Site 3

Contact Metamorphic Rock

Welcome to site 3! The GPS coordinates for this site are 0235805 6278249.

You will have noticed that you are now looking at a new rock type. You can confirm this by looking at your geological map and the location of site 3 relative to site 2.

Using the images of the outcrop for this site, make a quick sketch in your field book and record your observations.

Examine the model of a hand sample from site 3 in Pedestal3D below. Click on the < button to hide the side menu and give you more space to view the sample. Choose 'High' instead of 'Medium' from the drop down menu to maximise the quality of the sample image.

Task: Please choose an informative view direction in the Pedestal3D model and make a sketch in your field note book. You may zoom into interesting features and make multiple sketches rather than sketching the whole hand sample if needed. Please use the tools in Pedestal3D to measure grain size etc. for your sketches. It is important to recognise the difference between a fresh and weathered face of a sample. Sometimes weathering can conceal important features and the fresh face gives the best view, however the opposite can also be true. Study both faces in these samples to see which gives you the best view of the rock's microstructures.

About this rock

This is a contact metamorphic rock. That means that this rock didn't always look this way. The protolith of this rock was a thinly bedded and relatively homogeneous quartz-rich sandstone that has since been recrystallised during contact metamorphism.

When the magma intruded and formed the rocks at sites 1 and 2, the close proximity of the sandstone to that large heat source caused the minerals to bake.

The contact metamorphism alters the appearance and texture of the minerals within the rock, providing it with a sugary texture.

Question 1: This rock is dominated by one main mineral. By looking at the images of the rock, can you identify this mineral and provide some examples of its distinguishing properties and suggest the modal occurrence?

Question 2: What type of contact metamorphic rock has this become?

Two common approaches are taken to naming a contact metamorphic rock: (1) add 'meta' before the protolith rock name, e.g., meta-limestone can be used instead of marble; (2) use of the term hornfels, commonly with a mineral qualifier, e.g. andalusite hornfels. Hornfels is a term used to describe contact metamorphic rocks that contain randomly oriented minerals.

Question 3: Based on this information, what are the other names that you can use to name this rock?

Mineral Identification

This rock is made up of mostly quartz, however, there are some thin laminations with a higher proportion of a darker mineral (possibly an oxide or biotite) giving the rock a darker colour. Sandstone may contain variable types of sand grains (quartz, feldspar, lithic fragments, carbonate fragments) leading to differences in appearance. Can you identify the second mineral within this rock?

Question 4. Based on the mineral assemblage of the rock and previous information provided to you, what is the protolith/parent rock?

Planar Features

There are two main planar features that you may be able to identify in this outcrop. As you are unable to observe them in person, an explanation will be provided for you.

These two planar features can sometimes be confused with each other.

Planar feature 1 (photos not from Hartley):

When this rock underwent contact metamorphism, it was under a confining pressure. During exhumation, the stress or confining pressure is released. As stress is released, fractures can form within the rock. These fractures may appear as breaks or fractures in the rock.

One great example where you would see this planar feature is in columnar basalt which show a spectacular pattern of cooling fractures.

This first planar feature is not as important as the second as it doesn't tell us anything about the structure of the rock.

Question 5: What is the name of this first planar feature?

Planar feature 2 (photos not from Hartley):

The protolith of this contact metamorphic rock was a sandstone which contained layers of sand that were deposited in laminations which may be preserved in metamorphic rocks. As the quartz crystals were baked and recyrstalllised, this planar feature may have become less obvious compared to the protolith rock.

The best way to identify this planar feature is to look for differences in colour and grain size.

For this field site, some samples show a grey vs greenish layers, but a better example of this planar feature can be seen in field site 7.

Question 6: What is the name used to describe the layering in a sedimentary rock?

Site four this way -->

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