I have read somewhere that the size of the contact metamorphic aureole around an intrusion depends on the contact between the intrusion and the country rocks - the shallower the contact, the larger the aureole. Why is that? Does it have to do something with the heat transfer?
Thank you :)
Dear Rose, Goodmorning and Fraternal Greetings!
Regarding your question about the contact aureole,
Yes, you are correct. The size of the contact aureole depends upon the contact between the intrusion and country rock. Heat transfer is the main reason for the larger the aureole when it is at shallower the contact
To the best of my knowledge I have given you the following explanation
The contact aureole is the shell of metamorphosed or metasomatized rock enveloping the igneous body. Other contact aureoles develop at greater depths and may be physically emplaced to shallower levels along with the igneous body. These are termed dynamothermal aureoles. The aureole extends from the igneous contact, where the metamorphic effects are the greatest, out into the country rocks to where the temperature or heat energy is insufficient to effect any changes. This temperature lies between 400 and 750°F (200 and 400°C), and actual widths of contact aureoles range from several inches to miles. However, the nature of contact aureoles results in minerals characteristic of low to moderate pressures and moderate to high temperatures usually in common rock types: shales, basalt, limestone, and sandstone.
The progressive thermal metamorphism of the metapelites with pre-exiting foliation resulted in
a steady decrease of fissility as the grade progresses . The contact aureole constitutes three distinct zones, each characterized by specific mineral assemblages and textural and structural characteristics . The inner aureole with immediate contact with granite is characterized by high temperature minerals such as fibrolite, cordierite and garnet. The transition to the middle aureole, biotite and andalusite exhibit random orientation and decussate texture and the outer aureole have medium to fine grained metasedimentaries with shimmer texture. The outer and middle aureoles exhibit random orientation of biotite and andalusite showing decussated texture whereas the inner zone in the immediate neighbourhood of the contact with the granite shows hornfelsic texture. These textures are perhaps believed to be indicative of crystallization under the hydrostatic environment during the advanced stage of thermal metamorphism.
The temperature obtained from garnet-cordieritefibrolite- biotite hornfels (inner zone) estimates ranges between 650◦ and 685◦C The average temperature from Ti content in mica geothermometer proposed by Lal (1991) estimates 661 ± 20◦C (inner aureole) to 580 ± 15◦C (middle aureole) and 487 ± 30◦C (outer aureole).
Mineral assemblages delineate three metamorphic zones within the contact aureole: outer
aureole (Chl–And–Bt zone), middle aureole (And– Bt–Mus–Cd zone) and inner aureole (Sil/Fibr–Bt– Cd–Grt–zone). The internally consistent geothermobarometry estimates the 690◦C/3.4 kbar for the thermal peak of contact metamorphism in the inner aureole. The metamorphic temperatures based on different geothermometers are 487 ± 30◦C in the outer aureole, 580 ± 15◦C in the middle aureole and 661 ± 20◦C in the inner aureole at pressures between 1.6 and 2.58 kbar represent agreement of gradual thermal perturbation at shallow depth in the width between 600 and 800 mt.