1.1. Fluid -rock interactions in deep-seated crustal rocks
In recent years, the origin, nature and role of the fluid phase involved in granulite formation has again become a vital subject of scientific interest. In most Precambrian terranes, continental crust consists of granulites, with a difference between a (relatively) more superficial part (felsic, metasedimentary granulites) and a more igneous, intrusive deeper part (Touret, 1995). Affiliated peak metamorphism is suggested to be triggered by a sudden temperature increase, most probably related to intrusions of mantle derived melts (magmatic underplating) (Touret, 1995). Rocks generated or modified under granulite facies conditions are water deficient, and metamorphism has taken place at temperatures that would be sufficient to cause melting in the presence of water. The apparent dryness is reflected in the anhydrous mineralogy. Orthopyroxene-bearing members of the charnockite - anorthosite suite of rocks (cf. chapter 2) are abundant. Sheet silicates and amphiboles are absent or are present only as minor components. This implies that metamorphism has either occurred under fluid absent conditions, or the fluid must have been of other than predominantly aqueous composition. The absence or subordinate occurence of free H2O and dominance of CO2±N2±CH4 bearing fluids in granulitic lithologies has been demonstrated by several fluid inclusion studies from various granulite terranes worldwide (e.g. Raith et al., 1990; Santosh & Yoshida, 1992).
Prograde or peak-metamorphic inclusions have been shown to be preserved in varying metamorphic minerals that have undergone a metamorphic cycle (Blom, 1988;
Vry & Brown, 1991; Bakker & Mamtani, 2000). Nevertheless, the assumption that CO2 -rich fluids even of high density always reflect peak metamorphic conditions has been questioned, and shown to be misleading (e.g. Lamb et al., 1987; Lamb; 1990).
Furthermore, fluid inclusions detected in metamorphic rocks frequently reveal densities, which are incompatible with P-T constraints derived from solid phase equilibria (e.g. Swanenberg, 1980; Sterner & Bodnar, 1989; Phillipot & Selverstone, 1991). That inclusions undergo varying compositional and density changes during metamorphic history has been demonstrated by several findings in nature and experiment (e.g. Sterner & Bodnar, 1989; Hall & Sterner, 1993; Bakker & Jansen, 1994).
Küster & Stöckhert (1997) even presumed, that quartz is unable to preserve primary (metamorphic) inclusions that were captured above 300°C. It is thus very likely that fluid inclusions that formed during peak-metamorphic granulite facies conditions have experienced multiple retrograde modifications, including complete or partial decrepitation (failure by fracturing), stretching (failure by plastic creep), diffusion, or
1. Introduction
reactions of the fluid with its mineral host (so-called "back-reactions" in Heinrich &
Gottschalk, 1995). Additionally, the retrograde fluid evolution is characterised by a complicated regime of large and small scale fluid migration and influx, combined with fluid mixing and/or buffering. One example for large scale fluid migration is the pervasive influx or channelling of a carbonic fluid along shear zones, which some workers suggest to be responsible for "incipient" charnockite formation (e.g.
Srikantappa et al., 1985; Hansen et al., 1987). The free fluid phase might also be involved in ongoing alteration processes. Pineau et al. (1981) have described the formation of small carbonate particles at the emplacement of former inclusions through the reaction of a CO2-rich fluid with an incoming H2O-salt fluid. These "late"
carbonates are suggested to be very abundant in many granulites (Touret, 1995).
Aqueous fluids may also be involved in retrograde mineral reactions leading to the formation of hydrous phases (like sheet silicates) and remarkable variations in fluid salinities.
Potential host minerals react differently to the possible modification processes, and the mechanisms of local reequilibration are not systematic. As a result, the overall fluid movement was often not able to homogenise the fluid composition, not even in hand specimen scale (Touret, 1995), and samples may comprise a large variation in fluid compositions and densities.
It can be stated that fluid-rock interactions (involving modification of fluid inclusions and the interaction of an enclosed fluid with its mineral host) are abundant during granulite formation. The majority of fluid inclusions detected within granulite terranes are in fact characterised by the dominance of a CO2-rich and nearly complete absence of an H2O-bearing component. However, this does not necessarily imply, that the current fluid is identical with the fluid active during metamorphic reactions. In order to derive any useful data on former P-T conditions, fluid inclusion populations must be differentiated and related to specific stages of metamorphic history. Any possible mechanism of secondary change has to be taken into account during the evaluation and interpretation of derived data. A change in original fluid composition or density may specify reequilibration processes which are not yet completely understood, or give valuable information about particular reequilibration processes, which are known to occur only under certain conditions.
1.2. Previous studies and scope of the thesis
Hitherto, fluid inclusion data of metamorphic charnockites and associated granulitic lithologies from East Antarctica are rare, and have only been reported from the Lützow-Holm Bay (LHB) region (Santosh & Yoshida, 1991, 1992). The authors show that the fluid imprint on gneiss and metamorphic charnockite assemblages is dominantly pure CO2, and postulate an external, sub-lithospheric origin of the preserved fluid. Furthermore they combine fluid inclusion data with P-T-data derived from mineral phase equilibria and geochronologic information, and conclude that the LHB rocks followed a clockwise prograde and retrograde P-T-t-path.
The granulite facies basement complex exposed in the Petermannketten and the Otto-von-Gruber-Gebirge, central Dronning Maud Land, East Antarctica, comprises lithologies typical of Precambrian granulite terranes. In this study, basement gneisses that have obviously been subjected to metamorphic charnockitisation and subsequent leaching processes, massif-type anorthosites and associated shear zone samples, as well as anorogenic syenite and charnockite intrusives are investigated with regard to their modal and chemical mineral composition (using Electron Microprobe technique), and fluid content.
In a first step, the gneisses, anorthosites and shear zone samples are classified according to the recommendations of the IUGS subcommission for members of the charnockite-anorthosite suite of rocks. The main objective of this study is to examine the contemporary fluid content preserved in the different lithologies by microthermometry and Raman spectrometry. The data are evaluated in context of the nature of the fluid present during the early stage of granulitic metamorphism, charnockite formation and intrusion and deformation of the anorthosite body. It is illustrated that early-metamorphic fluids may be preserved in metamorphic minerals, although the influence of post-entrapment change is abundant and substantial.
Derived density data are used to calculate isochores, which are correlated with independent data to give further constraints on the character of the retrograde P-T-path.
Based on the frequent observation of carbonate and sheet silicate microcrystals in carbonic fluid inclusions, further emphasis is put upon micro-chemical reaction processes between an enclosed CO2-H2O fluid and its mineral host during retrogression. It is assumed that a fluid that originally contained an aqueous phase may react with surrounding plagioclase under complete consumption of the aqueous phase, and the formation of carbonates and sheet silicates. A quantitative model is established to describe volumetric and compositional changes caused by the possible reactions. The model is applied to hypothetical and actual fluid inclusions. It is shown
1. Introduction
that the combination of fluid inclusion data with thermodynamic modelling may provide crucial constraints on the volumetrical and compositional properties of the original fluid inclusion trapped during high-grade metamorphism. Isochores for an unmodified original inclusion can thus be reconstructed, leading to a more realistic estimation of P-T conditions during earlier metamorphic stages or fluid capturing.
The results are presented in two chapters devided on the basis of rock types (gneisses and anorthosites) and sample localities and (Petermannketten and Otto-von-Gruber-Gebirge). The detailed study using thermodynamic modelling techniques is presented in a separate chapter which consists of a manuscript that has been accepted for publication by the Journal of Metamorphic Geology.