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The Charnockite-Anorthosite Suite of Rocks Exposed in Central Dronning

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The Charnockite-Anorthosite Suite of Rocks Exposed in Central Dronning The charnockite-anorthosite suite of rocks exposed in central Dronning Maud Land, East Antarctica: a study on fluid-rock interactions, and post-entrapment change of metamorphic fluid inclusions Die charnockitischen und anorthositischen Gesteinsserien im zentralen Dronning Maud Land: Fluid-Gesteins-Wechselwirkungen und die Veränderung metamorpher Fluid-Einschlüsse nach ihrer Bildung Bärbel Kleinefeld Geologie der Polargebiete Fachbereich Geowissenschaften Universität Bremen Postfach 330440 28334 Bremen Die vorliegende Arbeit ist die inhaltlich unveränderte Fassung der Dissertation, die im Juli 2002 dem Fachbereich Geowissenschaften der Universität Bremen unter gleichem Titel vorgelegt wurde. Tableofcontents Table of contents Abstract.......................................................................................................................1 Zusammenfassung.................................................................................................... 3 1. Introduction ........................................................................................................5 1.1. Fluid-rockinteractionsindeep-seatedcrustalrocks...............................5 1.2. Previousstudiesandscopeofthethesis..................................................7 2. The charnockite – anorthosite suite of rocks..................................................9 2.1. Classifyingrocksofthecharnockiteseries ..............................................9 2.2. Massif-typeanorthosites.......................................................................... 10 2.3. "Incipient"or"arrested-type"charnockitisation.................................... 11 3. Principles of fluid inclusion studies ............................................................. 14 3.1. Theoreticalbackground -theidealmodel ............................................. 14 3.2. Practicalaspectsoffluidinclusionstudies ............................................ 16 3.2.1. Irreversiblepost-entrapmentchange..................................................................18 3.3. Nomenclature........................................................................................... 19 4. Analytical methods and data evaluation....................................................... 20 4.1. Polarisationmicroscopy .......................................................................... 20 4.2. Electronmicroprobeanalysis.................................................................. 20 4.3. Microthermometry................................................................................... 21 4.4. Ramanspectrometry................................................................................ 23 4.5. Bulkcomposition,molarvolumeandisochorecalculations................ 23 5. Geological setting............................................................................................. 27 5.1. ThepositionofcentralDronningMaudLandinrespecttoRodinia andGondwanareconstruction................................................................ 27 5.2. GeographyandgeologicalevolutionofcentralDronningMaud Land........................................................................................................... 29 5.3. NomenclatureofgeographicsitesinDronningMaudLand ............... 32 i Tableofcontents 6. The basement lithologies of the central Petermannketten - an example of secondary charnockitisation and leaching processes.............................. 34 6.1. Metamorphiccharnockitesandgneissesofthebasementlithologies. 35 6.1.1. Petrographyof"dark"rockvarieties...................................................................35 6.1.2. Petrographyof"light"rockvarieties...................................................................39 6.1.3. Fluidinclusionstudies...........................................................................................43 6.1.4. Mineralchemistryoffeldspars,pyroxenesandgarnets .................................49 6.2. Syn-andpost-kinematicintrusions........................................................ 52 6.2.1. Petrography .............................................................................................................52 6.2.2. Fluidinclusionstudies...........................................................................................54 6.3. SummaryandDiscussion -metamorphiccharnockitisationand successiveleaching .................................................................................. 55 7. The Otto-von-Gruber-Gebirge - fluid content of a massif-type anorthosite complex......................................................................................... 62 7.1. Massif-typeanorthositesamples ............................................................ 62 7.1.1. Petrography .............................................................................................................62 7.1.2. Fluidinclusionstudies...........................................................................................66 7.2. ShearzoneswithintheO.v.Gruberanorthositecomplex .................. 71 7.2.1. Petrography .............................................................................................................71 7.2.2. Fluidinclusionstudies...........................................................................................75 7.2.3. Mineralchemistryoffeldspars,pyroxenes,andgarnets ................................77 7.3. Discussion................................................................................................. 79 8. Fluid inclusions as micro-chemical systems: evidence and modelling of fluid-host interaction in plagioclase (separate paper) ............................ 87 9. Conclusion ...................................................................................................... 114 References .............................................................................................................. 118 Appendix.....................................................................................................................I 9.1. AppendixC................................................................................................ II 9.2. AppendixD .............................................................................................XX Acknowledgements ii Abstract Abstract The study areaof central Dronning Maud Land,East Antarcticais atypical example of a granulite facies Precambrian terrane that was exposed to substantial polymetamorphism during the late Neoproterozoic/early Palaeozoic.Fluid inclusion studies from typical representatives of the charnockite-anorthosite suite of rocks, associated gneisses and syenitic intrusives give new constraints on both peak metamorphic conditions and post-peak metamorphic processes during retrograde uplift. Detailed petrographical studies were supported by Electron Microprobe techniquesandcombinedwithmicrothermometryandRamanspectrometrydata. Three distinct fluid phases,either consisting of CO2±N2, H2O-salt or CO2±N2±H2O- salt were differentiated.All fluid inclusion types are hosted by plagioclase,quartzand garnet and display textural relationships indicative for a primary (metamorphic or magmatic) origin.The CO2±N2 fluid is most abundant,and it is assumed that it played an important role during metamorphic charnockite formation and anorthosite emplacement. However, evidence of post-entrapment change reveals that a large number of inclusions were subjected to profound reequilibration processes that resulted in amodification of original fluid properties,often accompanied by the partial tocompletelossofanaqueouscomponent. An important indicator for the residual character of some CO2±N2 fluid inclusions was the frequent observation of sheet silicate and carbonate microcrystals that were produced by a micro-chemical reaction of an originally CO2-H2O±N2 fluid with its plagioclase host.These observations from the anorthosite complex were used to model the fluid-host interaction with consideration of different original fluid compositions. Compared to an actual fluid inclusion it is obvious,that volume estimations of solid phases can be used as astarting point to reverse the retrograde reaction and recalculate the compositional and volumetrical properties of the original fluid.Isochores for an unmodified inclusion can thus be reconstructed,leading to amore realistic estimation ofP-Tconditionsduringearliermetamorphicstagesorfluidcapturing. Although CO2±N2 inclusions detected within the anorthosite body and associated shear zones reveal a large range in densities, isochoric calculations for the highest density inclusions are in accordance with independent P-T data for near peak- metamorphic conditions.This again illustrates that metamorphic minerals (plagioclase and garnet) are able to preserve the original metamorphic fluid, as substantial reequilibration processes do not take place uniformly within single crystals.A detailed fluid inclusion study can thus provide valuable constraints on the P-T conditions actingduringdifferentstagesoffluidentrapmentandreequilibration. A selection of representative isochores from the different basement lithologies have 1 Abstract been correlated with P-T constraints based on mineral-equilibriadataavailable from other studies.The gradual decrease in fluid densities best fits aclockwise P-T path and mineral-fluid equilibration during near isothermal decompression is postulated for the post-peak-metamorphic and retrograde development of the rocks exposed in central DronningMaudLand. 2 Zusammenfassung Zusammenfassung
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