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Insight Into a Fossil Plate Interface: Unraveling the P-T-t-d Evolution of the Akeyasi Metamorphic Belt in Southwest Tianshan (NW China).

Record Type:	Electronic resources : Monograph/item
Title/Author:	Insight Into a Fossil Plate Interface: Unraveling the P-T-t-d Evolution of the Akeyasi Metamorphic Belt in Southwest Tianshan (NW China)./
Author:	Bayet, Lea.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2018,
Description:	166 p.
Notes:	Source: Dissertations Abstracts International, Volume: 81-10, Section: B.
Contained By:	Dissertations Abstracts International81-10B.
Subject:	Plate tectonics. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=27605823
ISBN:	9781687930675

Insight Into a Fossil Plate Interface: Unraveling the P-T-t-d Evolution of the Akeyasi Metamorphic Belt in Southwest Tianshan (NW China).
Bayet, Lea.

Insight Into a Fossil Plate Interface: Unraveling the P-T-t-d Evolution of the Akeyasi Metamorphic Belt in Southwest Tianshan (NW China). - Ann Arbor : ProQuest Dissertations & Theses, 2018 - 166 p.

Source: Dissertations Abstracts International, Volume: 81-10, Section: B.

Thesis (Ph.D.)--Freie Universitaet Berlin (Germany), 2018.

This item must not be sold to any third party vendors.

Subduction zones represent important tectonic boundaries and are the scene of devastating geohazards. In subduction zones, most of the fluid and material transfer, deformation, and earthquakes occur at the plate interface. Although the geometry of the plate interface is partly constraint by geophysical data, the fate of deeply buried material and the processes occurring at depth remain uncertain. In particular, the deep burial of large volumes of sediments poses a conundrum on the role of buoyancy, coupling-decoupling transitions, and their origin. Indeed, material incorporated in a subduction zone may derive from various sources, depending for instance on the subduction type (accretion versus erosion), the nature of the trench infill, and the formation of melanges (tectonic or sedimentary). Fossil subduction zones provide key information on processes occurring at depth, recorded in rocks through their burial and exhumation. The Akezayi metamorphic complex (AMC) or southern Tianshan metamorphic belt (Tianshan, northwest China) represents such a fossil plate interface. Controversial models depicted the AMC as resulting from a tectonic melange, with juxtaposition of rocks from various depths or as consisting of two units: the high-pressure (HP) and the ultra-high-pressure (UHP) units. The AMC consists of a greenschist-facies unit in the north and south and a HP to UHP (HP-UHP) unit in its central part. The greenschist-facies unit consist of metasediments typically composed of green amphibole, chlorite, and albite ± white mica. The HP-UHP unit represents a ~4-5 km volcano-clastic sequence. Lithology in the HP-UHP unit is characterized by an intimate interlayering of micaschists and mafic rocks. Rock composition varies between 'pure' micaschist and 'pure' mafic components. The detailed mapping of the lithology highlighted a global compositional gradient from more pelitic in the north to more psammitic and mafic in the south. The HP-UHP sequence exhibits pervasive deformation with the identification of a minimum of five stages. The first, only visible on the microscopic level, is represented by a rotative schistosity of quartz inclusion in garnets, and is attributed to burial. This stage is followed by a pervasive schistosity, observed at various scales (from cm- to km-scale), subsequently reworked by crenulation schistosity and folding. Pervasive shearing of both, the schistosity and crenulation schistosity, occurred later during exhumation, as indicated