Neoproterozoic eclogite- to high-pressure granulite-facies metamorphism in the Mozambique belt of east-central Tanzania: A petrological, geochemical and geochronological approach

Abstract

This study investigated Neoproterozoic (Pan-African) eclogite- and high-pressure-granulite (E-HPG) facies rocks from the Mozambique belt of east-central Tanzania, collected close to the town of Ifakara and the adjacent Furua area from different tectonic settings, the Palaeoproterozoic Usagaran and the Neoproterozoic Mozambique belt. The studied rocks are E-HPG facies granite- and diorite-gneisses and a meta-gabbroic rock, which are retrogressed to amphibolite- and greenschist-facies conditions. Four different clockwise P-T paths were constructed. The first P-T path for a granodioritic gneiss displays peak metamorphic conditions at ~830°C and ~13.0kbar. The second P-T path for a quartz dioritic gneiss shows peak metamorphic conditions of ~920°C and ~14.9kbar. The third P-T path for a mafic granulite shows peak metamorphic conditions of ~820°C and ~13.2kbar. A fourth P-T path for a monzodioritic gneiss also displays peak metamorphic conditions of up to ~810°C and ~14.9kbar. Evidence for all four P-T paths is provided by mineral chemical and modal abundance calculations in combination with textural observations in thin sections. Zircon ages indicate that the east-central part of the Mozambique belt in Tanzania consists of granite-, granodiorite- and monzodiorite gneisses with Mesoarchaean (~2915Ma), Neoarchaean (~2637–2676Ma) and Palaeoproterozoic (~1873–1926Ma) protolith ages. Early Neoproterozoic (Tonian) igneous zircons were found in the mafic granulite with an age of ~989Ma. Late Neoproterozoic (Cyrogenian) igneous zircons were found in a dioritic and monzodiorite gneiss with ages of ~748Ma and ~718Ma, respectively. Metamorphic zircons extracted from Qtz-monzodiorite and granodiorite gneisses yielded ages of ~640Ma and are considered to approximate the peak of regional E-HPG metamorphism. We suggest that this high-grade metamorphic event was caused by the collision of fragments of East and West Gondwana during the Pan-African orogeny, associated with ocean closure and the formation of Andean-type continental arc domains, which ended in the formation of a collisional belt.