Spotted structures in gneiss and veins from Broken Hill, New South Wales, Australia

Abstract

SYNOPSISSPOTTED eye-like structures, made up of densely packed biotite 'pupils' mantled by quartz and feldspar, constitute about Io % by volume of a highgrade quartzofeldspathic gneiss located on the road to Purnamoota Homestead about IO km north of Broken Hill. These spotted structures have an uneven distribution within rocks exposed at this locality. In some parts of the outcrop they tend to be closely spaced ( fig. I), whereas elsewhere they are less abundant and the spotted gneiss merges into an essentially homogeneous gneiss composed of quartz (34~. by volume), plagioclase (24%), Kfeldspar (i 7 ~), biotite (20 %), and garnet, muscovite, and opaques (5 %). This homogeneous gneiss is believed to have developed into a 'matrix' gneiss phase holding the 'eyes' which have a felsic mantle to mafic clot ratio of 4: I.The whole eye-like structure appears to be a distorted prolate spheroid with its axis set close to the foliation plane of the gneiss. The internal biotite clot forms another prolate spheroid with its axis inclined at approximately 70 ~ to that of the enveloping quartz-feldspar mantle. Although most of the eyes are isolated within the matrix, some are linked to hold three or four biotite aggregates ( fig. S I) and rarely a vein-like patch which contains some ten or more partly linked spots of biotite may be found. In addition to the biotite, xenoblasts of almandine occur within some of the eye-like structures.Whole-rock chemical analyses (including both major oxides and trace elements) show that the homogeneous gneiss and the spotted gneiss are very similar. The most notable feature of the geochemistry of the spotted rocks is the close chemical correspondence between the matrix and the whole eyes. Moreover, the compositions of corresponding minerals in these phases are very similar. For example, plagioclase is consistently about An 35-40, the biotites are all very iron-rich (Fe/(Fe + Mg) 0.87) with compositions near siderophyllite, and Kfeldspar has the composition Or94Ab 6.It has been suggested that the biotite spots formed by alteration of garnet probably during a retrograde metamorphic event. However, studies of the microstructure of the biotite and garnet in the spotted gneiss show that biotite of the spots does not replace garnet. Furthermore, the chemical similarity between the spots and the matrix is best explained by an isochemical rearrangement of components of the matrix phase to form the eyes. Thus, a more likely origin is best related to the displacement of matter along chemical potential gradients possibly induced by deformation in the rock system--a process described by the term metamorphic differentiation.Veins in the Purnamoota Road gneiss are of two main contrasting types--regularly-disposed veinlets which are composed almost entirely of Kfeldspar and quartz, and irregularly shaped discontinuous trondhjemitoid variants rich in biotite spots and carrying rare garnet porphyroblasts. The field relationship between these two main types of veins is difficult to discern. In some parts of the ...