Исуа жасыл тас белдемі: құрамы мен геологиялық дамуы

Archean greenstone belt in southwestern Greenland

The Isua Greenstone Belt, also known as the Isua supracrustal belt since it is composed primarily of supracrustal rocks, is located in southwestern Greenland, in the Isukasia terrane, near the Nuuk capital region. It forms the largest supracrustal enclave in the Itsaq Gneiss Complex, which predominantly comprises 3850 3600 million year old (Ma) felsic orthogneisses. The greenstone belt comprises two major sequences of metamorphosed mafic volcanic and sedimentary rocks, which were divided on the basis of zircon uranium lead dating. These sequences are the 'southern terrane', which has an age of approximately 3800 Ma, and the 'northern terrane', which has an age of approximately 3700 Ma. structural analyses; geothermobarometry and metamorphic modelling using phase diagrams to determine metamorphic conditions; and a wide range of stable, radiogenic, and short lived isotope systems.

The Isua Greenstone Belt comprises many different lithologies. The most abundant rock types are mafic metavolcanic rocks with a range of compositions from boninite like to tholeiites and picrites. Though boninitic amphibolites at Isua are often interpreted as evidence for the action of plate tectonics, these are not true boninites

The mafic volcanic sequences contain abundant meta ultramafic rocks, including amphibolites, serpentinites, carbonated peridotites and peridotite. The majority of these are widely accepted to be intrusive in origin, representing ultramafic cumulates. and used as evidence to support the operation of plate tectonics during the formation of the Isua Greenstone Belt. However, this interpretation is contested, and some studies suggest that all peridotites at Isua are cumulates, representing shallow level magma chambers and conduits with the volcanic sequences. Metasedimentary rocks include banded iron formation and detrital quartzite, likely representing a metamorphosed siliciclastic sedimentary rock. Although they do not form part of the supracrustal belt itself, the belt is hosted in and in places intruded by tonalite trondhjemite granodiorite (TTG) orthogneisses. [[File:Timeline pic. jpg|500x400px|thumb|right|Cartoon of the age of various rock types formed during the evolution of the Isua Greenstone Belt. Note that the y axis does not have a scale and vertical placement of the different lithologies has no significance. and non plate tectonic or non uniformitarian models, in which the Isua Greenstone Belt formed in a tectonic regime that was different to the modern day Earth. formed in a subduction zone. Non plate tectonic models generally suggest a heat pipe or mantle plume origin for the belt. Other objections related to the composition of the dykes, which are unlike those found in modern day ophiolites. Despite the disagreement on the presence of a sheeted dyke complex at Isua, alternative lines of evidence have been proposed in support of an ophiolite origin for the belt. These are primarily based on the geochemistry of the volcanic rocks in the belt: tholeiitic amphibolites have been interpreted as metamorphosed island arc tholeiites, and boninite like amphibolites have been interpreted to represent metamorphosed boninites. and the presence of apparently high pressure minerals. rapid eruption of volcanic rocks and the corresponding removal of melt from the mantle below causes downward movement of the lithosphere and burial of mafic rocks. The buried mafic rocks eventually heat up and melt, producing the TTGs associated with the Isua Greenstone Belt.

Following its formation, the Isua Greenstone belt has undergone two major metamorphic episodes. The first predates the formation of the <3.5 Ga Ameralik dykes Though higher pressure conditions have been suggested locally on the basis of Ti humite group minerals in peridotites,

In August 2016, an Australia based research team presented evidence that the Isua Greenstone Belt contains the remains of stromatolite microbial colonies that formed approximately 3.7 billion years ago. However, their interpretations are controversial. If these structures are stromatolites, they predate the oldest previously known stromatolites, found in the Dresser Formation in western Australia, by 220 million years. However, some geologists interpret the structures as the result of deformation and alteration of the original rock. The ISB sedimentary layers containing the possible stromatolites overlay volcanic rocks that are dated to 3.709 billion years old and are capped by dolomite and banded iron formations with thorium uranium zircons dated to 3.695 ± 0.4 billion years old. All layers, including those bordering the stromatolites, experienced metamorphism and deformation after deposition, and temperatures not exceeding 550 °C (1022 °F). The identity of the ISB features as stromatolites is controversial, because similar features may form through non biological processes. Some geologists interpret the textures above the putative stromatolites as sand accumulation against their sides during their formation, suggesting that the features arose during the sedimentary process, and not through later, metamorphic deformation. However, others suggest that the rocks are so altered that any sedimentary interpretations are inappropriate. In 2016, geologist and areologist Abigail Allwood stated that the discovery of Isua stromatolites makes the emergence of life on other planets, including Mars early after its formation, more probable. However, in 2018, she and a team of additional geologists published a paper that raises significant questions as to the origin of the structures, interpreting them as arising from deformation. Thus, the ISB stromatolites remain a subject of ongoing investigation.