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Box Truck Rental is Helpful Box Truck Rental Customers Composition and evolution of the proterozoic Vioolsdrif batholith (including the Orange River group), Northern Cape province, South Africa
English: The Vioolsdrif Suite and Orange River Group represent genetically related calcalkaline plutonic and volcano-sedimentary assemblages of Palaeoproterozoic age formed during the Orange River orogeny. Together they occupy the largest part of the Richtersveld Subprovince – a unique tectono-stratigraphic terrane. Radiometric data indicate the period of formation roughly between 2.0-1.73 Ga. The subprovince has been vastly eroded and isolated from its original tectonic environment by subsequent tectonic processes, leaving a relatively small portion of its original extent for investigation. Previous studies have dealt with limited parts of the subprovince and although informal subdivisions of the Orange River Group and Vioolsdrif Suite are generally in use, some correlations and further subdivisions remained controversial. This study has two main aims, viz., to propose formal subdivisions of the two units and to investigate the magmatic processes and original tectonic environment of their formation. Geochemical evidence is presented here to support the proposed subdivisions, which were previously based entirely on field evidence. The subdivisions largely follow that of previous studies. The Orange River Group is subdivided into the Haib and De Hoop Subgroups. Geochemical evidence show that these two subgroups differ in the magmatic processes that led to their formation. The Haib Subgroup shows a genetic gradational relationship with the Vuurdood Subsuite, which is regarded as remnants of the primary magmas. The De Hoop Subgroup does not display this relationship with the Vuurdood Subsuite. The Vioolsdrif Suite is subdivided into the Vuurdood, Goodhouse and Ramansdrif Subsuites. Mafic-ultramafic bodies of the Vuurdood Subsuite are regarded as remnants of the primary magmas. This is based on multi-element variation diagram patterns and comparison to source magmas in modern island arcs (MORB). Previous studies have also shown that initial isotope ratios for the Vuurdood Subsuite are similar to those of the Goodhouse Subsuite and Orange River Group, relating them to a similar source. Dark mineral cumulate material are contained in the mafic-ultramafic bodies. The Goodhouse Subsuite is subdivided into the Khoromus Tonalite, Blockwerf Migmatite, Xaminxaip River Granodiorite, Gaarseep Granodiorite and Hoogoor Granite. The Khoromus Tonalite is identified as the oldest unit within the subsuite based on contact relationships as well as radiometric data, which continuously render older ages for the Khoromus Tonalite compared to the other units within the subsuite. Certain field and petrological observations in this unit may be interpreted as products of magma mixing processes. The Blockwerf Migmatite is distinguished from the other units by its migmatitic character and anomalous La/Yb ratios. The unit is identified as a possible remnant volcanic centre. The Xaminxaip River Granodiorite is interpreted as a subvolcanic unit and is characterised by the development of migmatite in places. This migmatite is attributed to metamorphic conditions which locally reached high grade in an orogenic geothermal regime. The Gaarseep Granodiorite represents the main phase of the Vioolsdrif Suite. Its compositional range includes all those represented by the other units individually from gabbro through diorite, tonalite and granodiorite to granite. Its development spans almost the entire evolutionary history of the Richtersveld Subprovince as a whole. The Hoogoor Granite is included in the Goodhouse Subsuite based on the geochemical variation patterns and available radiometric evidence. The Ramansdrif Subsuite is subdivided into the Ghaams and Sout Granites based on grain size variation and petrological evidence. The subsuite could have been formed by partial melting of the older plutonic phases of the Vioolsdrif Suite. The deformation associated with this partial melting event has not been identified. Previous studies have related the evolution of the Richtersveld Subprovince to modern subduction zone magmatism similar to that of the Andean volcanic arc. This is largely supported by the current study, however, a change in magmatic processes and the tectonic environment can be observed in the geochemical variation patterns. Multielement diagrams show patterns typical of subduction zone magmatism for both the Orange River Group and Vioolsdrif Suite. During the initial stages, primary magmas, now represented by the Vuurdood Subsuite, were derived from a depleted mantle reservoir. The first volcanic eruptions – those of the Haib Subgroup – represent fractional crystallization products off the primary magmas. With progressive development of the arc, newly formed crust was continuously recycled back into the mantle and crustal partial melting led to a change in magmatic processes with magma mixing and contamination becoming increasingly important. Tectonic discrimination diagrams suggest that the initial stages of the batholith development may be compared to a primitive continental arc, while the later stages may be be compared to a typical continental arc.
