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

摘要

The study area of central Dronning Maud Land, East Antarctica is a typical 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 techniques and combined with microthermometry and Raman spectrometry data.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, quartz and 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 largenumber of inclusions were subjected to profound reequilibration processes that resulted in a modification of original fluid properties, often accompanied by the partial to complete loss of an aqueous component.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 a starting 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 a more realistic estimation of P-T conditions during earlier metamorphic stages or fluid capturing.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 detailedfluid inclusion study can thus provide valuable constraints on the P-T conditions acting during different stages of fluid entrapment and reequilibration.A selection of representative isochores from the different basement lithologies have been correlated with P-T constraints based on mineral-equilibria data available from other studies. The gradual decrease in fluid densities best fits a clockwise 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 Dronning Maud Land.