Book contents
- Frontmatter
- Contents
- List of contributors
- Preface
- 1 Overview of the geology and tectonics of UHPM
- 2 Experimental and petrogenetic study of UHPM
- 3 Principal mineralogic indicators of UHP in crustal rocks
- 4 Structures in UHPM rocks: A case study from the Alps
- 5 Creation, preservation, and exhumation of UHPM rocks
- 6 The role of serpentinite melanges in the unroofing of UHPM rocks: An example from the Western Alps of Italy
- 7 Ultra-high-pressure metamorphic rocks in the Western Alps
- 8 HP and UHP eclogites and garnet peridotites in the Scandinavian Caledonides
- 9 Microcoesites and microdiamonds in Norway: An overview
- 10 UHPM terrane in east central China
- 11 A model for the tectonic history of HP and UHPM regions in east central China
- 12 Diamond-bearing metamorphic rocks of the Kokchetav massif (Northern Kazakhstan)
- 13 Orogenic ultramafic rocks of UHP (diamond facies) origin
- Index
5 - Creation, preservation, and exhumation of UHPM rocks
Published online by Cambridge University Press: 24 October 2009
- Frontmatter
- Contents
- List of contributors
- Preface
- 1 Overview of the geology and tectonics of UHPM
- 2 Experimental and petrogenetic study of UHPM
- 3 Principal mineralogic indicators of UHP in crustal rocks
- 4 Structures in UHPM rocks: A case study from the Alps
- 5 Creation, preservation, and exhumation of UHPM rocks
- 6 The role of serpentinite melanges in the unroofing of UHPM rocks: An example from the Western Alps of Italy
- 7 Ultra-high-pressure metamorphic rocks in the Western Alps
- 8 HP and UHP eclogites and garnet peridotites in the Scandinavian Caledonides
- 9 Microcoesites and microdiamonds in Norway: An overview
- 10 UHPM terrane in east central China
- 11 A model for the tectonic history of HP and UHPM regions in east central China
- 12 Diamond-bearing metamorphic rocks of the Kokchetav massif (Northern Kazakhstan)
- 13 Orogenic ultramafic rocks of UHP (diamond facies) origin
- Index
Summary
Abstract
Coesite-bearing eclogites exposed in the Western Alps and China record peak metamorphic temperatures of 550–900°C at pressures ≥ 2.5 GPa (≥25 kbar). Their presence as regionally metamorphosed rocks requires subduction of >400 km2 of upper crustal material to depths ≥90 km and subsequent exhumation. Existing experimental kinetic data suggest that pure coesite rock should not survive exhumation in the presence of a fluid, and this is corroborated by field observations. Partial survival of coesite is linked to its occurrence as inclusions in porphyroblasts that can maintain high internal pressures and prevent ingress of fluids. In the absence of coesite, reliable indicators of its former presence include subparallel, sometimes curving quartz subgrains and sets of quartz subgrains that truncate other sets. Polycrystalline quartz aggregates and cracks radiating outward from inclusions are not a priori evidence of ultrahigh pressure (UHP) metamorphism.
Petrologic constraints demand that coesite-bearing regional metamorphic rocks cooled during exhumation. Cooling during exhumation requires either (1) continued subduction beneath the eclogites that effectively chills the overlying UHP rocks during exhumation, or (2) transport toward the surface in the lower plate of an extensional structure such as a low-angle normal fault or shear zone. Both processes may occur; neither process demands rapid exhumation rates, although radiometric dating of different portions of the Dora-Maira pressure-temperature path indicate average long-term exhumation rates of 3.00 km Ma–1.
Introduction
Coesite is a high-pressure polymorph of SiO2 stable at pressures ≥ 2.5 GPa for metamorphic temperatures ≥500°C (Fig 5.1).
- Type
- Chapter
- Information
- Ultrahigh Pressure Metamorphism , pp. 159 - 181Publisher: Cambridge University PressPrint publication year: 1995
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