Sr-Nd isotopic disequilibrium of clinopyroxenes from the ultrapotassic effusive rocks of the East African rift system: Mixing of melts and source heterogeneity

Abstract

505 Ultrapotassic magmatism is the deepest type of magmatism showing a number of indicators of mantle source enrichment in incompatible trace elements (1). The western branch of the East African rift system is a classic area of the occurrence of ultrapotassic rocks (2, 3). The presence of rocks with different modal and chemical compositions within a relatively small area reflects kilometerscale heterogeneity in the upper mantle. The most popular model for the source of potassic magmas is a lherzolitic (harzburgitic?) mantle with numerous pyroxenite veins and layers (4). Addi� tional evidence for the heterogeneity of the source of potassic magmas in the western branch of the rift was obtained during the investigation of Sr and Nd isotopic systematics in phenocrysts from the volcanic rocks. Despite extensive geological work in this region, only a few Sr-Nd and Pb isotope analyses were reported for the kamafugite group of rocks (5, 6), and we are not aware of any published isotopic data for the major minerals of these rocks. Some studies were car� ried out for clinopyroxene from East African nephelin� ite lavas from Napak Volcano, eastern Uganda (7), and Mount Elgon Volcano, eastern Uganda-western Kenya (8). This paper reports the results of the investi� gation of Sr and Nd isotopic compositions of the major minerals (clinopyroxene and mica) of kamafugites and an attempt to estimate the composition of their mantle source. Our study focused on ultrapotassic volcanic rocks from the ToroAnkole and Virunga provinces in the northern part of the western branch of the East African rift system. The ToroAnkole volcanics are the youngest rocks of the East African rift system, and their K-Ar and 40 Ar/ 39 Ar age is no higher than 50 000 yr (9). Most samples for this study were collected within the Bun� yaruguru volcanic field. Samples of kamafugite (ugan� dite) and its probable derivatives, melaleucitite and leu� citite, from Visoke Volcano (Virunga Province) were studied for the sake of comparison. 1 The magmatism of the Virunga Province is of Miocene age (10). Among the characteristic features of the kamafugite rocks of the ToroAnkole Province are high Mg # values (up to 0.79), high contents of Ni (up to 780 ppm) and Cr (up to 1170 ppm), and extreme enrichment in incompatible trace elements: Ce/Y is up to 26.38; Zr/Y, up to 26.22; (La/Yb)n , up to 142,69; and (La/Sm) n , up to 7.27 (11).