TY - JOUR A1 - Kroner, Alfred A1 - Wilde, S. A. A1 - O'Brien, Patrick J. A1 - Li, J. H. A1 - Passchier, C. W. A1 - Walte, N. P. A1 - Liu, Dun Yi T1 - Field relationships, geochemistry, zircon ages and evolution of a late Archaean to Palaeoproterozoic lower crustal section in the Hengshan Terrain of northern China N2 - 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) Y1 - 2005 SN - 1000-9515 ER - TY - JOUR A1 - O'Brien, Patrick J. T1 - 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 N2 - 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 Y1 - 2006 UR - http://www.sciencedirect.com/science/journal/0012821X U6 - https://doi.org/10.1016/j.epsl.2006.03.033 ER - TY - JOUR A1 - O'Brien, Patrick J. A1 - Walte, N. P A1 - Li, J. H. T1 - The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications N2 - 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 Y1 - 2005 SN - 1367-9120 ER - TY - JOUR A1 - Konrad-Schmolke, Matthias A1 - Handy, Mark R. A1 - Babist, Jochen A1 - O'Brien, Patrick J. T1 - Thermodynamic modelling of diffusion-controlled garnet growth N2 - 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 Y1 - 2005 SN - 0010-7999 ER - TY - JOUR A1 - Konrad-Schmolke, Matthias A1 - Zack, Thomas A1 - O'Brien, Patrick J. T1 - Combining thermodynamic and trace element modeling : a tool to quantify mineral reactions and trace element budgets during metamorphism Y1 - 2009 UR - http://www.sciencedirect.com/science/journal/00167037 U6 - https://doi.org/10.1016/j.gca.2009.05.009 SN - 0016-7037 ER - TY - JOUR A1 - Möller, Andreas A1 - O'Brien, Patrick J. A1 - Kennedy, Allen A1 - Kröner, Alfred T1 - Linking growth episodes of zircon and metamorphic textures to zircon chemistry : an example from the ultra-high temperature granulites of Rogaland (SW Norway) Y1 - 2003 ER - TY - JOUR A1 - Carswell, D. A. A1 - Tucker, R. D. A1 - O'Brien, Patrick J. A1 - Krogh, T. E. T1 - Coesite Micro-Inclusions and the U-Pb Age of Zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway Y1 - 2003 ER - TY - JOUR A1 - Massonne, Hans-Joachim A1 - O'Brien, Patrick J. T1 - The Bohemian Massif and the NW Himalayas Y1 - 2003 ER - TY - JOUR A1 - Möller, Andreas A1 - O'Brien, Patrick J. A1 - Kennedy, Allen A1 - Kröner, Alfred T1 - Polyphase zircon in ultrahigh-temperature granulites (Rogaland, SW Norway) : constraints for Pb diffusion in zircon N2 - 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. Y1 - 2002 ER - TY - JOUR A1 - Carswell, D. A. A1 - Brueckner, H. K. A1 - Cuthbert, S. J. A1 - Mehta, K. A1 - O'Brien, Patrick J. T1 - The timing of stabilisation and the exhumation rate for ultra-high pressure rocks in the Western Gneiss Region of Norway Y1 - 2003 ER - TY - JOUR A1 - Cooke, R. A. A1 - O'Brien, Patrick J. A1 - Carswell, D. A. T1 - Garnet zoning and the identification of equilibrium mineral compositions in high-pressure-temperature granulites from the Moldanubian Zone, Austria Y1 - 2000 ER - TY - JOUR A1 - Cooke, R. A. A1 - O'Brien, Patrick J. T1 - Resolving the relationship between high P-T rocks and gneisses in collisional terranes : an example from the Gföhl gneiss-granulite association in the Moldanubian Zone, Austria Y1 - 2001 ER - TY - JOUR A1 - O'Brien, Patrick J. A1 - Zotov, Nikolay A1 - Law, Robin A1 - Khan, M. Azam A1 - Jan, M. Q. T1 - Coesite in Himalayan eclogite and implications for models of India-Asia collision Y1 - 2001 SN - 0091-7613 ER - TY - JOUR A1 - Guo, Jianbin H. A1 - O'Brien, Patrick J. A1 - Zhai, M. G. T1 - High pressure granulites in the Sanggan area, North China craton : metamorphic evolution, P-T paths and geotectonic significance Y1 - 2002 ER - TY - JOUR A1 - Treloar, P. J. A1 - O'Brien, Patrick J. A1 - Parrish, R. R. A1 - Khan, M. A. T1 - Exhumation of early Tertiary, coesite-bearing eclogites from the Pakistan Himalaya Y1 - 2003 ER - TY - JOUR A1 - O'Brien, Patrick J. T1 - The fundamental Variscan problem : high-temperature metamorphism at different depths and high-pressure metamorphism at different temperatures Y1 - 2000 ER - TY - JOUR A1 - Kröner, Alfred A1 - Wilde, Simon