Image showing predicted temperature changes between 10 and 12 Ma after emplacement of a radioactive granite of Sybella Batholith like dimensions and heat production. See discussion of following figures for more details.
High heat producing (HHP) granites raise fundamental questions about
the distribution of heat producing elements in the crust prior to granite generation, and
the effects of the redistribution of heat production during granite generation and ascent on the lithospheric thermal regimes.
Three distinct thermal effects influence the thermal evolution of lithosphere during the segregation and ascent of a high heat producing granite:
heating due to the advection of magma from source to emplacement level. This produces a short-lived, localised, high amplitude thermal perturbation in the immediate aureole of the granite
heating generated from the radioactive sources within the emplaced granite. This produces a long-lived, intermediate length-scale, low-amplitude thermal perturbation at crustal levels near the intrusion.
cooling due to the removal of heat producing elements from the source regions of the granite. This produces a long-lived, large length-scale cooling at deep crustal levels.
the way in which these various effects interact depends depends primarily on the amount of heat producing elements transported with the granite, but also :
the relative horizontal length scales of heat production distributions prior to, and following, magma ascent.
the depths of the source and emplacement levels
The figure below shows the thermal evolution of a crustal section (z = 5-40 km depth and some x=200 km wide) in which a composite granite (with dimensions of about 5 x 40 km) has been segregated from depths of 40 km and emplaced at 15 km depth at a temperature of 900°C. It carries an average heat production of ~ 5 5 & micro W m-3 and contributes a maximum of about ~24 mWm-2 to the surface heat flow, qc. In the top panel the contours represent the maximum temperatures attained, while the colours represent the time at which the maximum temperature is attained following granite ascent. The middle panel shows the change in temperature during the interval 8 Ma - 10 Ma (following initial granite emplacement. The bottom panel shows the long-term change in temperature due to the change in heat sources following granite segregation (it the steady state temperature with to the final heat source distribution minus the steady state temperature with the initial heat source distribution).
In the figure above the ratio of horizontal length scales for the source region heat production distribution to granite width is hx = 1.5. Note in the bottom panel the twofold effect of cooling at deep crustal (source) levels (of about 60 °) and heat at shallow (emplacement) levels (of about 20 °). The figure below shows the thermal structure when the source distribution is much more spread out (hx = 10), resulting in much greater heating at shallow emplacement levels and less cooling at source levels
In these calculations a number of approximations have been made:
More ../Images can be seen in my Numerical Image library under Sybella Granite