@article{CookeO'BrienCarswell2000, author = {Cooke, R. A. and O'Brien, Patrick J. and Carswell, D. A.}, title = {Garnet zoning and the identification of equilibrium mineral compositions in high-pressure-temperature granulites from the Moldanubian Zone, Austria}, year = {2000}, language = {en} } @article{O'Brien2000, author = {O'Brien, Patrick J.}, title = {The fundamental Variscan problem : high-temperature metamorphism at different depths and high-pressure metamorphism at different temperatures}, year = {2000}, language = {en} } @article{KroenerO'BrienNemchinetal.2000, author = {Kr{\"o}ner, Alfred and O'Brien, Patrick J. and Nemchin, A. A. and Pidgeon, R. T.}, title = {Zircon ages for high pressure granulites from South Bohemia, Czech Republic, and their connection to Carboniferous high temperature processes}, year = {2000}, language = {en} } @article{LiKroenerQianetal.2000, author = {Li, J. H. and Kr{\"o}ner, Alfred and Qian, X. L. and O'Brien, Patrick J.}, title = {Tectonic evolution of an early Precambrian high-pressure granulite belt in the North China Craton}, year = {2000}, language = {en} } @article{CookeO'Brien2001, author = {Cooke, R. A. and O'Brien, Patrick J.}, title = {Resolving the relationship between high P-T rocks and gneisses in collisional terranes : an example from the Gf{\"o}hl gneiss-granulite association in the Moldanubian Zone, Austria}, year = {2001}, language = {en} } @article{O'BrienZotovLawetal.2001, author = {O'Brien, Patrick J. and Zotov, Nikolay and Law, Robin and Khan, M. Azam and Jan, M. Q.}, title = {Coesite in Himalayan eclogite and implications for models of India-Asia collision}, issn = {0091-7613}, year = {2001}, language = {en} } @article{O'Brien2001, author = {O'Brien, Patrick J.}, title = {Subduction followed by Collision : Alpine and Himalayan examples}, year = {2001}, language = {en} } @article{O'BrienKryza2001, author = {O'Brien, Patrick J. and Kryza, R.}, title = {The Variscan basement of the Polish Sudetes}, year = {2001}, language = {en} } @article{MoellerO'BrienKennedyetal.2002, author = {M{\"o}ller, Andreas and O'Brien, Patrick J. and Kennedy, Allen and Kr{\"o}ner, Alfred}, title = {Polyphase zircon in ultrahigh-temperature granulites (Rogaland, SW Norway) : constraints for Pb diffusion in zircon}, year = {2002}, abstract = {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.}, language = {en} } @article{GuoO'BrienZhai2002, author = {Guo, Jianbin H. and O'Brien, Patrick J. and Zhai, M. G.}, title = {High pressure granulites in the Sanggan area, North China craton : metamorphic evolution, P-T paths and geotectonic significance}, year = {2002}, language = {en} } @article{MoellerO'BrienKennedyetal.2003, author = {M{\"o}ller, Andreas and O'Brien, Patrick J. and Kennedy, Allen and Kr{\"o}ner, Alfred}, title = {Linking growth episodes of zircon and metamorphic textures to zircon chemistry : an example from the ultra-high temperature granulites of Rogaland (SW Norway)}, year = {2003}, language = {en} } @article{CarswellTuckerO'Brienetal.2003, author = {Carswell, D. A. and Tucker, R. D. and O'Brien, Patrick J. and Krogh, T. E.}, title = {Coesite Micro-Inclusions and the U-Pb Age of Zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway}, year = {2003}, language = {en} } @article{MassonneO'Brien2003, author = {Massonne, Hans-Joachim and O'Brien, Patrick J.}, title = {The Bohemian Massif and the NW Himalayas}, year = {2003}, language = {en} } @article{CarswellBruecknerCuthbertetal.2003, author = {Carswell, D. A. and Brueckner, H. K. and Cuthbert, S. J. and Mehta, K. and O'Brien, Patrick J.}, title = {The timing of stabilisation and the exhumation rate for ultra-high pressure rocks in the Western Gneiss Region of Norway}, year = {2003}, language = {en} } @article{TreloarO'BrienParrishetal.2003, author = {Treloar, P. J. and O'Brien, Patrick J. and Parrish, R. R. and Khan, M. A.}, title = {Exhumation of early Tertiary, coesite-bearing eclogites from the Pakistan Himalaya}, year = {2003}, language = {en} } @article{O'BrienRoetzler2003, author = {O'Brien, Patrick J. and R{\"o}tzler, Jochen}, title = {High-Pressure Granulites : Formation, Recovery of Peak Conditions, and Implications for Tectonics}, year = {2003}, language = {en} } @article{KronerWildeO'Brienetal.2005, author = {Kroner, Alfred and Wilde, S. A. and O'Brien, Patrick J. and Li, J. H. and Passchier, C. W. and Walte, N. P. and Liu, Dun Yi}, title = {Field relationships, geochemistry, zircon ages and evolution of a late Archaean to Palaeoproterozoic lower crustal section in the Hengshan Terrain of northern China}, issn = {1000-9515}, year = {2005}, abstract = {The Hengshan complex forms part of the central zone of the North China Craton and consists predominantly of ductilely-deformed late Archaean to Palaeoproterozoic high-grade, partly migmatitic, granitoid orthogneisses, intruded by mafic dykes of gabbroic composition. Many highly strained rocks were previously misinterpreted as supracrustal sequences and represent mylonitized granitoids and sheared dykes. Our single zircon dating documents magmatic granitoid emplacement ages between 2.52 Ga and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses, possibly reflecting an older basement. A few granitic gneisses have emplacement ages between 2.35 and 2.1 Ga and show the same structural features as the older rocks, indicating that the main deformation occurred after similar to 2.1 Ga. Intrusion of gabbroic dykes occurred at similar to 1920 Ma, and all Hengshan rocks underwent granulite-facies metamorphism at 1.88-1.85 Ga, followed by retrogression, shearing and uplift. We interpret the Hengshan and adjacent Fuping granitoid gneisses as the lower, plutonic, part of a late Archaean to early Palaeoproterozoic Japan-type magmatic arc, with the upper, volcanic part represented by the nearby Wutai complex. Components of this arc may have evolved at a continental margin as indicated by the 2.7 Ga zircons. Major deformation and HP metamorphism occurred in the late Palaeoproterozoic during the Luliang orogeny when the Eastern and Western blocks of the North China Craton collided to form the Trans-North China orogen. Shear zones in the Hengshan are interpreted as major lower crustal discontinuities post-dating the peak of HP metamorphism, and we suggest that they formed during orogenic collapse and uplift of the Hengshan complex in the late Palaeoproterozoic (< 1.85 Ga)}, language = {en} } @article{O'BrienWalteLi2005, author = {O'Brien, Patrick J. and Walte, N. P and Li, J. H.