Wat is struktuurgeologie?
- Authors: Roering, C.
- Date: 2009-03-05T08:12:45Z
- Subjects: Structural geology , Gold mine geophysics
- Type: Inaugural
- Identifier: uj:14951 , http://hdl.handle.net/10210/2235
- Description: Inaugural lecture--Department of Geology, Rand Afrikaans University, 27 October 1980 , In general little is known by the public about the subject of structural geology. This subject has basically to do with deformation of rock on all scales, from the globe itself right down to dislocations on an atomic scale. Structural geology integrates the two disciplines of geology and physics into a sensible combination that is not only of academic importance but also of importance to the mineral industry. Several of the more interesting contributions to this subject indicate that mathematics and physics are now required in order to practice the discipline. A challenge of considerable importance is the role that structural geologists can play in the sphere of deep gold mine geophysics.
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Formation of major fold types during distinct geological events in the central zone of the Limpopo Belt, South Africa: new structural, metamorphic and geochronologic
- Authors: Boshoff, Rene
- Date: 2009-01-27T07:18:07Z
- Subjects: Geology , Structural geology , Metamorphism (Geology) , Folds (Geology) , Geological time , Limpopo Belt (South Africa)
- Type: Thesis
- Identifier: uj:14827 , http://hdl.handle.net/10210/1951
- Description: M.Sc. , The Limpopo Complex (LC) of southern Africa is one of the best-studied Precambrian granulite facies terrains in the world, yet workers still disagree on fundamental aspects of the geological evolution of this complexly deformed high-grade terrain. Most workers agree that the two marginal zones were exhumed in the late-Archaean, but disagree on the timing of major tectono-metamorphic events that affected the Central Zone (CZ) of Limpopo Belt, and the mechanism/s of its formation. There are currently two main schools of thought: The first school regards the LC as a late-Archaean orogenic zone that resulted from a north-south collision of the Zimbabwe and Kaapvaal cratons. Granitic plutons throughout the entire LC are considered to be accurate time-markers for this orogeny. The second school suggests that the CZ evolved as a result of a major Paleoproterozoic tectono-metamorphic event based mainly on the interpretation of metamorphic mineral ages. The present study focuses on two aims, namely (i) to provide a synthesis of published data as a basis to understand the ongoing age controversy concerning the evolution of the CZ, and (ii) to show that specific fold types in the CZ can be related to either the late-Archaean or the Paleoproterozoic event. New age, structural, metamorphic, and petrographic data are presented to show that (i) major sheath folds reflect the peak tectono-metamorphic event that affected the CZ in the late-Archaean, while (ii) major cross folds developed as a result of a transpressive event in the Paleoproterozoic. The age of formation of the Avoca sheath fold located about 40 km west of Alldays is accurately constrained by the age of emplacement of different structural varieties of precursors to the Singelele Gneiss: penetratively deformed syn- to late-tectonic Singelele gneisses with a zircon SHRIMP age of 2651 ± 8 Ma, date the time of formation of the sheath fold that is characterized by a single population of linear elements that define the central fold axis. The Avoca sheath fold documents top-to-the-NNE movement of material during the exhumation of the high-grade CZ rocks. Weakly foliated late-tectonic L-tectonites with a zircon SHRIMP age of 2626.8 ± 5.4 Ma, outcrop near the centre of the sheath fold, and provide a minimum age for the shear deformation event. An almost undeformed (post-tectonic) variety of the Singelele Gneiss was emplaced after the shear event. A detailed metamorphic study of metapelitic gneisses from the large Baklykraal cross fold, located about 20 km east of the Avoca sheath fold, documents a single decompression-cooling (DC) P-T path for the evolution of this structure. Three studied metapelitic samples characterized by a single generation of garnet provide a Pb-Pb age of 2023 ± 11 Ma, that accurately constrain the time of formation of this major fold to the Paleoproterozoic. A metapelitic sample characterized by two generations of garnet provide a slightly older Pb-Pb age of 2173 ± 79 Ma, that is interpreted to also reflect the late-Archaean event. The Baklykraal cross fold is characterized by two populations of linear elements: the one population defines the shallow N-S oriented fold axes, while the second population is associated with top-to-the-NNE movement of material during exhumation, resulting in folds with a nappe-like geometry. A DC P-T path for the Campbell cross fold (Van Kal, 2004) located just west of Musina, suggests that cross folds developed under significantly lower P-T conditions than is the case with sheath folds, providing an explanation for the lack of significant anatexis associated with the Paleoproterozoic event. The late-Archaean orogeny in contrast, was accompanied by widespread anatexis during a major magmatic event that is characterized by an abnormal high radiogenic signature. This study, for the first time, provides evidence that link specific fold types, and thus deformational events, to different tectono-metamorphic events. The main conclusion is that the CZ was exhumed as the result of two distinct orogenies, one in the late-Archaean, and the other in the Paleoproterozoic.
