Early and middle Proterozoic supracrustal successions and their tectonic settings

Quartzite-carbonate-shale, bimodal volcanics-quartzite-arkose and continuous (tholeiitic and calc-alkaline) volcanics-greywacke form three groups of contrasting lithological assemblages recognized in Early - Middle Proterozoic supracrustal successions. A table showing 79 of these successions is presented, with data on age, dominant rocks in order of abundance, thickness and key references. Interpretation of tectonic environments and regimes during development of the three groups of assemblages indicates rapidly changing tectonic regimes 1000 m.y. ago, at which time Phanerozoic-type plate tectonics first became important, and 1700 m.y. ago, at which time buoyancy-driven subduction began and lithosphere-activated rifting first became widespread. -M.S.New Mexico Inst. of Mining & Tech., Geoscience Dept. Socorro, NM, USA.