Afrikaans: Die Vioolsdrif Suite en Oranjerivier Groep verteenwoordig Palaeoproterosoïese kalkalkaliese plutoniese en vulkanies-sedimentêre opeenvolgings wat gevorm is tydens die Oranjerivier orogenese. Tesame beslaan hulle die grootste deel van die Richtersveld Subprovinsie – ‘n unieke tektonies-stratigrafiese terrein. Volgens radiometriese data was die tydperk van ontwikkeling tussen ongeveer 2.0-1.73 Ga. Die subprovinsie is grootliks verweer en latere tektoniese prosesse het dit van die oorspronklike tektoniese terrein geïsoleer sodat ‘n relatiewe klein gedeelte vir bestudering oorgebly het. Vorige studies het slegs oor beperkte gedeeltes van die subprovinsie gehandel. Alhoewel informele onderverdelings van die Oranjerivier Groep en Vioolsdrif Suite algemeen in gebruik is, is sekere korrelasies en verdere onderverdelings nog kontroversieël. Die doel van die huidige studie is veral tweërlei nl., om algemeen-aanvaarde onderverdelings van die twee eenhede voor te stel en om die tektonies-magmatiese prosesse waardeur hulle ontwikkel het te ondersoek. Geochemiese data word hier aangebied om die onderverdeling, wat voorheen geheel en al op veldverwantskappe berus het, te steun. Die voorgestelde onderverdeling wyk nie noemenswaardig af van dié in vorige studies nie. Die Oranjerivier Groep word onderverdeel in die Haib en De Hoop Subgroepe. Geochemiese data dui daarop dat dié twee subgroepe deur verskillende magmatiese prosesse ontstaan het en dat die tektoniese terreine waarin hulle ontstaan het nie heeltemal dieselfde was nie. Die Vioolsdrif Suite word onderverdeel in die Vuurdood, Goodhouse en Ramansdrif Subsuites. Die mafies-ultramafiese liggame van die Vuurdood Subsuite word beskou as oorblyfsels van die primêre magmas en dit bevat ook kumulate van donkerminerale. Die Vuurdood Subsuite toon ‘n genetiese verwantskap met die Haib Subgroep maar nie met die De Hoop Subgroep nie. Die verwantskap tussen die Vuurdood Subsuite en die Haib Subgroep kan moontlik die gevolg van fraksionele kristallisasie wees. Die Goodhouse Subsuite word onderverdeel in die Khoromus Tonaliet, Blockwerf Migmatiet, Xaminxaiprivier Granodioriet, Gaarseep Granodioriet en Hoogoor Graniet. Die Khoromus Tonaliet word beskou as die oudste eenheid in die subsuite op grond van kontakverwantskappe sowel as radiometriese data. Daar is veld- en petrografiese getuienisse wat daarop dui dat magma vermengingsprosesse ‘n rol gespeel het in die vorming van hierdie eenheid. Die Blockwerf Migmatiet word van die ander eenhede onderskei deur sy migmatitiese karakter en anomale La/Yb verhoudings. Die eenheid word geïdentifiseer as ‘n moontlike vulkaankeël-oofblyfsel. Die Xaminxaiprivier Granodioriet word geïnterpreteer as ‘n subvulkaniese eenheid en word gekenmerk deur die ontwikkeling van migmatiet op plekke. Hierdie migmatiet word geïnterpreteer as die produk van lokale hoëgraadse metamorfe toestande wat geheers het in die orogene subduksieregime. Die Gaarseep Granodioriet is die hooffase van ontwikkeling van die Vioolsdrif Suite. Dit oorheers in volume en oorvleuel met feitlik die hele vormingsgeskiedenis van die Richtersveld Subprovinsie. Dit bevat die volledige samestellingsreeks wat in die ander plutoniese eenhede individueel verteenwoordig word van gabbro deur dioriet, tonaliet en granodioriet tot graniet. Die Hoogoor Graniet word in die Goodhouse Subsuite ingesluit op grond van geochemiese variasiepatrone en beskikbare radiometriese data. Die Ramansdrif Subsuite word onderverdeel in die Ghaams en Sout Graniete op grond van korrelgroottevariasies en petrologiese getuienis. Die subsuite is skynbaar gevorm deur gedeeltelike insmelting van die ouer Goodhouse Subsuite plutoniese eenhede. Die vervormingsepsiode wat met hierdie gedeeltelike insmelting gepaard sou gaan, is nie geïdentifiseer nie. Vorige studies vergelyk die ontwikkeling van die Richtersveld Subprovinsie met tektoniesmagmatiese prosesse wat werksaam is in moderne vulkaanboë soos die Andesboog. Hierdie vergelyking word grootliks deur die huidige studie ondersteun maar ‘n verandering in die magmatiese prosesse en die tektoniese omgewing gedurende die ontwikkelingsgeskiedenis is waarneembaar in die geochemiese data. Multi-element diagramme vertoon tipiese subduksiesone variasiepatrone vir beide die Oranjeriviergroep en die Vioolsdrif Suite. Gedurende die aanvangstadium is primêre magmas gegenereer in die mantel, wat ‘n verarmde samestelling gehad het. Die Vuurdood Subsuite verteenwoordig oorblyfsels van die primêre magmas. Die eerste vulkaniese lawas – dié van die Haib Subgroep – was produkte van fraksionele kristallisasie vanaf die primêre magmas. Met die geleidelike ontwikkeling van die vulkaanboog is nuwe kors in groter-wordende volumes geproduseer. Hierdie kors is voortdurend in die mantel herwin en gedeeltelike insmelting het daartoe gelei dat prosesse soos magma vermenging en kontaminasie belangriker geword het as fraksionele kristallisasie in die bepaling van die magmasamestellings. Tektoniese diskriminasiediagramme dui daarop dat die aanvangstadiums van die batoliet se ontwikkeling moontlik plaasgevind het in ‘n primitiewe kontinentale boog, terwyl die latere stadiums in ‘n gewone kontinentale boog plaasgevind het.
Doctoral Degrees (Geology)
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