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Geological and geochemical study of the quartzofeldspathic rocks from the farm Gotha, Limpopo Province, South Africa
- Authors: Barnett, Martina
- Date: 2009-01-27T07:17:45Z
- Subjects: Geology , Geochemistry , Petrology , Mineralogy , Structural geology , Limpopo (South Africa)
- Type: Thesis
- Identifier: uj:14824 , http://hdl.handle.net/10210/1949
- Description: M.Sc. , This study has served to expand the geological map of surroundings of the Venetia Mine (Limpopo Province, South Africa) incorporating the area lying south of the kimberlite deposit and bounded in the south by the Dowe-Tokwe fault. The most significant structural conclusion stemming from this mapping project is that the Venetia Synform seems to be tectonically separate from the surrounding area and actually forms a klippe (shallowly dipping thrust) against the Krone Metamorphic terrane and the Gotha Complex. Petrographic descriptions of quartzofeldspathic lithologies found in the Krone Metamorphic Terrane to the west of the Venetia klippe (Mellonig, 2004) are identical suggesting that they belong to the Gotha igneous complex. There are no differences in geochemical compositions of monzogranite to granodiorite, tonalite and quartz diorite from Farms Gotha and Venetia. The rocks are I-type granitoids that generally form in continental magmatic arcs. The amount of U and Th in the igneous rocks of the Farms Gotha and Venetia (contained in minerals found within quartz, plagioclase, amphibole and K-feldspar crystal boundaries and the magmatic zircons of the Farm Gotha samples) and the pattern produced by heat producing elements (Council for Geoscience Radiogenic Map), indicate that that the unexpectedly high concentration of these elements are not the result of regional metamorphism, but is the remnant of the final crystallisation phase of the magma of the area. REE plots of the Venetia Mine samples show negative Eu anomalies, indicating the presence of plagioclase and K-feldspar in the magma source of the Venetia mine samples. The assumption is, that most samples retained their original chemical compositions having experienced only weak deuteric alteration and no dynamic metamorphism.
