Academic literature on the topic 'Barberton Granite-Greenstone Terrain'

Abstract We describe the multi-scale distribution of K, Th and U in the ca. 3.1 Ga Heerenveen batholith of the Barberton Granite-Greenstone Terrain. Data were obtained with a combination of tools, including a portable gamma-ray spectrometer from the scale of the whole batholith to the scale of outcrops, and autoradiography for the thin section scale. U is concentrated preferentially in minor phases in the border shear zones of the batholith and, within these shear zones, in late pegmatites as well as fractures. The processes responsible for the concentration of U in the Heerenveen batholith is discussed in terms of magmatism, hydrothermalism (redistribution of U in fissures associated with magmato-hydrothermal fluids), and supergene alteration. The statistical properties of K, Th and U concentrations are different. K shows spatial correlation over large distance, largely mirroring mappable rock types, with increased variability at larger scales. In contrast, U is dominated by small-scale variations (“nugget effect”) and its variability is, averaged and smoothed by large-scale integration. Spatial and statistical features thus offer useful and complementary insights on petrogenetic and metallogenic processes in granitoids in addition to standard approaches (petrography, geochemistry).

Abstract The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates. This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism.

Thesis (MSc)--Stellenbosch University, 2014
ENGLISH ABSTRACT: The compositions of tonalite-trondhjemite-granodiorite (TTG)-series rocks from the Barberton Granite-Greenstone Terrain (BGGT) of Southern Africa vary in terms of major element concentrations (leuco-trondhjemites to tonalites), trace elements (e.g. Sr, Rb, REE) and degree of HREE depletion [(La/Yb)N ~ 0.50 - 121]. These geochemical parameters have been interpreted to have significance for the geodynamic processes that produced TTG magmas, particularly the depth of the source. In the Nelshoogte pluton, trondhjemite emplacement occurred prior to and during (D2)-tectonism, and hornblende-bearing tonalites/granodiorites occur as intrusive plugs within the trondhjemite. The geochemical range portrayed by the ca. 3230 Ma Nelshoogte pluton covers almost the whole range portrayed by all the TTG rocks of the BGGT, formed in the time interval 3550 to 3230 Ma. U-Pb zircon dating reveals no significant differences in apparent ages between the trondhjemites and tonalites of the Nelshoogte pluton; all lie between ca. 3240 and 3220 Ma. A/CNK- and Ti-maficity correlations within the Nelshoogte pluton show behaviour typical of I-type granitic rocks, confirming the metamafic character of the source and the involvement of a peritectic garnet and clinopyroxene component in the magmas. The REE and traceelement patterns resemble those of both ca. 3450 Ma TTG plutons and other ~ 3230 Ma TTG plutons in the BGGT. This suggests that these different generations of TTG magmas were generated from similar sources and by similar processes. The geochemical details are consistent with a metamafic source which underwent melting at high pressure. This TTG source was fundamentally different from the more potassic source/s that produced the younger 3100 Ma granites. Additionally, high Zr/Sm and low Nb/Ta ratios in the Barberton TTG rocks suggest that the Nelshoogte pluton originated as a group of chemically distinct magmas, in a similar way to other ~ 3200 Ma TTGs. The protoliths of ~ 3450 Ma plutons and ~ 3200 Ma plutons might typically be rutile-bearing eclogite and/or 10 to 30 % garnet bearing amphibolite. The geochemical features of the Barberton TTGs suggest similar source compositions, but different pressures of partial melting. For the ca. 3230 Ma plutons, the rocks of the Badplaas pluton and the Nelshoogte trondhjemites/tonalites correspond to the high-pressure magmas, as reflected by high Sr content, as well as high Sr/Y and (La/Yb)N ratios. In contrast, the Kaap Valley TTGs and Nelshoogte granodiorites reflect the melting of a similar source at moderate pressure. Furthermore, the Hf isotope data; Hft (+ 0.1 to + 1.9) and TDM model ages (3330 - 3230 Ma), suggest that the source rocks are ~ 20 to 100 Myr older than the pluton.
AFRIKAANSE OPSOMMING: Die samestellings van tonaliet-trondhjemiet-granodioriet (TTG)-reeks gesteentes vanaf die Baberton Graniet-Groensteen terrien (BGGT) van Suiderlike Afrika verskil in terme van hoofelement konsentrasies (leuko-trondhjemiete tot tonaliete), spoorelemente (bv. Sr, Rb en seldsameaarde-elemente(SAE)) en ook van swaar seldsameaarde-element (SSAE) uitputting [(La/Yb)N ~ 0.50 - 121]. Deur interpritasie is daar vasgestel dat hierdie geochemiese beperkings betekenis dra vir die geodinamiese prosesse wat TTG magmas produseer het, en veral vir die diepte van die bron. In die Nelshoogte pluton het trondhjemiet inplasing voor en gedurende (D2)-tektonisme voorgekom en horingblende-draende tonaliet/granodioriet kom voor as intrusiewe proppe binne-in die trondhjemiet. Die geochemiese reeks wat uitgebeeld is deur die ~ 3230 Ma Nelshoogte pluton ondersteun byna die hele reeks wat uitgebeeld is deur al die TTG gesteentes van die BBGT wat gevorm het gedurende die tydsinterval 3550 tot 3230 Ma. U-Pb sirkoon datering openbaar geen merkwaardige verskille in ouderdomme tussen die trondhjemiete en tonaliete van die Nelshoogte pluton nie. Al die ouderdomme lê tussen 3240 en 3220 Ma. A/CNK- en Ti-mafisiteit korrelasies binne die Nelshoogte pluton toon die tipiese gedrag van I-tipe granitiese gesteentes, en bevestig sodoende die metamafiese karakter van die bron, asook die betrokkenheid van ‘n peritektiese granaat en klinopirokseen component in die magmas. Die seldsameaarde-element en spoorelement patroone lyk soos dié van albei ~ 3450 Ma plutons en ander ~ 3230 Ma TTG plutons in die BGGT. Dit stel voor dat hierdie verskillende generasies van TTG magmas genereer was vanaf soortgelyke bronne en ook deur soortgelyke prosesse. Die geochemiese besonderhede stem ooreen met ‘n metamafiese bron wat smelting teen hoë druk ondergaan het. Hierdie TTG bron het fondamenteel verskil van die meer kalium-ryke bron/ne wat die jonger 3100 Ma graniete prodiseer het. In ‘n eenderse manier as ander ~ 3200 Ma TTG gesteentes, stel hoë Zr/Sm en lae Nb/Ta verhoudings in die Baberton TTG gesteentes verder voor dat die Nelshoogte pluton ontstaan het as ‘n groep chemies verskillende magmas. Die protoliete van ~ 3450 Ma plutons en ~ 3200 Ma plutons mag dalk tipies rutiel-draende eklogiet en/of 10 tot 30% granaat draende amfiboliet wees. Die geochemiese aspekte van die Baberton TTGs stel eenderse bron komposisies, maar verskillende drukke van gedeeltelike smelting voor. Vir die ~ 3230 Ma plutons stem die gesteentes van die Badplaas pluton en die Nelshoogte trondhjemiete/tonaliete ooreen met die hoë druk magmas, soos weerspieël deur hoe Sr inhoud, sowel as hoe Sr/Y en (La/Yb)N verhoudinge. In kontras weerspiel die Kaap Valley TTGs en Nelshoogte granodioriet die smelting van ‘n eenderse bron teen matige druk. Verder stel die Hf isotoop data; Hft (+ 0.1 to + 1.9) en TDM model ouderdomme (3330 - 3230 Ma) voor dat die bron gesteentes ~ 20 to 100 Mjr ouer is as die pluton.