Um das Verständnis der geologischen Entwicklung des größten bekannten Vorkommens von ultra-hochdruck (UHP) Gesteinen auf der Erde, des Dabie Shan im östlichen China, zu erhöhen, wurde eine multidisziplinäre Studie durchgeführt.<br /> Geophysikalische Daten wurden entlang einer ca. 20 km langen seismischen Linie im östlichen Dabie Shan gesammelt. Diese reflektionsseismischen Daten zeigen, dass die Kruste aus drei Lagen besteht. Die Oberkruste besitzt eine durchgehend niedrige Reflektivität und meist subhorizontale Reflektoren bis in eine Tiefe von ca. 15 km. Aufgrund dieser Charakteristika wird diese Zone als UHP-bezogener krustaler Keil interpretiert, der auf nicht UHP Kruste überschoben wurde. Ein abrupter Wechsel in der Geometrie aber auch Intensität der Reflektoren markiert die Grenze zu einer mittel- bis unterkrustalen Zone, die sich bis ca. 33 km Tiefe erstreckt. Diese Zone repräsentiert wahrscheinlich kratonale Yangtze Kruste, die von der triassischen UHP-Orogenese nicht erfasst wurde, aber während der Exhumierung das Liegende relativ zum UHP Keil war. Starke und kontinuierliche Reflektoren im Tiefenintervall von 33-40 km bilden höchstwahrscheinlich die Moho an der Basis der Kruste ab. Relikte einer Krustenwurzel, die sich wahrscheinlich während der Kollisionstektonik gebildet hatte, sind nicht sichtbar. Ein flaches tomographisches Geschwindigkeitsmodell, das auf der Inversion der Ersteinsätze gründet, konnte zusätzlich erstellt werden. Dieses Modell bildet deutlich die unterschiedlichen Lithologien auf beiden Seiten der Tan Lu Störung ab. Sedimente östlich der Störung zeigen Geschwindigkeiten von 3.4 - 5.0 km* s^-1, wohingegen die Gneise im Westen 5.2 - 6.0 km*s^-1 aufweisen. Die Geometrie der Geschwindigkeits-Isolinien kann als Ausdruck der Strukturen der Gesteine angenommen werden. Somit zeigen die Sedimente ein nordwestliches Einfallen zur Störung hin, wohingegen isoklinale Falten in den Gneisen abgebildet werden.<br /> Geländedaten aus der UHP Einheit des Dabie Shan ermöglichen die Definition von Grundgebirgs- und Deckeinheiten, die Teile des ehemaligen passiven Kontinentalrandes des Yangtze Kratons repräsentieren. Eine der Deckeinheiten, die Changpu Einheit, besitzt nach wie vor einen stratigraphischen Kontakt zu den Grundgebirgs-Gneisen. Der anderen Einheit hingegen, der Ganghe Einheit, fehlt ein entsprechendes Grundgebirge. Diese Einheit steht vielmehr über einen Blasto-Mylonit in tektonischem Kontakt zum Grundgebirge der vorherigen. Die Changpu Einheit baut sich aus kalk-arenitischen Metasedimenten auf, die mit Metabasalten assoziiert sind. Die Ganghe Einheit wird von arenitisch-vulkanoklastischen Metasedimenten, die ebenfalls mit metabasaltischen Gesteinen vergesellschaftet sind, dominiert. Das Grundgebirge baut sich aus diversen felsischen Gneisen auf, die von reliktisch eklogitfaziell bis grünschieferfaziell ausgeprägt sind, und in denen, zusätzlich zu Metabasalten, sporadisch mafisch-ultramafische Meta-Plutone auftreten. Mit Ausnahme der Ganghe Einheit, führen die Metabasite Coesit und belegen somit das UHP Ereignis. Die Mineralchemie der analysierten Proben dokumentiert deutliche Variationen in der Zusammensetzung der Hauptminerale, Granat und Omphazit, was entweder unterschiedliche Protolithe oder unterschiedliche Grade von Stoffaustausch mit den Wirtsgesteinen reflektiert. Gehalte von dreiwertigem Eisen in Omphaziten mit geringen Gesamteisengehalten, wurden mittels Titration bestimmt, wobei sich Werte von 30-40 % ergaben. Dennoch wurde ein noch konservativerer Wert von 50% dreiwertigem Eisen in den entsprechenden Berechnungen angenommen, hauptsächlich, um mit anderen Arbeiten vergleichbar zu sein. Texturen und chemische Zonierungen in den Mineralen sind kompatibel mit Gleichgewichtsbedingungen während dem Höhepunkt der Metamorphose, der retrograd überprägt wird. P-T Daten wurden mit deutlicher Betonung auf das Granat-Omphazit-Phengit Barometer, das mit Fe-Mg Austausch-Thermometern kombiniert wurde, berechnet. Höchstdrucke reichen von 42-48 kbar (für die Changpu Einheit) bis ca. 37 kbar (für das Grundgebirge und die Ganghe Einheit). Während der eklogitfaziellen Metamorphose wurden Temperaturen von ca. 750 °C erreicht. Obwohl die maximalen Drucke deutlich schwanken, sind die Temperaturbestimmungen in guter Übereinstimmung. Die Druckschwankungen können zum einen durch deutlich Ca-dominierte Granate (bis zu 50 mol% Grossular in der Changpu Einheit) und/oder zum anderen durch Modifikationen der Mineralzusammensetzungen während der retrograden Metamorphose erklärt werden.