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The origin of the Kheis Terrane and its relationship with the Archean Kaapvaal Craton and the Grenvillian Namaqua province in Southern Africa
- Authors: Van Niekerk, Hermanus Stephanus
- Date: 2009-01-29T12:08:52Z
- Subjects: Groups (Stratigraphy) , Sedimentary rocks , Structural geology , Plate tectonics , Namaqualand (South Africa)
- Type: Thesis
- Identifier: uj:14852 , http://hdl.handle.net/10210/1974
- Description: D.Phil. , The tectonic history of the Kheis Terrane and its relationship with the Namaqua-Natal Metamorphic Province (NNMP) along the western margin of the Kaapvaal Craton were the focus of this study. Major issues addressed in this study are the origin and timing of formation of the Kheis Terrane and the recognition and definition of terrane boundaries in the area. Results of detailed measured sections across the Kheis Terrane, heavy mineral provenance studies, 40Ar/39Ar analyses of metamorphic muscovite, U-Pb SHRIMP dating of detrital zircon grains from 12 samples from the Kheis- and Kakamas Terranes and one igneous body from the Kakamas Terrane are presented. A new stratigraphic unit, the Keis Supergroup, comprising the Olifantshoek-, Groblershoop- and Wilgenhoutsdrif Groups, is defined. The base of the Keis Supergroup is taken at the basal conglomerate of the Neylan Formation. The Mapedi- and Lucknow Formations, previously considered part of the Olifantshoek Group, are now incorporated into the underlying Transvaal Supergroup. The Dabep Fault was found not to represent a terrane boundary. Rather, the Blackridge Thrust represents the boundary between the rocks of the Kheis Terrane and the Kaapvaal Craton. Provenance studies indicate that the rocks of the Keis Supergroup were deposited along a passive continental margin on the western side of the Kaapvaal-Zimbabwe Craton with the detritus derived from a cratonic interior. Detrital zircon grains from the rocks of the Keis Supergroup of the Kheis Terrane all gave similar detrital zircon age populations of ~1800Ma to ~2300Ma and ~2500Ma to ~2700Ma. The Kaapvaal Craton most probably never acted as a major source area for the rocks of the Keis Supergroup because of the lack of Paleo- to Mesoarchean zircon populations in the Keis Supergroup. Most of the detrital zircon grains incorporated into the Keis Supergroup were derived from the Magondi- and Limpopo Belts and the Zimbabwe Craton to the northeast of the Keis basin. The rock of the Kakamas Terrane was derived from a totally different source area with ages of ~1100Ma to ~1500Ma and ~1700Ma to ~1900Ma which were derived from the Richtersveld- and Bushmanland Terranes as well as the ~1166Ma old granitic gneisses ofthe Kakamas Terrane. Therefore the rocks of the Kheis- and Kakamas Terranes were separated from each other during their deposition. Detrital zircon populations from the Sprigg Formation indicate that it this unit was deposited after the amalgamation of the Kheis- and Kakamas Terranes and therefore does not belong to the Areachap Group. Results provide clear evidence for a tectonic model characterised by the presence of at least two Wilson cycles that affeected the western margin of the Kaapvaal Craton in the interval between the extrusion of the Hartley lavas at 1.93Ga and the collision with the Richtersveld tectonic domain at ~1.13Ga. According to the revised plate tectonic model for the western margin of the Kaapvaal- Zimbabwe Craton, the Neylan Formation represents the initiation of the first Wilson Cycle, with rifting at ~1927Ma ago, on the western margin of the Kaapvaal-Zimbabwe Craton. The metasedimentary rocks of the Olifantshoek Group were deposited in a braided river environment which gradually changed into a shallow marine environment towards the top of the Olifantshoek Group in the Top Dog Formation. The metasedimentary rocks of the Groblershoop Group were deposited in a shallow, passive or trailing continental margin on the western side of the Kaapvaal-Zimbabwe Craton. The rocks of the Wilgenhoutsdrif Group overlie the Groblershoop Group unconformably. This unconformity is related to crustal warping as a volcanic arc, represented by the metavolcanics of the Areachap Group, approached the Kaapvaal-Zimbabwe Craton from the west. The rocks of the Keis Supergroup were deformed into the Kheis Terrane during the collision of the Kaapvaal-Zimbabwe Craton, Areachap Arc and the Kgalagadi Terrane to form the Kaapvaal-Zimbabwe-Kgalagadi Craton. This event took place sometime between 1290Ma, the age of deformed granites in the Kheis Terrane and 1172Ma, the initiation of rifting represented by the Koras Group. This is supported by 