Onverwacht Group

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The Onverwacht group of the Swaziland super group or Barberton super group is the oldest supracrustal group of the Kaapvaal craton . It is assigned to the time interval 3530 to 3230 million years BP and thus comes from the Paleoarchean and Mesoarchean ( Isuum and Vaalbarum ).

etymology

The word Onverwacht is used to describe several localities and locations in South Africa. In Afrikaans or Dutch it means unexpected, unexpected .

Occurrence

The Onverwacht Group is found in the Barberton greenstone belt in northeastern South Africa and in the northwestern border area of Swaziland .

stratigraphy

Komatiitic lava of the Onverwacht Group with characteristic spinifex texture, Komati River

Stratigraphically the deposited under the sea Onverwacht group is divided into two sub-groups ( engl. Subgroups ), each with three formations divided (from the hanging wall of the footwall ):

The mostly magmatic Onverwacht Group has a bimodal petrological character ( ultramafic / mafic - rhyolitic ) and has a total thickness of over 16,000 meters.

Its basal, ultramafic emphasized, around 8,700 meter thick Tjakastad subgroup consists mainly of pillow lavas and massive lava flows in which ultramafic komatiites and mafic, magnesium-rich basalts and tholeiites are embedded. Their contact with the underlying basement , which is established further south in the direction of Central Wasiland from the 3644 to 3538 million year old Ancient Gneiss Complex ( AGC for short ) with the Stolzburg Pluton intruded from 3470 to 3437 million years ago , is not visible.

The Sandspruit Formation lying on its side reaches a thickness of 3200 meters. It is very strongly deformed and metamorphosed and exists as mafic / ultramafic slate . In the southwest of the Barberton-Grünstein belt, it is flanked by TTG rocks , into which it was partially folded. It is followed concordantly by the 1980 meter thick, heterolithic Theespruit Formation , which, in addition to the magmatites (komatiites and basalts) already mentioned, also contains metamorphosed, acidic pyroclastics (tuffs) and acidic lavas. The formation is intruded by the approximately 3510 million year old Steynsdorp pluton . The final, 3500 meter thick Komati Formation lies with a 600 to 800 meter wide shear zone contact - the Komati Fault or the Komati Schist Zone or KSZ for short - over the two preceding formations. The formation is made up of komatiites and komatiitic basalts. It can be divided into a lower and an upper section. The lower section is made up of 48% layers of olivine komatiite, which are either solid (61%), in olivine spinifex texture (37%) or blister-rich (2%), and 52% of komatiitic basalt. Cooling units are predominantly layered lava flows ( sheet flows ), but channeled lava flows ( channel flows ) and pahoehoe lava flows also occur . 84% of the upper section is occupied by pillow lavas made of komatiitic basalt, the rest are layered lava flows with pyroxene- spinifex texture. The Komati formation is interspersed with corridors and storage corridors made of komatiite, komatiitic basalt, diabase , tonalite and wehrlite .

The overlying, mafic to acidic Geluk subgroup , around 7,700 meters thick , is separated from the Tjakastad subgroup by the Middle Marker - a largely silicified clastic unit of accretionary lapilli , chert , volcaniclastic sand and carbonates . In contrast to the Tjakastad subgroup, which can only present pyroclastics in the Theespruit Formation, sediments of biochemical and orthochemical origin as well as pyroclastics and volcaniclastics are widespread in the Geluk subgroup. Among the sediments of the Geluk subgroup, pyroclastics and volcaniclastics make up around 70 to 80%, the remaining 20 to 30% are made up of biochemical and orthochemical deposits. The pyroclastics / volcaniclastics include monolithological debris flow breccias, conglomerates and sandstones reclaimed by the current, as well as tuffs made from pyroclastic fall deposits and pyroclastic flow deposits .

The basal formation of the Geluk subgroup, the Hooggenoeg Formation , which is up to 4850 meters thick , has tholeiitic cushion basalts and tholeiitic massive basalts in its lower section, which are traversed by gabbro ducts . This is followed by komatiitic rocks (komatiitic basalts and komatiites) in the middle and upper section. The Hooggenoeg Formation is closed off by acidic volcanic rocks (lava flows and sub-volcanic reservoirs). In the entire formation, chert layers with carbon-rich material that have emerged from silicified ultramafites lie between the individual lava flows. In the hanging wall of the formation, localized erosion phenomena appear above a jaspilite , which are concordantly filled by lava flows and volcanic plastics of the Kromberg Formation and the Mendon Formation. The 1920-meter-mighty Kromberg Formation begins with the Buck Reef Chert , an important 300-meter-thick Chertlage. This is followed by mafic volcanic plastics and basalt rivers. Another thin chert layer, the Footbridge Chert, delimits the formation to the final, 920 meter thick Zwartkoppie Formation / Mendon Formation , which again consists of komatiites and is covered by the Msouli Chert .

The Geluk subgroup is concordant with the continental metasediments of the 3,258 to 3,226 million year old Fig Tree group - pelitic and clastic sediments such as slate , sandstones , conglomerates , chert and jaspilite - and the discordant following, 3230 to 3110 million years old and also superimposed on continental moodies group .