<br /> Die präsentierten integrativen geologischen Daten ermöglichen die folgenden Schlussfolgerungen<br /> i) Grundgebirgs- und Deckeinheiten treten im Dabie Shan auf und wurden beide UHP metamorph überprägt<br /> ii) Der Dabie Shan ist das metamorphe Äquivalent des früheren passiven Kontinentalrandes des Yangtze Kratons<br /> iii) felsische Gneise, die eine UHP Metamorphose durchlaufen, sind von Volumenver-änderungen betroffen, die durch großräumige Phasenumwandlungen (Quarz <-> Coesit) hervorgerufen werden, was direkt die tektono-metamorphen Prozesse beeinflusst<br /> iv) Initiale Unterschiede in der Temperatur sind möglicherweise dafür verantwortlich, dass generell Unterkrustengesteine in UHP Fazies fehlen / A multidisciplinary study has been carried out to contribute to the understanding of the geologic evolution of the largest known occurrence of ultra-high-pressure (UHP) rocks on Earth, the Dabie Shan of eastern China. <br /> Geophysical data, collected along a ca. 20 km E-W trending seismic line in the eastern Dabie Shan, indicate that the crust comprises three layers. The upper crust has a homogeneously low reflectivity and exhibits roughly subhorizontal reflectors down to ca. 15 km. It is therefore interpreted to portray a crustal UHP slab thrust over non-UHP crust. An aprubt change in intensity and geometry of observed reflectors marks the boundary of a mid- to lower crustal zone which is present down to ca. 33 km. This crustal zone likely represents cratonal Yangtze crust that was unaffected by the Triassic UHP event and which has acted as the footwall during exhumation of the crustal wedge. Strong and continuous reflectors occurring at ca. 33-40 km depth most likely trace the Moho at the base of the crust. Any trace of a crustal root, that may have formed in response to collision tectonics, is therefore not preserved. A shollow tomographic velocity modell based on inversion of the first arrivals is constructed additionally. This model clearly images the distinct lithologies on both sides of the Tan Lu fault. Sediments to the east exhibit velocities of about 3.4 - 5.0 km* s^-1, whereas the gneisses have 5.2 - 6.0 km*s^-1. Geometry of velocity isolines may trace the structures present in the rocks. Thus the sediments dip shallowly towards the fault, whereas isoclinal folds are imaged to occur in the gneisses.<br /> Field data from the UHP unit of the Dabie Shan enables definition of basement-cover sequences that represent sections of the former passive margin of the Yangtze craton. One of the cover sequences, the Changpu unit, still displays a stratigraphic contact with basement gneisses, while the other, the Ganghe unit, includes no relative basement exposure. The latter unit is in tectonic contact with the basement of the former unit via a greenschist-facies blastomylonite. The Changpu unit is chiefly constituted by calc-arenitic metasediments intercalated with meta-basalts, whereas the Ganghe unit contains arenitic-volcanoclastic metasediments that are likewise associated with meta-basalts. The basement comprises a variety of felsic gneisses, ranging from preserved eclogitic- to greenschist-facies paragenesis, and locally contains mafic-ultramafic meta-plutons in addition to minor basaltic rocks. Metabasites of all lithologies are eclogite-facies or are retrogressed equivalents, which, with the exception of those from the Ganghe unit, bear coesite and thus testify to an UHP metamorphic overprint. Mineral chemistry of the analysed samples reveal large compositional variations among the main minerals, i.e. garnet and omphacite, indicating either distinct protoliths or different degrees of interaction with their host-rocks. Contents of ferric iron in low Fetot omphacites are determined by wet chemical titration and found to be rather high, i.e. 30-40 %. However, a even more conservative estimate of 50% is applied in the corresponding calculations, in order to be comparable with previous studies. Textural constraints and compositional zonation pattern are compatible with equilibrium conditions during peak metamorphism followed by a retrogressive overprint. P-T data are calculated with special focus on the application of the garnet-omphacite-phengite barometer, combined with Fe-Mg exchange thermometers. Maximum pressures range from 42-48 kbar (for the Changpu unit) to ~37 kbar (for the Ganghe unit and basement rocks). Temperatures during the eclogite metamorphism reached ca. 750 °C. Although the sample suite reveals variable peak-pressures, temperatures are in reasonable agreement. Pressure differences are interpreted to be due to strongly Ca-dominated garnet (up to 50 mol % grossular in the Changpu unit) and modification of peak-compositions during retrogressive metamorphism.