40Ar/39Ar analyses of metamorphic muscovite from the Kheis Terrane that did not provide any evidence for a ~1.8Ga old Kheis orogeny (an age commonly suggested in the past for this orogeny). This collisional event resulted in the deformation of the rocks of the Keis Supergroup into the Kheis Terrane sometime between 1290Ma and 1172Ma.The second Wilson cycle was initiated during rifting along the Koras-Sinclair-Ghanzi rift on the Kaapvaal-Zimbabwe-Kgalagadi Craton at ~1172Ma. It was followed soon after by the initiation of subduction underneath the Richtersveld cratonic fragment at ~1166Ma after which the rocks of the Korannaland Group were deposited. The closure of the oceanic basin between the Kaapvaal-Zimbabwe-Kgalagadi Craton and the Richtersveld cratonic fragment occurred about 50Ma later (~1113Ma, the age of neomorphic muscovite in the metasedimentary rocks of the Kakamas Terrane) and resulted in the large open folds characterising the Kheis terrane and NNMP. Detrital zircon populations in the Sprigg Formation show that this formation does not belong to the Areachap Group and that it was deposited after the closure of the oceanic basin between the Kaapvaal-Zimbabwe-Kgalagadi Craton and the Richtersveld cratonic fragment at ~1113Ma. The Areachap Group can be extended towards the north and into Botswana along the Kalahari line where it forms the boundary between the Kaapvaal-Zimbabwe Craton to its east and the Kgalagadi Terrane to its west. The Areachap Terrane is thus related to the collision of the Kaapvaal-Zimbabwe Craton and Kgalagadi Terrane and was deformed a second time during the oblique collision of the Richtersveld cratonic fragment with the combined Kaapvaal-Zimbabwe-Kgalagadi Craton. The extension of the Areachap Group to the north along the Kalahari line opens up new exploration prospects for Coppertontype massive sulphide deposits underneath the Kalahari sand.
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Structure, stratigraphy and sedimentology of the paleoproterozoic Nsuta manganese deposit, Ghana
- Authors: Van Bart, Adrian
- Date: 2008-07-18T13:42:03Z
- Subjects: Manganese ores , Stratigraphic geology , Structural geology , Ghana
- Type: Thesis
- Identifier: uj:7367 , http://hdl.handle.net/10210/812
- Description: The Nsuta manganese deposit is located in the Western Region of Ghana, approximately five kilometers south of Tarkwa Goldfields. The deposit has been an important source of manganese ore since mining began in 1916. The purpose of this project was to produce a concise model of the stratigraphy, sedimentology and structural evolution of the deposit in support of future exploration projects. The manganese ores occur as an up to 45m thick carbonate bed in a thick turbidite-greenstone succession that is part of the ~2.2 Ga Birimian Supergroup. Calc-alkaline volcanics, volcaniclastics, turbidites, argillites and phyllites are thought to have been deposited in a backarc basin environment. The entire sedimentary succession, including the manganese orebody, is a thick turbidite package hosted between an upper and lower greenstone unit consisting predominantly of volcaniclastic material. The entire lithological succession at Nsuta is interpreted to have been deposited within the middle to lower reaches of a submarine fan environment. Field evidence suggests a simple stratigraphy, commencing with a lower greenstone unit composed largely of volcaniclastic material. This is followed by an upward-fining lower turbidite unit deposited in response to a marked transgression and sea level rise. Maximum rate of sea level rise provided ideal conditions for manganese precipitation and concentration, as detrital influx ceased. The central portion of the carbonate orebody that formed hosts the manganese orebody. An upward-coarsening turbidite unit follows above the carbonate unit. This upward-coarsening succession reflects a regression and a highstand systems tract in terms of sequence stratigraphic principles. It is capped by an unconformity that formed during a period of rapid relative sea level fall. It is overlain by a second upward-fining turbidite succession. This succession is not fully preserved as there is a sheared contact between it and the overlying upper greenstone unit. Post-depositional deformation and metamorphic alteration are largely attributed to the Paleoproterozoic Eburnean Orogeny. A first phase of compression was directed along a NW-SE axis and produced a series of isoclinal anticlines and synclines (F1) with NE-SW striking axial planes. This was followed by thrusting between the anticlines and synclines. The age of this deformation and closely associated greenschist