Meteorite impact

Immediately above Chertlage H4c , the Hooggenoeg Formation contains a horizon of spherules that are interpreted as quenched silicate melt droplets from a meteorite impact . The location was dated to 3470.4 ± 2.3 million years BP using the lead-lead method ( 207 Pb / 206 Pb). A corresponding location with practically identical age (3470.1 ± 1.9 million years BP) was also found in the West Australian Warrawoona Group .

metamorphosis

The Onverwacht group was generally affected by a submergence metamorphosis, and in the vicinity of intrusions it was also changed contact metamorphically. The formations above the Komati Schist Zone (KSZ) are only low metamorphic (pumpellyite-prehnite facies). In the shear zone, the degree of metamorphosis to the green slate facies increases and reaches the amphibolite facies in the two lying formations (Sandspruit and Theespruit Formations) , and even the granulite facies in very deeply folded keel regions.

The maximum physical conditions achieved in septa between the Theespruit pluton and the Stolzburg pluton were 0.9 GPa and 700 ° C; to the west of the Stolzburg Pluton, comparable values ​​(0.8 to 1.1 GPa and 650 to 700 ° C) could be determined. The base of the Komati formation was 0.39 GPa and 490 to 530 ° C and only 0.19 GPa and 320 to 420 ° C were determined for the upper edge of the Hooggenoeg formation.

The metamorphosis and the associated deformations took place in two phases:

  • 3418 million years BP
  • 3236 to 3219 million years BP

The early phase around 3418 million years BP is associated with syntectonic acid magmatism. The major phase around 3230 million years BP reflects the stretch tectonics that the Onverwacht Group began to exhume.

Overall, the maximum metamorphic conditions show a relatively low geothermal gradient of around 20 K / km.

Age dating

Concordant zircons from arcoses of the basal Sandspruit Formation could be dated by Dziggel and colleagues (2002) using the lead | lead-lead method in the period 3540 to 3521 million years BP. For the Theespruit Formation, Kröner and colleagues (1996) also found age of 3548 ± 3 to 3544 ± 3 million years BP using the lead-lead method on zircons. Very similar to Armstrong and colleagues (1990), who determined 3538 + 4 - 2. The Komati formation provided ages between 3490 (Lopez-Martinez and colleagues, 1992) and 3481 million years BP (Dann, 2000) and for the final middle marker Armstrong and colleagues (1990) found 3472 million years BP.

The Hooggenoeg Formation could be determined by Byerly and colleagues (2002) with 3470 million years BP. According to de Vries and colleagues (2006), their final acidic volcanic plastics gave the interval 3457 to 3428 million years BP. For the Kromberg Formation and the Mendon Formation, Lowe and Byerly (2007) give the time span ≤ 3416 to> 3298 million years BP.

Overall, the Onverwacht group uses these age dating to cover the period from 3550 to 3300 million years BP, with the boundary between the two subgroups being around 3470 million years BP.

Geodynamic development

In the period 3530 to 3416 million years BP, the Onverwacht Group deposited a mighty package of green stones (Tjakastad subgroup including the Hooggenoeg Formation) on the substrate of an older continental crust (including the approximately 3,640 million year old Ancient Gneiss Complex ). Further crust thickening occurred as a result of the settlement of TTG melts originating from infracrustal basalts in the interval 3470 to 3437 million years BP. Resuming mantle magma activity from 3334 to 3298 million years ago BP (deposition of the Kromberg and Mendon Formations) finally resulted in a thickening of the crust and an associated gravitational instability - triggered by the superposition of more than 10,000 meters thick and relatively heavy komatiitic volcanic rocks on lighter granitic middle crust ( consisting of TTG and AGC). This resulted in a partial convective overturning around 3230 million years BP , which happened relatively quickly. It was accompanied by an isothermal pressure relief of the lower green stone package, which had been initiated by a general exhumation proceeding under strain. As a result, the granitoids on the edges of the greenstone belt began to rise. They poured their erosion products - the Fig Tree and Moodies groups - into the sinking core area of ​​the greenstone belt, which was simultaneously deformed during sediment accumulation. The horizontal movements of the green stones in the direction of the sinking zone took place via detachment zones such as the KSZ in the central crustal area. As a result, the upper green stone package was deformed horizontally down to horizontal isoclinal folds .

