Places I've been: You Yangs granite, Victoria, Australia

It's that time of year again - yes the end of May. This comes with the inevitable two or three weekends of teaching the 1st year geology igneous field trip. Our first stop is the You Yangs granite. One of the biggest, and closest examples of a granite to CBD Melbourne (WGS84 37° 57′ 0″ S, 144° 25′ 48″ E), it towers over the basaltic plain to the west of the city. On a clear day the views are lovely, and you can even just see the Menzies building towering over Clayton in the far distance.

The You Yangs are a small granite body, more of a stock than a batholith. The granite is rich in quartz, oligoclase and orthoclase with minor mafic minerals such as biotite and hornblende. The You Yangs are porphyritic with large K-feldspar (orthoclase) crystals up to 7cm long and a course grained groundmass. The granite also contains numerous xenoliths and enclaves.

 

A granitic tor located behind the picnic ground bathrooms.

 

An enclave in the tor - note the concentration of dark minerals and the presence of large K-feldspar crystals suggesting a similar chemistry to the granite. This is a large example of an enclave and spans 25cm. Some of the enclaves I've seen in this granite are 60cm wide.

 

A close up of the side of the granite

 

Closer still - a xenolith is bisected by two aplite veins. Xenoliths are pieces of country rock that have been surrounded by the magma during intrusion. They are then metamorphised by the heat produced by the cooling granite. The xenoliths seen in the You Yangs are typically hornfels, formed from metamorphising the Ordovician siltstones and sandstones (marine). Aplite is a quartz rich vein with small amounts of feldspar. It is thought to be the residual melt that solidifies in the cracks formed in the cooling granite.