Zircon U‐Th‐Pb and mineral K‐Ar and 40Ar/39Ar isotopic studies indicate that the maximum deposition age of the Mt Narryer quartzite (which contains detrital zircons up to 4200 Ma old) is 3280 Ma, or by association with other sequences possibly 3100 Ma. This postdates a major episode of high‐grade metamorphism, granite emplacement and deformation at 3300 Ma, which affected adjacent gneiss terranes and which previously had been considered to have affected metasediments and basement gneisses alike. Prograde metamorphism of the Narryer metasediments to amphibolite facies evidently took place during a younger event culminating ca 2700 Ma, prior to injection of granite sheets ca 2650 Ma in age, by which time the present tectonic framework had been assembled.
Abstract Sm‐Nd and Rb‐Sr isotopic data for Archaean gneisses from three localities within the eastern Yilgarn Block of Western Australia indicate that the gneisses define a precise Rb‐Sr whole rock isochron age of 2780 ± 60 Ma and an initial 87Sr/86Sr of 0.7007 ± 5. The Sm‐Nd isotopic data do not correspond to a single linear array, but form two coherent groups that are consistent with a c. 2800 Ma age of crust formation, with variable initial Nd. These results indicate that the gneiss protoliths existed as continental crust for a maximum period of only c. 100 Ma, and probably for a much shorter time, prior to the formation of the 2790 ±30 Ma greenstones.
Abstract Titiraupenga is an early Pleistocene volcano sited approximately 20 km northwest of Lake Taupo, North Island, New Zealand, at the junction of the Taupo Volcanic Zone and a line of andesite volcanoes extending from Ohakune in the south to Waiheke Island in the north. Exposures on the volcano are restricted to a central plug (type 1 andesite) and rare exposures of flows on the lower slopes (type .2 andesite). Both type 1 and type 2 andesites are porphyritic with phenocrysts of orthopyroxene, augite, plagioclase, and scarce olivine in a groundmass of plagioclase, pyroxene, and iron OXIde. Pigeonite occurs only in the groundmass and as reaction rims to orthopyroxene phenocrysts in type 1 andesites. Chemically, most samples are low-Si andesites, and variation is probably due to varying degrees of pyroxene accumulation within the magma reservoir. Type 2 andesites are inhomogeneous and probably result from mixing of magmas from different parts of the reservoir. Aphyric lenses (type 3 andesite) within type 2 andesites are thought to represent a wholly liquid portion of the reservoir which was incorporated in the type 2 magma during eruption. Slight differences in isotopic composition of type 3 magma may indicate some crustal contamination.
Ion microprobe analysis of zircons from three sites (Watersmeet Dome in northern Michigan, Mount Sones in eastern Antarctica, and Mount Narryer in western Australia) is discussed. Implications of the results to Archean geochronology and early Earth crust composition are addressed.
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