Isotopic data are used to determine a new chronology for the Torridonian of NW Scotland. The calcite fraction of the Stoer Group Limestone Member yields a Pb–Pb age of 1199 ± 70 (2 σ ) Ma (MSWD = 1.1). Rb–Sr whole-rock regressions from siltstones of the Diabaig and Applecross Formations (both Torridon Group) produce ages of 994 ± 48 (2 σ ) Ma (MSWD = 17) and 977 ± 39 (2 σ ) Ma (MSWD = 10) respectively. These dates, which agree well with palaeomagnetic estimates, are interpreted as the time of early diagenesis which is probably within error of the time of deposition. Previous isotopic age determinations are shown to be in error. Size fractionsfrom shales of the Stoer Group and Applecross Formation have been analysed for Rb–Sr and produce ages of 689 ± 87 (2 σ ) Ma (MSWD = 14) and 667 ± 66 (2 σ ) Ma (MSWD = 55) respectively. They are used, together with Sm-Nd isotopic data, to suggest a thermal event affecting the entire Torridonian at this time. Our new chronology for the Torridonian is consistent with the Proterozoic geology of Scotland and Laurentia. It suggests that deposition of the Stoer Group is contemporaneous with uplift of the Lewisian and the Torridon Group may be associated with the extensional collapse of the Grenville orogen. The c. 700Ma diagenetic event is possibly related to Neoproterozoic rifting of the Laurentian margin.
Irreversible chemical differentiation of the mantle’s essentially infinite reservoir for at least the past 3800 Ma has produced new continental, sialic crust during several relatively short ( ca . 100-300 Ma) episodes which were widely separated in time and may have been of global extent. During each episode (termed 'accretion—differentiation superevent’), juvenile sial underwent profound igneous, metamorphic and geochemical differentiation, resulting in thick ( ca . 25-40 km), stable, compositionally gradational, largely indestructible, continental crust exhibiting close grouping of isotopic ages of rock formation, as well as mantle-type initial Sr and Pb isotopic ratios for all major constituents. Isotopic evidence suggests that within most accretiondifferentiation superevents — and especially during the earlier ones - continental growth predominated over reworking of older sialic crust. Reworking of older sialic crust can occur in several types of geological environment and appears to have become more prevalent with the passage of geological time. It is usually clearly distinguishable from continental growth, by application of appropriate age and isotope data.
Augustine Volcano, a Quaternary volcanic centre of the eastern Aleutian Arc, produces predominantly andesites and dacites of low- to medium-K calc-alkaline composition. Mineralogical and major element characteristics of representative lavas suggest that magmatic evolution has been influenced by both crystal fractionation and magma-mixing processes. However, incompatible trace element variations (e.g. K/Rb) indicate that these evolved lavas have been contaminated by the mafic arc crust of the underlying Talkeetna accreted terrane. The limited range of isotope compositions also supports the assimilation of non-radiogenic mafic crust (e.g. 87Sr/86Sr = 0.7032−0.7034; 143Nd/144 Nd = 0.51301−0.5130). In addition, Pb-isotope compositions parallel the North Pacific mean oceanic trend (206Pb/204 Pb = 18.3−18.8; 207Pb/204Pb = 15.5−15.6; 208Pb/204Pb = 38.2−38.3) and do not require a subducted sediment component in the source. Relatively high (Ba/La) N (0.79−18.10) and B/Be (14.5) ratios do, however, suggest a metasomatic fluid component derived from the dehydration of the subducting plate.
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