Palaeoproterozoic eclogites record lithobaric mixing during subduction and exhumation

Abstract

The interrogation of mineral assemblages that preserve evidence of having reached eclogite-facies conditions provides insight into the thermal state of subduction regimes. One of the first appearances of such assemblages in the geological record is documented in the Palaeoproterozoic Usagaran Belt in central Tanzania, where ca. 2000 Ma relic eclogite-facies assemblages are preserved. The eclogites contained the assemblage garnet + omphacite + rutile + quartz and have subsequently been overprinted by diopside + plagioclase + hornblende + ilmenite ± orthopyroxene. The eclogitic domains are preserved within low-strain domains, and these are encased by comparatively high-strain garnet-kyanite metapelitic gneisses and garnet-bearing mafic gneisses. Mineral equilibria forward modelling indicate that the eclogites reached minimum peak pressures of ca. 15–18 kbar and Zr-in-rutile thermometry applied to armoured rutile in garnet yields peak temperatures of 755–768°C. Peak pressure–temperature (P–T) conditions are consistent with a cool geothermal gradient of 460°C/GPa. The retrograde history of the eclogite facies rocks is characterised by post-peak near-isothermal decompression to granulite facies conditions of 6.5–7 kbar and 800°C. Elevated Zr concentrations in fine-grained rutile and the preservation of prograde compositional zoning in garnet indicates that peak metamorphic conditions and near-isothermal exhumation occurred within ca. 1 Ma. The comparatively high-strain metapelitic domains record lower peak pressures (7.3–8.3 kbar and 683–700°C) than the relic eclogites, indicative of a separate P–T history where the maximum P–T conditions coincide approximately with the post-peak assemblages in the eclogites. In light of the results from mineral equilibria forward modelling, the regional scale association between the relic eclogites and high-strain gneisses is consistent with an exhumation model involving either extrusion or extension, or a combination of both. This model permits the juxtaposition of deeply buried rocks with mid-crustal material.