01whole.pdf (11.01 MB)
Download file

Shenglikou and Zedang peridotite massifs, Tibet (China): upper mantle processes and geodynamic significance

Download (11.01 MB)
thesis
posted on 28.03.2022, 11:02 authored by Qing Xiong
Kilometer-scale peridotite massifs in collision or suture zones provide optimal targetsfor investigating the structure, composition, origin and evolution of the Earth’s mantle. In thisstudy, I have investigated the Shenglikou garnet-facies peridotite body in the North Qaidamorogen of NE Tibet, and the Zedang spinel-facies peridotite massif in the Yarlung Zangbosuture of south Tibet, using petrographic, geochemical and multi-isotopic techniques toconstrain their origins and geodynamic processes. The Shenglikou peridotite massif is enclosed by ultrahigh-pressure (UHP) crustal rocks. Zircon U-Pb ages and Hf-isotope compositions show that the UHP crustal rockscontain Neoarchean (~2.8-2.5 Ga) components and experienced magmatic rejuvenation at~2.1-2.0 Ga and ~1.1-0.8 Ga. The rejuvenation made the Qaidam continent more mafic inbulk composition, which facilitated its subduction to reach UHP mantle depths. The Shenglikou peridotite massif mainly consists of pyroxene-rich and olivine-rich ultramafic rocks, and is locally crosscut by phlogopite-bearing pyroxenite dykes. Chemicaland isotopic data suggest that the Shenglikou massif originated as Archean continental lithospheric mantle, which experienced melt refertilization events (at ~1.5-1.4 Ga and ~0.7Ga) and fluid metasomatism during early Paleozoic time. The pyroxenite dykes were formedas cumulates within the lithospheric mantle wedge at ~500 Ma, and the parental magmascame from fluid-metasomatized convective mantle wedge beneath an Andean-type convergent margin. The subsequent continental subduction slab entrained the mantle-wedge fragments,undergoing UHP metamorphism (~440 Ma) and later exhumation (~400 Ma). The Zedang peridotite massif mainly comprises lherzolite (spinel Cr# = 0.17-0.30) inthe west and harzburgite (spinel Cr# = 0.33-0.62) in the east. The lherzolitic domain showsstronger plastic deformation with equilibration temperatures (up to 1260 °C) ~200-300 Khigher than the harzburgitic domain. The harzburgites experienced melt metasomatism andcontain chromitite-dunite associations, but the lherzolites did not. The mineral compositionsshow that the disseminated chromitites and dunites were derived by reaction betweenharzburgites and island-arc tholeiitic magmas. Grains of pure SiC, SiC + K-rich glasses ± zircon, SiC + Si + SiOX, SiC + Si + Fe-V-Ti-Mn alloys and some glasses have been found inthe Zedang harzburgitic domain. The super-reduced assemblages may have formed in themantle by local infiltration by CH₄ and/or H₂ fluids from subducting slabs or the deep mantle.The coexistence of the super-reduced phases (fO₂ of 5-7 log units lower than the IW buffer)with oxidized glasses and silica phases suggests their very short residence time in the hightemperatureambient mantle. Combining the new data from this study with those from previous studies, all evidencesuggests that the Zedang harzburgites were emplaced at shallow levels (after their deepsubduction) at ~200-150 Ma in a forearc spreading center. They ascended rapidly from themantle Transition Zone in a process triggered by slab rollback and driven by their buoyancy.The super-reduced phases formed rapidly and locally in the upwelling channel, as didthe diamonds in the appropriate pressure range. The harzburgites cooled in the Tethyanoceanic basin, and were then underplated by lherzolites, due to transient upwelling of fertile asthenosphere, triggered by a later subduction at ~130-120 Ma. After this underplating, theperidotites were affected locally by reaction with seawater until they were exposed as part ofthe Yarlung Zangbo ophiolite belt in the late Cretaceous. The Shenglikou and Zedang peridotite massifs and related rocks reveal two styles ofmantle geodynamic evolution: the first massif records SCLM processes from Archean to earlyPaleozoic, while the second one reflects upper-mantle processes during the evolution of theTethyan oceans and the formation of Himalayan-Tibetan orogenic system in Phanerozoictime.

History

Table of Contents

1. Introduction -- 2. Methodology -- 3. Geological setting -- 4. Continental deep subduction in the Paleozoic and magmatism in the Proterozoic: a case of the North Qaidam UHP belt -- 5. pt. 1. Episodic refertilization and metasomatism of Archaean mantle: evidence from an orogenic peridotite in North Qaidam (NE Tibet, China) -- 5. pt. 2. Pyroxenite dykes in orogenic peridotite from North Qaidam (NE Tibet): tracking metasomatism and melt segregation in the mantle wedge -- 5. pt. 3. Tectonic history of the Shenglikou peridotite massif and its UHP country rocks, North Qaidam (NE Tibet, China) -- 6. pt. 1. Origin of large-scale mantle heterogeneities in the Zedang peridotite massif, Yarlung Zangbo suture, (S. Tibet, China) -- 6. pt. 2. Origin of chromitite-dunite associations in the Zedang peridotite massif, Yarlung Zangbo suture, (S. Tibet, China) -- 6. pt. 3. Moissanite (SiC) and associated phases in the Zedang peridotite massif, Yarlung Zangbo suture, (S. Tibet, China) and tectonic implications -- 6. pt. 4. Tectonic history of the Zedang peridotite massif and related Yarlung Zangbo ophiolites, S. Tibet (China) -- 7. Conclusions.

Notes

Includes bibliographical references This thesis is submitted to both Macquarie University (Australia) and China University of Geosciences (Wuhan), under the conditions of the Cotutelle agreement for this PhD program between these two universities.

Awarding Institution

Macquarie University

Degree Type

Thesis PhD

Degree

PhD, Macquarie University, Faculty of Science and Engineering, Department of Earth and Planetary Sciences

Department, Centre or School

Department of Earth and Planetary Sciences

Year of Award

2015

Principal Supervisor

Sue O'Reilly

Additional Supervisor 1

Bill Griffin

Additional Supervisor 2

Norm Pearson

Rights

Copyright Qing Xiong 2015 Copyright disclaimer: http://www.copyright.mq.edu.au

Language

English

Jurisdiction

Tibet (China)

Extent

1 online resource (xviii, 268, 98 pages) illustrations, maps

Former Identifiers

mq:44532 http://hdl.handle.net/1959.14/1070073