Site 2

Felsic plutonic rock with fine-grained inclusions of darker rock

Welcome to your second stop! You are currently at field site 2. The GPS coordinates for this site are 0235749 6277979. Take some time to look at the map on google earth pro and consider the following questions:

Where is site 2 relative to site 1?

Are you in the same geological unit as site 1?

Are you towards the centre or the edge of a geological unit?

You have already learnt about the assemblage of minerals making up this rock at site 1 and have looked at it in the Sketchfab model (revisit by clicking on the site 1 link at the top of the page if you wish).

You will have noticed by looking at the geological map that site 2 is within the same rock unit as the previous site. However, site 2 has some interesting features making it appear slightly different to the previous site!

Have a look around this example of a similar outcrop from sketchfab.com What is different from site 1? Make a sketch in your field note book noting any interesting features.

Plutons

The rock outcrop observed at site 2 is a part of a large felsic pluton. The large size of the pluton and the fact that it cooled deep in the Earth means that the minerals cool slowly and crystallise at roughly the same rate across the entire pluton body. However, much smaller igneous intrusions such as dykes or sills, tend to loose heat quickly from their margins, forming what is commonly called a 'chilled margin'.

Question 1: Given what has just been described to you, would the margins of this felsic plutonic rock likely have a finer average grain size compared to closer the centre of the pluton at site 1?

Xenoliths & Microgranitoid enclaves

Most igneous rocks appear to be quite uniform made up of an assemblage of minerals. Sometimes these rocks can have inclusions that stand out from the regular appearance of the rock. There are two common types of inclusions that you may see in plutonic rocks: xenoliths and microgranitoid enclaves.

Xenoliths

A xenolith is a piece of pre-existing foreign rock, generally of metamorphic origin, which has been trapped inside an igneous rock. This usually occurs when a piece of rock gets entrained into a cooling magma body.

Enclaves

An enclave is an aggregate of minerals or rock observed inside a larger rock body. A description of how these may form can be found in your field trip worksheet. Question 2a. Estimate how wide the enclave is.

Think about a lava lamp (video) and how the blobs of lava move around. This analogy can be applied to understand what happens during magma mixing to form enclaves. Can you visualise the idea of blobs of one magma type forming globules and moving around like a lava lamp?

Question 2b. Describe the shape of the enclaves in this image?

Examine the model of a hand sample from site 2 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.

Look at this enlarged image of the rock. You can see that there is a difference in grain size in the host rock compared to the enclave (lower right).

In the field, you would describe the larger minerals as coarser-grained, and the smaller ones as fine-grained. Using the 50 cent coin as a guide, estimate of the grain size of the minerals within the enclave?

Question 2c. What is your estimate for the average size of the minerals in the enclave inclusion?

Look at the edges of the inclusion to check for a quenched or chilled margin.

Question 2d. Does the margin of the enclave appear to have a quenched margin (finer grains)?

The minerals that make up the enclave form a microgranite. What are some of the minerals that you can identify within the inclusion? To help you, a chart has been provided. Do you recognise any of the minerals from the chart within the images of the rock samples?

Question 2e. List in your field trip worksheet some of the names of the minerals you can identify in the enclave.

In the field at Hartley, you would have had the chance to thoroughly investigate all of the inclusions. As you are unable to check for yourself, the following two questions will be answered for you.

Question 2f. Are all of the inclusions the same?

Yes, pretty much all of the inclusions within this field site are similar to those you have investigate above.

Based on the content of this page you should be able to answer this next one fairly easily.

Question 2g. Are the inclusions most commonly xenoliths or microgranitoid enclaves?

Unconformities

During the semester you have learnt about the different types of unconformities. In the images below you can see examples of the different unconformities.

Question 3: If the rocks at sites 1 and 2 were to be covered by sediments that lithify into sedimentary rocks, what type of unconformity would be created in this location?

Angular Unconformity

Rocks lie on an older deformed (tilted, faulted and/or folded) basement. The relationship between the two groups of layered rocks is angular.

Disconformity

There is a time break between parallel strata, and an irregular surface has been developed on top of the older units.

Paraconformity

Similar to a disconformity, yet with parallel strata and break surface.

Nonconformity

The underlying unit lacks layering e.g. granite, homogeneous gneiss.

On to site 3....

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