Magnetofabrics of shear zones and ultrahigh-pressure rocks from the Chinese Continental Scientific Drilling (CCSD)
Zusammenfassung der Projektergebnisse
This study provides rock magnetic and magneto-mineralogical data of exhumed ultrahigh-pressure rocks from the CCSD-mainhole. In the first part a summary of the results of the mafic and ultramafic rocks are presented and in the second part those of the quartzofeldspathic gneisses. Mafic and ultramafic rocks: Mafic and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) in the Maobei eclogite body of the Sulu ultrahigh-pressure metamorphic belt in eastern China were studied for their magnetofabrics. Variably retrogressed eclogites and serpentinized ultramafic rocks were retrieved from the depth interval of 100 to 1000 m of the CCSD borehole. A network of irregular shaped and oriented veins containing a retrograde metamorphic assemblage cuts across the eclogite foliation at both low and high angles. SEM imaging of the eclogites documents that magnetite associated with retrograde pargasitic amphiboles developed around shape-preferred garnet. SEM imaging of the serpentinized ultramafic rocks documents that magnetite rims grew around shape-preferred garnet and that magnetite formed within a mesh texture related to serpentinization. Syn-serpentinization magnetite growth increased bulk susceptibilities, but reduced anisotropy. Maximum susceptibility axes from both eclogites and serpentinized garnet-peridotites trend N-S, i.e. parallel with the stretching lineation as defined by olivine and omphacite grains. This implies that the magnetic fabric mimics the UHP fabric and survived retrogression and that the fabric is inherited from the UHP stage. As a consequence, retrogression was not associated with substantial ductile deformation and the mafic-ultramafic Maobei body behaved as a rigid body within a ductile deforming quartzo-feldspathic matrix during exhumation. Internal strain is restricted to brittle fracturing, associated fluid circulation and vein formation facilitating retrograde reactions in the mafic-ultramafic rocks. The magnetofabrics of the eclogites is controlled by the primary eclogite texture as obtained during UHP-metamorphism, the primary eclogite composition and the retrograde formation of pargasitic amphibole-magnetite coronas around garnet. The magnetofabrics of the serpentinized ultramafic rocks is controlled by the primary olivine texture as obtained during UHP-metamorphism, syn-serpentinization formation of magnetite along former grain boundaries and microcracks of shape-preferred olivine in a mesh texture, and the formation of magnetite rims around shape preferred garnet. The preservation of primary retrograde features in eclogites and serpentinites such as non-stoichiometric magnetite, serpentinite mesh textures, primary grain boundaries of omphacite, garnet and olivine, irregular veins with retrograde mineral assemblages documents that ductile deformation during retrogression was insignificant and that temperatures did not exceed 580°C during and after retrograde magnetite formation. The Maobei eclogite body thus behaved as a rigid block within a ductile deforming felsic gneiss matrix. Fluid expulsion from crystallizing partial melts are considered as the most likely process that enabled vein formation and serpentinization with associated retrograde mineral reactions in the mafic-ultramafic rocks. Quartzofeldspathic gneisses: Quartzofeldspathic gneisses of the Chinese Continental Scientific Drilling (CCSD) project were investigated by the anisotropy of magnetic susceptibility (AMS) and magnetic mineralogy in combination with geochemical data in order to constrain their textural evolution during exhumation. The gneisses show intra- and inter-sample variations of the bulk susceptibility (Kmean) at the cm- to dm-scale and thus variable contributions of para- and ferrimagnetic mineral phases independent of the sampling depth and the protolith. These variations depend on the distribution of Fe in paramagnetic or ferrimagnetic mineral phases. The stability of paramagnetic Febearing silicates or ferrimagnetic Fe-oxides strongly depends on oxygen fugacity and fluid activity. We therefore interpret the strong intra-sample variations as a result of fluid-rock interaction under medium-grade retrogression rather than due to a primarily irregular distribution of ferrimagnetic minerals. Magnetite inclusions in biotite indicate that some biotite