TTG Gneisses Research Articles

An Sm-Nd isotopic dating study of the Archean Kongling Complex in the Huangling area of the Yangtze Craton

Sm-Nd isotopic compositions of associated amphibolites, TTG gneisses and granitic gneisses from the Archean Kongling Complex of the Yangtze Craton, South China are systematically analyzed. The results show that the amphibolites and TTG gneisses have a similar age of ca. 2 700 Ma, which well agrees with zircon U-Pb dating of previous studies. However, their eND(t) are different, suggestive of possible dissimilarity in source composition.

Ion microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland

We present ion microprobe U-Pb zircon geochronology for three samples from the late Archaean Lewisian gneiss complex at Gruinard Bay, northwestern Scotland, in an attempt to resolve the discrepancy between Sm-Nd and Pb-Pb isochron ages for trondhjemite and hornblendite components. Whole-rock samples imply ca. 2.8 Ga. ages for both components (Whitehouse et al., 1996), but separated minerals have yielded ca. 2.4–2.5 Ga for trondhjemite and 3.3 Ga for hornblendite (Burton et. al., 1994). Zircons separated from three representative lithologies at Gruinard Bay (trondhjemite, hornblendite, and banded tonalite) yield concordant or near concordant U-Pb data with 207Pb206Pb ages mostly between ca. 2.85 Ga and ca. 2.75 Ga, and a few outliers at ca. 2.65 Ga. Backscattered electron images of zircons from the trondhjemite allow differentiation between ca. 2.85 Ga cores and ca. 2.75 Ga overgrowths; the latter are interpreted as igneous on the basis of field relationships of the trondhjemite to the other samples. Morphology of the hornblendite zircons is more consistent with crystallisation under metamorphic rather than igneous conditions at ca. 2.8 Ga, although the previous arguments of Whitehouse et al. (1996) for a protolith age not greatly in excess of this remain valid. The new zircon data suggest that magmatism at Gruinard Bay occurred over a relatively short time interval, in accord with the previous observations from whole-rock isotopic data. There is no evidence from the analysed zircons for ca. 2.4–2.5 Ga or 3.3 Ga magmatic ages in the trondhjemite and hornblendite, respectively. The mineral isochron ages for the trondhjemite probably represent metamorphic resetting related to the regional ca. 2.5 Ga high-grade event recorded in the central region. For the hornblendite, the mineral Sm-Nd ages are confirmed as spurious, resulting from post-crystallisation open-system isotope behaviour on the mineral scale. Regional comparison of ages within the Lewisian Complex suggests that the ≤5 ca. 2.85 Ga Gruinard Bay TTG gneisses might have been amalgamated with ≥ 2.96 Ga gneisses of the central region before or during a ca. 2.76 Ga event and subsequently have experienced a similar geological evolution.

Geochemistry and single zircon geochronology of Archaean TTG gneisses in the Taihua high-grade terrain, Lushan area, central China

Gneisses of tonalite-trondhjemite-granodiorite (TTG) composition, typical rocks found in Archaean terrains all over the world, also occur in the Taihua high-grade terrain of the Lushan area, central China. These rocks underwent high-amphibolite to granulite facies metamorphism and intense deformation. Their petrology, geochemistry and single zircon geochronology suggests that they are derived from granitoid plutons which were emplaced 2806 ± 7 Ma, and 2841 ± 6 Ma, respectively. Geochemically, the gneisses show the calc-alkaline trend and have strongly fractionated rare earth element (REE) patterns. We suggest that the original granitoids formed through partial melting of garnet-granulite which is now represented by inclusions within the gneisses.