by the pervasive glaucophane lineation. Some of the shear zones reach a km-scale extent and structurally divide the HP-UHP unit in five sub-units. Finally, C' structures define the latest observed deformation stage and formed during greenschist-facies conditions. The determination of the peak pressure-temperature (P-T) conditions was conducted in a systematic way, on spatially distributed samples using robust thermobarometric methods. The peak temperature was estimated by applying the Raman spectroscopy on carbonaceous material (RSCM; TC) and Zr in rutile thermometry (TZr). Results yield a homogeneous distribution of peak temperature combining both TZr and TC, with an average of 538 ± 12 °C for the HP-UHP unit. Minimum peak pressure was estimated using quartz in garnet (QuiG) and phengite barometry. Coesite identified in several samples indicates a minimum pressure of 2.7 GPa for a given temperature of ~540 °C. Combining coesite occurrence, QuiG and phengite barometry revealed a rather coherent pressure distribution of 2.52 ± 0.25 GPa, i.e., widespread UHP metamorphic conditions. A review of the previous metamorphism age estimates revealed peak condition ages of 320.5 ± 6.6 Ma (2σ) using garnet Lu-Hf and Sm-Nd, zircon and rutile U-Pb, and Rb-Sr white mica dating. The present study yielded similar ages with three well-constrained isochrons giving a date of 321.6 ± 1.9 Ma (2σ). A fourth sample yielded an age of 315.9 ± 1.8 Ma, still in agreement with the peak metamorphism estimates within error. Post-peak conditions have been estimated to around 311 Ma, mainly based on Rb-Sr, Ar-Ar and K-Ar white mica dating. We thus interpret the age of 310.0 ± 2.5 Ma of the last sample as reflecting post-peak conditions, which is supported by the presence of retrograde paragonite finely interlayered with phengite. Combining field data, peak P-T estimates, and Rb-Sr dating lead to the conception of a new geodynamic model for the AMC. We suggest that the volcano-clastic rocks derive from the sedimentary cover of the oceanic plate, parts of an eroded accretionary prism and trench infill containing ashes from arc volcanism. The material was buried, detached, and stacked, near greenschist-facies levels and at HP-UHP conditions. The deeper stacking occurred in several successive slices thrusted on top of each other at a depth of ~80km (2.5-2.7 GPa), previously inferred as a decoupling-coupling transition. Although buoyant and given that a duration of a few million years was estimated for such a stacking, this volcano-clastic sequence remained relatively cold (540 °C) and did not undergo diapirism. The temperatures were likely shielded by the presence of a mantle wedge cold nose, intensive fluid circulation and/or a thickened overriding plate. We suggest that diapirs most likely form deeper down and/or farther away from the plate interface. This stacking formed a coherent and single unit, as attested by the rather homogeneous peak P-T-t conditions between the structural sub-units. The sequence was subsequently detached from the upper plate and exhumed to blueschist-facies conditions, where thrusts were reactivated in major shear zones with a normal movement. The thickness of each slice (< 1km), and the shearing and thinning of the ~4-5 km sequence during exhumation may reflect a thin plate interface thickness, as suggested by geophysical data (~2-5 km). The pillow basalt unit, located in the southernmost part of the belt, was detached at last, together with the rest of the sequence at UHP conditions, or during exhumation of sequence at HP conditions. The present-day geometry reveals a dome-like structure of the greenschist-facies unit, suggesting its exhumation prior to or simultaneous with the exhumation of the HP-UHP unit.

