First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France)

We report new LA-ICP-MS U-Pb zircon ages of varied crystalline rocks occurring in the Serre Chevalier crystalline terrain, an allochtonous basement unit located at the top of the Briançonnais nappes stack ∼5 km west of Briançon city. Mapped as an undifferentiated metasedimentary basement on the geological map at 1/50,000 scale, this unit actually displays varied lithologies including alternating micaschist, paragneiss, quartzite, coarse-grained conglomerate and felsic (leptynite) to mafic (amphibolite) gneiss of magmatic origin. All rocks were metamorphosed in response to a dominant pre-Alpine event under amphibolite facies conditions. Partial recrystallization under low-grade amphibolite to greenschist facies conditions was associated with alpine events. Zircon ages were obtained on four types of rocks: (i) a micaceous quartzite from the core of the metasedimentary unit displays a dominant age population around 610 Ma, with a minimal age of 580 Ma, and subordinated age populations at c. 940 Ma, 1825 Ma and 2100-2560 Ma ; (ii) a granite boulder from a coarse-grained conglomerate yields a well-defined age of 582 ± 5 Ma and subordinated inherited ages between 1800 and 2200 Ma. Zircon rims of probable metamorphic origin provide a concordant age of 492 ± 4 Ma ; (iii) a gneissic band in the vicinity of the conglomerate is dated at 597 ± 4 Ma ; (iv) a coarse-grained garnet amphibolite yields an age of 517 ± 3 Ma interpreted as the emplacement age of the protolith, either as intrusive unit cutting accross the sedimentary succession or lava intercalated within the sediments. The mainly Ediacarian record in the metasediments (quartzite and granite boulder in conglomerate) lead us to conclude that this material resulted from the erosion of a nearby Ediacarian (Cadomian ?) basement. The age of 517 Ma obtained on the amphibolite provides a minimum age for the sedimentary succession which deposition is therefore bracketed between 582 Ma and 517 Ma. This interval is comparable to that of the metasedimentary units of the Brioverian group of the Armorican Massif. Based on their low Th/U ratio, the 492 Ma-old zircon rims in the granite boulder are attributed to an Upper Cambrian metamorphic event

Holocene geology of central Africa

N° Outcrop: B000101. View of the Stone Line showing jasper fragments and well-rounded quartz pebble at all levels of the profile

Holocene geology of central Africa

N° Outcrop: B000101. Same comment as preceding. Four samples have been collected in the Cover Horizon on this outcrop (plastic bags)

Holocene geology of central Africa

N° Outcrop: DOL4017. A typical "immature" quartz-bearing Stone Line: ~ 2 metres of accumulated angular quartz blocks of all sizes overlying altered schists

Holocene geology of central Africa

N° Outcrop: DOL4050. View of the Stone Line and Cover Horizon overlying altered schists about 40 m above the present river drainage. Note the rounded aspect and large size (~ up to 0.5 m) of many blocks within the Stone Line

Holocene geology of central Africa

N° Outcrop: DOL4035-4036. General view of a typical profile of superficial formations in western Congo. Note the blanketing of the landscape by the Cover Horizon. The underlying Stone Line is made of resedimented ferricrrust blokcs. The altered basement (pink color) is visible in the lower part of the profile