Warrawoona Group

from Wikipedia, the free encyclopedia

The Warrawoona Group , which was deposited in the period 3525 to 3426 million years BP, is a geological stratification of the Paleoarchean ( Isuum and beginning of the Vaalbarum ), which occurs in the area of ​​the Pilbara Craton in northern Western Australia . The supposedly oldest cyanobacteria in the fossil record were discovered in it, dating back to 3465 million years BP.

introduction

Geology of the Pilbara Craton. The distribution area of ​​the Warrawoona Group is circled in red

The 3530 to 3165 million year old East Pilbara Terrane (also East Pilbara Granite Greenstone Terrane ) represents the ancient nucleus of the archaic Pilbara craton . The up to 3720 million year old Kraton, the maximum age of which can be proven by inherited zircons of an acid magmatism and which existed from 3720 to 2830 million years BP (then from 2780 million years BP was then covered by sediments of the Hammersley Basin ), grew by three Mantle diapiric events (vertical tectonics) that occurred in periods 3530 to 3430, 3350 to 3290, and 3270 to 3240 million years BP.

As a result of the mantle diapirism, the supracrustal, volcanic sedimentary Pilbara Supergroup deposited itself over the leveled basement in the period from 3530 to 3000 million years BP - an approximately 20,000 meter thick, predominantly basaltic sequence that can be divided into four cycles separated by discordances . As early as 3200 million years BP, an upwelling, non-subducible continent had grown due to the melting processes of the mantle and the associated production of TTG magmas in the lower crust.

The basis of the Pilbara Supergroup is formed by the approximately 12,000 meters thick "Warrawoona Group", which was formed in the period 3525 to 3426 million years BP. It consists mainly of magnesium -rich, tholeiitic and komatiitischen pillow lava and peridotite Komatiiten together, subordinate it also contains tuffs , Chertlagen , banded iron formation and clastic sediments. The rocks were extracted during several ultramafic, mafic and acidic volcanic cycles.

The Pilbara Supergroup represents the oldest, well-preserved, autochthonous basalt series on earth, which has only been moderately deformed and metamorphosed. The deposits were mostly submarine, with the exception of the Dresser Formation , which was sedimented shallowly or aerically (recognizable by ripples and dry cracks ), and the shallow marine panorama formation .

stratigraphy

The Warrawoona Group can be structured stratigraphically as follows (from hanging to lying ):

The “Coonterunah Subgroup”, which is only accessible in the Pilgangoora greenstone belt , begins with the up to 2500 meters thick “table-top formation” - predominantly tholeiitic basalts , which are discordantly laid over foiled granites of the basement. The acidic volcanic rocks ( Dazite ) of the “Coucal Formation” and a chert horizon lie above it, and they are partly interlocking with the basalts . This is followed by a second basalt layer (“double bar formation”), which is closed off by acidic volcanic rocks. Age dates for the acidic volcanic rocks of the Coucal Formation were 3515 million years BP and for the base of the Double Bar Formation it was 3508 million years BP. The final acidic volcanic rocks returned 3496 million years BP.

The “North-Star-Basalt” of the “Talga Talga Subgroup” consists of a basalt layer at the base of which ultramafic lavas occur and which is covered by a thin layer of acidic volcanic rocks. The basalt has been dated to around 3490 million years BP, the final volcanic rocks at 3480 to 3477 million years BP. The "McPhee-Formation / Dresser-Formation" begins with a chertlage over which ultramafic lavas poured discordantly.

This is followed by the "Coongan Subgroup" with the "Mount Ada Basalt", which is assigned an age of 3474 to 3463 million years BP. In the hanging wall of the basalt, the acidic volcanic rocks (dazites) of the “duffer formation” interlock.

The “Salgash Subgroup”, which closes the Warrawoona Group, begins with a layer of Chert (“Marble-Bar-Chert”) and ultramafic lavas of the “Towers Formation”. The "apex basalt" and the acidic volcanic rocks of the "panorama formation" are placed on top. The sequence of the hanging wall then fell victim to erosion. The acidic volcanic rocks (dazites, andesites to rhyolites ) of the Panorama Formation were between 3458 and 3426 million years old BP.

The Warrawoona Group is discordantly covered by the Kelly Group , which only begins after a hiatus of around 75 million years against 3350 million years BP with the Strelley Pool Formation and ultramafic lavas of the Euro basalt . For the first time, volcaniclastic rubble layers (agglomerates) and sandstones were deposited over an angular discordance, which were sealed by a chert and jasper layer of the Strelley Pool Formation.

In older works, the Coonterunah subgroup is not counted as part of the Warrawoona Group, rather the group begins here with the Talga Talga subgroup . The final Salgash subgroup then extends into the Kelly Group and ends around 3325 million years BP with the euro basalt. As a result, the Warrawoona Group defined in this way covers the period 3490 to around 3325 million years BP.

