The Hvalfjördur area, 30 km north of Iceland's capital Reykjavik, belongs to the sequence of Late Tertiary to early Quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites. The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjördur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near-surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the Laxárvogur and Hvalfjördur central volcanoes. Altogether 11 different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled ''zeolite zones'' occur in the Hvalfjördur area.
The phonolitic Limberg t3 tephra (Kaiserstuhl Volcanic Complex, Germany) was previously dated by the conventional K/Ar method yielding inconsistent results. We have re‐dated this tephra layer with three independent methods. Fission Track (FT) external detector analyses on single apatite crystals (16.8 ± 1.3 Ma, 2s) and (U‐Th)/He measurements on titanite and apatite (16.5 ± 1.0 Ma, 2s and 16.8 ± 1.0 Ma, 2s, respectively) are in close agreement with laser Ar/Ar dates on incrementally heated single crystals of sanidine (16.3 ± 0.4 Ma, 2s). Due to very rapid cooling, the He, FT and Ar thermochronometers provide one single age representing the eruption event. The different minerals are characterised by favourable properties with respect to their chemical composition, grain size and shape. In particular for the t3 sanidine, homogeneity has been demonstrated by electron microprobe analysis and on a grain‐to‐grain and grain‐internal scale by single crystal incremental laser heating. Based on the agreement between independent methods and the mineral yield of this unit, the Limberg t3 tephra is proposed as multi‐method age reference material for single grain laser Ar/Ar, FT and (U‐Th)/He dating.
The Skattøra migmatite complex in the north Norwegian Caledonides consists of migmatized slightly nepheline-normative metagabbros that are net-veined by numerous (up to 90%) anorthositic and leucodioritic dykes. The average chemical composition of 17 anorthosite dykes is (wt %) 58.4 % SiO 2 , 0.2 % TiO 2 , 23% Al 2 O 3 , 1.8% FeO t , 0.7% MgO, 6.3% CaO, 7.8% Na 2 O, 0.2% K 2 O. A migmatite leucosome and a dyke have been dated by the U/Pb method on titanite to 456±4 Ma. In low melt fraction areas minor leucosomes are orientated parallel to the foliation. More intense anatexis formed stromatic to schlieric migmatites. The leucosomes are commonly connected to dykes, suggesting that melt segregated and left its source. Dyke thicknesses range from a few centimetres up to several metres. In general, early dykes are parallel to the foliation in the host rock, while the later dykes cut the foliation. Plagioclase (An 20-50 ) is the dominant mineral (85-100%) in the dykes and the leucosome, but 0-15 % amphibole is generally present. Field relations, geochemistry and preliminary melting-experiments strongly suggest that the anorthosites originated by H 2 O-fluxed anatexis of the gabbroic host rock.
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