Gabbro and Rhyolite

Having started with granite, there is no doubt in my mind where we should turn next in this exploration of the geology of the Nantais, and the relationship that exists between the region’s rocks and its wines. We turn now to gabbro, which alongside granite is one of the Muscadet region’s most important igneous rocks. If wines from granite are all about substance, breadth and longevity, than those from gabbro are all about focus, minerality and tense nervosity. The gabbro terroirs in the south-eastern corner of the vineyard, between Gorges, Le Pallet and Vallet can yield wines in this very style, and as such have given me some of the most exciting experiences with Muscadet I can recall, particular from André-Michel Brégeon and more recently Fred Lailler (pictured below) of Domaine Brégeon.

Along with gabbro one other igneous rock found in this corner of the Loire Valley also cries out for discussion here, and that is rhyolite. It is not a rock of any great relevance to the wines of Muscadet, but let us not forget that the Nantais is not just about Muscadet; there are also the wines of the Fiefs-Vendéens (among others), to the south of the Muscadet vineyards, and this is where we can find rhyolite. So in this guide I complete my examination of the region’s igneous rocks with both gabbro and rhyolite, after which I will move on to the metamorphic rocks in the next instalment of this guide.

Gabbro

Like granite, gabbro is an igneous rock which formed when molten rock cooled slowly and crystallised underground, so in other words it is a plutonic or intrusive rock. It is commonly found in mid-ocean ridges, or in ancient rock formations formed of compressed and uplifted oceanic crust. This is exactly the origin of the gabbro found in the Massif Armoricain, as illustrated in my introduction to the geology of this region.

Unlike granite which tends to be quite pale in colour, gabbro is a dark rock, its colour ranging from charcoal grey through to very dark green or jet black; if those descriptions seem to be the wrong way round, it might be because much of the dark stone referred to as granite used in construction isn’t really granite; other stones magically assume this name, including gabbro itself which is occasionally used as a facing stone, when it is often referred to as ‘black granite’. In terms of its composition gabbro is in fact identical to basalt, but gabbro develops a much coarser grain as it cools slowly underground, whereas basalt is defined by its much finer crystalline structure, formed during rapid cooling at the surface.

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