Fossil finds

In 1983 were at Warrawoona of Arthur Hugh Hickman around 3.465 billion years old microstructures in Apex Chert discovered. This find was interpreted as cyanobacteria by James William Schopf in 1993 . These would be the oldest fossils in the fossil record. However, not all experts follow this point of view, as an abiological origin of the structures is also possible. Martin Brasier and colleagues (2004) see the structures as chemical-hydrothermal formations. A final decision in the so-called Schopf-Brasier debate is still pending.

Wacey, Saunders, Brasier and Kilburn recently reported the discovery of laminated, high-carbon materials from the basal sandstones of the strelley pool formation (3426 to 3350 million years old) , which is only marginally younger but already part of the Kelly Group Microbial films on pyrite grains . This is the first evidence of microbial oxidation reactions in shallow water, in which reduced forms of iron and sulfur acted as electron donors.

The allegedly oldest stromatolites are reported from the Dresser Formation, which is around 3490 million years old. They grew up in one of growth faults ( Engl. Growth faults ) surrounded Caldera complex, which was crossed at its base by an extensive network of hydrothermal veins.

Petrology

Ultramafic rocks

The ultramafic komatiites occur as thin, bubble-rich, with Spinifex structures, 1 to 5 meters thick flow units. In contrast to comparable units in the Kelly Group, they are not depleted in aluminum . Their MgO content is between 22 and 30 percent by weight.

Mafische rocks

The mafic rocks (basalts with a tholeiitic trend) can generally be divided into two magma sequences, one into a group with a high titanium content (> 0.8 percent by weight TiO 2 ), which makes up around 65% of all basalts, and into a group with low Ti Content (<0.8 weight percent TiO 2 ) for the remaining 35%. The low Ti basalts come from a more depleted mantle region than the high Ti basalts, with the depletion becoming accentuated with increasing time.

Intermediate and acidic rocks

In the Coucal formation of the Coonterunah subgroup, two acidic, sodium- emphasized magma series (K 2 O / Na 2 O = 0.050-0.045) can be distinguished. Members of the CF-1 series have lower Ti and Fe contents and their trace element composition is more fractionated . A low Yb content and a high La / Yb ratio indicate that there is only minor contamination from TTG magma components. The CF-2 series is much more typical of an origin from fractionated tholeiites (and not from calcareous or archaic TTG magma series), recognizable by a strong Fe emphasis, low K 2 O content (<1.0 percent by weight) and increased concentrations heavy rare earths (HREE) and Y , which in turn are positively correlated with the SiO 2 content and the La / Yb ratio. A Dazit that very likely emerged from a TTG magma is outside of these two series. It is characterized by very low Yb contents and a very low La / Yb ratio.

The duffer formation consists of three different magma series. Rare rhyolites are typical for their origin from very clearly fractionated tholeiites with high contents of Fe, HREE, Zr and Nb . Frequent basalts and dazites as end links, however, form two sodium-emphasized series (K 2 O / Na 2 O <0.50) with a large overlap area. These two series are similar to archaic TTG magmas, but have a higher Na ratio and significantly fewer basalts compared to today's Andean island arch rocks.

The composition of the volcanic rocks in the Panorama Formation reveals distinct volcanic centers. They are generally very similar to the rocks of the Duffer Formation, but tend to have a higher K 2 O / Na 2 O ratio (0.35-0.70). Nevertheless, among them are the oldest volcanic representatives of TTG rocks with very high La / Yb and Yb concentrations of 0.6 ppm.

Magmatism

During the deposition of supracrustal, predominantly volcanic Warrawoona Group Two occurred magmatic episodes in the period from 3500 to 3460 million a BP the Callina Super Suite , and between 3450 and 3.42 billion years BP the Tambina Super Suite produced by melting in deeper crustal areas. The Callina Supersuite was created at about the same time as the Duffer Formation and the Tambina Supersuite at the same time as the Panorama Formation.

The granitic Callina Supersuite consists of a TTG complex typical of the Paleoarchean. Compared to today's adakites bound to subduction zones , these TTG rocks have a significantly lower magnesium number (Mg #), and their chromium and nickel contents also contradict a subduction origin. Despite their simultaneous extraction, the acidic volcanic rocks of the Duffer Formation differ in terms of their trace elements and their neodymium isotopic composition. The duffer formation is likely to be traced back to the fractionation of a tholeiitic parent magma which had been contaminated by TTG crust components. In contrast to this, the TTG rocks of the Callina Supersuite, which can be seen from their neodymium isotopic composition and their Pb-Pb data , emerged from the melting of basaltic crust with partly very old TTG complexes (> 3.7000 million years BP) .

