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 - 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 - O'Brien, Patrick J. T1 - Subduction followed by Collision : Alpine and Himalayan examples Y1 - 2001 ER - TY - JOUR A1 - O'Brien, Patrick J. A1 - Kryza, R. T1 - The Variscan basement of the Polish Sudetes Y1 - 2001 ER - TY - JOUR A1 - Konrad-Schmolke, Matthias A1 - O'Brien, Patrick J. A1 - Zack, Thomas T1 - Fluid Migration above a Subducted Slab-Constraints on Amount, Pathways and Major Element Mobility from Partially Overprinted Eclogite-facies Rocks (Sesia Zone, Western Alps) JF - Journal of petrology N2 - The Western Alpine Sesia-Lanzo Zone (SLZ) is a sliver of eclogite-facies continental crust exhumed from mantle depths in the hanging wall of a subducted oceanic slab. Eclogite-facies felsic and basic rocks sampled across the internal SLZ show different degrees of retrograde metamorphic overprint associated with fluid influx. The weakly deformed samples preserve relict eclogite-facies mineral assemblages that show partial fluid-induced compositional re-equilibration along grain boundaries, brittle fractures and other fluid pathways. Multiple fluid influx stages are indicated by replacement of primary omphacite by phengite, albitic plagioclase and epidote as well as partial re-equilibration and/or overgrowths in phengite and sodic amphibole, producing characteristic step-like compositional zoning patterns. The observed textures, together with the map-scale distribution of the samples, suggest open-system, pervasive and reactive fluid flux across large rock volumes above the subducted slab. Thermodynamic modelling indicates a minimum amount of fluid of 0 center dot 1-0 center dot 5 wt % interacting with the wall-rocks. Phase relations and reaction textures indicate mobility of K, Ca, Fe and Mg, whereas Al is relatively immobile in these medium-temperature-high-pressure fluids. Furthermore, the thermodynamic models show that recycling of previously fractionated material, such as in the cores of garnet porphyroblasts, largely controls the compositional re-equilibration of the exhumed rock body. KW - fluid migration KW - subduction KW - fluid-rock interaction KW - Sesia Zone Y1 - 2011 U6 - https://doi.org/10.1093/petrology/egq087 SN - 0022-3530 VL - 52 IS - 3 SP - 457 EP - 486 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Konrad-Schmolke, Matthias A1 - Zack, Thomas A1 - O'Brien, Patrick J. A1 - Barth, Matthias T1 - Fluid migration above a subducted slab - Thermodynamic and trace element modelling of fluid-rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps) JF - Earth & planetary science letters N2 - The amount and composition of subduction zone fluids and the effect of fluid-rock interaction at a slab-mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Stand Pb distribution during mineral growth, -breakdown and modification induced by fluid-rock interaction. Our results show that Li and 13 budgets are fluid-controlled, thus acting as tracers for fluid-rock interaction processes, whereas Stand Pb budgets are mainly controlled by the fluid-induced formation of epidote. Our calculations show that fluid-rock interaction caused significant Li and B depletion in the affected rocks due to leaching effects, which in turn can lead to a drastic enrichment of these elements in the percolating fluid. Depending on available fluid-mineral trace element distribution coefficients modelled fluid rock ratios were up to 0.06 in weakly deformed samples and at least 0.5 to 4 in shear zone mylonites. These amounts lead to time integrated fluid fluxes of up to 1.4-10(2) m(3) m(-2) in the weakly deformed rocks and 1-8-10(3) m(3) m(-2) in the mylonites. Combined thermodynamic and trace element models can be used to quantify metamorphic fluid fluxes and the associated element transfer in complex, reacting rock systems and help to better understand commonly observed fluid-induced trace element trends in rocks and minerals from different geodynamic environments. KW - fluid-rock interaction KW - subduction zone KW - fluid migration KW - slab-mantle interface KW - trace element transport Y1 - 2011 U6 - https://doi.org/10.1016/j.epsl.2011.09.025 SN - 0012-821X VL - 311 IS - 3-4 SP - 287 EP - 298 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Schmidt, Alexander A1 - Mezger, Klaus A1 - O'Brien, Patrick J. T1 - The time of eclogite formation in the ultrahigh pressure rocks of the Sulu terrane Constraints from Lu-Hf garnet geochronology JF - Lithos : an international journal of mineralogy, petrology, and geochemistry N2 - Eclogites from the main borehole of the Chinese Continental Scientific Drilling project yield highly precise Lu-Hf garnet-clinopyroxene ages of 216.9 +/- 1.2 Ma (four samples) and 220.5 +/- 2.7 Ma (one sample). The spatial distribution of the rare earth elements in garnet is consistent with the preservation of primary growth zoning, unmodified by diffusion, which supports the interpretation that the Lu-Hf ages date the time of formation of garnet, the major rock forming mineral in the eclogites. The preservation of primary REE-zoning, despite peak metamorphic temperatures around 800-850 degrees C. indicates that the Lu-Hf chronometer is perfectly suitable to date garnet-forming reactions in high grade rocks. The range of Lu-Hf ages for eclogites in the Dabie-Sulu UHP terrane point to episodic rather than continuous growth of garnets and thus punctuated metamorphism during the collision of the North China Block and the Yangtze Block. The U-Pb ages and Hf-isotope systematics of zircon grains from one eclogite sample imply a protracted geologic history of the eclogite precursors that started around 2 Ga and culminated in the UHP metamorphism around 220 Ma. KW - Lu-Hf KW - Eclogite KW - Garnet KW - Geochronology KW - Ultrahigh-pressure Y1 - 2011 U6 - https://doi.org/10.1016/j.lithos.2011.04.004 SN - 0024-4937 VL - 125 IS - 1-2 SP - 743 EP - 756 PB - Elsevier CY - Amsterdam ER -