Both the primary and the metamorphic characters of the lithologies represent outstanding petrographic features within the Beigua Geopark.
Scientific investigations across several decades allowed inferring the birth and orogenic stacking of the Alpine Thetys in the Piedmont – Ligurian domain. The present setting of the alpinized ophiolites outcropping in the park results from a complex, multy phase history, started already in the Early Jurassic and developed until the Oligocene.
The Beigua Geopark contains a rather complete sequence of oceanic lithosphere, and two important metamorphic overprints, respectively ocean floor and alpine, can be identified.
The oceanic lithosphere is represented by mantle lherzolites, intruded by gabbric rocks dated at about 160 Ma. Evidence for the unroofing of the gabbro-peridotitic basement to the Jurassic ocean floor is given by serpentinites, tectonic breccias, and rodingites. The gabbro-peridotitic sequence was covered by basalt flows, elsewhere in the Apennine preserved as pillow lavas and sills; basalt dikes also cut the serpentinized lherzolites. Pelagic sediments, presently cherts, marbles, calcschists, quartzschists, were originally associated with the volcanic cover.
The alpine subduction dismembered the pristine, likely continuous, ocean basin into several tectonic-metamorphic units that attained different depths of subduction, thus developing blueschist parageneses up to Grt-Omph eclogites and Lws – eclogites, along a relatively continuous series of facies (Vara, Bric Tarinè). The orogenic built-up resulted in the exhumation and partial re-equilibration (e.g. antigorite, olivine in the serpentineschists) of the high-pressure units (Bric dell'Uomo Morto). The subsequent orogenic collapse produced post-orogenic tectonic breccias and was related to the erosion of the structural higher and deepest-in-the subduction units.
In the Oligocene, the marine transgression on the whole basement deposited molasses, that recycled the highest structural units, resulting in the selection and enrichment of jadeite and omphacite-bearing rocks, originated in late-intruded diorite or plagiogranite dikes across the ophiolites sequence.
In Beigua Geopark, several sites of petrography interest present also mineralogical or geomorphological significant features.