Early Precambrian granitic gneiss, mafic schist, and granite, outlined by prominent east-northeast-trending magnetic and gravity anomalies, are extensive in the buried basement of northeastern South Dakota, the southwestern-most extension of the Superior geological province. These rocks were involved in widespread orogenesis about 2500 m.y. ago. In central and western South Dakota equally prominent geophysical anomalies define the trend of middle Precambrian rocks. A middle Precambrian sedimentary succession in the Black Hills was deformed and intruded by granite approximately 1700 m.y. ago. Four distinct metamorphic belts which possibly date mainly from middle Precambrian time are recognized. A large granite terrane in southern South Dakota is tentatively interpreted as having formed 1430 to 1460 m.y. ago. The basement complex in southeastern and central South Dakota is overlain by a sheet of Sioux Quartzite. A new Rb-Sr age determination on a rhyolite suggests that the Sioux Quartzite is at least 1470 m.y. old.

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