A contact aureole in limestone and shale: where is the granite?

This is a low-pressure, high-temperature contact aureole.

• The sequence from shale to spotted slate to andalusite hornfels to wollastonite-bearing marble is a hornfels facies series. Andalusite and chiastolite indicate low pressure; at higher pressure, andalusite would give way to kyanite or sillimanite.
• Wollastonite in calcite marble forms by the reaction calcite + quartz = wollastonite + CO2. It requires temperatures of roughly 600-700 degrees C and, critically, a low CO2 activity in the pore fluid so that the reaction proceeds to the right.
• The wollastonite is restricted to the first 10 m because that is where temperature was highest and where infiltrating water from the cooling granite lowered XCO2. Farther out, temperature fell and the fluid remained CO2-rich, so calcite + quartz stayed stable.
• The unaffected conodont colour at 400 m suggests temperatures below about 50-80 degrees C, so the aureole is roughly 400 m wide.

Conditions: about 600-700 degrees C near the contact and less than about 3 kbar, i.e., a shallow epizonal setting. The mineral assemblage is not compatible with regional greenschist or amphibolite facies, which would require a foliation and a different P-T path.

The evidence favours a shallow intrusion, but the drill hole must test the geometry.

• An aureole about 400 m wide with andalusite and wollastonite is typical of an epizonal intrusion at less than about 5-8 km depth. A deeper pluton would produce a wider aureole and higher-pressure minerals such as kyanite or sillimanite.
• A sill would give a symmetric aureole above and below the intrusion. The traverse is only east of the covered strip, so the data cannot yet distinguish a sill from a stock or inclined sheet. The 20 m covered strip west of the marble is the likely contact zone.
• The marble at 0 m is already at the contact, so the granite may lie immediately west under the soil. The absence of exposed granite does not mean the intrusion is far away.

Drill plan: collar in the covered strip and drill east at about 45-60 degrees, or drill vertically just west of the marble. A 150-250 m hole should intersect the granite, pass through the contact and enter the marble. Log the contact attitude, the width of the marble, and whether there is a second marble horizon above the granite. That will distinguish a sill from a pluton.

Caution: one hole cannot prove a sill; it can only confirm the contact depth and the intrusive relationship.

The absence of foliation supports contact metamorphism rather than weakening it.

• Contact metamorphism is driven by heat from an intrusion. It is essentially static: directed stress is not required, so new minerals grow in random orientations and the rock remains massive or hornfelsic.
• The regional cleavage 8 km away is a tectonic fabric formed under directed stress. Its absence here means this outcrop escaped that regional strain, or the contact metamorphism post-dated it and obliterated the fabric near the intrusion.
• Bedding is still recognizable in the shale and spotted slate, which is typical of low-strain thermal metamorphism. If this were regional metamorphism, bedding would commonly be transposed or strongly flattened.
• The mineral sequence itself is the strongest evidence: andalusite and wollastonite are not regional-facies indicators at this scale, and the zonation is centred on the covered contact.

Conclusion: the lack of foliation is expected. The student should not reject the contact aureole because the rock is not schistose. The correct description is a hornfels aureole around a shallow granite, with a separate regional cleavage developed elsewhere.