Area: 49.9 hectares.
This is a new site.
The site is listed in the Geological Conservation Review as of national importance.
Description and Reasons for Notification:
Luxulyan Quarry provides excellent exposures of the coarse-megacrystic biotite granite of the St Austell granite.
This rock is the earliest variety intruded at St Austell, and is important for comparative study to show how it relates to the more complex lithium and volatile-rich types in the area which succeeded it at the time. The rock is also typical of the Cornubian coarse-grained biotite granites of the Cornubian Batholith and, despite its early emplacement in the St Austell sequence, its chemistry and mineralogy show it to be highly evolved, with high level variety.
Rounded xenoliths of pelitic and semi-pelitic composition are common and the exact contribution they made to influencing the composition of the granite magmas and to the latter mineralisation remains the subject of debate.
The site is also important for rare in-situ occurrences of the tourmaline-rich rock luxullianite. This is found as near-vertical sheets, up to 2 m in thickness, which sometimes anastomose and are often discontinuous. Their formation is attributed to the action of post-magmatic metasomatism, although the details of this process remain controversial.