Abstract

The c. 800 km 2 Tete Complex of northwestern Mozambique is located at the eastern end of the Neoproterozoic, east-west-trending Zambezi Belt, near the transition zone into the north-south-trending Mozambique Belt. Country rocks are dominated by supracrustal lithologies, including pelitic schist and gneiss, marble and calc-silicate, quartzite and psammitic gneiss, amphibolite, and biotite- + or - amphibole-bearing quartzofeldspathic gneiss. The northern margin of the Complex is marked by the east-west-trending, south-dipping Sanangoe Shear Zone (SSZ). Available contact exposures and relationships do not allow distinction between magmatic and tectonic emplacement mechanisms for the Tete Complex. The vast majority of Tete Complex rocks are unmetamorphosed plutonic rocks with excellent preservation of magmatic textures and mineralogy, although garnet-bearing meta-anorthosite occurs near the SSZ, indicating that an amphibolite- to granulite-grade metamorphic event, probably of Late Neoproterozoic age, affected marginal parts of the Complex. Medium- to fine-grained gabbroic rocks are dominant, and contain extensively zoned plagioclase (An (sub 55-87) ) and olivine (Fo (sub 60-82) ), and augite with Mg (super *) = 70-79. Textures and mineral compositions indicate relatively rapid cooling and a normal magmatic fractionation trend. The stable coexistence of olivine and intermediate plagioclase limits the pressure of crystallization to <7-8 kbar (<20-25 km); the complex was likely emplaced at shallow depths in the upper crust. Minor pyroxenite occurs as cumulate layers up to 2 m thick, and is composed of up to 88% of Al-rich clinopyroxene (Al 2 O 3 up to 9 wt.%), with subordinate Al-rich orthopyroxene (Al 2 O 3 up to 5.8 wt.%) and plagioclase (An (sub 56-75) ). These pyroxenes resemble high-Al pyroxene megacrysts common in massif-type anorthosite, and as such, may represent high-pressure crystallization products entrained by gabbroic magmas to a shallow crustal emplacement site. Coarse-grained (up to 7 cm) anorthosite and leucotroctolite are concentrated in the 30 km 2 Nyangoma region, representing <5% of the surface area of the Complex, and resemble massif-type anorthositic rocks in terms of textures and mineralogy (An (sub 47-57) , En (sub 59-75) , Fo (sub 58-64) ). Mineralogy and isotopic compositions indicate that Tete gabbroic, pyroxenitic, and anorthositic rocks crystallized from a common mafic magma. The magmatic crystallization age of the Tete Complex is best constrained by a 9-point Sm-Nd whole-rock regression of 1025+ or -79 Ma (MSWD = 4.0). Initial isotopic ratios (calculated at 1 Ga) are: epsilon Nd = +3.5 to +4.5 (mean = +4.1) and I Sr = 0.702758-0.702878 (mean = 0.702818), indicating derivation from depleted mantle, with little or no involvement of Archaean crustal contaminants. Assimilation of small amounts of Meso- or Palaeoproterozoic crust is permissible, if not likely. Cross-cutting dolerite dykes have epsilon Nd = +5.8 to +6.9 (at 1 Ga), indicating lesser crustal contamination, and may be equivalent to the upper mantle depleted source of the Tete magmas. The Chipera anorthosite/gabbro body north of the SSZ has epsilon Nd = +0.9 to +2.7 (at 1 Ga), indicating possible contamination with older (Archaean?) crustal components. Even lower epsilon Nd values (to -4.3), implying assimilation of Late Archaean crust, occur in the Uluguru anorthosites of Tanzania. The SSZ, therefore, may represent a major boundary between crustal age provinces.

You do not currently have access to this article.