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GeoRef Categories
Era and Period
Epoch and Age
Book Series
Date
Availability
In situ Pb–Pb garnet geochronology as a tool for investigating polymetamorphism: a case for Paleoarchean lateral tectonic thickening Open Access
Abstract The Barberton Granite–Greenstone Belt remains a key location in the debate concerning the nature of Archean tectonic processes. Much work has focused on deciphering the tectonic significance of the c. 3.23 Ga metamorphism, as this has been correlated with lower geothermal gradient conditions potentially indicating Archean subduction. However, several studies also found evidence of an earlier, 3.45 Ga metamorphic episode, overprinted by the 3.23 Ga event. Here we apply in situ Pb–Pb dating and P–T modelling to a large (3 cm diameter) garnet crystal, allowing for the direct dating of the metamorphic conditions obtained from the garnet. The garnet core produced an isochron age of 3435 ± 45 Ma, corresponding to an increase in P and T evolution reaching peak conditions of at least 7 kbar and 700°C. Analyses obtained from the garnet rim give an isochron age of 3245 ± 41 Ma, corresponding to P–T conditions reaching 8–9 kbar and 700°C. The preservation of two moderate- to high-pressure events occurring 200 million years apart is consistent with lateral tectonic processes producing crustal thickening at 3.2 Ga and may also be a viable process for the earlier event.
S- to I- to A-type magmatic cycles in granitic terranes are not globally recurring progressions. The cases of the Cape Granite Suite of Southern Africa and central Victoria in southeastern Australia Available to Purchase
De Kraalen and Witrivier Greenstone Belts, Kaapvaal Craton, South Africa: Characterisation of the Palaeo-Mesoarchaean evolution by rutile and zircon U-Pb geochronology combined with Hf isotopes Available to Purchase
Metamorphic evolution for the Inyoni shear zone: Investigating the geodynamic evolution of a 3.20 Ga terrane boundary in the Barberton granitoid greenstone terrane, South Africa Available to Purchase
Magmatic clasts in the Saldanha ignimbrites, and Trekoskraal beach pebbles: missing pieces from the volcanic puzzle in the Cape Granite Suite Available to Purchase
The Saldanha Bay Volcanic Complex: Clarifying the Cambrian geology of the Postberg - Saldanha Area, West Coast, South Africa Available to Purchase
AGE AND FIELD RELATIONSHIPS OF MAHAMBA ORTHOGNEISSES AND MKHONDO VALLEY METAMORPHIC SUITE PARAGNEISSES FROM THE MKHONDO RIVER, ANCIENT GNEISS COMPLEX, SWAZILAND. Available to Purchase
Successive midcrustal, high-grade metamorphic events provide insight into Mid-Archean mountain-building along the SE margin of the proto–Kaapvaal craton Available to Purchase
Radiation and extinction patterns in Permian floras from North China as indicators for environmental and climate change Available to Purchase
Chemical structure in granitic magmas – a signal from the source? Available to Purchase
Though typically exhibiting considerable scatter, geochemical variations in granitic plutons and silicic volcanic deposits are commonly modelled as products of differentiation of originally homogeneous magmas. However, many silicic igneous bodies, particularly those classified as S-types, are internally heterogeneous in their mineralogy, geochemistry and isotope ratios, on scales from hundreds of metres down to one metre or less. The preservation of these heterogeneities supports recent models for the construction of granitic magma bodies through incremental additions of numerous batches (pulses) of magma derived from contrasting sources. Such pulses result from the sequential nature of the melting reactions and the commonly layered structure of crustal magma sources. Internal differentiation of these batches occurs, but not generally on the scales of whole magma chambers. Rather than being created through differentiation or hybridisation processes, at or near emplacement levels, much of the variation within such bodies (e.g. trace-element or Mg# variation with SiO 2 or isotope ratios) is a primary or near-source feature. At emplacement levels, the relatively high magma viscosities and slow diffusion rates of many chemical components in silicic melts probably inhibit processes that would lead to homogenisation. This permits at least partial preservation of the primary heterogeneities.