TY - JOUR A1 - Wilke, Franziska Daniela Helena A1 - Sobel, Edward A1 - O'Brien, Patrick J. A1 - Stockli, Daniel F. T1 - Apatite fission track and (U-Th)/He ages from the Higher Himalayan Crystallines, Kaghan Valley, Pakistan: Implications for an Eocene Plateau and Oligocene to Pliocene exhumation JF - Journal of Asian earth sciences N2 - Apatite fission track and apatite and zircon (U-Th)/He ages were obtained from high- and ultra high-pressure rocks from the Kaghan Valley, Pakistan. Four samples from the high altitude northern parts of the valley yielded apatite fission track ages between 24.5 +/- 3.7 and 15.6 +/- 2.1 Ma and apatite (U-Th)/He ages between 21.0 +/- 0.6 and 5.3 +/- 0.2 Ma. These data record cooling of the formerly deeply-subducted high-grade metamorphic rocks induced by denudation and exhumation consistent with extension and back sliding along the reactivated, normal-acting Main Mantle Thrust. Overlap at around 10 Ma between fission track and (U-Th)/He ages is recognised at one location (Besal) showing that fast cooling occurred due to brittle reactivation of a former thrust fault. Widespread Miocene cooling is also evident in adjacent areas to the west (Deosai Plateau, Tso Moran), most likely related to uplift and unroofing linked to continued underplating of the Indian lower crust beneath Ladakh and Kohistan in the Late Eocene to Oligocene. In the southernmost part of the study area, near Naran, two significantly younger Late Miocene to Pliocene apatite fission track ages of 7.6 +/- 2.1 to 4.0 +/- 0.5 Ma suggest a spatial and temporal separation of exhumation processes. These younger ages are best explained by enhanced Late Miocene uplift and erosion driven by thrusting along the Main Boundary Thrust. KW - NW Himalaya KW - Kaghan Valley KW - Thermochronology KW - AFT KW - (U-Th)/He KW - Cooling rates Y1 - 2012 U6 - https://doi.org/10.1016/j.jseaes.2012.06.014 SN - 1367-9120 VL - 59 IS - 3 SP - 14 EP - 23 PB - Elsevier CY - Oxford 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 - 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 - 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 - GEN A1 - Borghini, Alessia A1 - Ferrero, Silvio A1 - O'Brien, Patrick J. A1 - Laurent, Oscar A1 - Günter, Christina A1 - Ziemann, Martin Andreas T1 - Cryptic metasomatic agent measured in situ in Variscan mantle rocks BT - Melt inclusions in garnet of eclogite, Granulitgebirge, Germany T2 - Postprints der Universität Potsdam : Mathematisch Naturwissenschaftliche Reihe N2 - Garnet of eclogite (formerly termed garnet clinopyroxenite) hosted in lenses of orogenic garnet peridotite from the Granulitgebirge, NW Bohemian Massif, contains unique inclusions of granitic melt, now either glassy or crystallized. Analysed glasses and re‐homogenized inclusions are hydrous, peraluminous, and enriched in highly incompatible elements characteristic of the continental crust such as Cs, Li, B, Pb, Rb, Th, and U. The original melt thus represents a pristine, chemically evolved metasomatic agent, which infiltrated the mantle via deep continental subduction during the Variscan orogeny. The bulk chemical composition of the studied eclogites is similar to that of Fe‐rich basalt and the enrichment in LILE and U suggest a subduction‐related component. All these geochemical features confirm metasomatism. In comparison with many other garnet+clinopyroxene‐bearing lenses in peridotites of the Bohemian Massif, the studied samples from Rubinberg and Klatschmühle are more akin to eclogite than pyroxenites, as reflected in high jadeite content in clinopyroxene, relatively low Mg, Cr, and Ni but relatively high Ti. However, trace elements of both bulk rock and individual mineral phases show also important differences making these samples rather unique. Metasomatism involving a melt requiring a trace element pattern very similar to the composition reported here has been suggested for the source region of rocks of the so‐called durbachite suite, that is, ultrapotassic melanosyenites, which are found throughout the high‐grade Variscan basement. Moreover, the Th, U, Pb, Nb, Ta, and Ti patterns of these newly studied melt inclusions (MI) strongly resemble those observed for peridotite and its enclosed pyroxenite from the T‐7 borehole (Staré, České Středhoři Mountains) in N Bohemia. This suggests that a similar kind of crustal‐derived melt also occurred here. This study of granitic MI in eclogites from peridotites has provided the first direct characterization of a preserved metasomatic melt, possibly responsible for the metasomatism of several parts of the mantle in the Variscides. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 976 KW - clinopyroxenite KW - eclogite KW - melt inclusions KW - metasomatism KW - orogenic peridotite Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-474592 SN - 1866-8372 IS - 976 SP - 207 EP - 234 ER - TY - JOUR A1 - Kotkova, Jana A1 - O'Brien, Patrick J. A1 - Ziemann, Martin Andreas T1 - Diamond and coesite discovered in Saxony-type granulite solution to the Variscan garnet peridotite enigma JF - Geology N2 - The pressures required for diamond and coesite formation far exceed conditions reached by even the deepest present-day orogenic crustal roots. Therefore the occurrence of metamorphosed continental crust containing these minerals requires processes other than crustal thickening to have operated in the past. Here we report the first in situ finding of diamond and coesite, characterized by micro-Raman spectroscopy, in high-pressure granulites otherwise indistinguishable from granulites found associated with garnet peridotite throughout the European Variscides. Our discovery confirms the provenance of Europe's first reliable diamond, the "Bohemian diamond," found in A.D. 1870, and also represents the first robust evidence for ultrahigh-pressure conditions in a major Variscan crustal rock type. A process of deep continental subduction is required to explain the metamorphic pressures and the granulite-garnet peridotite association, and thus tectonometamorphic models for these rocks involving a deep orogenic crustal root need to be significantly modified. Y1 - 2011 U6 - https://doi.org/10.1130/G31971.1 SN - 0091-7613 VL - 39 IS - 7 SP - 667 EP - 670 PB - American Institute of Physics CY - Boulder 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 - 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 - 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 - 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 -