Maybe we can summurize the discussion as:
To(observed)= Tr(regional heat) + Tm(magmatic heat)
(regional heat resulting from mantle heat flow and crustal radioactive
Tm and Tr will be rarely equal to zero. In subducted material, Tm is low.
In batholitic zones, Tm is high. But Tm effects will be particularly well
seen if Tr is low, i.e. close to the surface. At depth, Tm can be even
higher (Bob's example) but partly hidden by a higher Tr.
In addition, tectonic effects are fundamental, when brittle, the effects on
the rock itself will be very localized.
The initial definition of hornfels is then when Tm>>Tr (implying low P),
then in brittle conditions (then no schistosity).
This is a different concept compared to contact metamorphism that is indeed
often regional when deep. The best heat vector in the crust is magmas, it is
then not a surprise to have them as the cause of regional HT metamorphism.
Ce qui est simple est toujours faux, ce qui ne l'est pas est inutilisable.
(What is simple is always false, what is not is unusable).
Head of Section, Isotope Geology
B-3080 Tervuren, Belgium
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