ISBN: 9781687930675Subjects--Topical Terms:

542702
Plate tectonics.
Subjects--Index Terms:

Metamorphic rocks

Insight Into a Fossil Plate Interface: Unraveling the P-T-t-d Evolution of the Akeyasi Metamorphic Belt in Southwest Tianshan (NW China).
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100 1 $a Bayet, Lea. $3 3550344
245 1 0 $a Insight Into a Fossil Plate Interface: Unraveling the P-T-t-d Evolution of the Akeyasi Metamorphic Belt in Southwest Tianshan (NW China).
260 1 $a Ann Arbor : $b ProQuest Dissertations & Theses, $c 2018
300 $a 166 p.
500 $a Source: Dissertations Abstracts International, Volume: 81-10, Section: B.
500 $a Advisor: John, Timm.
502 $a Thesis (Ph.D.)--Freie Universitaet Berlin (Germany), 2018.
506 $a This item must not be sold to any third party vendors.
520 $a Subduction zones represent important tectonic boundaries and are the scene of devastating geohazards. In subduction zones, most of the fluid and material transfer, deformation, and earthquakes occur at the plate interface. Although the geometry of the plate interface is partly constraint by geophysical data, the fate of deeply buried material and the processes occurring at depth remain uncertain. In particular, the deep burial of large volumes of sediments poses a conundrum on the role of buoyancy, coupling-decoupling transitions, and their origin. Indeed, material incorporated in a subduction zone may derive from various sources, depending for instance on the subduction type (accretion versus erosion), the nature of the trench infill, and the formation of melanges (tectonic or sedimentary). Fossil subduction zones provide key information on processes occurring at depth, recorded in rocks through their burial and exhumation. The Akezayi metamorphic complex (AMC) or southern Tianshan metamorphic belt (Tianshan, northwest China) represents such a fossil plate interface. Controversial models depicted the AMC as resulting from a tectonic melange, with juxtaposition of rocks from various depths or as consisting of two units: the high-pressure (HP) and the ultra-high-pressure (UHP) units. The AMC consists of a greenschist-facies unit in the north and south and a HP to UHP (HP-UHP) unit in its central part. The greenschist-facies unit consist of metasediments typically composed of green amphibole, chlorite, and albite ± white mica. The HP-UHP unit represents a ~4-5 km volcano-clastic sequence. Lithology in the HP-UHP unit is characterized by an intimate interlayering of micaschists and mafic rocks. Rock composition varies between 'pure' micaschist and 'pure' mafic components. The detailed mapping of the lithology highlighted a global compositional gradient from more pelitic in the north to more psammitic and mafic in the south. The HP-UHP sequence exhibits pervasive deformation with the identification of a minimum of five stages. The first, only visible on the microscopic level, is represented by a rotative schistosity of quartz inclusion in garnets, and is attributed to burial. This stage is followed by a pervasive schistosity, observed at various scales (from cm- to km-scale), subsequently reworked by crenulation schistosity and folding. Pervasive shearing of both, the schistosity and crenulation schistosity, occurred later during exhumation, as indicated by the pervasive glaucophane lineation. Some of the shear zones reach a km-scale extent and structurally divide the HP-UHP unit in five sub-units. Finally, C' structures define the latest observed deformation stage and formed during greenschist-facies conditions. The determination of the peak pressure-temperature (P-T) conditions was conducted in a systematic way, on spatially distributed samples using robust thermobarometric methods. The peak temperature was estimated by applying the Raman spectroscopy on carbonaceous material (RSCM; TC) and Zr in rutile thermometry (TZr). Results yield a homogeneous distribution of peak temperature combining both TZr and TC, with an average of 538 ± 12 °C for the HP-UHP unit. Minimum peak pressure was estimated using quartz in garnet (QuiG) and phengite barometry. Coesite identified in several samples indicates a minimum pressure of 2.7 GPa for a given temperature of ~540 °C. Combining coesite occurrence, QuiG and phengite barometry revealed a rather coherent pressure distribution of 2.52 ± 0.25 GPa, i.e., widespread UHP metamorphic conditions. A review of the previous metamorphism age estimates revealed peak condition ages of 320.5 ± 6.6 Ma (2σ) using garnet Lu-Hf and Sm-Nd, zircon and rutile U-Pb, and Rb-Sr white mica dating. The present study yielded similar ages with three well-constrained isochrons giving a date of 321.6 ± 1.9 Ma (2σ). A fourth sample yielded an age of 315.9 ± 1.8 Ma, still in agreement with the peak metamorphism estimates within error. Post-peak conditions have been estimated to around 311 Ma, mainly based on Rb-Sr, Ar-Ar and K-Ar white mica dating. We thus interpret the