Vishnu Basement Rocks

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As Vishnu Basement Rocks or Vishnu Group (Vishnu-basement or Vishnu Group) - named after the Hindu deity Vishnu - is paläoproterozoische polymetamorphic crystalline basement referred that in the US state of Arizona situated Grand Canyon is pending. The Vishnu Basement Rocks are the deepest and oldest geological unit of the Inner Gorge . The basement in the Grand Canyon owes its formation mainly to the Yavapai mountain range , which pressed BP rocks of a backarc basin with the associated igneous island arc (the Yavapai terran ) under the Mojave terran in the northwest from 1720 to 1680 million years ago . The parent rocks were heavily metamorphosed and tectonically deformed in several phases . The metamorphosis usually reached the upper green slate facies / lower amphibolite facies, in some crust blocks even the lower granulite facies with migmatite formation were realized. Because of these orogenic changes, the Vishnu Basement Rocks are understandably quite complex and can be divided into many individual geological units. The rather heterogeneous rocks can still be assigned to one of the following groups:

  • A continent fragment of the Mojave Terran
  • Metamorphic backarc sediments
  • Island arc magmatism
  • Anorogenic intrusions

A continent fragment of the Mojave Terran

Grand Canyon with Vishnu Basement Complex below flat rocks of the Unkar Group. The steep foliation and light granitoid intrusives can be seen

The oldest rock of the Vishnu Basement Rocks is the Elves Chasm gneiss , radiometrically dated to 1840 million years BP. It is an orthogneiss whose protoliths (parent rock) were a mafic tonalite ( hornblende - biotite- tonalite) and an intermediate quartz diorite . In addition, flat amphibolite layers can be found in the gneiss, which probably represent former dike rocks. The Elves Chasm gneiss thus emerged from a pluton, which was then metamorphosed into an orthogneiss. Its old age brings it close to the rocks of the continental Mojave Terran a little further to the west, to which many geologists also count it.

The Mojave Terran in the Grand Canyon reaches up to the Crystal shear zone , a 2 km wide highly plastic mylonitic zone - the border to the Yavapai Terran. Rocks immediately to the west such as Tuna Creek Granodiorite lead z. B. recycled zircons with Mojave-typical age, whereas the rocks of the Yavapai Terran following eastwards are all much younger.

Metamorphic backarc sediments

The (now metamorphic) sediment pile of the former Backarc basin includes the following units (from young to old):

  • Vishnu Schist
    Purely sedimentary siliciclastic origin. Consists of quartz mica slate, clay slate and metamorphosed arenite. These are probably metamorphosed turbidites .
  • Brahma Schist
    Former volcanic rocks of mafic to intermediate composition. They are now available as amphibolite, hornblende-biotite-plagioclase slate, biotite-plagioclase slate, orthoamphibol-leading slate and gneiss. The occurrence of metamorphosed sulphides is remarkable . The Brahma Schist has been dated to 1742 Ma BP.
  • Rama Schist
    Also of volcanic origin, mostly Felsic composition. Occurs as massive, fine-grain quartz feldspar slate and gneiss and is around 1750 million years old.

These supracrustal sedimentary rocks were deposited in the period 1750 ± 2 to 1741 ± 1 Ma BP and reached a total thickness of around 13,000 meters. Their stratigraphic sequence is not always clear because they can interlock. However, the above sequence is generally observed in anticline areas.

Island arc magmatism

Immediately after the end of the sedimentation process in the Backarc basin, the first phase of magmatic intrusions took place . It was granitic to granodioritic composition:

With the onset of the first orogenic movements of the Yavapai orogeny (accretion of the arch of the island), a second phase of igneous (synorogenic) calcareous-alkaline intrusions was accompanied, which, however, was somewhat mafic overall. The following were formed:

A third phase of a very rocky nature took place after the climax of the metamorphosis. In the period 1698-1662 Ma BP, pegmatite and aplite dikes , peraluminous granites and the phantom pluton formed around 1662 Ma BP.

Anorogenic intrusions

Much later, anorogenic igneous rocks intruded around 1400 Ma BP, granite intrusions, granite dikes and pegmatites formed. But they are in no way related to the accretion processes.

Metamorphosis and deformation during the accretion process

After the sedimentation of the 13-kilometer-thick sediment package ended around 1740 Ma BP, it was submerged metamorphosed, which took place on two paths. A pressure stressed with the formation of thistle and garnet and a more temperature stressed with the formation of andalusite , sillimanite and garnet. In both cases, 0.7 GPa (or 7 kilobars) of pressure was reached and there was anatectic melting and migmatite formation in places - corresponding to a final depth of around 25 kilometers (lower middle crust). The gradual pressure increasing had during the period 1730-1700 Ma BP, a first deformation phase causes (D1) in the sediments, which are characterized by a relatively flat-lying cleavage , to north west aligned structure and to northwest carried thrusts and isoklinalem Faltenbau distinguished. If the sediment package continued to narrow in a south-east-north-west direction, from 1713 Ma BP, before the metamorphic peak was reached (1707-1698 Ma BP), a second deformation phase (D2) began, which was a penetratively steep, northeast-trending foliation with the associated steep to overturned folds imprinted on the existing structures. This second phase lasted until 1685 Ma BP - long after the climax of the metamorphosis had passed and decompression had already started. The decompression phase then let the rocks of the Vishnu Basement Complex emerge to a depth of approximately twelve kilometers (0.3-0.4 GPa). From 1685 Ma BP isobaric cooling from 650 ° C to 450 ° C finally started in this depth range.

Block architecture

Structurally, the basement of the Vishnu Basement Complex is characterized by its block architecture. It consists of a total of six lithotectonic crustal blocks, up to ten kilometers wide, which are separated from one another by steep, northeast-trending shear zones . The shear zones can be up to two kilometers wide and represent a very high degree of deformation. During the dynamometamorphosis, these crustal blocks behaved differently. Although they all document the same maximum pressure and were therefore at the same depth, they have very different maximum temperatures, which can be between 520 and 750 ° C. This very high temperature difference can mainly be explained by the different heat input by means of pegmatitic intrusives - blocks with a high proportion of pegmatites were heated to a much greater extent. The pegmatite intrusions were particularly noticeable during the decompression phase in the period 1685–1660 Ma BP and caused secondary garnet growth and thus a temporary reversal in the general cooling trend.

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  • G. Dumond, KH Mahan, ML Williams, KE Karlstrom: Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic field gradients: Upper Granite Gorge, Grand Canyon, USA. In: GSA Bulletin. V. 119, 2007, pp. 202-220. doi: 10.1130 / B225903.1
  • DP Hawkins, SA Bowring, BR Ilg, KE Karlstrom, ML Williams: U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona. In: GSA Bulletin. V. 108, 1996, pp. 1167-1181. doi: 10.1130 / 0016-7606

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