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dc.contributor.advisor Wörner, Gerhard Prof. Dr. Albrecht, Nina 2017-05-15T09:09:28Z 2017-05-15T09:09:28Z 2017-05-15
dc.language.iso eng de
dc.subject.ddc 910 de
dc.subject.ddc 550 de
dc.title Metamorphic fluids at extreme pressure conditions and their significance for element transfer in subduction zones de
dc.title.alternative A multidisciplinary study on metamorphic veins in UHP/HP eclogites from Dabieshan, China de
dc.type doctoralThesis de
dc.contributor.referee Wörner, Gerhard Prof. Dr. 2017-04-05
dc.description.abstracteng The present study provides a comprehensive geochemical dataset on ultra-/high pressure (UHP/HP) eclogites and enclosed, fluid-deposited metamorphic veins. It targets to broaden our understanding of the nature and extent of fluid-rock interaction in deep subduction environments, and to evaluate the capability of metamorphic fluids at high pressures to producing elemental fractionations which may account for global mass imbalances such as the enigmatic lead- and niobium paradoxes. This is approached by bringing together geochemistry, isotope geology and a fluid inclusion study to reconstructing fluid origin and compositional evolution as well as fluid-assisted element transport and fractionation in subducting basaltic rocks. The occurrence of different vein generations records multiple stages of fluid flow during deep subduction and exhumation of continental crust. 1 st generation quartz-rutile veins are identified to represent internal remnants of prograde, virtually “dry” eclogite dehydration. The precipitating Si-, and HFSE-bearing aqueous fluids solidify in a closed system at UHP peak conditions, without evidence of retrogression in the host eclogite or detectable compositional fluid evolution. Fluids that precipitate mineralogical complex 2 nd generation veins are found to form at the onset of exhumation from UHP eclogite facies depths. Based on chemical and isotopic fingerprinting, their initial source is determined to be external fluids that are derived from prograde dehydration of bedrock gneisses. iGiven the starting situation of a felsic crust with intercalated basaltic bodies being conjointly subducted, a petrogenetic multistage model in consistence with the collected data is reconstructed: (1) Shallow fluid-rock interaction of comparatively “soft” felsic crust and isotopically light meteoric glaciation waters prior to subduction, while “resistant” basaltic blocks remain widely unaffected. (2) Prograde dehydration of felsic bedrock (later gneisses) during subduction, resulting in fluid- assisted Ca-, Al-, REE-, LILE-, HFSE-, OH- metasomatism of basalt, that selectively occurs at the gneiss-basalt interface and is most likely explained by lawsonitization. (3) Conjoint eclogitization of partly metasomatized and partly pristine basalt bodies, producing epidote- and phengite-bearing eclogites that adopt gneissic signatures, alongside pristine grt + omp ± qtz eclogites that retain their precursors geochemical fingerprints. (4) In-situ formation of 1 st generation veins in pristine, ±dry eclogites at the metamorphic UHP peak. (5) Formation of 2 nd generation veins and concurrent pervasive eclogite retrogression, restricted to metasomatized eclogite portions, and supplied by decompressional lawsonite breakdown and by external remnants of gneissic fluids at the onset of exhumation. (6) Occasional replacement of 2 nd generation veins during late retrograde stages, resulting in 3 rd generation veins with amphibolite facies mineralogy and eventually greenschist facies overprint. Fluid inclusion characteristics, vein mineralogy and mineral chemistry document a compositional evolution of post-peak 2 nd generation vein-forming fluids, proceeding in three stages. The initial fluid, derived from lawsonite breakdown, is dominated by Ca, Al, OH, and is rich in REE and most trace elements. It is trapped in primary and possibly early pseudosecondary fluid inclusions found in eclogite and vein minerals during an early retrograde crystallization stage at the onset of exhumation. This fluid is successively diluted towards the amphibolite facies stage due to element partitioning into newly forming, retrograde (REE-rich) epidote group minerals, either in veins or in retrogressed eclogite. It is trapped at variable stages in pseudo-secondary fluid inclusions. Secondary fluid inclusions and late (REE-poor) epidote record highly evolved, low salinity fluids that experienced negligible influx of meteoric waters. iiElement fractionation is common in the investigated UHP/HP rocks. Nb/Ta is profoundly elevated in rutile of retrogressively affected eclogites relative to Nb/Ta in pristine samples. Fluid-affected eclogites, i.e. rutile therein, are thus identified to be a potential high-Nb- high Nb/Ta candidate to theoretically balance the global niobium deficiency. However, whether or not the crust-mantle vs. eclogite mass balance can be closed depends on the mass of such a reservoir, which is presently unknown. U/Pb is lowered in retrogressively affected eclogites relative to pristine samples, resulting in shifting of retrogressed eclogites towards unradiogenic 206 Pb/ 204 Pb compositions. High pressure retrogressed eclogites, as described in this study, are thus a potential candidate to balancing the global lead- paradox. A theoretical residuum after fluid-rock interaction could provide a conceptual explanation for the enigmatic HIMU source. de
dc.contributor.coReferee Pack, Andreas Prof. Dr.
dc.subject.eng UHP/HP metamorphism de
dc.subject.eng Dabieshan de
dc.subject.eng deep subduction de
dc.subject.eng HIMU de
dc.subject.eng Nb/Ta paradox de
dc.subject.eng metamorphic fluid evolution de
dc.subject.eng metamorphic veins de
dc.subject.eng high pressure fluid-rock interaction de
dc.subject.eng fluid inclusions in metamorphic rocks de
dc.identifier.urn urn:nbn:de:gbv:7-11858/00-1735-0000-0023-3E42-C-8 Fakultät für Geowissenschaften und Geographie de
dc.subject.gokfull Geologische Wissenschaften (PPN62504584X) de
dc.identifier.ppn 887410375

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