metamorphism can be accurately constrained between 2.09 Ga and 2.07 Ga. E-W oriented oblique listric faulting has a prominent effect on the appearance of the Nsuta manganese deposit, as it produced a series of imbricate fault blocks dipping to the north. Associated with this period of deformation is small-scale cross folding with axes plunging to the east (F2). The faults post-date the Eburnean Orogeny and must be associated with a second major tectonic event. Finally, a NNE-SSW striking normal fault, locally known as the German Line, caused further block rotation, notably in the northern parts of the mining concession. Late Mesozoic deep lateritic weathering and incision of the lateritic peneplane by modern rivers have resulted in the complex dissected appearance of the Nsuta orebody. However, based on the detailed structural analysis provided in this study, a feasible target for future exploration of manganese ore buried beneath Late Mesozoic and Cenozoic sediments and soils, has been identified. This target is located to the west of Hills A and B. , Dr. J.M. Huizenga Prof. Nic Beukes Prof. J. Gutzmer
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Mesoproterozoic volcanism, metallogenesis and tectonic evolution along the western margin of the Kaapvaal Craton
- Authors: Bailie, Russell Hope
- Date: 2010-06-07T06:52:22Z
- Subjects: Geology , Volcanism , Geochemistry , Structural geology , Kaapvaal Craton (South Africa)
- Type: Thesis
- Identifier: uj:6866 , http://hdl.handle.net/10210/3298
- Description: D.Phil. , The western margin of the Archean Kaapvaal Craton, at its contact with the polydeformed and metamorphosed Proterozoic Namaqua Province, is host to four volcanosedimentary successions of Mesoproterozoic age (1.1-1.3 Ga) that occur in close spatial and temporal association to each other. These are the Areachap Group, the Leerkrans Formation of the Wilgenhoutsdrif Group and the two volcanosedimentary successions that comprise the Koras Group. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of these successions, particularly as they occur immediately adjacent to an important crustal suture. A comprehensive whole rock and isotope geochemical study, complemented by zircon-based geochronology where necessary, was thus carried out to characterize and compare the volcanic rocks associated with these four successions. The results are used to assess the role of the four volcanosedimentary successions during the development of the Mesoproterozoic suture between the Kaapvaal Craton and the Namaqua Province during the ~1.2-1.0 Ga Namaquan Orogeny. The geochemical study of the Areachap Group examined a suite of lithologies from different locations along the ~280km long outcrop belt, with the aim of testing the lateral continuity and integrity of this highly metamorphosed and deformed succession. As the bulk of the samples collected were from diamond drill core intersecting volcanogenic massive sulphide (VMS) Zn-Cu deposits it was only appropriate to extend the investigation to assess the metallogenesis and relation of these deposits to their host rock sequences. This included a survey of the sulphur isotope composition of sulphides and sulphates that comprise the Zn-Cu deposits. Furthermore, the architecture and origin of the world-class Copperton deposit, the largest Zn-Cu deposit of the Areachap Group, was examined. For this purpose, available literature data were collated and complemented by new geochemical and geochronological information. Sm-Nd isotopic systematics and U-Pb zircon ages suggest a coeval origin and close genetic link between the metavolcanic rocks of the Leerkrans Formation of the Wilgenhoutsdrif Group and the Areachap Group. Both successions record the establishment of an eastward-directed subduction zone on the western margin of the Kaapvaal Craton. The Areachap Group represents the highly metamorphosed and deformed remnants of a Mesoproterozoic (ca. 1.30-1.24 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton at ~1.22-1.20 Ga, during the early stages of the Namaquan Orogeny. The igneous protoliths within the Areachap Group are low- to medium-K tholeiitic to calc-alkaline in composition ranging in composition from basaltic through to rhyolitic. Tholeiitic basalts, represented by volumetrically minor amphibolites within the succession have Sm-Nd isotopic characteristics indicative of derivation from a depleted mantle source as denoted by their positive Nd(t) values. The lithogeochemical results highlight the fact that, despite differences in lithological architecture on a local scale, the Areachap Group exhibits coherent geochemical characteristics along its entire strike length.
