The largest plutonic body in the terrane has limited outcrop and is sheared by the Llyn Shear Zone in the west and covered by later (Arenig) sediments to the east.[1] Altered to greenschist facies the pluton contains a bimodal suite of gabbro-diorite, monzogranite (Sarn granite) and granodiorite.
The Sarn granite is leucocratic and covers an expanse of about 6 km2 in contrast to the gabbro and diorite that exist as small and scattered exposures. The dioritic component of the complex has been confirmed as having a Neoproterozoic age of 614 ±2 Ma using U-Pb zircon dating.[3] Therefore, the shearing of the Llyn is also temporally constrained by the date.