<br /> The integrated geological data presented in this thesis allow it to be concluded that, <br /> i) basement and cover rocks are present in the Dabie Shan and both experienced UHP conditions<br /> ii) the Dabie Shan is the metamorphic equivalent of the former passive margin of the Yangtze craton <br /> iii) felsic gneisses undergoing UHP metamorphism are affected by volume changes due to phase transitions (qtz <-> coe), which directly influence the tectono-metamorphic processes<br /> iv) initial differences in temperature may account for the general lack of lower crustal rocks in UHP-facies

Oriented needle-shaped or rod-shaped quartz precipitates occur in clinopyroxenes have been commonly observed in eclogites or garnet peridotites from the high pressure or ultra-high pressure (HP/UHP) metamorphic belts, and their occurrence has been used as an indicator of UHP metamorphism. However, the origin of those quartz precipitates and their crystallographic orientation relationships with clinopyroxene hosts are still not clear. In order to understand the formation mechanisms and environments of the quartz precipitates, the present study has used electron backscattered diffraction (EBSD) analysis, petrographic and scanning electron microscopy, and electron microprobe analysis to study textural features, mineral assemblages, mineral compositions, and crystallographic orientation relationships of mineral precipitates in the omphacite from Xitieshan, North Qaidam UHP metamorphic belt. The results show that the oriented rod-like precipitates in the omphacite hosts are mainly composed of quartz + edenite, and the rods are 5~20

This dissertation research addresses the tectonism of continental crust during ocean basin closure, suturing between continental landmasses, and collisional orogenesis. The new data and insights presented here were gathered through localized geologic investigations of the Tibetan Plateau of central Asia. This area of central Asia is an ideal location to study these fundamental tectonic processes because it has been the locus of numerous Tethyan ocean basins and terminal collisions between continents during Phanerozoic accretion of Gondwana-derived landmasses onto the southern margin of Eurasia. In this work, I propose, in many orogens, that high-pressure (HP) metamorphism of continental rocks may mark the early stages of the suturing process between continental landmasses rather than the culmination of suturing. This insight has been acquired from a geologic-, geochronologic-, and thermochronologic-based investigation of the HP-near ultrahigh-pressure bearing Triassic metasedimentary metamorphic belt in central Tibet. This work shows near synchronous continent-continent collisions between landmass adjacent to the Paleo-Tethys ocean prior to its final closure in Late Triassic time. In addition, this work shows that Mediterranean-style tectonics may be more widespread during accretionary tectonics than previously thought. A comparison between the distribution of the HP bearing metamorphic belt, autochthonous crystalline basement, and geophysical images of Tibet suggests that a Mesozoic tectonic feature may be controlling the structure and distribution of melt within the middle crust of the Tibetan Plateau. This concept underscores the importance of inherited tectonic frameworks on the evolution of orogenic plateaus. Work in southwest Tibet, along the India-Asia suture zone, highlights the complex behavior of continental crust during collisional orogenesis. This work identifies previously undocumented magmatism, crustal antexis, and high-grade metamorphism along the India-Asia suture. In this work I attribute these observations to the initial interactions between Indian, Asian, and subducting Neo-Tethys oceanic lithosphere.

