The History of Granulite-Facies Metamorphism and Crustal Growth from Single Zircon U-Pb Geochronology: Namaqualand, South Africa

Robb, L. J.; Armstrong, Richard; Waters, David J.

doi:10.1093/petroj/40.12.1747

Cited by 137 publications

(69 citation statements)

References 26 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Omission of the analysis exhibiting most lead loss (spot 3.1) permits calculation of a normally weighted regression line with an upper intercept date of 1053 +29 / −25 Ma (MSWD = 1.75, probability of fit = 0.058). The lower intercept date of 245 +81 / −84 Ma is similar to ages expected for activity (Thomas et al, 1996;Robb et al, 1999) and of the 1.06-1.03 Ga Oribi Gorge Suite megacrystic granitoids in the Natal sector (Thomas et al, 1993). All spots analysed from sample WE 1/66 cluster close to concordia (Fig.…”

Section: Resultssupporting

confidence: 76%

“…The geochronological and isotopic evolution of crust in the Namaqua-Natal Belt is dominated by mantle extraction events at ∼2 and 1.4 Ga (Rogers and Hawkesworth, 1982;Barton, 1983;Eglington et al, 1989;Thomas and Eglington, 1990;Thomas et al, 1993;Clifford et al, 1995;Reid, 1997;Reid et al, 1997;Wareham et al, 1998;Robb et al, 1999; and references quoted therein). This two-stage evolution is well illustrated on plots of initial 87 Sr/ 86 Sr relative to date (Fig.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Geochronological and isotopic constraints on the Mesoproterozoic Namaqua–Natal Belt: evidence from deep borehole intersections in South Africa

Eglington

Armstrong

2003

Precambrian Research

View full text Add to dashboard Cite

Section: Resultssupporting

confidence: 76%

Section: Discussionmentioning

confidence: 99%

Geochronological and isotopic constraints on the Mesoproterozoic Namaqua–Natal Belt: evidence from deep borehole intersections in South Africa

Eglington

Armstrong

2003

Precambrian Research

View full text Add to dashboard Cite

“…The MRPSZ is localized in mid-crustal rocks of the Namaqua Metamorphic Complex that experienced high-temperature (T )-low-pressure (P ) amphiboliteto granulite-facies peak metamorphic conditions during the Mesoproterozoic at ca. 1200-1050 Ma (Jackson, 1976;Blignaut, 1977;Waters and Whales, 1984;Waters, 1986Waters, , 1988Robb et al, 1999;Clifford et al, 2004;Cornell et al, 2009;Diener et al, 2013;Bial et al, 2015). The development of the MPRSZ post-dates peak metamorphism, with deformation having occurred under retrograde amphibolite-to greenschist-facies conditions, at ca.…”

Section: Regional and Outcrop Geologymentioning

confidence: 99%

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

2016

View full text Add to dashboard Cite

Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks-Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40 Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the P -T -fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the P -T conditions of deformation at 2.7-4.2 kbar and 450-480 • C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T -M H 2 O diagrams show that the mineral assemblages were fluid saturated and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localized in strongly dehydrated granulites, the fluid must have been derived from an external source, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of pervasive hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that, following initiation, fluid infiltration caused a positive feedback that allowed weakening and continued strain localization. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain-size reduction. Features such as tectonic tremor, which are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

show abstract

“…Moore et al, 1986;Sangster et al, 1992;Thomas et al, 1994;Robb et al, 1999). However, no geophysical comparisons have been made because until recently, no complementary seismic profiles have been available.…”

Section: Rodinian Connectionsmentioning

confidence: 99%

Deep Crustal Profile Across the Southern Karoo Basin and Beattie Magnetic Anomaly, South Africa: An Integrated Interpretation With Tectonic Implications

Lindeque

Wit

Ryberg

et al. 2011

South African Journal of Geology

View full text Add to dashboard Cite

Two geophysical onshore-offshore lines on the southern margin of Africa form the Agulhas-Karoo Geophysical Transect (AKGT) and cross prominent geological features such as the Karoo Basin, Cape Fold Belt (CFB) and the Beattie Magnetic Anomaly (BMA). Geophysical data acquired along this AKGTransect between 2004 and 2007 within the Inkaba yeAfrica (lyA) framework, provide the platform for constructing a deep crustal section (IyA-200501) for the centre 100 km of the western AKGT-transect in order to resolve these features at depth. We present a detailed deep crustal model constructed from the joint interpretation of: i. archive data comprising surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions and regional seismic reflection profiles, and ii. line coincident newly acquired high-resolution geophysical data consisting of near vertical seismic reflection data, shallow P-and S-wave velocity data, wide-angle refraction data, high resolution magnetotelluric data and impedance spectroscopy measurements on borehole samples. Our model differentiates four components in the up to 45 km thick crust: 1. a ~2 to 5 km thick folded Karoo Supergroup, disrupted by low-angle thrust faults rooted in a zone of local décollements in the lower Ecca Group and resting paraconformably on 2. a continuous undeformed sub-horizontal ~1.5 to 10 km thick wedge of the Cape Supergroup (CSG). This CSG wedge stretches from the Escarpment in the north to the tectonic front of the CFB in the south, and rests on an unconformity that dips about three degrees to the south. The angular unconformity is interpreted as an erosional peneplain that separates the CSG wedge from component 3. the ~13 to 21 km mid-crust basement below. The mid-crust contains a distinct north-dipping seismic fabric, here interpreted as 1.4 to 1.0 Ga Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) crust. A south-dipping mid-crustal detachment, interpreted as a ductile thrust zone, separates the mid-crust from component 4. a highly reflective ~10 to 24 km thick lower crust. The latter is interpreted as an older Palaeoproterozoic section of the NNMB (or even Archean cratonic basement), and bounded by a ~2 to 5 km thick, highly reflective bottom layer below that lies subparallel to a clear Moho. This bottom layer is interpreted as a mafic underplate, metasomatic reaction zone, or lower-crust to mantle transition zone. Collectively the seismic reflection and wide-angle refraction data support an interpretation that the NNMB mid-crustal layer contains the BMA source, possibly connected to two zones of strong reflectivity: a ~10 to 12 km wide northern zone and a ~5 to 7 km wide southern zone, both about 5 km thick and 7 to 8 km below surface. We interpret the BMA source to be at least in part, a Namaqua-like massive to disseminated, deformed/metamorphosed stratiform sulphide-magnetite ore body with metasomatic overprint.

show abstract

The History of Granulite-Facies Metamorphism and Crustal Growth from Single Zircon U-Pb Geochronology: Namaqualand, South Africa

Cited by 137 publications

References 26 publications

Geochronological and isotopic constraints on the Mesoproterozoic Namaqua–Natal Belt: evidence from deep borehole intersections in South Africa

Geochronological and isotopic constraints on the Mesoproterozoic Namaqua–Natal Belt: evidence from deep borehole intersections in South Africa

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

Deep Crustal Profile Across the Southern Karoo Basin and Beattie Magnetic Anomaly, South Africa: An Integrated Interpretation With Tectonic Implications

Contact Info

Product

Resources

About