Individual evidence

  1. ^ A b Van Kranendonk, MJ et al .: Age, lithology and structural evolution of the 3.53 Ga Theespruit Formation in the Tjakastad area, southwestern Barberton Greenstone Belt, South Africa, with implications for Archean tectonics . In: Precambrian Research . tape 261 , 2008, p. 115-139 .
  2. ↑ Viljoen, MJ and Viljoen, RP: Archaean volcanicity and continental evolution in the Barberton region, Transvaal . Ed .: TN Clifford and IG Gass, African Magmatism and Tectonics. Oliver and Boyd, Edinburgh 1970, p. 27-49 .
  3. ^ A b c Dann, JC: The 3.5 Ga Komati Formation, Barberton Greenstone Belt, South Africa, Part I: New maps and magmatic architecture . In: South African Journal of Geology . tape 103 , 2000, pp. 47-68 , doi : 10.2113 / 103.1.47 .
  4. ↑ Lanier, WP and Lowe, DR: Sedimentology of the Middle Marker (3.4 Ga), Onverwacht Group, Transvaal, South Africa . In: Precambrian Research . tape 18 , 1982, pp. 237-260 .
  5. ↑ Jackson, MPA, Eriksson, KA and Harris, SW: Early Archean foredeep sedimentation related to crustal shortening: a reinterpretation of the Barberton Sequence, Southern Africa . In: Tectonophysics . tape 136 , 1987, pp. 197-221 .
  6. ↑ Lowe, DR: Comparative sedimentology of the principal sequences of Archean greenstone belts, in South Africa, Western Australia, and Canada: implications for crustal evolution . In: Precambrian Research . tape 17 , 1982, pp. 1-29 .
  7. ↑ Lowe, D. and Byerly, G .: Stratigraphy of the west-central part of the Barberton Greenstone Belt, South Africa . In: Lowe, D. and Byerly, G., Geological evolution of the Barberton Greenstone Belt, South Africa (Eds.): Geological Society of America Special Paper . tape 329 , 1999, pp. 1-36 .
  8. ↑ a b Byerly, GR et al .: An Archean Impact Layer from the Pilbara and Kaapvaal Cratons . In: Science . tape 297 , 2002, pp. 1325-1327 .
  9. ↑ a b Dziggel, A. et al .: Metamorphism of the granite-greenstone terrane accretion in the Barberton greenstone belt . Ed .: Cassidy, KF et al., 4th International Symposium, Extended Abstracts. record 2001/37. AGSO-Geoscience Australia, 2002, p. 39-41 .
  10. ^ Cloete, M .: Aspects of volcanism and metamorphism in the 3.47 Ga Barberton Greenstone Belt . In: Memoir Geological Survey of South Africa . tape 84 , 1999, pp. 232 .
  11. ^ A b Diener, JG et al: High pressure, low temperature metamorphism in the southern Barberton granite-greenstone terrane, South Africa: a record of overthickening and collapse of Mid-Archean Continental crust . In: Benn, K. et al., Archean Geodynamic Processes (Ed.): Monograph American Geophysical Union . vol. 164, 2006, p. 239-254 .
  12. ↑ Dziggel, A. include: Metamorphism of the granite-greenstone terrane south of the Barberton greenstone belt, South Africa: an insight into the tectono-thermal evolution of the lower portion of the Onverwacht Group . In: Precambrian Research . tape 314 , 2002, pp. 221-247 .
  13. ↑ Lopez-Martinez, M. et al: A 40 Ar / 39 Ar geochronological study of komatiites and kolatiitic basalts from the lower Onverwacht volcanics: Barberton Mountain land, South Africa . In: Precambrian Research . tape 57 , 1992, pp. 481-526 .
  14. ↑ Armstrong, RA et al .: The stratigraphy of the 3.5-3.2 Ga Barberton Greenstone Belt revisited; a singke zircon ion microprobe study . In: Earth and Planetary Science Letters . tape 101 , 1990, pp. 90-106 .
  15. ^ De Vries, ST et al .: Growth-fault structure and stratigraphic architecture of the Buck Ridge volcano-sedimentary complex, upper Hooggenoeg Formation, Barberton Greenstone Belt, South Africa . In: Precambrian Research . tape 149 , 2006, pp. 77-98 .
  16. ^ Lowe, DR and Byerly, GR: An overview of the geology of the Barberton greenstone belt and vicinity: implications for early crustal development . In: Van Kranendonk, MJ, Smithies, RH and Bennett, VC, Earth's oldest rocks (eds.): Developments in Precambrian Geology . vol. 15. Elsevier, Amsterdam 2007, p. 481-526 .
  17. ↑ Kröner, A .: The Ancient Gneiss Complex of Swaziland and environs: records of Early Archean crustal Evolution in southern Africa . In: Van Kranendonk, MJ, Smithies, RH and Bennett, VC, Earth's oldest rocks (eds.): Developments in Precambrian Geology . vol. 15. Elsevier, Amsterdam 2007, p. 465-480 .
  18. ↑ Moyen, J.-F. among others: TTG plutons of the Barberton granitoid-greenstone Terrain, South Africa . In: Van Kranendonk, MJ, Smithies, RH and Bennett, VC, Earth's oldest rocks (eds.): Developments in Precambrian Geology . vol. 15. Elsevier, Amsterdam 2007, p. 607-668 .
  19. ^ Kisters, AFM et al .: Extensional detachment faulting and core complex formation in the southern Barberton granite-greenstone terrain: evidence for a 3.2 Ga orogenic collapse . In: Precambrian Research . tape 117 , 2003, p. 355-378 .
  20. ↑ Ramberg, H .: Gravity Deformation and the Earth's Crust . Academic Press, London 1967, pp. 214 .