Robert Anczkiewicz

Early Miocene continental subduction and rapid exhumation in the western Mediterranean

New Lu-Hf garnet growth ages from high-pressure rocks from the structurally lowest terrane (Nevado-Filabride Complex) in the Betic Cordillera of southern Spain indicate early Miocene subduction of continental crust, requiring significant revision of the timing and processes of orogeny in the western part of the Alpine orogenic system. Mafic eclogite and pelitic schist give Lu-Hf ages in the range 18–14 Ma, and fission-track data show that the terrane was exhumed by 12–8 Ma. The overlying terrane (Alpujarride Complex) shows early Tertiary high pressure-temperature metamorphism; this was followed by a thermal event that accompanied rapid crustal extension at 22–18 Ma, leading to the formation of the Alboran Sea backarc basin. The evidence therefore suggests two phases of continental subduction separated by a phase of large-scale intraorogenic extension. The two terranes are now separated by a mid-late Miocene low-angle extensional fault. The timing and paleogeographic setting suggest that the Nevado-Filabride terrane originally was close to the rifted South Iberian margin, and was subducted during westward motion of the extending upper terrane between the converging African and Iberian plates.

Structural evidence for back sliding of the Kohistan arc in the collisional system of northwest Pakistan

In the Naran region of the Pakistan Himalayas, a regionally distributed second generation of folds results from northward-directed shear deformation. These folds are collapse structures associated with back sliding of the hanging wall, namely the Kohistan paleo–island arc. They are explained by a geometrical model that combines coeval vertical and horizontal shortening. Accordingly, they are synconvergence collapse features that indicate at least 5 km of vertical shortening of the imbricate thrust slices derived from the Indian continent in the northwest Himalayas.

Improving precision of Sm-Nd garnet dating by H2SO4 leaching: a simple solution to the phosphate inclusion problem

Abstract Sm-Nd garnet geochronology is often hampered by the presence of submicroscopic inclusions of rare earth element-rich phosphates, which lower age precision, lead to inaccurate ages or make dating impossible. We propose a single-step sulphuric acid leaching technique as a very efficient tool in eliminating phosphate inclusions, which helps to achieve more precise and more accurate Sm-Nd garnet dates. Examples from silimanite grade metapelites demonstrate the much higher efficiency of this method in comparison with previously proposed techniques based on HF and HCl. 147Sm/144Nd ratios obtained on garnets leached by sulphuric acid were twice as high as those obtained by HF and HCl leaching. This led to age precision better than 3% for Tertiary samples. Comparison of leached and unleached nearly inclusion-free garnets from high pressure granulites, demonstrates that there is no Sm/Nd fractionation induced by H2SO4 leaching. Our new technique eliminates phosphates, but does not attack garnet. This considerably reduces the necessity for hand-picking and lowers the amount of sample required for analysis making Sm-Nd garnet dating a more easily applied geochronometer.

Stratigraphy and structure of the Indus Suture in the Lower Swat, Pakistan, NW Himalaya

Abstract We present a geological map, detailed structural description and new subdivisions of the Indus Suture Zone lithologies in northeast Pakistan (Lower Swat region). The Indus Suture Zone is a dominantly fore-arc related assemblage obducted onto the Indian Plate. Initial southward thrusting was followed by the reactivation of the Indus Suture as a ductile-brittle normal fault. Significant strike-slip faulting may have taken place at this stage. Later brittle reverse faults demonstrate that shortening outlasted normal faulting. The comparison of the structural evolution of the Indus Suture Zone and the Indian Plate shows that both terrains were sharing the deformational history, possibly since the earliest stage of collision. The Lower Swat area records a full orogenic cycle from early rifting through continent-arc collision to orogenic collapse. We present a geodynamic model that integrates all these elements.

Isotopic constraints on the evolution of metamorphic conditions in the Jijal-Patan complex and the Kamila Belt of the Kohistan arc, Pakistan Himalaya

Abstract Pressure-temperature data and Sm-Nd and Rb-Sr garnet ages are presented for retrogressed granulitic rocks of the Jijal-Patan complex and the Kamila Amphibolite Belt. Despite the retrogression and hydration, the two samples contain garnet and hydrous minerals that yield pressures and temperatures similar to previous estimates for pristine granulite facies rocks from the same area. The Sm-Nd and Rb-Sr garnet ages are concordant at 95–100 Ma for the two samples and for both isotopic systems. These ages are interpreted as dating cooling through 700–800 °C following magmatic crystallization and granulite facies metamorphism. In the case of the garnet amphibolites from the Kamila Belt, the garnets retain prograde major element zonation. In addition, the closure temperature for the Rb-Sr system is very close to the recorded temperature. For these reasons, the age of 100 Ma must represent a time close to that when the pressure and temperature preserved in the mineralogy and its chemistry was recorded. The isotopic equilibrium between garnet and paragonite at 90–100 Ma suggests that the regional hydration event that affected the lower crust of the Kohistan arc also occurred at this time. Cooling rates calculated from the Rb-Sr and Sm-Nd ages for the partially retrogressed granulite give a minimum of 3–6°C Ma−1 and imply a different tectonic mechanism for the exhumation of the lower crust than is typical for granulites. This process may be related to early regional decompression following the collision of the Kohistan arc with Eurasia.

