Physical property variations within archaean granite-greenstone terrane of the Yilgarn Craton, Western Australia: The influence of metamorphic grade

Abstract

Petrophysical properties (magnetic susceptibility, density) and gravity data are presented from greenstone belts of the Yilgarn Craton which have been subjected to greenschist and amphibolite facies metamorphism. The study areas were the Weebo/Wildara greenstone belt (greenschist facies) and Southern Cross greenstone belt (amphibolite facies). Comparison of similar rock types from these areas reveals systematic changes in their magnetic susceptibility and density interpreted as reflecting mineralogical changes associated with metamorphism. Bulk rock chemical analyses from each area are similar. Increased density of ultramafic rocks from greenschist to amphibolite facies results from replacement of serpentine/talc (2.7 g/cm3) by olivine (3.3 g/cm3). The prograde reaction of actinolite/tremolite and plagioclase to hornblende causes increased density of the mafic rocks. Changes in magnetic susceptibility are closely related to magnetite abundance and therefore extremely sensitive to local alteration processes (e.g. talc-carbonate alteration of ultramafic rocks). In this study, an increase in magnetic susceptibility with increasing metamorphic grade is noted for both mafic and ultramafic rocks. These effects may reflect increased magnetite content of the amphibolite facies rocks of approximately 0.005 volume % and 0.5 volume %, respectively. Additionally, the average grainsize of magnetite was observed to increase with metamorphic grade, thereby further increasing the magnetic susceptibility. Gravity modelling suggests a maximum vertical thickness of mafic greenstones of < 2 km at Weebo/Wildara and Southern Cross. The absence of a thick mafic pile below the Weebo/Wildara greenstone belt is interpreted to reflect tectonic detachment at shallow depth.