Microstructural controls and the role of graphite in matrix/ porphyroblast exchange during synkinematic andalusite growth in a granitoid aureole

Abstract In the contact metamorphic aureole of the Tinaroo Batholith (north Queensland, Australia), mylonitic rocks were metamorphosed during a regional folding/crenulation event (D₂) synchronous with the emplacement of muscovite-bearing granitoids. Prismatic and skeletal andalusite porphyoblasts grew in carbonaceous schists, mainly from the dissolution of staurolite. Muscovite, quartz and biotite played a dual role in this reaction, acting in a catalytic capacity as well as reactants or products. Staurolite was replaced by coarse-grained muscovite ± biotite, whereas andalusite locally replaced quartz ± muscovite ± biotite, with diffusion of H, Al, Si, Mg, Fe and K ionic species linking sites of dissolution and growth. Graphite contributed to the reaction mechanism in a number of ways. Accumulations of graphite in front of advancing andalusite crystal faces led to skeletal growth and the formation of chiastolite structure, where incremental growth occurred on adjacent {110} faces, with subsequent filling in and inclusion of graphite along the diagonal zones. The presence of graphite in some layers in the schist matrix prevented recrystallization of strained muscovite grains. The muscovite grains in these layers, in contrast to adjacent thin non-graphitic layers, were preferentially replaced by quartz. This resulted in muscovite-depletion haloes in graphitic layers around andalusite porphyroblasts. Somewhat arcuate zones of graphite, concentrated during dissolution of quartz along a crenulation cleavage, occur on some andalusite faces. Reactivation of the mylonitic foliation during the formation of D₂ crenulations led to a preferential dissolution of quartz in zones of progressive shearing localized near andalusite porphyroblasts and hence the accumulation of graphite. Lack of deflection of the pre-existing mylonitic foliation and anastomosing of the axial planes of D₂ crenulations around andalusite porphyroblasts demonstrate not only the timing of growth, but also that growing porphyroblasts do not push aside existing foliations.     

A COMPARATIVE STUDY OF GLAUCONITE AND THE ASSOCIATED CLAY FRACTION IN MODERN MARINE SEDIMENTS

The crystallinity and mineralogy of both the glauconite and the clay fraction of samples from six contemporary marine environments were investigated by X-ray diffraction. In those areas where glauconite is now forming, the mineralogy and the degree of crystallinity of both the glauconite pellets and the associated clay fraction are similar. In contrast, detrital and relic glauconites are observed to have mineralogies that are different from their clay fractions. No consistent relationship was observed between degree of crystallinity and color of the pellets. Further, only two classes of glauconite as defined by BURST (1958) were common: expandable, interlayered clays and two or more clay minerals in a mixed assemblage. Based on the clay fraction-glauconite similarities and other supporting evidence, glauconite on the Scotia Ridge is concluded to be authigenic. Glauconite from Santa Monica Bay, California and from the continental shelf off Morocco appear to be detrital. Glauconite pellets in the shelf sediments off Guinea and the southeastern Atlantic Shelf of the United States are both detrital and authigenic. The poor crystallinity exhibited by the Chatham Rise glauconite is in contrast to the well-crystallized associated clay fraction and indicates that they are not genetically related. However, the origin of this glauconite remains in doubt.