Accuracy of laser-ablation (LA)-MC-ICPMS Sr isotope analysis of (bio)apatite – a problem reassessed

Apatite is a key mineral whose Sr-isotope record has a wide range of applications including palaeofluid flow studies from inorganic apatite, and past faunal/human mobility or palaeoecology using bioapatite. The incremental growth of mammalian enamel bioapatite potentially allows extraction of Sr isotopic compositions at sub-annual time-resolution using laser-ablation plasma mass spectrometry (LA-MC-ICPMS). However, existing apatite LA-MC-ICPMS Sr-isotope data have yielded mixed results. Here we assess the achievable accuracy/precision of (bio)apatite LA-MC-ICPMS Sr-isotope analysis and evaluate sources of inaccuracy. Using robust plasma conditions (ThO+/Th+ < 0.2%), we obtain long-term (4 year) accurate and precise Sr-isotope data for modern shark teeth for both radiogenic 87Sr/86Sr (0.709171 ± 0.000053, 2 SD), and naturally invariant 84Sr/86Sr (0.056500 ± 0.000040, 2 SD). Based on our accurate 84Sr/86Sr-results also for low-Sr enamel, we deduce that interferences are successfully corrected (Kr) or negligible (Ca-argide/dimer), leaving 87Rb and 40Ca31P16O isobaric interferences as key potential sources for 87Sr/86Sr inaccuracy. Our (pseudo)high-resolution mass scans using a virtually Rb–Sr-free, concentrated Ca–P-solution simulating apatite LA analysis show no evidence for 40Ca31P16O at the required intensities to explain previously observed 87Sr/86Sr offsets. Rather, using the same Ca–P solution with varying Rb + Sr isotope standard additions, we accurately constrain the mass bias-corrected 85Rb/87Rb-ratio, and using apatite glasses assess the extent of Rb/Sr elemental fractionation during laser-ablation. Finally, we present concordant LA-MC-ICPMS and microsampled-TIMS 87Sr/86Sr results for low-Sr tooth enamel with highly variable inter & intra-87Sr/86Sr ratios and comparatively high Rb/Sr-ratios. This archaeological example also illustrates well the problem of defining equivalent sample volumes that allow unequivocal comparison between LA and TIMS data.

A simple mechanism for mid-crustal shear zones to record surface-derived fluid signatures

Ion microprobe analyses of garnet porphyroblasts from three separate splays of the mid-crustal Walter-Outalpa shear zone, Curnamona Province, South Australia, indicate homogeneous δ 18 O values of 18 O values in deeply exhumed shear zones may therefore be indicative of fault structures that have a prior history of surface exposure, weathering, burial and re-exposure.

Plutonium isotopes in the atmosphere of Central Europe: Isotopic composition and time evolution vs. circulation factors

This paper reports evidence of Pu isotopes in the lower part of the troposphere of Central Europe. The data were obtained based on atmospheric aerosol fraction samples collected from four places in three countries (participating in the informal European network known as the Ring of Five (Ro5)) forming a cell with a surface area of about 200,000km(2). We compared our original data sets from Krakow (Poland, 1990-2007) and Bialystok (Poland, 1991-2007) with the results from two other locations, Prague (Czech Republic; 1997-2004) and Braunschweig (Germany; 1990-2003) to find time evolution of the Pu isotopes. The levels of the activity concentration for (238)Pu and for ((239+240))Pu were estimated to be a few and some tens of nBqm(-3), respectively. However, we also noted some results were much higher (even about 70 times higher) than the average concentration of (238)Pu in the atmosphere. The achieved complex data sets were used to test a new approach to the problem of solving mixing isotopic traces from various sources (here up to three) in one sample. Results of our model, supported by mesoscale atmospheric circulation parameters, suggest that Pu from nuclear weapon accidents or tests and nuclear burnt-up fuel are present in the air.

Multiple high-pressure metamorphic events and crustal telescoping in the NW Highlands of Scotland

Abstract: The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian orogen in NW Scotland. A Western Unit consists of trondhjemite–tonalite–granodiorite (TTG) gneisses and subordinate mafic intrusions with locally preserved mafic high-pressure granulite and eclogite assemblages. An Eastern Unit comprises TTG gneisses, Grenville-age (c. 1.1 Ga) eclogite and metasediments. U–Pb zircon ages from the Western Unit TTG gneisses are highly disturbed with Neoarchaean upper intercepts and Palaeo- to Mesoproterozoic lower intercepts, suggesting strong reworking at these times. U–Pb zircon, Sm–Nd and Lu–Hf garnet–clinopyroxene dates from the Western Unit high-pressure granulite and eclogite yield Neoarchaean (c. 2.6–2.8 Ga) and Palaeoproterozoic (c. 1.75 Ga) ages, respectively. These ages correspond to the ages of partial resetting of the TTG gneisses. The eclogite in the Western Unit may represent the high-pressure convergent margin to the lower-pressure Laxfordian events within the Lewisian Gneiss Complex, whereas the Mesoproterozoic eclogites in the Eastern Unit may represent a farther eastward Grenville-age margin. Further east, Ordovician high-pressure granulites within the Moine Supergroup may represent another, later, Grampian convergent margin. These high-pressure belts were developed sequentially towards the east and telescoped westwards from Palaeoproterozoic to Early Palaeozoic times. Supplementary material: U–Pb analytical data and LA-ICP-MS trace element data are available at http://www.geolsoc.org.uk/SUP18389.

Fluorapatite from a remarkable occurrence of graphite and associated minerals

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