Mafic, ultramafic, and anorthositic rocks of the Tete Complex, Mozambique:petrology, age, and significance

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 th e transition zone into the north-south-trending Mozambique Belt. Country ro cks are dominated by supracrustal lithologies, including pelitic schist and gneiss, marble and calc-silicate, quartzite and psammitic gneiss, amphibol ite, and biotite- +/- amphibole-bearing quarlzofeldspathic gneiss. The nort hern margin of the Complex is marked by the east-west-trending, south-dippi ng Sanangoe Shear Zone (SSZ). Available contact exposures and relationships do not allow distinction between magmatic and tectonic emplacement mechani sms for the Tete Complex. The vast majority of Tete Complex rocks are unmet amorphosed plutonic rocks with excellent preservation of magmatic textures and mineralogy, although garnet-bearing meta-anorthosite occurs near the SS Z, indicating that an amphibolite- to granulite-grade metamorphic event, pr obably of Late Neoproterozoic age, affected marginal parts of the Complex. Medium- to fine-grained gabbroic rocks are dominant, and contain extensivel y zoned plagioclase (An(55-87)) and olivine (Fo(60-82)), and augite with Mg * = 70 - 79. Textures and mineral compositions indicate relatively rapid co oling 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 d epths 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 (Al2O3 up to 9 wt.%), with subordinate Al-rich orthopyroxene (A1(2)O(3) up to 5.8 wt. %) and plagioclase (An(56-75)). These pyroxenes resemble high-Al pyroxene m egacrysts common in massif-type anorthosite, and as such, may represent hig h-pressure crystallization products entrained by gabbroic magmas to a shall ow crustal emplacement site. Coarse-grained (up to 7 cm) anorthosite and le ucotroctolite are concentrated in the 30 km(2) Nyangoma region, representin g <5% of the surface area of the Complex, and resemble massif-type anorthos itic rocks in terms of textures and mineralogy (An(47-57), En(59-75), Fo(58 -64)) Mineralogy and isotopic compositions indicate that Tete gabbroic, pyr oxenitic, and anorthositic rocks crystallized from a common mafic magma. Th e magmatic crystallization age of the Tete Complex is best constrained by a 9-point Sm-Nd whole-rock regression of 1025 +/- 79 Ma (MSWD = 4.0). Initia l isotopic ratios (calculated at 1 Ga) are: epsilon(Nd) = +3.5 to +4.5 (mea n = +4.1) and I-Sr = 0.702758 - 0.702878 (mean = 0.702818), indicating deri vation from depleted mantle, with little or no involvement of Archaean crus tal contaminants. Assimilation of small amounts of Meso- or Palaeoproterozo ic crust is permissible, if not likely. Cross-cutting dolerite dykes have e psilon(Nd) = +5.8 to +6.9 (at 1 Ga), indicating lesser crustal contaminatio n, and may be equivalent to the upper mantle depleted source of the Tete ma gmas. 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 (Arc haean?) crustal components. Even lower epsilon(Nd) values (to -4.3), implyi ng assimilation of Late Archaean crust, occur in the Uluguru anorthosites o f Tanzania. The SSZ, therefore, may represent a major boundary between crus tal age provinces.