Narryer Gneiss Terran

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The Narryer Gneiss Terran is a very old crustal segment of the Western Australian Yilgarn craton from the Paleoarchean ( Isuum ) . From the Terran nearby Jack Hills previously oldest known worldwide originate zircons whose radiometric dating to 4.404 billion years BP go back. In the approximately 3780 million year old Manfred complex , the Terran is home to the oldest rocks in Australia .

etymology

The name of the terran, engl. Narryer Gneiss Terrane or Narryer Gneiss Complex , abbreviated NGC , is derived from the 514 meter high Mount Narryer in Western Australia.

description

Satellite image of the Jack Hills in the Narryer Gneiss Terran

The Narryer Gneiss Terran consists of granitoids , mafic intrusions and supracrustal metasediments . It has been deformed several times and metamorphically overprinted. The high-grade metamorphic conditions reached the amphibolite and granulite facies. This often led to a complete destruction of the original igneous and sedimentary structures .

The rocks of the Narryer Gneiss Terran are all older than 3300 million years BP, most of them even older than 3600 million years BP. As already mentioned at the beginning, very old zircons were isolated in the Jack Hills, which have the record age of 4404 million years, but their age range is mainly between 3800 and 3600 million years BP.

The Narryer Gneiss Complex lies on the northwestern edge of the Yilgarn Craton and is touched to the north by the Gascoyne Complex , a younger orogen consisting of metagranites and metasediments.

Geological structure

The Narryer Gneiss Terran can be divided into four rock units:

Manfred complex

The Manfred complex is a very reduced and discontinuous sequence ultramafic to mafic Kumulatgesteine , which are embedded in a matrix from each other and mixed Dugel- Meeberrie-gneiss. The rocks are predominantly pyroxene gabbros and amphibolites ; serpentinized peridotites and dunites , which contain occasional relics of igneous or metamorphic olivine , also occur less frequently .

The Manfred complex is boudinized, whereby the size of the individual boudins can vary from a centimeter to a hundred meters depending on the structural location (within anti- and synclines). In the area of ​​Mount Narryer it is assumed that the cumulative rocks of the Manfred Complex penetrated parallel to the layers and were then sheared.

The Manfred complex can thus be interpreted as a paleoarchaic, mafic to ultramafic layered intrusion , which was later disrupted in its original association. This was only partly due to tectonic causes, namely, there are also outcrops that suggest its fragmentation by the layered or bedrock-like penetration of the Dugel and Meeberrie gneisses.

Geochronological studies of the Manfred complex have shown an age of 3730 million years BP using the lead-lead method on zirconia. This is the highest generally recognized age for a sheet intrusion on earth to date. It contains the oldest known magmatic structures and mineral compounds and also the oldest anorthosite in the world.

Meeberrie gneiss

Meeberrie gneiss is a ductile deformed ribbon gneiss of monzogranitic composition. The original rock is interpreted as monzogranitic bed dikes or lopolithic intrusions.

The gneiss shows a clear ribbon structure with amphibolite facies of potassium feldspar and quartz of different grain sizes. The rock is mostly very strongly deformed, but pressure shadow areas reveal a relictual, porphyry and even grain structure.

Dugel gneiss

The leucocrate, depleted in biotite and muscovite, Dugel gneiss has a syenogranitic or monzogranitic composition, which reflects the chemistry of the original parent rock . Metamorphic bands can be seen in the rock, which are expressed as variable grain size . The Dugel gneiss is very badly deformed at its edge. The rock has preserved an amphibolite facial, metamorphic mineralogy and is continuously penetrated by pegmatite veins.

In tectonically less stressed sections, the Dugel gneiss occurs as a medium-grain, leucocrate, potassium feldspar - phenocrystals - bearing metagranite, which was recrystallized under granulite-facial conditions. The rock has a greasy look that it annealed (Engl. Annealed ) quartz - and feldspar owes. Under granulite conditions resulting leucosomes cross the metamorphic banding and were deformed dynamically later.

It is assumed that the Dugel gneiss intruded in layers in the older Meeberrie gneiss, but most of the contact areas are unfortunately ductile overprinted by metamorphic bands or mylonite zones .

Metasediments

Supracrustal metasediments occupy approximately 10% of the exposed surface in the Narryer Gneiss Terran. They are unevenly deformed, but in their entirety have reached the metamorphic degree of the amphibolite facies. They appear in narrow syncline belts on Mount Narryer and the Jack Hills. The metamorphosis of the metasediments, which took place between 2700 and 2600 million years BP, reached green slate to amphibolite facies in the Jack Hills, whereas the conditions of the granulite facies were realized at Mount Narryer.

Among the metasediments, quartzites and band ores are most frequently represented, with gneisses, metaconglomerates and pelitic to semipelitic quartz-muscovite slates also occurring . At Mount Narryer only quartzites, conglomerates and pelites are exposed, whereas chemical sediments such as chert and ribbon ores also appear at the Jack Hills .

The metaconglomerates are predominantly monomictic, with the veined pebbles consisting of ortho quartzite; polymictic varieties are also present. In zones with a low degree of deformation, the metaconglomerates can even have primary sedimentary structures such as graded stratification, oblique stratification and heavy mineral layers .

Geodynamics

The Narryer Gneiss Terran has been affected by several phases of deformation . The first phase took place in the time interval 3730 to 3680 million years BP before the formation of the Meeberrie gneiss, but after the intrusion of the Manfred complex. The second phase is at 3350 million years BP; it reached the conditions of the amphibolite facies with simultaneous recalibration of the chronometric isotope ratios. The phase between 2700 and 2600 million years ago was the most important, so there was strong magmatism in the Yilgarn Kraton , which led to the formation of granite- greenstone belts . This last phase has thoroughly overprinted the earlier phases, so that the original structures in the northeast-trending, steep fold structure and parallel to the metamorphic banding were adjusted. Nevertheless, structures from the older phases were able to survive in areas of lower tectonic tension.

