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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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