Tectonic setting of gabbroic rocks in the western part of the Lupa Gold Field, south-west Tanzania

Abstract

There is a pronounced occurrence of gabbroic rocks in the western part of the Lupa gold field, SW Tanzania. These rocks occur in the peripheral part around the Saza-Chuya granodiorite with which it makes a gradiational, unchilled contact. The rocks have been altered or metamorphosed to low-grade greenschist facies as indicated by the presence of secondary epidote, penninite, chlorite, sericite and calcite. The majority of major-element oxides show scattered variation trends against immobile elements such as Zr, indicating some degree of element mobility during post-crystallization processes of alteration or metamorphism. Major-trace-element data show that the gabbroic rocks in the study area are calc-alkaline. REE patterns show that the rocks are highly evolved with a high (La/Yb)N ratio of about 30 and (La/Sm)N ratios of between 18-40. The highly evolved nature of the studied rocks is shown also by low values of magnesium between 56-62%. The gabbroic rocks in the western part of the Lupa gold field seem to have been formed in the continental arc setting.