Contrasted patterns of k/rb distribution in precambrian high grade metamorphic rocks from central Australia

Abstract

Precambrian high‐grade metamorphic rocks (granulite facies, transitional granulite to amphibolite facies, and amphibolite facies) from the Amata area, Mus‐grave Ranges, central Australia, display contrasting weighted average K/Rb ratios (628, 310, and 203 respectively). This pattern of higher K/Rb ratios in medium to high‐pressure granulite facies rocks than in amphibolite facies rocks is in keeping with patterns found elsewhere in areas of metamorphosed Precambrian continental crust. The weighted average values are strongly influenced by the predominantly quartzo‐feldspathic (acidic to sub‐acidic) nature of the three metamorphic terrains. The ratios therefore reflect extensive control exerted by host‐rock mineralogy on the distribution of K and Rb.

The K/Rb ratios displayed by the quartzo‐feldspathic and certain of the mafic rocks of the area are probably the result of crustal fractionation processes, whereby Rb is relatively depleted in the granulite facies rocks and relatively concentrated in the lower grade rocks. However, the K/Rb ratios in the mafic granulites may represent original igneous patterns, preserved as a result of isochemical metamorphism.