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Nd whole-rock data from the Windmill Islands area yield early Proterozoic to middle Archaean Nd model ages. These crustal residence times are consistent with regional correlations with other parts of Antarctica (Bunger Hills, Denman Glacier area) and the Albany-Fraser Orogen of south-western Australia during the Mid-Proterozoic and thus support reconstructions with a continuous Mid-Proterozoic orogen in these areas. The new Nd isotope data provide strong evidence that no age boundary exists between the higher- and lower-grade parts of the Windmill Islands area, and that the metamorphic complex represents a single terrane with a common crustal history. The data support the notion of a time- link between the occurrence of intrusive charnockites (C-type magmas) and high-grade metamorphism. The magmatic rocks and orthogneisses in the area are interpreted to have a mixed source consisting of older crustal components, i.e. older sediments (ca. 3.2-2.6 Ga) and a younger mafic component (ca. 1.9 Ga). Two garnet Sm-Nd isochrons yield ages of 1156±17 Ma and 1137±2.5 Ma and are identical to SHRIMP U-Pb results on monazite from these samples. A garnet Sm-Nd age of 1123±13 Ma for the Ford granite is significantly younger than the SHRIMP U-Pb zircon age for this sample. The difference relates to the different closure temperature of each isotopic system and is thus interpreted as initial cooling after granulite facies metamorphism. Keywords. East Antarctica - Granulites - Garnet-whole rock isochrons - Intrusive charnockite - Nd model ages
SHRIMP U-Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite-norite intrusive complex, both from the ultrahigh temperature (UHT; >900 °C pigeonite-in) zone and from outside the hypersthene-in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2{sigma}, n = 8) and 1052 ± 5 Ma (1{sigma}, n = 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma.In contrast, samples from the UHT zone exhibit four major age groups:(1) magmatic cores yielding ages over 1500 Ma(2) magmatic cores giving ages of 1034 ± 13 Ma (2{sigma}, n = 4) and 1056 ± 10 Ma (1{sigma}, n = 1)(3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1{sigma}) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and(4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2{sigma}, n = 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2{sigma}, n = 4).This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950 °C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th/U-Y systematics indicate that the main processes affecting zircon were dissolution-reprecipitation in a closed system and solid-state recrystallization during and soon after M1.
In order to monitor the seismic activity of Mt. Merapi (Indonesia) over a long period of time, we installed a permanent array of both broadband and short-period seismometers during the summer of 1997. Considering the requirements of an automatic classification and localization system for seismic monitoring and surveillance at active volcanoes, we split this network into three small aperture arrays distributed around the volcano. We introduce here a newly developed method to determine the hypocenters in an automatic, non-linear manner using the coherence of seismic waves observed at the different arrays. To test this method, we analyze a swarm of VT-B events recorded by the network. The first step in this algorithm is based on a modified smoothed coherence transform. In the second step we perform a semblance analysis applied to the 3D problem, evaluating the quality of the estimated relative onset-times. After more than one year of dormancy, Mt. Merapi renewed its activity at the end of June 1998. This gave us the opportunity to analyze all stages of dome growth, collapse and new intrusion of magma using the associated seismicity in a post-processing sense. This also allowed us to calibrate and test our newly developed automatic monitoring system using the more pronounced waveforms of VT-B events. By detecting and classifying different event types automatically, we are able to localize a large number of VT-B events which occurred just before the initial eruption. We are also able to resolve some properties of the wavefield at Mt Merapi which are essential for further interpretations. Finally, the results show that the source region of the VT-B type seismicity just before the 1998 eruption is closely related to the region of subsequent high volcanic activity and therefore may represent a promising tool to forecast future eruptions.