Granites and Earth Evolution.
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METAMORPHISM CAUSED BY MAGMA DEEP INTRUSIONS

Reverdatto V.V.

Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, rever@uiggm.nsc.ru


In geodynamical view, it is possible to differentiate the following main varieties (types) of metamorphism at present, i.e. : (1) metamorphism caused by supply of the Earth’s crust with additional heat by magmatic intrusions or flows of deep fluid; as a result, thermal gradient significantly exceeds average crust value; (2) metamorphism caused by expansion/extension of lithosphere, accumulation of sediments in depressions, their subsidence and heating; in addition, the thermal gradient was close to the average crust value ~25oC/km; (3) collisional metamorphism associated with horizontal movements that initiate convergence, deformation and subduction of lithospheric plates, over- and underthrusts in the Earth’s crust; the thermal gradient fixed in rocks was lower than the average crust value 7-20oC/km that is related to the relative short duration of events and unachievement of balance among blocks of rocks.

In the present report, the metamorphic type caused by supply of the Earth’s crust with additional heat has been considered. The contact metamorphism and zonal one of moderate pressures belong to this type; the latter is also known as “metamorphism of low pressures and high temperatures”. They have the same physical nature, being genetically and spatially associated with intrusive magmatism but differed by PT-conditions of formation and combinations of metamorphic facies; they can have different width of metamorphic zonation, duration of formation, etc. at the same size of magmatic intrusive bodies and other similar conditions. It is recognized that initial temperature of host rocks (coming from average crust value of geothermal gradient ~25oC/km) does not usually reach 100-120oC at the contact metamorphism. The geothermal gradient can achieve more than 100oC/km at contact metamorphism; in addition, the thermal gradient is not linear: its steep dip increases when approaching to the contact. The boundary between contact and zonal metamorphisms of moderate pressures is obscure, however it is recognized that contact metamorphism is limited by depth ~5 km and pressure ~1.5 kbar. At such depth, the contact aureoles are quite clearly observed in unmetamorphosed rocks that is characteristic feature of low pressure metamorphism. Zonal metamorphism of moderate pressures occurs at depths more than 5 km, where the initial temperature of host rocks is more than 100oC. Therefore, temperatures are higher near deep magmatic intrusive bodies than at contact metamorphism, when other similar conditions are available; duration of host rock heating increases, and zones of thermal transformation become wider. The latter circumstance also causes less steep thermal gradient than at contact metamorphism. The question arises: what are maximal depth and lithostatic pressure where zonal metamorphism is still possible due to temperature of intrusive magma and initial temperature of host rocks? Since the temperature of zonal metamorphism of moderate pressures does not exceed, as a rule, ~900oC (that is testified by stability of mineral association of granulite facies rocks in inner parts of zonation), its maximal depth is to be limited by 25-28 km in the Earth’s crust. It results from temperature estimates of intrusive contacts at the account of host rock initial temperature 600-700oC (at initial thermal gradient 25oC/km) and temperature of basic magma ~1100oC.The maximal depth of zonal metamorphism must be limited by 20-23 km under the same conditions and at the initial geothermal gradient 30oC/km. At the same time, the depth where temperature at intrusive contact is hardly differed from the one of host rocks, is maximal for metamorphism that is caused by granite magma introduction. This depth for granite magma intrusives equals 26-28 km at initial geothermal gradient 25oC/km. The outer boundary of metamorphic zonation fixed by mineral transformations gets obscure at such big depths (in middle part of the Earth’s crust). From above, depth 25-28 km is suggested to be considered maximal for zonal metamorphism of moderate pressures. At such depth, the maximal lithostatic pressure is limited by 7-8 kbar. In these conditions, acid rocks partially melt, and migmatites form due to regionally higher level of temperatures, however it is evident that metamorphism caused by magma intrusions also occurs in basic rocks. It is supported by specific examples, where such metamorphism realized at 5-7 kbar (De Yoreo et al. 1989; Hermes, Murray 1988; Schumacher et al. 1989; Zen et al. 1968) or 8 kbar (Elan 1985; Ross 1985; Sams, Saleeby 1988). The pressure up to 7-8 kbar and depth up to 25-28 km are additionally the reason, why the name “metamorphism of low pressures and high temperatures” should hardly be used for zonal metamorphism of moderate pressures (PT-conditions of amphibolite facies).

Metamorphism of rocks that compose ancient shields has a number of peculiarities. Nearly uniform high pressures and temperatures were usually achieved in big areas at granulite metamorphism. The range of pressures was limited by 7-5 kbar, and that of temperatures – 700-1000oC. Numerous stages of metamorphism are hardly differentiated, but on the whole, the general thermal gradient in the progressive stages is likely to have been sufficiently close to the middle crust value. Within one stage metamorphism at lithostatic control of pressure, the mechanism taking into account conductive heat transfer can be offered with continuous warming up the rock strata by constant heat source, i.e. the convecting mantle magma. The model calculations show that stationary distribution of temperatures is established during 30-40 mln. years for continental crust 40-60 km thick at constant temperatures 1000-1100oC at the lower boundary. Changes in temperature occur only in the lower part of the crust during the first millions years. Then the changes comprise the middle part of the crust as well, particularly during the period 3-30 mln. years. The melting of acid rocks becomes possible at depth more than 20 km. In other explanations of granulite formation PT-conditions in ancient shields, heat transfer near large basic/ultrabasic plutons should be taken into account together with deformation of the Earth’s crust.

Magmatic intrusions are the most important mechanism of rock supply with additional heat in the Earth’s crust at moderate and big depths. The geological data and analysis of models attribute to the fact that heat transfer associated with fluid filtration played sharply subordinate role.

The work has been carried out with financial support of Russian Foundation for Basic Research (grant 08-05-00166) and grant of Russian Federation President for the leading scientific schools NSH-258.2008.5.

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