age of 310.0 ± 2.5 Ma of the last sample as reflecting post-peak conditions, which is supported by the presence of retrograde paragonite finely interlayered with phengite. Combining field data, peak P-T estimates, and Rb-Sr dating lead to the conception of a new geodynamic model for the AMC. We suggest that the volcano-clastic rocks derive from the sedimentary cover of the oceanic plate, parts of an eroded accretionary prism and trench infill containing ashes from arc volcanism. The material was buried, detached, and stacked, near greenschist-facies levels and at HP-UHP conditions. The deeper stacking occurred in several successive slices thrusted on top of each other at a depth of ~80km (2.5-2.7 GPa), previously inferred as a decoupling-coupling transition. Although buoyant and given that a duration of a few million years was estimated for such a stacking, this volcano-clastic sequence remained relatively cold (540 °C) and did not undergo diapirism. The temperatures were likely shielded by the presence of a mantle wedge cold nose, intensive fluid circulation and/or a thickened overriding plate. We suggest that diapirs most likely form deeper down and/or farther away from the plate interface. This stacking formed a coherent and single unit, as attested by the rather homogeneous peak P-T-t conditions between the structural sub-units. The sequence was subsequently detached from the upper plate and exhumed to blueschist-facies conditions, where thrusts were reactivated in major shear zones with a normal movement. The thickness of each slice (< 1km), and the shearing and thinning of the ~4-5 km sequence during exhumation may reflect a thin plate interface thickness, as suggested by geophysical data (~2-5 km). The pillow basalt unit, located in the southernmost part of the belt, was detached at last, together with the rest of the sequence at UHP conditions, or during exhumation of sequence at HP conditions. The present-day geometry reveals a dome-like structure of the greenschist-facies unit, suggesting its exhumation prior to or simultaneous with the exhumation of the HP-UHP unit.
520 $a Subduktionszonen stellen wichtige tektonische Grenzen dar und sind Schauplatze verheerender Naturkatastrophen. In den Subduktionszonen treten die meisten Fluid- und Materialtransfers, Deformationen und Erdbeben entlang der Plattengrenzflache auf. Obwohl die Geometrie der Plattengrenzflache durch geophysikalische Daten teilweise bekannt ist, sind die Entwicklung tief subduzierter Gesteine und die zugrunde liegenden Prozesse kaum erforscht. Insbesondere die tiefe Subduktion groser Sedimentvolumina wirft Fragen nach den Rollen von Auftrieb, Kopplungs-Entkopplungs-Ubergangen und ihrer Ursprunge auf. Tatsachlich kann das Material, das in eine Subduktionszone eingebracht wird, aus verschiedenen Quellen stammen - abhangig vom Subduktionstyp (Akkretion versus Erosion), der Art der Tiefseegrabensedimente und der Bildung von Melangen (tektonisch oder sedimentar). Fossile Subduktionszonen liefern wichtige Informationen uber Vorgange in der Tiefe, welche in Gesteinen wahrend der Versenkung und Exhumierung aufgezeichnet wurden. Der Akezayi Metamorphic Complex (AMC) oder sudliche Tianshan Metamorphic Belt (Tianshan, Nordwestchina) reprasentiert eine solche fossile Plattengrenze. Umstrittene Modelle interpretieren den AMC als das Ergebnis einer tektonischen Melange, in der Gesteine aus verschiedenen Tiefen vermengt wurden, oder welche aus zwei Einheiten besteht: einer Hochdruck- (HP) und einer Ultrahochdruck- (UHP) -Einheit. Der AMC besteht aus einer Grunschiefer-faziellen Einheit im Norden und Suden, und einer HP-UHP-Einheit (HP-UHP) im zentralen Teil. Die Grunschiefer-fazielle Einheit besteht aus Metasedimenten, die typischerweise grunen Amphibol, Chlorit und Albit ± weisem Glimmer fuhren. Die HP-UHP-Einheit ist eine ~ 4-5 km vulkanisch-klastische Abfolge. Die Lithologie in der HP-UHP-Einheit ist durch eine Zwischenschicht von Glimmerschiefern und mafischen Gesteinen charakterisiert. Die Gesteinszusammensetzung variiert zwischen rein pelitischen und rein mafischen Komponenten. Die detaillierte Kartierung der Lithologie offenbarte einen globalen Zusammensetzungsgradienten von pelitisch im Norden zu psamititisch und mafisch im Suden. Die Sequenz der HP-UHP-Einheit zeugt von einer durchdringenden Deformation, welche in mindestens funf Stufen unterteilt werden kann. Die erste, welche nur mikroskopisch sichtbar ist, wird durch eine rotierende Schieferung von Quarzeinschlussen in Granaten dargestellt und wird der Versenkung zugeschrieben. Diesem Stadium folgte eine durchdringende Schieferung, die auf verschiedenen Skalen (von cm- bis km-Skala) beobachtet wird und anschliesend durch Krenulations-Schieferung und Faltung uberpragt wird. Die durchdringende Scherung von beidem, der Schieferung und der Krenulations-Schieferung, erfolgte spater wahrend der Exhumierung wie durchdringende Glaukophan-Lineationen