K/Rb ratios and metasomatism in metabasites from a Precambrian amphibolite–granulite transition zone

Abstract

Potassium and rubidium values are reported for a suite of 181 metabasites from the amphibolite–granulite transition zone in the Precambrian Bamble sector of south Norway. Both alkalis are more abundant, and the K/Rb ratios lower, in the rocks from the amphibolite facies. Rubidium has been strongly fractionated with respect to potassium. The K/Rb ratios exhibit a regular and systematic variation and this defines a linear trend which is markedly different from established igneous fractionation trends. The chemical fractionation occurred as a result of the metamorphism, and the chemical changes are in keeping with the concept of a vertical chemical fractionation within the continental crust. Similar linear trends established in other high grade metamorphic terrains may have the same (metasomatic ?) origin.

The trace element geochemistry of some high‐grade Bergen‐Jotun kindred gneisses from the Jotun Nappe around Gjendebu, central southern Norway

“…This positive correlation between potash and K/Rb is unusual since a negative correlation is often apparent in high-grade terrains (e.g. Sighinolfi, 1969;Field & Clough, 1976;Allen, 1979) and in igneous rocks (Shaw, 1968). For comparative purposes the regression lines of these various authors are shown in Fig.…”

“…Selected regression lines (see text) shown against the plotted data from this study (shaded area, with outlying points shown). 1, Rocks from the Gjendebu region (excluding ultrabasic rocks); 2, high-grade rocks from the Valle Strona, Western Alps (Sighinolfi, 1969); 3, south Norwegian metabasites (Field & Clough, 1976); 4, igneous 'main trend' (Shaw, 1968). C .…”

“…Fields for Talasea, Unalaska, and the high-K Deccan Traps are shown. Regression lines: 2, rocks from the Valle Strona (Sighinolfi, 1969); 3, south Norwegian metabasic rocks (Field & Clough, 1976); 4, the igneous 'main trend' of Shaw (1968). Lines a and b are Twist's regression lines for Jotun kindred pyroxene gneisses and peripheral gabbros respectively (see text and Twist, 1979).…”

P–T conditions for the Arendal granulites, southern Norway: implications for the roles of P, T and CO2 in deep crustal LILE‐depletion

“…This area has been the subject of numerous detailed geochemical and isotopic studies (e.g. Clough, 1977;Field & Clough, 1976; Field, 1977;Field & RBheim, 1979, 1980Field er al., 1980, in press;Smalley et al, 1983;Lamb, 1981). However, recently calibrated geothermometers and geobarometers based on chemical partitioning between minerals have not been applied to these rocks.…”

“…The LILE-depletion was. initially, seen as the result of scavenging by a fluid phase during granulite facies metamorphism of a pre-existing suite of orthogneiss precursors with normal LILE contents (Field & Clough, 1976;Cooper & Field, 1977). More recently, however, trace element modelling calculations for the acid-intermediate charnockitic gneisses (Field ef al., 1980) have revealed that the distribution patterns of the REEs and LlLEs in these rocks could be explained by a primary igneous fractionation model, without recourse to a discrete metamorphic depletion event.…”