Metamorphic evolution of the Sittampundi Layered Complex, India, during the Archaean–Proterozoic boundary: insight from pseudosection modelling and zircon U–Pb SHRIMP geochronology

In the Palghat–Cauvery Shear/Suture Zone of the Southern Granulite Terrane, the Sittampundi Layered Complex occurs as a mappable unit. The Sittampundi Layered Complex consists of mafic, ultramafic and anorthositic rocks with chromitite layers and has been interpreted as an arc/ophiolite complex that formed in an Archaean suprasubduction zone arc setting. In the Sittampundi Layered Complex, reddish-black metabasites occur as layers or boudins with a rim of amphibolite within anorthosite. The peak metamorphic assemblage of the metabasites is garnet + clinopyroxene + quartz + rutile ± plagioclase ± orthopyroxene, and symplectite consisting of amphibole and plagioclase formed around the garnet during retrograde metamorphism. The protolith of metabasite may have intruded in a suprasubduction zone arc setting during the late Neoarchaean (c. 2540–2520 Ma), and then underwent high-pressure granulite-facies peak metamorphism (900–800 °C and 11–14 kbar) in the early Palaeoproterozoic (c. 2460–2440 Ma), followed by amphibolite-facies metamorphism (550–480 °C and 5.5–4.5 kbar) in the middle Palaeoproterozoic (c. 1900–1850 Ma). The results obtained in this study, together with previous studies, indicate that: (1) the high-pressure granulite-facies metamorphism in the study area indicates that subduction occurred during the Archaean–Proterozoic boundary with a higher apparent average geothermal gradient (∼20–16 °C km−1) than the modern-day Earth, and (2) the apparent average geothermal gradient of the subduction zone was ∼29–14 °C km−1 during the Archaean–Proterozoic boundary, which was still too high to enter the realm of eclogite-facies metamorphism.