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Contact!
Contact!
Welcome to site 4! The GPS coordinates for this site are 0236019 6278310. Site 4 exposes a narrow (~1m wide), coarse to very grained, light coloured rock which cuts across a metamorphosed quartz-rich rock (very similar to that at site 3). Although not from Hartley, a similar key relationship can be observed in the Sktechfab.com model below, where a light coloured rock is surrounded by darker rock. Make a sketch of this in your notebook. It is important to note here that not all outcrops are great to look at. This sketchfab being an example where we only see a small portion of the darker rock on the 'upper' edge of the light rock.
When observed closely, the dark rock at site 4 is very similar to the contact metamorphosed rock from site 3 (Figure A). However, the light coloured rock at site 4 is something new to us today. You can see this rock up close in Figures C–F. Looking at the Sketchfab.com model for context and the relative position of each rock, which rock do you think formed first?
Examine the model of a hand sample from site 4 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.
Pegmatite
Pegmatite
The lighter coloured, coarse to very coarse grained rock is called a 'Pegmatite'. Pegmatites are coarse grained igneous rocks (commonly felsic) found in sheets of rock called a dyke near plutonic batholiths. The magma that crystallises pegmatite is very volitile-rich and the volitiles (e.g. water) suppress nucleation of crystals such that only a few very large crystals grow. Felsic pegmatites contain the same minerals as granite (quartz, feldspar and mica), while intermediate to mafic pegmatites contain mineral assemblages typical of mafic igneous rocks.
The pegmatite exposed at Hartley is a composite dyke, where pulses of enriched fluids originated from the felsic intrusion we saw at site 1.
Pegmatite dykes usually represent products of the final stages of crystallisation of the pluton. Therefore, pegmatites are typically rich in incompatible elements (dominated by the large ion lithophile elements that did not easily fit into the crystal structure of minerals in the felsic pluton – boron, beryllium, lithium, uranium thorium, cesium, etc.). As such, pegmatites may form economic deposits.
Carefully examine Figures C and D.
Question 1: The minerals shown by the red, white and yellow arrows are common in the pegmatite. What are they?
Question 2: Have a go at filling out the table in your field trip worksheet.
In figures E and F you can see two interlocking minerals. They give an appearance of runic script. When we see lots of these graphic textures the rock can be termed granophyric.
Question 3: Name the two minerals which are intergrown and interlocking in Figures E and F.
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