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Structural-metamorphic studies of distinct fold types related to distinct tectono-metamorphic events in the central zone of the Limpopo Complex, South Africa
- Authors: Van Kal, Shaun Michael
- Date: 2009-01-28T09:43:40Z
- Subjects: Geology , Structural geology , Folds (Geology) , Metamorphism (Geology) , Petrology , Limpopo (South Africa)
- Type: Thesis
- Identifier: uj:14850 , http://hdl.handle.net/10210/1972
- Description: M.Sc. , The Central Zone of the Limpopo Complex displays two major structural features: the roughly east-west oriented Tshipise Straightening Zone Paleoproterozoic in age and a “Cross Folded Zone” to the north of the Straightening Zone comprising large-scale sheath and cross folds suggested to have developed during a Late- Archaean high grade tectono-metamorphic event. This study presents and discusses structural-metamorphic data showing that two closely associated folds (Ga-Tshanzi and Campbell) in the eastern part of the Cross Folded Zone near Musina, record different structural and metamorphic histories that may be applied to the evolution of the entire Central Zone of the Limpopo Complex. The Ga-Tshanzi structure has an ovate-shaped closed outcrop pattern approximately 4km long, and 3km wide with the long axis of the fold pattern oriented in a westerly direction. The fold geometry, characterized by a central fold axis that plunges steeply to the SSW, is very similar to other closed folds in the Central Zone previously interpreted as sheath folds. The Ga-Tshanzi fold deforms rocks of the Beit Bridge Complex (calc-silicate, metaquartzite, metapelite and magnetite quartzite and quartzofeldspathic Singelele Gneiss), and members of the Messina Layered Suite. The ovate structure is characterised by a gneissic fabric comprising peak metamorphic mineral assemblages. This regional gneissic fabric that occurs throughout the Central Zone also defines the shape of the neighbouring Campbell fold. Mineral lineations and fold hinges in the Ga-Tshanzi fold mainly present within metaquartzites and calc-silicates, plunge steeply to the southwest, parallel to its central fold axis indicating a NNE-SSW transport direction during fold formation. A decompression-cooling P-T path calculated for metapelitic gneisses from the Ga-Tshanzi fold shows that the closed fold developed under high-grade, deep crustal conditions. Peak P-T conditions of 7.5kbar/799ºC were followed by decompression and cooling down to 5.23kbar/605ºC. Water activity during this event was low, ranging from 0.122 at peak conditions, and decreasing to 0.037 at the minimum calculated conditions. The Ga-Tshanzi closed fold and the closely associated Campbell cross fold were thus formed at deep crustal levels and partially exhumed along a similar decompression-cooling P-T path to mid-crustal levels during the early orogenic event. The Campbell fold, described as a cross fold in the literature, is approximately 15km long and has a V shaped outcrop pattern that tapers from 12km in the southeast to 2 km in the northwest. This fold is developed in lithologies similar to those of the Ga-Tshanzi fold as well as in Sand River Gneisses. It has a near isoclinal fold geometry with both limbs dipping towards the southwest and a fold axis that plunges moderately to the west-southwest. This fold, that is interpreted to have developed during the same deformational event as the Ga-Tshansi structure has, however, subsequently been affected at mid- to upper crustal levels by shear movement along the Tshipise Straightening Zone displaying widespread development of younger planar and linear structural features. Planar features include north-south-trending high temperature shear zones that crosscut the regional fabric and flexural slip planes particularly evident in quartzites. Linear features from the Campbell fold that are mainly developed in younger shear and flexural slip planes, indicate, in contrast to the Ga-Tshanzi fold, an ENE-WSW directed crustal movement that is in accordance with the sense of movement suggested for the Tshipise Straightening Zone. The calculated decompression-cooling P-T path for sheared metapelitic gneisses from discrete high temperature shear zones deforming rocks of the Campbell cross fold shows that this superimposed shear deformational event occurred under peak P-T conditions of 4.98kbar/681ºC, followed by decompression and cooling down to 3.61kbar/585ºC. Water activity during this shear event was high, ranging from 0.217 at peak conditions and decreases to 0.117 at minimum calculated conditions. Structural and metamorphic data for the two folded areas thus indicate two distinct tectono-metamorphic events: (i) a late Archaean peak metamorphic and deformational event responsible for the formation of the Ga-Tshanzi fold, and similar folds throughout the Central Zone including the Campbell cross fold that was accompanied by steep NNE-SSW transport of crustal material, and (ii) a shear deformational event linked to the Paleoproterozoic Tshipise Straightening Zone that partially obliterated the early structural and metamorphic history of the Campbell fold during mid to upper crustal conditions during relatively shallow ENE-WSW directed movement of crustal material. The fact that this superimposed event had no apparent metamorphic effect on the studied metapelitic rocks of the closely associated Ga-Tshanzi closed fold, suggests that shearing was constrained to discrete north-south orientated zones.
