ALBERT

Notes: [Auszug] Fig. 1 Conchordia diagram showing U-Pb ages by ion micro-probe for 140 grains of detrital zircon from Jack Hills conglomerate sample W74. All points on the main diagram and the open symbols in the inset are reconnaissance analyses. The filled symbols in the main diagram denote concordant ages for ...

Notes: Abstract Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U−Pb age. Recrystallisation provides a mechanism for resetting zircon U−Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U−Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.

Notes: Abstract Petrological and isotopic investigations were undertaken on high pressure granulites of granitic to mafic composition from the Prachatice and Blanský les granulite complexes of southern Bohemia, Czech Republic. The predominant felsic granulites are quartz + ternary feldspar (now mesoperthite)-rich rocks containing minor garnet, kyanite and rutile, and most show a characteristic mylonitic fabric formed during retrogression along the exhumation path. Three high temperature reaction stages at distinctly different pressures are recognized. Rare layers of intermediate to mafic composition, containing clinopyroxene, best record a primary high pressure–high temperature stage (〉15 kbar, 〉900 °C), and a well-defined overprint at medium pressure granulite facies conditions (6–8 kbar, 700–800 °C) during which orthopyroxene (+plagioclase) formed from garnet and clinopyroxene. A further high temperature overprint at lower pressure (ca. 4 kbar) is reflected in the development of cordierite- and/or andalusite-bearing partial-melt patches in some felsic granulites. Conventionally separated zircons from the granulites were measured on a SHRIMP II ion microprobe. Near-spherical, multifaceted grains interpreted to be metamorphic, and short prismatic grains from the cordierite-bearing melt patch, are all concordant and yielded indistinguishable results producing an average age, for 83 individual grain spots, of 339.8 ± 2.6 Ma (2σ). Metamorphic grains from a meta-granodiorite associated with the granulites gave the same age (339.6 ± 3.1 Ma, mean of 9), whereas inherited magmatic grains of the same sample yielded 367.8 ± 1.4 Ma. A mean age of 469.3 ± 3.8 Ma was obtained for two short prismatic concordant grains in one of the granulites, whereas several of the rounded grains with ca. 340 Ma metamorphic zircon overgrowths had much older (207Pb/206Pb minimum ages up to 1771 Ma) discordant cores. In addition to analysis of conventionally separated grains, ion-microprobe measurements were also made on zircons extracted from thin sections (drilled-out, mounted and repolished) such that a direct relationship between the dated zircons and petrographic position could be made. Identical results were obtained from both preparation methods, thus showing that the considerable advantage in petrological control is not offset by any appreciable lack of precision when compared to conventionally prepared ion-microprobe samples. All these isotopic results are identical to those previously obtained by conventional multigrain and single-grain evaporation techniques, but rather than allowing a greater resolution of the age of the petrographically obvious different metamorphic stages the results document, for the first time, the apparent short time scale for high, medium and low pressure metamorphism in the granulites. The short time period between the 340 Ma age for the high pressure granulites, as derived here and from studies of similar rocks elsewhere in the European Variscides, and the 320–330 Ma ages for regional low pressure–high temperature metamorphism, migmatization and granite magmatism, strongly suggests an important link between these two high temperature processes.

Notes: Abstract Internal structures in zircons from granitoids from the late Archaean Darling Range Batholith show secondary features revealed by HF etching, which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid. Zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation. The most striking feature is the development of high trace element concentration areas found in zircons from a number of granites. These structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band, about half way between the outer edge and the centre of the zircon. In any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim. This suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones. This explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger SHRIMP ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres. Also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon. This structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands. All structures can be transgressed by generally rounded lobes and patches of low U, weakly nebulously zoned zircon. This is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid, resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon.

Notes: Abstract Conventional and SHRIMP U-Pb analyses of zircon, monazite, titanite and apatite from the high grade rocks of the Northampton Complex in Western Australia provide constraints on the timing of metamorphic processes and deformation events in the northern Darling Mobile Belt (western margin of the Archean Yilgarn Craton). Paragneisses and mafic volcanics and/or intrusions have undergone granulite facies metamorphism in a probable extensional tectonic setting prior to formation of W- to NW-verging folds and thrusts cut by normal shears (interpreted as late collapse structures) during the main deformation event (D1). These structures are folded by open to tight folds with NW-striking axial surfaces developed in a second, NE-SW contractional event (D2). Zircons from a mafic granulite provide an age of 1079 ± 3 Ma attributed to new zircon growth prior to, or at the peak of regional granulite facies metamorphism. Metamorphic monazites extracted from a paragneiss yield an identical age of 1083 ± 3 Ma. The similarity of ages between zircons from the mafic granulite (1079 ± 3 Ma) and monazites from the paragneiss (1083 ± 3 Ma) is interpreted to reflect fast cooling and/or rapid uplift, which is consistent with thrusting of the gneissic units during the first deformation event (D1) associated with the onset of retrograde metamorphism. Granitic activity at 1068 ± 13 Ma was followed by intrusion of post-D2 pegmatite (989 ± 2 Ma), which constrains the end of metamorphism and associated deformation. Cooling of the complex to about 500 °C is timed by the apatite age of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons from a paragneiss sample yield a maximum age of 2043 Ma, with no evidence of an Archean Yilgarn signature. A majority of ages between 1.6 and 1.9 Ga are consistent with derivation from the Capricorn Orogen on the northern margin of the Yilgarn Craton. Younger detrital zircons with 1150–1450 Ma ages, however, indicate an additional source that had undergone early Grenvillian igneous or metamorphic event(s) and also places a maximum age constraint upon deposition. The source of this clastic material may have been from within the southern Darling Mobile Belt or from Greater India (adjacent to the Northampton Complex in Rodinia reconstructions). This study documents an extended Grenvillian history, with basin formation, sedimentation, granulite facies metamorphism, contractional tectonics (two periods with orthogonal directions of shortening) and late pegmatite emplacement taking place between 1150–989 Ma on the western margin of the Yilgarn Craton. Ages recorded in this study indicate that the proposed global distribution of Grenvillian belts during assembly of the Rodinia supercontinent should be reassessed to include the Darling Mobile Belt.

Notes: Abstract U-Pb isotopic analyses have been determined on four size splits from an orthogneiss zircon population and on seven size and magnetic splits from a paragneiss zircon population from the Ceneri Zone, Southern Switzerland. The orthogneiss zircon results suggest that a severe isotopic disturbance occurred in this region between 400–450 m. y. ago which, besides initiating new zircon growth, almost completely reconstituted older zircon. The U-Pb systems of the orthogneiss zircons are nearly concordant, except the coarsest size split where some older lead is present. The paragneiss zircon U-Pb systems have Precambrian apparent ages and appear to be an old detrital zircon suite which has been severely isotopically disturbed by the same Caledonian event which affected the orthogneiss zircons. Zircon U-Pb data-points of paragneiss size-and-magnetic fractions are linearly correlated on a concordia plot. The age significance of this is not fully understood, however these results indicate that Precambrian crustal rocks, at least 1,500 m. y. old, contributed detritus to the original sediments of the Alpine gneisses. The gneissic rocks in Southern Switzerland have been postulated as a possible source or root zone for petrologically and chemically similar gneisses in the Silvretta Nappe to the north. The close similarity of zircon U-Pb systems of orth- and para-gneisses between the two regions supports this hypothesis.

Notes: Summary We report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 μm3 and 80 μm3, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 μm. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral “age distribution” within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 μm, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.