The 2 Ga eclogites of Central Tanzania: directly linking age and metamorphism

Abstract

The ca. 2 Ga retrogressed eclogites in the Usagaran Belt in central Tanzania are among the oldest documented eclogites in the world. As such, they have been used to pinpoint the thermal conditions of the onset of modern style subduction on Earth. Two samples of retrogressed mafic eclogite have been interrogated to reconstruct the metamorphic history of the subducted crust. The samples are several kilometres apart, separated by a mylonitic upper amphibolite-transitional granulite foliation. Lu--Hf garnet geochronology pinpoints eclogite metamorphism at 1994 ± 9 Ma, supported by rutile U--Pb geochronology. Zircon U--Pb geochronology supports subduction metamorphism at ca. 2000 Ma. Mineral equilibria modelling and Zr in rutile thermometry suggests the retrogressed eclogites reached a minimum pressure of 18 kbar at approximately 800 °C, consistent with a warm subduction thermal gradient. These conditions were followed by amphibolite-granulite-facies overprinting. Proposed tectonic models for the formation of the eclogites involve either east-dipping subduction of the Tanzanian Craton margin, or west-dipping subduction of oceanic crust under the Tanzanian Craton. The retrogressed eclogites were likely exhumed via slab breakoff driven buoyancy of the oceanic crust and transferred to shallow crustal levels in a back arc position. This data, combined with data from eclogites from the Eburnian-Transamazonian and Ubende orogens, demonstrate the possible existence of a large subduction system on the margin of west Africa from ca. 2.1–1.9 Ga. This system has led to the preservation of several eclogitic rocks, which suggests that modern style subduction was operating on significant geographic and temporal scales at this time.