Petrography and geochemistry of a meta-anorthosite and associated rocks in the Mozambique Belt in central Tanzania

The Mozambique Belt in Tanzania is a part of the East African Orogen (EAO) and represents the collision zone between East and West Gondwana. The formation of Gondwana took place during the Pan-African Orogeny (peak at 640 Ma) and formed a nappe sequence that was thrust on the Archean Tanzania craton from east to west. The grade of metamorphism increases eastwards from nearly unmetamorphic sedimentary rocks to lower crustal granulite facies rocks. The formation ages of the granulites are mainly between 2.5 -2.7 and 1.8 2.0 Ga but Pan-Arican granulite facies metamorphism occurred at 640 Ma. The granulites are intruded by smaller bodies of anorthosite of various size. From these bodies there is one age of 690 Ma and we relate these anorthosites to comparable bodies in southwest Madagascar, where magmatic crystallization ages are Pan-African. (Ashwal et al. 1998). Here, we present a geochemical study of a meta-anorthosite body of the Uluguru Mountains in central Tanzania. The rock type comprises plagioclase (XAn = 0.54), almandine-rich garnet, pyroxene (predominately diopsid, subordinate hypersthene), Ca-rich hornblende, biotite and scapolite. Spinel is a minor constituent and accessory minerals are apatite, ilmenite, magnetite, epidote and zoisite. The anorthositic rocks are associated with granitoids containing a fine-grained matrix of plagioclase (XAn=0.26-0.37) and K-feldspar. Garnet (XAlm = 0.6) occurs of large aggregates of smaller subgrains. Accessory minerals are magnetite, Zn-rich spinel, ilmenite and zircons of mm-size. The intense fractionation of the anorthosite is demonstrated from highly spiked chondrite-normalized spider patterns. Sr is enriched due to Plagioclase fractionation and Ba is included in small grains of K-feldspar in the matrix. Two troughs are characteristic - one at Rb Th and one at Nb Ta both indicating (lower) crustal contamination. REE show generally a decrease from La to Lu with a highly pronounced positive Eu anomaly. This study is supported by FWF P15599.