Genesis of migmatites from the palmer region of south Australia

Abstract

Migmatites of the veined gneiss type are associated with granites and granitic gneisses within the highest grade region of an Early Palaeozoic metamorphic terrain around Palmer on the eastern flank of the Mount Lofty ranges, South Australia. The four main hypotheses previously proposed for the genesis of the granitic portions of migmatites, viz., bodily injection of granitic magma, metasomatism, partial melting and metamorphic differentiation are discussed in the light of new chemical data. Structural and stratigraphic interpretations of the region indicate that migmatites were formed from quartzofeldspathic rocks without disturbance of the structural style. Bulk analyses of three selected migmatites are compared with six analyses of lower-grade quartzofeldspathic schists collected from an equivalent stratigraphic horizon 10 km from the migmatites. Differences in major element chemistry can be explained in terms of variation in proportions of the constituent minerals. Critical trace-element ratios are similar, and it is suggested that there is no necessity to introduce material from adjacent igneous intrusions or from unknown sources. Four granitic veins from the migmatites are richer in potassium than the associated large bodies of granitic rocks (twelve analyses). The veins also have high K Rb ratios (average 454) compared with these other granitic rocks in which the K Rb ratios are normal (average 229). Abnormally high K Rb , Ba K , Ba Sr , Sr Ca and Ba Rb ratios, all of which have been shown to decrease with fractionation, are the reverse of those expected if the granitic portions were produced by regional metasomatism since “emanations” coming either from granitic magmas or from any other source should be highly fractionated. Metamorphic temperature estimates are such that partial melting cannot be discounted. A plot of the normative percentages of quartz, albite and orthoclase onto the synthetic granite system show that the granitic veins are rich in potassium compared with the “ternary minimum” composition. The normative Ab An ratios are too high to explain this as a projection of a quaternary minimum in the tetrahedron Q, Ab, Or, An as suggested by Von Platen (1965). Partial melting would also require an abnormally equilibrium high water pressure (greater than 10 kbar) and could not explain the presence of “dents du cheval”. Three new analyses of biotite from the Palmer migmatites are compared with two from an associated intrusive granite and two from an associated granitic gneiss. The three migmatite biotites are identical in major and minor element composition: the mole % annite is constant and similar to the mole % annite in the granitic gneiss but different from the values for biotites of the intrusive granite. From this data it is suggested that the f H 2O - f O 2 - T conditions within the migmatite zone were constant. Four complete analyses of potassium feldspars separated from the migmatites show no differences in major element concentrations from the twelve potassium feldspars separated from the rocks of the major granitic bodies, but minor elements in both potassium feldspar and biotite reflect the minor element variations in the total rocks. The distribution of Ba Rb in coexisting biotite and potassium feldspar from seven rocks of the migmatite zone is close to theoretical predictions, indicating that these minerals are in equilibrium even in the migmatites in which this feldspar is virtually confined to the granitic veins and biotite concentrated in the selvedges. Barium and strontium are preferentially enriched in the potassium feldspar relative to biotite, whereas Rb is concentrated in biotite relative to potassium feldspar so that by concentration of potassium feldspar within the veins and concentration of biotite in the selvedges, the abnormally high K Rb , Ba Rb , Ba K , Sr Ca and Ba Sr can be adequately explained. It is concluded that the granitic portions of the Palmer migmatites were formed in place without introduction of material from granitic magma or any other source. The data is most readily explained by production of granitic veins by metamorphic differentiation.