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Provenance of ordovician to silurian clastic rocks of the Argentinean precordillera and its geotectonic implications
- Authors: Abre, Paulina
- Date: 2009-03-31T09:20:36Z
- Subjects: Structural geology , Stratigraphic geology , Historical geology , Argentina
- Type: Thesis
- Identifier: uj:8232 , http://hdl.handle.net/10210/2344
- Description: D. Phil. , A Mesoproterozoic basement and a Cambrian-Ordovician carbonate platform characterize the Precordillera terrane. These characteristics and its distinct geologic history mark a difference between this suspected exotic-to-Gondwana terrane and the Gondwanan autochthonous, leading to speculation that the Precordillera was derived from Laurentia. The surprising similarities of the carbonate sequences between the Precordillera and certain parts of southeast Laurentia suggest a common geological history. However, other models interpret the origin of the Precordillera terrane as being para-autochthonous with respect to Gondwana. All these models are still controversial. A combination of several methodologies including petrography and heavy minerals analysis, geochemistry, Sm-Nd and Pb-Pb isotopes and zircon dating were applied to several Ordovician and Ordovician to Silurian units of the Precordillera terrane. Geochemistry and petrography indicates that all the Formations studied have similar characteristics, with at least two sources providing detritus to the basin. The dominant source has an unrecycled upper continental crust composition whereas the other component is more depleted. The study of detrital chromian spinels suggests that mid-ocean ridge basalts, continental intraplate flood basalts and ocean island basaltrelated rocks were among the sources for the detrital record of the Precordillera terrane. Nevertheless, the mafic sources and their ages remain unknown. Nd isotopes account for negative εNd values and TDM ages in a range of variation found elsewhere within Gondwana and basement rocks of the Precordillera. The Sm/Nd ratios of certain samples indicate fractionation of LREE. Pb isotopes indicate that a source with high 207Pb/204Pb was important, and point to Gondwanan sources. Detrital zircon dating constrain the sources as being dominantly of Mesoproterozoic age (but with a main peak in the range 1.0 to 1.3 Ga), with less abundant populations of Neoproterozoic (with a main peak in the range 0.9 to 1.0 Ga), Palaeoproterozoic, Cambrian and Ordovician ages in order of abundance. i The uniformity shown by the provenance proxies indicate that there were no important changes in the provenance from the Lower Ordovician until the Early Silurian. Several areas are evaluated as sources for the Precordillera terrane. The rocks that fit best all the provenance constraints are found within the basement of the Precordillera terrane and the Western Pampeanas Ranges. Basement rocks from the Arequipa-Antofalla area (Central Andes) also match the isotopic characteristics, but a northern source is less probable, except for the Western tectofacies. On the other hand, areas such as Antarctica, Falklands/Malvinas Microplate, the Natal-Namaqua Metamorphic belt and the Grenville Province of Laurentia can be neglected as sources. The proposal of these areas as sources is in agreement with palaeocurrents and facies analyses and suggests proximity between them and the Precordillera since at least the Late Arenig to Early Llanvirn. This has important implications for the proposed models regarding the geotectonic evolution of the Precordillera terrane. The models would need to be adjusted to the here proposed youngest timing of collision.
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