Bryher

The entirety of the Isles of Scilly is comprised of Permian-aged two mica granite. On Bryher, the granite is grey and fine- to coarse-grained with quartz, plagioclase feldspar, biotite, muscovite and white alkali feldspar phenocrysts up to 25 mm. Due to the presence of both biotite and muscovite micas, the granite is therefore termed a “two mica granite”. Accessory minerals in the granite include cordierite, tourmaline, topaz, ilmenite and rutile; these are typically best observed using a microscope.

Along the norther edge of Brhyer a wide range of granite textures can be observed. Enclaves, or rounded inclusions, of fine-grained granite are present within the coarse-grained granite. There are also angular xenoliths of killas, or metasedimentary rocks, with sharp boundaries included within the coarse-grained granite. There is evidence for mingling of different granite melts, with undulose mixtures of fine- and coarse-grained granite, along with areas of alkali feldspar accumulation and zones of pegmatite. Finally, there are several fine-grained sheets, with pegmatitic boundaries that crosscut the granite outcrops.

The Isles of Scilly Granite is part of the large mass of granite that lies underneath southwest England termed the Cornubian Batholith. The Bryher granite formed in the Early Permian and is approximately 290 million years old. All of the granites in southwest England formed at the end of the Variscan Orogeny, a mountain building event. During mountain building events, heat and pressure results in melting of the crust. As the earth’s crust was stretching apart at the end of the Variscan Orogeny, these melts were emplaced through fault systems over millions of years.

The textures show evidence for co-existence of several different granite melts at the same time. As the granite melts cooled, globules or fine-grained granite will have been entrained in the coarse-grained granite. Fine-grained granite sheets have exploited weaknesses within the cooled coarse-grained granite. Where the granite has cooled and later been sheared, this manifests as the gneissose texture seen along some of the coastal exposures of the Isles of Scilly. This also provides an importance time constraint on the evolution of the Cornubian Batholith; the Isles of Scilly Granite was emplaced, and cooled, before crustal extension and the emplacement of later granite such as Dartmoor.