Siberian Craton Aldan And Anabar Shields

The Siberian craton has Archean crust exposed in the Aldan and Anabar shields and in some of the Pha-nerozoic fold belts that surround the craton. Much of the craton is covered by thick platform sediments, and these are also underlain by Archean crust.

The Aldan shield contains a gneissic and migma-titic basement deformed into large oval gneiss domes. Granulite facies gneisses, known as the Aldan Supergroup, have yielded ages of 3.4 to 3.2 billion years old. Interspersed greenstone and schist belts known as the Subgan or Olondo Group have been dated to be about 3.0 billion years old. Both of these rock suites are cut by granitoid plutons that fall into two

East Russian Craton

Map showing the simplified geology and tectonics of Russia and surrounding areas, including the Russian (East European) craton, Siberian (Angara) craton, Urals, Altaids, and the Verkhoyansk Kolyma region.

age groups, 3.1-2.9 billion years and 2.6-2.5 billion years. The gneisses form domes that separate the belts of greenstones and schist, which are typically complexly deformed and show evidence of polyphase folding.

The Aldan Supergroup consists mostly of granu-lite facies gneiss with interleaved slivers of quartz-ites, marble, and calc-silicate rocks, reflecting that a deformed and highly metamorphosed shallow-water sedimentary type sequence is deformed along with the gneiss. The Aldan supergroup can be divided into three main belts: the Aldan-Timpton block contains a lower assemblage of thick quartzite overlain by pyroxene-amphibole gneiss; this is unconformably overlain by hypersthene-tonalite gneiss, marble and cal-silicate rocks, with quartzite and mica-gneiss, and an upper sequence consisting of interlayered calcsili-cates and graphitic gneiss.

The Subgan Group includes greenstone belts made predominantly of mafic to felsic metavolca-nic rocks along with minor amounts of quartzites, banded-iron formation, and schist metamorphosed to greenschist to amphibolite facies. There are approximately 30 greenstone belts in the Aldan shield, most 15-100 miles (30-150 km) long, and 1.5-2 miles (2.5-4 km) wide. The structural geology of the greenstone belts is difficult to decipher, but it is clear that they are cut by many shear zones and have been through polyphase deformation. Their relationships with older gneisses are not well established.

The Aldan shield is cut by abundant granitoid plutons, including a 3.4-3.2 billion-year-old tonal-ite-charnockite series and a 2.6-2.5 billion-year-old group of biotite-rich granites and pegmatites.

The Anabar shield forms a plateau sitting 1,500 to 2,800 feet (500-900 m) above sea level. Geologically, the shield is divided into Archean granulites and anorthosites of the Anabar Complex and the early Proterozoic (1.9 billion years old) Lamyuka Complex. Most interpretations of the Lamyuka Complex suggest that it represents amphibolite-facies reworking of the Anabar Complex along 6-20 mile- (10-30 km-) wide and 120 mile- (200 km-) long belts of deep shearing and basement reactivation.

The Anabar Complex consists of high grade gran-ulitic gneiss, schist, and migmatite that are strongly folded about north-northwest axes and deformed into broad domes. The gneisses have yielded ages of about 3.2 billion years old and show strong meta-morphism at 2.7 billion years ago. Granodiorite plu-tons cut the older rocks and are elongate parallel to regional foliations and structural trends.

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Responses

  • Darlene
    Are craton underlain by ocean?
    2 years ago

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