Nd,Sr, Pb,Ar, and 0 isotopic systematics of Sturgeon Lake kimberlite,Saskatchewan, Canada: constraints on emplacement age,alteration, and source composition

Rb-Sr isotopic dating of phlogopite megacryst samples separated from Sturgeon Lake kimberlite, Saskatchewan, yields a crystallization age of 98±1 Ma (2 , MSWD=1.2; ⁸⁷Sr/⁸⁶Sr(t)=0.7059). The ⁴⁰Ar/³⁹Ar analyses of a phlogopite megacryst sample indicate the presence of large amounts of excess ⁴⁰Ar and yield an excessively old age of 410 Ma. Assessment of the Ar data using isotope correlation plots indicates clustering of the data points about a mixing line between the radiogenic ⁴⁰Ar component at 98 Ma and a trapped component with uniform ³⁶Ar/⁴⁰Ar and Cl/⁴⁰Ar. Values of {ie212-1} as high as +20%. (VSMOW) for calcite from the groundmass and a whole-rock sample indicate pervasive lowtemperature alteration. The {ie212-2} of matrix carbonate is-11.3%. (PDB), slightly lighter than typical values from the literature. The {ie212-3} values of about +5%. (VSMOW) for brown phlogopite megacrysts may be primary, green phlogopites are interpreted to be an alteration product of the brown variety and are 2%. heavier. Initial Nd-Sr-Pb isotopic ratios for a whole-rock sample {ie212-4}; ⁸⁷Sr/⁸⁶Sr=0.7063, ²⁰⁶Pb/²⁰⁴Pb=18.67, ²⁰⁷Pb/²⁰⁴Pb=15.54, ²⁰⁸Pb/²⁰⁴Pb=38.97) suggest an affinity with group I kimberlites. Initial {ie212-5} values of +1.7 and +0.5 (⁸⁷Sr/ ⁸⁶Sr(t)=0.7053 and 0.7050) for eclogitic and lherzolitic garnet megacryst samples, and values of 0.0 for two phlogopite megacryst samples reflect an origin from an isotopically evolving melt due to assimilation of heterogeneous mantle. Lilac high-Cr lherzolitic garnet megacrysts give an unusually high {ie212-6} of +28.6 (⁸⁷Sr/⁸⁶Sr=0.7046) indicating a xenocrystic origin probably from the lithospheric mantle. The very radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of the kimberlite are consistent with melting of EM II (enriched) mantle components.     

Rb-Sr isochron study of the Thorr and Main Donegal Granites,Ireland

A Rb-Sr whole-rock isochron study indicates that the entire Donegal granite suite was emplaced into orthotectonic Caledonian (Dalradian) rocks over a short interval during mid-Silurian to earliest-Devonian times. The Thorr pluton, probably the earliest member of the suite, yields an age of 418 ± 26 Myr and initial ⁸⁷Sr/⁸⁶Sr ratio of 0·7055 ± 4, while the latest member, the Main Donegal pluton has an age of 407 ± 23 Myr and initial ⁸⁷Sr/⁸⁶Sr ratio of 0·7063 ± 5 (Λ⁸⁷Rb = 1·42 ± 10⁻¹¹ yr⁻¹). Errors on both the age and initial Sr isotope ratios incorporate both a priori and geological scatter components and are quoted at the 2-sigma level. The low and restricted range of initial Sr isotope ratios suggests small but significant differences in the composition of the parental granitic magmas which were derived from a low Rb/Sr, low ⁸⁷Sr/⁸⁶Sr source.     

The radiometric age of the Mount Keith Granodiorite,a maximum age estimate for an Archaen greenstone sequence in the Yilgarn block,Western Australia

The ‘granite’ pluton previously shown by Durney (1972) to be unconformably overlain in the Jones Creek area of Western Australia by a boulder conglomerate and a younger greenstone sequence has a Rb-Sr total rock age of $\text{2689 ± 17}$ Ma. Two boulders analysed from the conglomerate are not distinguished isotopically from the underlying ‘granite’. Biotite ages from the ‘granite’ reflect the final effects of a regional upper greenschist facies metamorphism at approximately $\text{2622 ± 15}$ Ma. Potassium feldspars in the ‘granite’ indicate ages less than 2600 Ma and are presumably related to late stage tectonic adjustments.Thus the isotopic dating shows that the ‘granite’, now formally named the Mount Keith Granodiorite, belongs to the early part of the 2750–2600 Ma Yilgarn age group (Compston and Arriens, 1968) and places an older limit on the age of the upper greenstone sequence. This greenstone was laid down within a period of 70 Ma or less and was followed by metamorphism which had ended by 2622 Ma, the time of closure of the biotite Rb-Sr system.Details of the petrography, particularly the metamorphic textures and paragenesis, indicate that despite their externally massive appearance, all the samples except the pegmatite have been recrystallized to varying degrees. Major-element analyses are given also, to aid in the chemical classification of the Granodiorite and in understanding its petrogenesis.