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Figure 7.19. The shaded region denotes a large basalt body, which intruded at
1100 C: (a) 1 Ma after intrusion the basalt body is cooling, and the country rock is
being heated; (b) 20 Ma after intrusion the basalt body has solidified and cooled.
(From Fowler and Nisbet (1982).)
effects near the source of heat because that is where convection is most active. In
granites, radioactive heat generation may prolong the action of convection cells.
The presence of water also has profound effects on the mineralogical course of the
metamorphism. Nevertheless, simple conductive models are useful for a general
understanding of metamorphism around plutons.
Basic intrusion
The igneous body is assumed to intrude at 1100 C and to have radioactive heat
generation of 0.42
10 5 Jkg 1 ,
is released over a 1-Ma cooling interval. Figure 7.19 shows the temperature field
after 1 Ma and after 20 Ma. Contact metamorphism of the country rock is an
important transient phenomenon, but, if such basic intrusions are to be a major
cause of regional as opposed to local metamorphism, the intrusions must form a
large proportion of the total rock pile.
Wm 3 ; the latent heat of crystallization, 4.2
×
Granitic intrusion
The granite is assumed to intrude at 700 C and to have radioactive heat generation
of 4.2
10 5 Jkg 1 ,isreleased over
2 Ma. Figure 7.20 shows the thermal evolution of this model. It is clear that there
is less contact metamorphism than for the basic intrusion, but there is extensive
deep-level or regional metamorphism. Indeed, massive lensoid granitic bodies
are common in calc-alkaline mountain chains such as the Andes, and may be an
Wm 3 ; the latent heat of crystallization, 4.2
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