This thesis seeks to improve our understanding of the processes involved in continental collision zones, with a particular focus on subduction-exhumation. The main objective of this work has been to define the tectonometamorphic evolution of the deeply subducted Seve Nappe Complex (SNC) in the Scandinavian Caledonides. I utilize mineralogy, petrology and geochronology to constrain the P-T-t paths of the SNC rocks in Sweden. The research has focused on the high grade rocks of the SNC and resulted in the discovery of metamorphic diamonds within the gneisses in west-central Jämtland and southern Västerbotten. Microdiamonds provided evidence for the ultra-high pressure metamorphism (UHPM) and subduction of continental rocks to mantle depths. The UHPM in these rocks was confirmed by calculations of the P-T conditions. The UHPM is further recorded by eclogites and garnet pyroxenites from northern Jämtland and eclogites from Norrbotten. All these findings provide compelling evidence for regional UHPM of vast parts of the SNC (at least 400 km along the strike of this allochthonous unit). The SNC rocks followed nearly isothermal decompression paths and paragneisses have locally experienced partial melting during exhumation. Formation of the peculiar Ba- and Ti-enriched dark mica in the Tväråklumparna metasediments is related to the latter stage. In-situ monazite dating of the diamond-bearing gneisses from west-central Jämtland supports previous geochronological data inferring that the peak of metamorphism is probably Middle Ordovician and was followed by Early Silurian partial melting. The exact timing of the UHPM here still remains to be resolved. The Lu-Hf garnet and U-Pb zircon dating of eclogite and gneiss from northern Jämtland confirms the Middle Ordovician age of the UHP-HP metamorphism of the SNC rocks. The chemical dating of monazite from the Marsfjället gneiss suggests an earlier UHP history of the Seve rocks in southern Västerbotten as a post-UHP uplift is dated to ca. 470 Ma. Based on the P-T-t data obtained in this thesis, particularly on the evidence for Middle Ordovician UHPM and subsequent Silurian exhumation, a new tectonic model for the Scandinavian Caledonides has been proposed. The outcomes of this thesis therefore improve our understanding of the tectonometamorphic history of the Caledonides.

M.Sc. (Geology) / This treatise presents the first comprehensive investigation in petrography and geochemistry of a mantle-derived eclogite xenolith suite from the Balmoral kimberlite. Eclogites form a minor component of the Earth’s mantle however they play a vital role in our understanding of geodynamic processes, i.e. the subduction of oceanic crust (Jacob, 2004) and the crystallization of diamond within the sub-cratonic lithosphere. A large portion of eclogites from the Balmoral kimberlite pipe is comprised of bimineralic (garnet and clinopyroxene) rocks with the rest being corundum-bearing. Mica with average modal abundances ≤10 vol% is observed as an accessory phase in bimineralic xenoliths. Modal abundances of corundum in corundum-bearing samples range between 1 and 6 vol%. Textures are ambiguous in Balmoral eclogites and thus chemical criteria of McCandless and Gurney (1989) places all Balmoral eclogites into Group II. As typically observed in garnets from eclogites (Hills and Haggerty, 1989; Jacob, 2004), garnets from Balmoral eclogites are chromium- and manganese-poor. They have a general trend from pyrope-rich towards grossular-rich compositions, with some almandine. Garnets from the bimineralic eclogites have disparate suites of low- and high-MgO samples. High-MgO bimineralic garnets are pyropic in composition with averages at Pyr63Gros22Alm15, whereas garnets from the low-MgO suite are widespread from relatively less pyropic towards grossular-rich compositions with average compositions of Pyr49Gros40Alm11. Garnets from the corundum-bearing eclogites are homogeneous and characterised by the highest grossular component (averaging at Gros47Pyr28Alm25). The clinopyroxenes for Balmoral eclogites are omphacitic in composition. Jadeite content is highly variable (ranging between 8 and 58 wt%) in these clinopyroxenes. The clinopyroxenes in bimineralic eclogites are characterised by a wide variation from diopside-rich towards jadeite-rich compositions. Clinopyroxenes in corundum-bearing eclogites have the highest jadeite levels.

The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host. I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase. SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of our study suggest portions of the Yukon-Tanana terrane were subducted to high-pressure and ultrahigh-pressure conditions. This is the first report of ultrahigh-pressure metamorphism in the accreted terranes of the North American Cordillera. Petrological, geochemical, geochronological, and structural relationships link the eclogites at St. Cyr to other eclogite localities in Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon-Tanana terrane.

