The Ploumanac'h granitic pluton (Brittany, France)
The Ploumanac'h granitic pluton, on the Northern coast of Brittany, comprises a concentric series of granitic intrusions dated at 303 +/- 15 Ma. Perfect outcropping conditions (low coast and high amplitude tides) makes it an excellent case study of magmatic processes. Three granitic intrusions have been successively emplaced, from the outside to the inside:
- a coarse-grained pink granite with potassic calcalkaline affinities, including two different facies: the Ploumanac'h granite proper and the La Clarté granite, interpreted as a cumulative facies of the former (enrichment in K-feldspar megacrysts and in mafic minerals, biotite and amphibole). Mafic rocks (gabbros) at least partly of mantle origin are associated wth this first intrusion.
- a fine-grained pink granite, also with a potassic calcalkaline chemistry, the Toénot / Canton granite
- a peraluminous grey leucogranite, containing either biotite or biotite + muscovite, the Ile Grande granite, with a geochemistry clearly indicating a crustal source.
Ile Millau - external intrusive contact
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The contact between the Ploumanac'h granite and surrounding sediments is exposed on the South shore of Ile Millau. The intrusive contact is brittle, and the sedimentary layering is cut by the granitic intrusion.
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The surrounding sediments are characterized by an alternance of greywacke and arkose, both transformed into hornfels by the contact metamorphism. The greywacke layers contains cordierite (dark grains on the picture) and andalousite.
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Granitic dykes have been injected under pressure between sedimentary layers, breaking the hornfels. Hornfels fragments have then been entrained by the granitic magma.
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Ploumanac'h granite and associated gabbros
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The Ploumanac'h granite proper is the dominant facies in the outter part of the pluton. It is a potassic calcalkaline monzogranite containing quartz, pink K-feldspar, plagioclase, biotite, hornblende and accessory minerals (titanite, zircon, apatite, ilmenite, magnetite, allanite).
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In the Anse Sainte-Anne area, numerous mafic bodies are included in the granite. The least evolved rocks are norites, containing plagioclase, ortho- and clino-pyroxene, and rare olivine. Numerous evidence of mingling are visible at the outcrop scale (lobate contact, pillows...) indicating that the two magmas where emplaced simultaneously in the liquid state.
The granite in-between the mafic enclaves is enriched in alkali feldspar megacrysts and dark mafic minerals. A residual melt enriched in quartz and plagioclase has likely been expelled by filter-pressing.
The mafic enclave next to the center of the right-hand picture contains pink crystals of alkali feldspar surrounded by a rim of white plagioclase, reminiscent of the rapakivi texture. This texture is a consequence of chemical disequilibrium between the mafic magma and crystals of alkali feldspar mechanically injected from the granitic magma.
The granite in-between the mafic enclaves is enriched in alkali feldspar megacrysts and dark mafic minerals. A residual melt enriched in quartz and plagioclase has likely been expelled by filter-pressing.
The mafic enclave next to the center of the right-hand picture contains pink crystals of alkali feldspar surrounded by a rim of white plagioclase, reminiscent of the rapakivi texture. This texture is a consequence of chemical disequilibrium between the mafic magma and crystals of alkali feldspar mechanically injected from the granitic magma.
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Quartz crystals from the granite have also been mechanically injected in the mafic enclaves. They are in chemical disequilibrium with the mafic magma and are surrounded by a dark reaction rim (likely clinopyroxene, center of picture). The composition of this enclave enriched in quartz and feldspar xenocrysts is intermediate between the composition of a gabbro and that of a granite.
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Cumulate granite from La Clarté
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A transition zone between the Ploumanac'h granite proper and the La Clarté granite showcases spectacular igneous layering, interpreted as deposits from gravity currents during cristallisation of the granitic magma (Barrière, Contrib. Mineral. Petrol. 77: 214-224). Igneous layering is common in mafic rocks, but rare in granitic intrusions. Mafic minerals (biotite, amphibole) are concentrated in schlierens, often truncated by the effect of magmatic currents.