The Tambina Supersuite differs from the Callina Supersuite primarily in its younger age, its lower degree of deformation and its much less migmatization . Leukogranites occur very often in it (such as in the Shaw granite complex ), the melts of which had emerged directly from the protoliths of the Callina Supersuite.

tectonics

The East Pilbara Terrane is a classic cathedral-and-keel structure , which is characterized by oval granite bulges, on average 60 kilometers wide, with keel-like recessed greenstone belts in between. Since no stratigraphic repetitions are recognizable, thrust tectonics can be ruled out. Rather, the structuring that can now be observed is essentially due to a general, regional doming around 3460 million years BP with subsequent convective upheavals towards 3320 and 3240 million years BP.

Of the four main tectonizations of the kraton, two phases took place during the Warrawoona Group's deposit. The first deformation phase (D 1) was associated with the magmatism of the Callina Supersuite already mentioned above, whereas the second deformation phase (D 2) can be correlated with the Tambina Supersuite (3458 to 3420 million years BP). In addition to simple shifting of layers and extensive folding , the Warrawoona Group was also folded tightly, isoclinally or even over during the D 1 deformation. During the weaker D 2 deformation occurred again tilts, foliation in fine Pyrophylliten and incipient out curvature of the Shaw-granite.

Individual evidence

  1. ↑ Lunar and Planetary Science XXXV: Characterization Of The Organic Matter In An Archean Chert (Warrawoona, Australia) . 2004.
  2. ^ MJ Van Kranendonk, among others: Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia . In: Terra Nova . tape 19 , 2007, pp. 1-38 , doi : 10.1111 / j.1365-3121.2006.00723.x .
  3. ^ AH Hickman, MJ Van Kranendonk: Diapiric processes in the formation of Archaean continental crust, East Pilbara Granite-Greenstone Terrane, Australia . In: Developments in Precambrian geology . tape 12 . Elsevier, 2004, p. 118-139 .
  4. ^ MJ Van Kranendonk, AH Hickman, DL Huston: Geology and mineralization of the East Pilbara - a field guide . Western Australia Geological Survey, Record 2006/16, 2006, pp. 94 .
  5. ^ A b c R. H. Smithies, among others: It started with a plume - early Archaean basaltic proto-continental crust . In: Earth and Planetary Science Letters . tape 238 , 2005, pp. 284-297 .
  6. ^ R. Buick, et al: Record of emergent continental crust ~ 3.5 billion years ago in the Pilbara Craton of Australia . In: Nature . tape 375 , 1995, pp. 574-577 .
  7. ^ MJ Van Kranendonk, inter alia: Geology and tectonic evolution of the Archaean North Pilbara Terrain, Pilbara Craton, Western Australia . In: Econ. Geol. Band 97 , 2002, p. 695-732 .
  8. ↑ TE Zegers, among others: Extensional structures during deposition of the 3460 Ma Warrawoona Group in the eastern Pilbara Craton, Western Australia . In: Precambrian Research . tape 80 , 1996, pp. 89-105 .
  9. ↑ J. William Schopf: Micro Fossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life . In: Science . tape 260 , no. 5108 , 1993, pp. 640-646 , JSTOR : 2881249 .
  10. ↑ HJ Hofmann: Archean Micro Fossils and Abiomorphs . In: Astrobiology . tape 4 (2) . McGill University 2004.
  11. ^ MD Brasier, among others: Questioning the evidence for earth's oldest fossils . In: Nature . tape 416 , 2002, pp. 76-81 .
  12. ↑ D. Wacey, M. Saunders, MD Brasier, MA Kilburn: Earliest microbially mediated pyrite oxidation in 3.4 billion-year-old sediments . In: Earth and Planetary Science Letters . tape 301 , 2011, pp. 393-402 .
  13. ^ MR Walter, R. Buick, JSR Dunlop: Stromatolites, 3,400-3,500 Myr from the North Pole area, Western Australia . In: Nature . tape 284 , 1980, pp. 443-445 .
  14. ↑ P. Hollings, R. Kerrich: An Archean arc basalt-Nb-enriched-adakite association: the 2.7 Ga Confederation assemblage of the Birch-Uchi greenstone belt, Superior Province . In: Contrib. Mineral. Petrol. tape 139 , 2000, pp. 208-226 .
  15. ↑ RH Smithies: The Archaean tonalite-granodiorite-trondhjemite (TTG) series is not an analogue of Cenozoic adakite . In: Earth Planet. Sci. Lett. tape 182 , 2000, pp. 115-125 .
  16. ↑ MJ Bickle, inter alia: origin of the 3500-3300 Macalk-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith . In: Precambrian Research . tape 60 , 1993, pp. 117-149 .
  17. ^ MJ Van Kranendonk: Geology of the Tambourah 1: 100,000 sheet . In: 1: 100,000 Geological Series Explanatory Notes . Western Australia Geological Survey, 2003, p. 59 .
  18. ↑ WJ Collins, inter alia: Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications . In: Journal of Structural Geology . tape 20 , 1998, pp. 1405-1424 .