Crustal age gap

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As crustal age gap in the will Geology of the period 2450 to 2200 million years Before Present called because during this period lasting 250 million years in the Earth's crust almost no igneous rocks were longer produced. It ran roughly parallel to the Great Oxygen Disaster and the Paleoproterozoic Glaciation .

designation

The crust age gap ( engl. Crustal age gap ) is in the technical literature as a magmatic age gap (Magmenalterslücke) or as magmatic shutdown designated (Magmenstop).

introduction

Clear minimum age of new crust formation in the period 2200 to 2500 million years BP (Geon 23 and Geon 24), according to Condie (2006)

A gap in the distribution of radiometric dating of rocks between the late Archean and the Paleoproterozoic , which was established early on, remained with the increasing sample size. The formation of crusts since the Archean Era was not a steady, linear process of growth, but was irregular and episodic. Clear maxima were found in the Neo-Archean at 2700 million years BP and again at the end of the Paleoproterozoic with the formation of the supercontinent of Columbia around 1890 million years BP. When exactly the plate tectonics started remains speculative. However, due to the spread of archaic cratons , it is clear that an efficient recycling mechanism was already in place back then.

Findings

What is remarkable about the crustal age gap of the Paleoproterozoic Era is that it is not only reflected in the zirconia ages of granitoids , but can also be found in detritic (ie eroded and subsequently sedimentary deposited) zircons and their age distributions. In both analysis methods, the zirconia ages in the period from 2450 to 2200 million years BP pass through a globally detectable, pronounced minimum frequency (ie on all cratons of that time). The following rock associations were affected by this minimum:

  • TTG granitoids and calcareous plutons of the island arcs - missing
  • Greenstone belt of the island arches - missing
  • Komatiite - absent
  • Magmatic Greater Provinces , engl. large igneous province (LIP) - BP missing in the period 2400 to 2200 million years. The only exception is an occurrence in the south of India ( Dharwar Kraton ) around 2365 million years BP. By 2,450 million years BP, LIP had still been formed.
  • Ribbon ores - missing. By 2,450 million years BP, a record amount of ribbon ore was deposited in the Hammersley Basin . The ore formation then stopped practically until 1900 million years BP (between 2200 and 1900 million years BP, very little iron ore was sedimented).
  • Orogenic gold deposits - reach an absolute minimum between 2300 and 2100 million years BP.

Discordances

In addition to greatly reduced or absent magma production, the crustal age gap is characterized by discordances that occur worldwide :

Since levels of the middle earth's crust were partially exposed below these supra-regional discordances, maximum erosion thicknesses in the range 10,000 to 15,000 meters can be assumed. Such leveling implies a significant decline in global sea ​​levels .

Exceptions

The following examples may be given as exceptions to the general absence of igneous ages in the period from 2450 to 2200 million years BP:

However, only small amounts of new, juvenile crust were formed in the Arrowsmith ore as well as in the Borborema orogen and primarily older crust sections were amalgamated.

Explanation

Such a long absence of magmatic activities is unique in the history of the earth and can actually only be explained by a fundamental disturbance of the convection system in the earth's mantle , probably caused by an impairment of ascending mantle diapirs . The magma stop lasted a good 250 million years. This very long period corresponds roughly to the cycle (lifespan) of our current oceanic crust , the maximum age of which in the western Pacific does not exceed 200 million years.

Measurements on komatiites showed that lavas mined in the Paleoproterozoic have significantly lower formation temperatures than in the Archaic, recognizable by the MgO content , which had clearly decreased from over 30% to 22% MgO. This suggests much lower temperatures of the upper jacket.

The lower mantle temperatures and thus the reduced spreading activity on oceanic ridges explain the drop in sea level observable worldwide during the crustal age gap. It is very likely that an above-average amount of cold, subducted, oceanic crust had accumulated as a result of the supercontinent formation in the period from 2700 to 2500 million years BP. In this context it is interesting that immediately after the end of the crustal age gap in the period 2200 to 2000 million years BP, the breaking up of the newly created supercontinent Superia (or Kenorland) was initiated by the regenerated and powerful magmatism.

Individual evidence

  1. ^ RL Armstrong: Radiogenic isotopes. The case for crustal recycling on a near-steadystate no-continental-growth Earth . In: Philos. Trans. R. Soc. Lond. A301, 1981, p. 443-472 .
  2. ↑ KC Condie, C. O'Neill, RC Aster: Evidence and implications for a widespread magmatic shutdown for 250 My on Earth . In: Earth and Planetary Science Letters . tape 282 , 2009, pp. 294-298 .
  3. ↑ VR Pradhan, MK Pandit, JG Meert: A cautionary note on the age of the Paleomagnetic pole obtained from the Harohalli dyke swarms, Dharwar craton, southern India . In: RK Srivastava, C. Sivaji, NVC Rao (eds.): Indian Dykes . Narosa Ltd, New Delhi 2008, p. 1-13 .
  4. ^ AE Isley, DH Abbott: Plume-related mafic volcanism and the deposition of banded iron formation . In: J. Geophys. Res. Band 104 , 1999, pp. 15461-15477 .
  5. ↑ DI Groves, RM Much rich, RJ Goldfarb, KC Condie: Controls on the heterogeneous distribution of mineral deposits through time . In: Geological Society, London, Special Publication . tape 248 , 2005, pp. 71-101 .
  6. ^ RP Hartlaub, LM Heaman, T. Chacko, KE Ashton: Circa 2.3-Ga magmatism of the Arrowsmith orogeny, Uranium City Region, Western Churchill craton, Canada . In: J. Geol. Volume 115 , 2007, p. 181-195 .
  7. ^ AH Fetter, WR Van Schmus: Geologic history and framework of Ceara State: NW Borborema province, NE Brazil . In: Programs-Geol. Soc. At the. 29 (6), A-49, 1997.
  8. ^ NT Arndt, SJ Barnes, CM Lesher: Komatiite . Cambridge University Press, Cambridge, United Kingdom 2008, pp. 488 .
  9. ^ W. Bleeker: The late Archean record: a puzzle in approx. 35 pieces . In: Lithos . tape 71 , 2003, p. 99-134 .