}, title = {The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications}, issn = {1367-9120}, year = {2005}, abstract = {The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende + plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the complex, relict high-pressure mafic granulites are garnet + clinopyroxene-bearing rocks with a secondary development of orthopyroxene around both garnet (kelyphites) and clinopyroxene (coronas). South of the newly defined central, E-W-trending, Zhujiafang shear zone, numerous mafic boudins and less-deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet (full of quartz inclusions) between the magmatic plagioclase and pyroxene domains. Additional orthopyroxene (after magmatic augite) and sodic rims to magmatic plagioclase clearly indicate medium-pressure granulite-facies metamorphism. These findings suggest tectonic juxtaposition in this area of three different structural levels of the same Proterozoic-imprinted crust: high-pressure granulite grade in the northern Hengshan, medium-pressure granulite grade in the southern Hengshan and amphibolite- to greenschist-facies grade in the Wutaishan to the SE. (c) 2004 Elsevier Ltd. All rights reserved}, language = {en} } @article{KonradSchmolkeHandyBabistetal.2005, author = {Konrad-Schmolke, Matthias and Handy, Mark R. and Babist, Jochen and O'Brien, Patrick J.}, title = {Thermodynamic modelling of diffusion-controlled garnet growth}, issn = {0010-7999}, year = {2005}, abstract = {Numerical thermodynamic modelling of mineral composition and modes for specified pressure-temperature paths reveals the strong influence of fractional garnet crystallisation, as well as water fractionation, on garnet growth histories in high pressure rocks. Disequilibrium element incorporation in garnet due to the development of chemical inhomogeneities around porphyroblasts leads to pronounced episodic growth and may even cause growth interruptions. Discontinuous growth, together with pressure- and temperature-dependent changes in garnet chemistry, cause zonation patterns that are indicative of different degrees of disequilibrium element incorporation. Chemical inhomogeneities in the matrix surrounding garnet porphyroblasts strongly affect garnet growth and lead to compositional discontinuities and steep compositional gradients in the garnet zonation pattern. Further, intergranular diffusion-controlled calcium incorporation can lead to a characteristic rise in grossular and spessartine contents at lower metamorphic conditions. The observation that garnet zonation patterns diagnostic of large and small fractionation effects coexist within the same sample suggests that garnet growth is often controlled by small-scale variations in the bulk rock chemistry. Therefore, the spatial distribution of garnet grains and their zonation patterns, together with numerical growth models of garnet zonation patterns, yield information about the processes limiting garnet growth. These processes include intercrystalline element transport and dissolution of pre-existing grains. Discontinuities in garnet growth induced by limited element supply can mask traces of the thermobarometric history of the rock. Therefore, thermodynamic modelling that considers fractional disequilibrium crystallisation is required to interpret compositional garnet zonation in terms of a quantitative pressure and temperature path of the host rock}, language = {en} } @article{O'Brien2006, author = {O'Brien, Patrick J.}, title = {The age of deep, steep continental subduction in the NW Himalaya : relating zircon growth to metamorphic history. Comment on: "The onset of India-Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya" by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005) 83-97}, doi = {10.1016/j.epsl.2006.03.033}, year = {2006}, abstract = {Leech et al. [Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005) 83-97], present 3 clusters of ages for growth stages in zircon from quartzo- feldspathic gneisses hosting coesite-bearing eclogite from the Tso Morari Complex, NW India. These age clusters, from oldest to youngest, are interpreted to represent the age of ultrahigh-pressure metamorphism, a subsequent eclogite facies overprint and a later amphibolite facies retrogression and require subduction of Indian crust to have started earlier than previously accepted. However, no petrographic evidence, such as inclusions in the zircons relating to particular metamorphic events, is presented to substantiate the proposed sequence of metamorphic stages. Previously published data from eclogites of the same area indicate that coesite-eclogite is not the first but at least the second eclogite facies stage. In addition, the newly proposed time interval between coesite-eclogite and the amphibolite facies overprint is longer than previously indicated by diffusion modelling of natural garnet-garnet couples in eclogite. Neither the age of ultrahigh-pressure metamorphism nor the timing of initiation of subduction is reliably constrained by the presented data}, language = {en} }