originated during decomposition of magnetite. It is the cause of the observed variations of Kmean at the cm- to dm-scale. Decomposition of magnetite and the concomitant formation of biotite are also indicated by a negative correlation between Kmean and the shape factor T of the AMS ellipsoids and the generally coaxial and relatively moderate anisotropic AMS-ellipsoids within each unit. Enrichment of Ba in low susceptibility, paramagnetic gneisses also imply fluid-rock interaction. Borehole susceptibility logs may thus be used to estimate the degree of fluid activity during retrogression of high-grade metamorphic rocks, provided that the Fe content is sufficiently high. The protolith- and depth-invariant intra- and inter-sample variations of the magnetic susceptibility result from primary compositional variations as well as fluid induced magnetite decomposition into Fe-bearing silicates, represented in most cases by biotite. Biotite gives rise to an oblate fabric. Accordingly, the variations in shape factor T due sensitive for phyllosilicate formation during magnetite decomposition. At the intra-unit scale, the orientation of the principal susceptibility axes and thus the AMS-ellipsoids are independent of Kmean, indicating that fluid induced magnetite decomposition was not accompanied by major ductile deformation of the magnetic minerals within the units. Inter-unit variation of the orientation of the AMS-ellipsoid is probably related to early ductile shearing prior to exhumation.
Projektbezogene Publikationen (Auswahl)
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(2007) Anisotropy of the magnetic susceptibility of UHP-rocks from the Chinese Continental Scientific Drilling (CCSD) – first results from the mainhole. ICDP-IODP-Kolloquium Potsdam
Grimmer JC, Qi X, Xu Z
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(2008) Magnetofabrics of eclogites and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) project: evidence for ultrahigh-pressure (UHP) texture inheritance throughout retrogression. ICDP-IODP-Kolloquium Hannover
Grimmer JC, Qi XX, Xu ZQ
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(2008) Magnetofabrics of eclogites and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) project: evidence for ultrahigh-pressure (UHP) texture inheritance throughout retrogression. TSK12 Karlsruhe, Geotectonic Research 95, Special Issue 1, 59-61
Grimmer JC, Qi XX, Xu ZQ
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(2009) Magnetofabrics of eclogites and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) project: evidence for ultrahigh-pressure (UHP) texture inheritance throughout retrogression. YORSGET 2008 Oviedo, Trabajos de Geologica 29
Grimmer JC, Qi XX, Xu ZQ
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2008: Chinese Academy of Geological Sciences (CAGS) Beijing. Magnetofabrics of eclogites and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) project: evidence for ultrahigh-pressure (UHP) texture inheritance throughout retrogression
Grimmer JC
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(2009) Magnetofabrics of ultrahigh-pressure gneisses from the Chinese Continental Scientific Drilling (CCSD) project: Evidence for a genetic link between ferromagnetic and paramagnetic gneisses. ICDP-IODP-Kolloquium Potsdam
Grimmer JC, Qi XX, Xu ZQ
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(2009) Magnetofabrics of ultrahigh-pressure gneisses from the Chinese Continental Scientific Drilling (CCSD) project: Retrogression of ferromagnetic gneisses. EGU Vienna, Geophysical Research Abstracts 11
Grimmer JC, Qi XX, Xu ZQ
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(2009) Ultrahigh pressure texture inheritance during retrogression: evidence from magnetofabrics in eclogites and ultramafic rocks (Chinese Continental Scientific Drilling project). Tectonophysics 475/2: 267-278
Qi XX, Grimmer JC, Xu ZQ
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2009: Chinese Academy of Geological Sciences (CAGS) Beijing. Rock magnetic textures of UHP-gneisses from the Chinese Continental Scientific Drilling (CCSD) project, China
Grimmer JC
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2009: Institut für Geowissenschaften, Goethe-Universität Frankfurt. Exhumierung von ultrahochdruckmetamorphen Gesteinen in Ostchina: magnetische Gefügeentwicklung während der retrograden Metamorphose, Chinese Continental Scientific Drilling (CCSD) project, China
Grimmer JC
