Archean basement inliers within the Northern Highland terrane (NHT), Scottish Caledonides, have been correlated with the Lewisian Gneiss Complex of the Laurentian foreland. New zircon U-Pb ages indicate that the NHT basement contains evidence for magmatism at 2823–2687 Ma and 1772–1655 Ma. The first group compares with crystallization ages of the foreland Archean gneisses. However, the second group, and a supracrustal unit, formed ~100–250 m.y. after the youngest major phase of juvenile magmatism and sedimentation in the foreland. Also, there is no indication within the NHT basement of the Paleoproterozoic mafic and felsic intrusions common within the foreland, leading us to conclude that there is no firm basis for correlation of the two crustal blocks. The Caledonian Moine thrust, which separates the foreland and the NHT basement, is thought to have reworked a Grenvillian suture indicated by the presence of the ca. 1100–1000 Ma Eastern Glenelg eclogites. On the basis of the new isotopic data, we propose that the NHT basement was a fragment of Baltica that was emplaced onto Laurentia during the Grenvillian orogeny, representing a further example of basement terrane transfer in the circum–North Atlantic orogens.
The continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare >3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.
The enigmatic occurrence of Neoarchean- and Proterozoic-aged zircon grains in some samples of metasedimentary rocks from the Jack Hills, Western Australia, that otherwise appear to have been deposited before 3.0 Ga is explained by contamination with embedded zircon grains sourced from transported regolith. Zircon U-Pb age spectra obtained by laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of modern alluvium in creek beds draining the metasedimentary belts of Mount Narryer and Jack Hills are dominated by these exotically sourced younger components, as is the red soil that partly covers and infiltrates fractures in the exposed Hadean zircon-bearing bedrock at Jack Hills. The consistency of the age patterns found in superficial deposits sampled over some 100 km implies a dominance of colluvium transported from the wider Yilgarn craton, adjacent Capricorn orogen, and more distal sources rather than material produced from the local (Mesoarchean and older) bedrocks. The potential for similar cases of “environmental contamination” of surface rocks by exogenous zircons needs to be considered more widely, particularly in situations where interpreted maximum deposition ages are based on only a small number of outlying youngest analyzed grains.
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