Raman spectroscopy is an irreplaceable method for chemical and structural characterization of many substances. This is especially true for carbonaceous matter. It is non-destructive and capable of determining the crystallinity of the studied carbonaceous phase, and subsequently, with the aid of empirical relations, also the temperature of formation of the rock under study. Until very recently, the most of the Raman spectroscopy measurements was conducted only single point-wise on grains chosen on purpose, e.g., in an optical microscope. Alongside with the evolution and affordability of the Raman spectroscopic technique it is now very appealing to utilize the possibility of Raman mapping coupled to the automated analysis of large quantities of acquired spectra. The aim of this study was to critically evaluate the pros and cons of such an approach on selected rocks with varying carbon content and crystallinity of the carbonaceous phase. The samples range from graphite, Karelian shungite, elemental carbon-rich carbonates, alpine schist to eclogite. In general, we can rate the large area mapping as suitable in many cases, however, there are certain issues associated with the method, especially concerning the sample preparation and automated analysis. Several actions are proposed to limit the pitfalls...

English abstract This work focuses on high pressure atoll garnet-bearing metabasites from the central part of Krušné hory Mts. in Saxothuringian zone of Bohemian Massif. Eclogite bodies are interpreted as a part of alochtonnous units, which were dragged into the high pressure conditions of subduction zone during a subduction of Saxothuringian oceanic crust under the Teplá-Barrandien Unit and subsequently they were exhumed into upper parts of subduction- collisional system. Main mineral assemblage of eclogites consists of omphacite, garnet, quartz and amphibole which replaces grains of omphacite. Minor minerals present are rutile, ilmenite, talc a chlorite. Zircon, apatite, paragonite, pyrite, plagioclase, albite and monazite are enclosed in porphyroblasts of garnets. Grains of garnet frequently form the atoll structures, where its central part of a grain is replaced by new minerals of matrix and rim part stays preserved. Based on compositional profiles and mapping of major and trace elements, two evolution phases of garnet were distinguished. Older garnet (I) forms mainly preserved cores of garnet and on the contrary younger garnet (II) is present on rims or also replaces garnet I in the core part of a grain. Garnet I has higher amounts of Ca and Mn but lower Mg and Fe than garnet II. Preserved grains of...

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Indiana University-Purdue University Indianapolis (IUPUI) / The Tso Morari terrane within the Himalayan orogenic belt underwent ultrahigh-pressure (UHP) coesite-eclogite metamorphism due to northward subduction of the Indian continent under the Eurasian continent during the early Eocene. In this study we optimized a best protocol for thermodynamically modelling pressure-temperature (P-T) paths of high-grade metabasites using the Tso Morari eclogite as a case study through evaluating the effects of employing commonly used thermodynamic modeling techniques (e.g., programs, thermodynamic datasets, a-X relations). A “fishhook” shaped clockwise P-T path was obtained with a peak pressure of ~28.5 kbar at ~563 °C, followed by a peak temperature of ~613 °C at ~24.5 kbar. The peak pressures predicted by modelling protocols are consistent with the conventional thermobarometry results and petrographic observations from the Tso Morari eclogites. Secondly, thermodynamic modelling using P-M(H2O) pseudosections on Tso Morari UHP rocks indicates three distinct fluid events during the prograde and retrograde metamorphism. Fluid Event 1 caused the fluid-assisted homogenization of prograde garnet cores in eclogite at ~18.5 kbar and ~555 °C; Fluid Event 2 is evidenced by the formation of poikiloblastic epidote (~23.5 kbar and ~610 °C, at the expense of lawsonite) and amphibole (from ~19.0 to ~14.5 kbar at ~610 °C, at the expense of omphacite and talc), and symplectite association (~8.7 kbar and ~625 °C) in the eclogite matrix without external fluid supply. Fluid Event 3 was determined through modelling the amphibolitization of eclogites with external fluid infiltration at ~9.0–12.5 kbar and ~608 °C. This fluid phase most likely derived from the mixing of dehydrated host orthogneiss and/or metasediments during exhumation through the amphibolite-facies zone in the subduction channel. This study demonstrates the need for using careful petrographic observations in parallel with thermodynamic modelling to achieve realistic results. / 2023-12-02