematisch Nautrwissenschaften Fakultat; AlemaniaFil: Trumbull, Robert. German Research Centre for Geosciences; AlemaniaFil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Altenberger, Uwe. Universitat Potsdam. Mathematisch Nautrwissenschaften Fakultat; AlemaniaFil: van Schijndel, Valby. German Research Centre for Geosciences; Alemani
The Rehoboth Province; a buried craton in southern Africa
No full textWe are investigating the origin and history of the Rehoboth Province, which is an enigmatic area due to the extensive Kalahari sand cover. Outcrops are mainly located in Namibia and Botswana along the borders of the magnetically quiet crustal terrane (Fig. 1). Palaeoproterozoic and Mesoprotorozoic stratigraphic successions are found at the northern boundary in the Rehoboth Basement Inlier (RBI), a thrust complex in the Damara front. The relationships between Paleoproterozoic and Mesoproterozoic formations or even within formations are often complicated by thrusting. Using U-Pb zircon geochronology we have now documented the existence of an hitherto unrecognized 3.4-2.4 Ga Archaean crustal component in Palaeoproterozoic metasediments, which was probably exposed as granitoids in the northern Rehoboth Province. This indicates a much longer geological history for the Rehoboth Province than previously known. Published zircon geochronology in the south-eastern corner of the Rehoboth Province has also indicated the presence of ancient crust. Zircon xenocrysts dated in Koras bimodal intrusives and volcanic rocks, north of Upington are in the range 1.7 to 2.1 Ga reflecting Paleoproterozoic basement (Pettersson et al., 2007). Glacial diamictites of the Permian Dwyka Group exposed at Rietfontein west of the Kalahari carry cobbles plucked from the bedrock by the ice sheet which covered the Gondwana supercontinent about 300 Ma ago. Microbeam U-Pb zircon dating of these granitic cobbles indicate the presence of a 2.5 to 2.9 Ga Archean crust beneath the Kalahari, with a lesser ~2050 Ma component (Cornell et al., 2010). Our data shows that there were at least three crustal development events at 3.4-2.4 Ga, 2.1-1.7 and 1.32-1.1 Ga. The volcanic and sedimentary rocks of the Mesoproterozoic Sinclair Supergroup (SS) lie in the Konkiep Subprovince between the Rehoboth Province and Namaqua Province. The basement of the Sinclair Supergroup is not yet constrained. In a preliminary study zircon xenocrysts in ~1.2 Ga migmatites gave an age of ~1.75 Ga, indicating a Palaeoproterozoic protolith and possibly an extension of the Rehoboth Province basement. The timing of accretion of the Rehoboth Province to the Kaapvaal Craton is still unclear and data is slim. Additional 39Ar-40Ar on hornblende and muscovite in the Groblershoop and Olifantshoek areas will increase the dataset and hopefully provide more constraints
40Ar-39Ar geochronology and PT estimations on garnet-hornblende muscovite-plagioclase schists from the Kheis Belt, South Africa
No full textThe Kheis Belt on the western margin of the Kaapvaal Craton, South Africa, is often referred to as a thin-skinned fold and thrust belt. However, Humphreys et al. [1] indicated that the schists of the Palaeoproterozoic Groblershoop Formation within the Kheis Belt reached equilibrium conditions between 600-700°C and 8-11 kBar. The schists of this study contain a main mineral assemblage of grt+hbl+plag+musc+epi+qtz with accessory chlorite, biotite, ilmenite and rutile. The peak metamorphic mineral assemblage is represented by grt+hbl+musc and the plagioclase has an Ab content of 0.83. The garnet and hornblende have inclusions of quartz, epidote and ilmenite, indicating a greenschist facies mineralogy assemblage. This is oldest assemblage preserved in the sample. Preliminary modeled results, obtained by equilibrium phase diagrams based on bulk composition were calculated for the NaCaKFMASH system with the program Theriak/Domino [2, 3]. These diagrams indicate that the garnet and hornblende growth started at 520°C and 9.5-10 kBar. An increase in Mg in the rims of the garnets points to a prograde reaction. PT conditions went up during garnet growth and reached a possible maximum at 660°C and 12.5 kBar. At higher temperatures kyanite becomes stable and this is not present in the samples. It is unlikely that the temperature reached above 660°C. Retrograde chlorite postdates all other minerals. 40Ar- 39Ar dating on both muscovite and hornblende gave 1147±4 Ma and 1141±3 Ma. These ages are related to the Namaqua Orogeny and the agreement between the cooling ages of muscovite and hornblende arguments for a rapid uplift. Therefore are these ages thought to represent cooling ages that can be directly correlated with the peak metamorphic conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Current applications using key mineral phases in igneous and metamorphic geology: perspectives for the future
Get PDFThe study of magmatic and metamorphic processes is challenged by geological complexities like geochemical variations, geochronological uncertainties and the presence/absence of fluids and/or melts. However, by integrating petrographic and microstructural studies with geochronology, geochemistry and phase equilibrium diagram investigations of different key mineral phases, it is possible to reconstruct insightful pressure–temperature–deformation–time histories. Using multiple geochronometers in a rock can provide a detailed temporal account of its evolution, as these geological clocks have different closure temperatures. Given the continuous improvement of existing and new in situ analytical techniques, this contribution provides an overview of frequently utilized petrochronometers such as garnet, zircon, titanite, allanite, rutile, monazite/xenotime and apatite, by describing the geological record that each mineral can retain and explaining how to retrieve this information. These key minerals were chosen as they provide reliable age information in a variety of rock types and, when coupled with their trace element (TE) composition, form powerful tools to investigate crustal processes at different scales. This review recommends best applications for each petrochronometer, highlights limitations to be aware of and discusses future perspectives. Finally, this contribution underscores the importance of integrating information retrieved by multi-petrochronometer studies to gain an in-depth understanding of complex thermal and deformation crustal processes
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