page_title: Génis Unit
section_title: Génis porphyroid
hierarchical_section_title: Génis Unit / Stratigraphy / Génis porphyroid
caption_attribution_description: English: Sawcut of a lower Ordovician Génis porphyroid (metaignimbrite) from the Génis unit, northwestern Massif Central, France. The rock has been dextrally sheared.
context_page_description: The Génis Unit is a Paleozoic metasedimentary succession of the southern Limousin and belongs geologically to the Variscan basement of the French Massif Central. The unit covers the age range Cambrian/Ordovician till Devonian.
context_section_description: Below a pronounced angular unconformity one encounters the Génis porphyroid. This rock represents alkaline, rhyolitic ignimbrites (metaignimbrites) of lower Ordovician age (Tremadocian). Its mineralogy is composed of phenocrysts of quartz, alkali feldspar and plagioclase (albite) and a very fine-grained matrix (grain-size 5  μ) made of quartz, feldspars, sericite and rare chlorite. Original fiamme are hard to discern, but welded glassy layers are recognisable. The metaignimbrite is rich in potassium and contains more than 70% of SiO₂.
  Underlying the Génis porphyroid are the Donzenac schist and the Thiviers sandstone, the main formations of the Thiviers-Payzac Unit. In the Fugeyrollas anticline the Thiviers sandstone appears on the surface and thus also crops out within the Génis Unit. Both formations are supposed to cover the age range Neoproterozoic to Cambrian.
caption: Sawn sample of the Génis porphyroid showing a dextral shear sense.
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