anzeigen. Einige der Scherzonen erreichen eine Ausdehnung im km-Bereich und teilen die HP-UHP-Einheit strukturell in funf Untereinheiten. Schlieslich zeigen C'-Strukturen das letzte beobachtbare Deformationsstadium an, welches unter Grunschiefer-faziellen Bedingungen ablief. Die Bestimmung der Peak-Drucke (P) und -Temperaturen (T) wurde systematisch an Hand von raumlich verteilten Proben mittels robusten thermobarometrischen Methoden durchgefuhrt. Die Peak-Temperaturen wurde durch Raman-Spektroskopie an kohlenstoffhaltigem Material (RSCM; TC) und Zr in Rutil-Thermometrie (TZr) bestimmt. Die Ergebnisse zeigen eine homogene Verteilung der Peak-Temperaturen mit durchschnittlich 538 ± 12 ° C fur die HP-UHP-Einheit aus der Kombination von TZr und TC an. Der minimale Spitzendruck wurde unter Verwendung von Quarz in Granat- (QuiG) und Phengit-Barometrie geschatzt. Coesit, der in mehreren Proben identifiziert wurde, zeigt einen Mindestdruck von 2,7 GPa fur die gegebene Temperatur von ca. 540 ° C an. Durch die Kombination des Vorhandenseins von Coesit, QuiG- und Phengit-Barometer konnte eine ziemlich koharente Druckverteilung von 2.52 ± 0.25 GPa bestimmt werden, d.h. UHP-metamorphe Bedingungen. Eine Uberprufung fruherer Altersschatzungen ergab ein Alter von 320,5 ± 6,6 Ma (2σ) fur die metamorphen Peak-Bedingungen unter Verwendung von Granat Lu-Hf- und Sm-Nd-, Zircon und Rutil U-Pb-, und Rb-Sr-Hellglimmer-Datierungen. Die vorliegende Studie erzeugte ahnliche Daten mit drei gut definierten Isochronen, welche ein Alter von 321,6 ± 1,9 Ma (2σ) definieren. Eine vierte Probe ergab ein Datum von 315,9 ± 1,8 Ma, welches innerhalb des Fehlers ebenfalls mit den Schatzungen des Alters der metamorphosen Peak-Bedingungen ubereinstimmen. Post-Peak-Bedingungen wurden in alteren Studien auf etwa 311 Ma geschatzt und die Alter basieren hauptsachlich auf Rb-Sr-, Ar-Ar- und K-Ar-Weisglimmer-Datierungen. Wir ordnen somit das Alter von 310,0 ± 2,5 Ma der letzten Probe den Post-Peak-Bedingungen zu, was durch die Anwesenheit von retrogradem Paragonit, welcher mit Phengit verwachsen ist, unterstutzt wird. Die Kombination aus Gelandedaten, Peak-P-T-Abschatzungen und Rb-Sr-Datierungen fuhrte zur Konzeption eines neuen geodynamischen Modells fur den AMC. Wir vermuten, dass die vulkanisch-klastischen Gesteine von der sedimentaren Bedeckung der ozeanischen Platte, Teilen eines erodierten Akkretionsprismas, und der Tiefseegrabenfullung, welche Asche vom Bogenvulkanismus enthalt, stammen. Das Material wurde in der Folge subduziert, abgeschert und gestapelt, jeweils unter Grunschiefer-faziellen- und HP-UHP-Bedingungen. Die tiefere Stapelung erfolgte durch die Uberschiebung mehrerer aufeinanderfolgenden Schollen in einer Tiefe von ca. 80 km (2,5-2,7 GPa), welche zuvor als Entkopplungs-Kopplungs-Ubergang gedeutet wurde. Trotz Auftriebs und der Tatsache, dass fur eine solche Stapelung eine Dauer von einigen Millionen Jahren angenommen wird, blieb diese vulkanisch-klastische Abfolge relativ kalt (540 ° C) und erfuhr keinen Diapirismus. Die relativ geringen Temperaturen wurden wahrscheinlich durch das Vorhandensein einer "kalten Nase" des Mantelkeils, einer intensiven Fluidzirkulation und/ oder einer verdickten, hangenden Platte begunstigt. Wir vermuten, dass Diapire sich wahrscheinlich tiefer und/ oder weiter von der Plattengrenzflache entfernt bilden. Die gestapelten Schollen bildeten eine koharente und einzelne Einheit wahrend der Subduktion, was durch die homogenen Peak-P-T-t Bedingungen in den strukturellen Untereinheiten gestutzt wird. Die Sequenz wurde anschliesend von der hangenden Platte gelost und bis in blauschiefer-fazielle Tiefen exhumiert, wo Uberschiebungen in grosen Scherzonen mit einer normalen Bewegungsrichtung reaktiviert wurden. Die Machtigkeit jeder tektonischen Scholle (<1km), und das Scheren und die Ausdunnung der 4-5 km machtigen Sequenz wahrend der Exhumierung kann eine dunne Plattengrenzflachenmachtigkeit widerspiegeln, wie auch durch geophysikalische Daten (ca. 2-5 km) angedeutet wurde. Die Kissenbasalteinheit, die sich im sudlichsten Teil des Gurtels befindet, wurde schlieslich zusammen mit dem Rest der Sequenz unter UHP-Bedingungen oder wahrend der Exhumierung der Sequenz bei HP Bedingungen abgeschert. Die heutige Geometrie zeigt eine domartige Struktur der Grunschiefer-faziellen Einheit, was auf eine Exhumierung vor oder zusammen mit der HP-UHP-Einheit hindeutet.
590 $a School code: 0693.
650 4 $a Plate tectonics. $3 542702
650 4 $a Petrology. $3 535210
650 4 $a Geochemistry. $3 539092
650 4 $a Geology. $3 516570
653 $a Metamorphic rocks
653 $a Subduction zones
690 $a 0996
690 $a 0584
690 $a 0592
690 $a 0372
710 2 $a Freie Universitaet Berlin (Germany). $3 1900748
773 0 $t Dissertations Abstracts International $g 81-10B.
790 $a 0693
791 $a Ph.D.
792 $a 2018
793 $a English
856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=27605823

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