A. A1 - Zhao, Guochun A1 - O'Brien, Patrick J. A1 - Sun, Min A1 - Liu, Dun Yi A1 - Wan, Yusheng A1 - Liu, S. W. A1 - Guo, Jianbin H. T1 - Zircon geochronology and metamorphic evolution of mafic dykes in the Hengshan Complex of northern China: Evidence for late Palaeoproterozoic extension and subsequent high-pressure metamorphism in the North China Craton N2 - Magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the Hengshan Complex of the North China Craton, These dykes now occur as boudins and deformed sheets within migmatitic tonalitic, trondhjemitic, granodioritic and granitic gneisses and are conspicuous due to relics of high-pressure granulite or even former eclogite facies garnet + pyroxene-bearing assemblages. SHRIMP ages for magmatic zircons from two dykes reflect the time of dyke emplacement at similar to 1915 Ma, whereas metamorphic zircons dated by both SHRIMP and evaporation techniques are consistently in the range 1848-1888 Ma. The Youngest granitoid gneiss yet dated in the Hengshan has an emplacement age of 18 2 17 Ma. These results complement recent geochronological studies from the neighbouring Wutai and Fuping Complexes, to the SE of the Hengshan, showing that a crustal extension event Occurred in the late Palaeoproterozoic. This preceded a major high-pressure collision- type metamorphic event in the central part of the North China Craton that occurred in the Palaeoproterozoic and not in the late Archaean as previously thought. Our data support recent suggestions that the North China Craton experienced a major, craton-wide orogenic event in the late Palaeoproterozoic after which it became cratonized and acted as a stable block. Y1 - 2006 U6 - https://doi.org/10.1016/j.precamres.2006.01.008 SN - 0301-9268 ER - TY - JOUR A1 - O'Brien, Patrick J. T1 - Type-locality granulites: high-pressure rocks formed at eclogite-facies conditions N2 - The type-locality granulites from the Granulitgebirge of Saxony, Germany, are rocks of broadly granitic composition containing minor garnet and kyanite within a commonly mylonitised matrix of feldspars and quartz. Petrographic evidence indicates a primary assemblage of ternary feldspar + quartz + garnet + kyanite + rutile, most likely resulting from partial melting of a granitic protolith, for which equilibrium temperature and pressure conditions of > 1000 degrees C and > 1.5 GPa have been deduced. These extreme (for crustal rocks) conditions, and the inferred peak assemblage, are supported by the newly-developed Zr-in-rutile geothermometer and experimental studies on the same bulk composition, respectively. As these conditions lie above those required for plagioclase stability in quartz tholeiites, they are thus in the eclogite facies. Widespread modification of the peak assemblage, for example mesoperthite formation after ternary feldspar, deformation-induced recrystallisation of perthites to two-feldspar + quartz aggregates, biotite replacing garnet, Ca-loss at garnet rims, sillimanite replacing kyanite or secondary garnet growth, makes reliable interpretation of equilibrium assemblages and compositions very difficult and explains the spread of published pressure- temperature values and consequent confusion about formation depths and the validity of tectonometamorphic models. Such extreme metamorphic conditions in rock compositions typical for the upper continental crust, reflecting a hot subduction environment, has important consequences for understanding some collisional orogens Y1 - 2006 UR - http://www.springerlink.com/content/105515 U6 - https://doi.org/10.1007/s00710-005-0108-2 ER - TY - JOUR A1 - Kröner, Alfred A1 - O'Brien, Patrick J. A1 - Nemchin, A. A. A1 - Pidgeon, R. T. T1 - Zircon ages for high pressure granulites from South Bohemia, Czech Republic, and their connection to Carboniferous high temperature processes Y1 - 2000 ER - TY - JOUR A1 - Li, J. H. A1 - Kröner, Alfred A1 - Qian, X. L. A1 - O'Brien, Patrick J. T1 - Tectonic evolution of an early Precambrian high-pressure granulite belt in the North China Craton Y1 - 2000 ER -