In the contact area with neighboring orogenes and thrust belts of the Proterozoic , the Narryer Gneiss Terran was again affected by their later deformations.

Dating

Most of the age determinations for the Narryer-Gneiss Complex come from the metasediments. A clear maximum in the age distribution between 3750 and 3500 million years BP could be determined on detritic zircons. Secondary maxima are at 4200 to 4100 million years BP and at 3450 to 3350 million years BP. The record ages reaching beyond 4,400 million years BP are relatively rare.

Schedule

According to AF Trendall (1991), the development of the Narryer Gneiss Terran was as follows:

  • > 4100 million years BP: Formation of the host rock ( paragneiss ) for the Manfred complex.
  • approx. 3780 million years BP: Penetration of the Manfred complex, an ultramafic-mafic layer intrusion.
  • 3730 to 3680 million years old BP: first deformation phase.
  • approx. 3680 to 3600 million years old BP: Formation of the Meeberrie gneiss.
  • 3680 to 3400 million years old BP: second deformation phase.
  • 3490 to 3440 million years old BP: The tonalitic to monzogranitic Eurada gneiss forms.
  • 3400 million years BP (also 3380 to 3350 million years BP): intrusion of the syenogranite from which the Dugel gneiss emerges, but also intrusion of mafic and ultramafic dikes .
  • 3400 to 3350 million years old BP: Deposition of sediments, the later metasediments.
  • 3350–3300 million years old BP: amphibolite to granulite facial metamorphosis (growth of ortho- and clinopyroxenes).
  • 2700-2600 million years old BP: Formation of granitic layer intrusions and docking on the Yilgarn Kraton ( Murchison Terran ) under green slate to granulite facial conditions.
  • 2000–1600 million years old BP: Invasion of mafic veins associated with the Gascoyne Complex and the Capricorn Orogen .

meaning

The zircons found in the metasediments show maximum abundance at 4150 million years BP and in the interval 3600 to 3300 million years BP as well as individual finds at 4100 and 4130 million years BP. The most significant is certainly the discovery of a 4404 million year old zircon crystal from the metaconglomerates of the Jack Hills, which is the world's oldest known mineral to date. The δ 18 O values ​​of the zirconia are all significantly increased compared to the jacket value of 5.3 ‰ SMOW , which suggests assimilation processes during the melting process. In order to achieve the high isotope ratio, sediments or hydrothermally affected rocks had to be incorporated. This suggests that the Earth already had a hydrosphere at this early point in its evolutionary history. In less than 100 million years since the formation of the Earth's core and the moon , it must have cooled down enough to hold liquid water. This conclusion falls within the range of the cool early herd theory (English. Early Cool Earth , ECE). The 4004 million year old zircon is zoned with regard to rare earths and δ 18 O values. This fact also suggests magmatic processes during their growth, with the zoning expressing different proportions of supracrustal assimilates.

Individual evidence

  1. ↑ IR Fletcher u. a .: Sm-Nd, Pb-Pb and Rb-Sr geochronology of the Manfred Complex, Mount Narryer, Western Australia . In: Precambrian Research . tape 38 , 1988, pp. 343-354 .
  2. ↑ SA Wilde u. a .: Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago . In: Nature . tape 409 , no. 6817 , 2001, p. 175-178 .
  3. ^ IR Williams, JS Myers: Archean geology of the Mount Narryer region Western Australia . In: Report 22, Geological Survey of Western Australia . Department of Mines, Perth 1987.
  4. ↑ JS Myers: Western Gneiss Terrane . In: Geology and Mineral Resources of Western Australia: Western Australia Geological Survey . Memoir 3, 1990, pp. 13-31 .
  5. ↑ JS Myers: Oldest known terrestrial anothosite at Mount Narryer, Western Australia . In: Precambrian Research . tape 38 , 1988, pp. 309-323 .
  6. ^ R. Maas, MT McCulloch: The provenance of Archean clastic metasediments in the Narryer Gneiss Complex, Western Australia: Trace element geochemistry, Nd isotopes and U-Pb ages of detrital zircons . In: Geochim. Cosmochim. Acta . tape 55 , 1991, pp. 1915-1932 .
  7. ↑ a b A. P. Nutman et al. a .: SHRIMP U-Pb zircon geochronology of the Narryer Gneiss Complex, Western Australia . In: Precambrian Research . tape 52 , 1991, pp. 275-300 .
  8. ↑ a b P. D. Kinny et al.: Early Archaean zircon ages from orthogneisses and anorthosites at Mount Narryer, Western Australia . In: Precambrian Research . tape 88 , 1988, pp. 325-341 .
  9. ^ JS Myers, IR Williams: Early Precambrian evolution at Mount Narryer, Western Australia . In: Precambrian Research . tape 27 , 1985, pp. 153-163 .
  10. ↑ JS Myers et al. a .: Excursion 1: Narryer Gneiss Complex . In: SE Ho u. a. (Ed.): University of Western Australia Publication . tape 21 , 1990, pp. 61-95 .
  11. ↑ a b W. H. Peck u. a .: Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons: ion microprobe evidence for high δ 18 O continental crust and oceans in the Early Archean . In: Geochimica et Cosmochimica Acta . tape 65 , no. 22 , 2001, p. 4215-4229 .
  12. ↑ JW Valley et al. a .: Oxygen isotopes in zircon: a new look at crustal evolution . In: EOS . tape 81 , 2000, pp. 25 .
  13. ↑ JW Valley et al. a .: A cool early Earth . In: Geology . tape 30 , 2002, pp. 351-354 .