Geochemistry and Nd - isotopic composition of high silica rhyolites in the neoarchaean Musoma-Mara greenstone belt, northern Tanzania: evidence for the presence of older continental crust

Abstract

The high silica rhyolites of the Kibasuka Hill, although volumetrically minor, constitute a part of the greenstone sequence in the northern Musoma-Mara Greenstone Belt (MMGB). They are characterized by high silica contents (63.38 – 81.65 wt %), very low CaO (! 0.05 wt %),variable to unusually high K2O (2.24 – 13.43 wt %) most likely attributed to strong sericite alteration, and their major element composition are similar to those of melts derived from partial melting of metapelites. The samples have very low concentration of the transition elements, Sr (3.13 – 48.4 ppm) and very large negative Europium anomalies (Eu/Eu* = 0.27 –0.48). Their Nd isotopic composition reveals that the T-depleted mantle (DM) model ages of the samples are highly variable and range from 2867 Ma to 4015 Ma. These geochemical features are consistent with generation of the high silica rhyolites by partial melting of a heterogeneous source composed of juvenile Archaean rocks of the MMGB and older sedimentary rocks. The later imparts to the rocks very old mean crustal residence ages. Partial melting of this suite took place at low-pressure conditions within the continental crust where plagioclase was a stable phase. Such old TDM model ages of up to 4015 Ma recorded by the high silica rhyolites suggest the presence of very old continental crust in the MMGB which has so far not been identified anywhere else in the Tanzania Craton.