2022/02/15 by Jonas Vanardois, Françoise Roger, Pierre Trap +9
Earth and Planetary Sciences · #Amphibole #Continental crust #Crust #Eclogite #Facies #Geochemistry #Geochemistry and Elemental Analysis #Geological and Geochemical Analysis #Geology #Geomorphology #Mafic #Massif #Metamorphic facies #Omphacite #Paleontology #Petrology #Quartz #Subduction #Tectonics #Zircon #earthquake and tectonic studies
paper · pdf · doi:10.1111/jmg.12659
openalex publication_date 2022/02/15 · crossref created 2022/02/15 · crossref issued 2022/03/02 · crossref published 2022/03/02 · crossref published-online 2022/03/02 · crossref published-print 2022/08/01 · crossref deposited 2023/08/24 · openalex created_date 2025/10/10 · crossref indexed 2026/08/04 · openalex updated_date 2026/08/05
Abstract Mafic eclogites are found in many orogens as lenses embedded in quartzofeldspathic migmatites. These high‐pressure relics are interpreted either as remnants of ancient sutures and thus formed during oceanic subduction or as fragments of lower crust exhumed from the root of orogenic thickened crust. It is critical to distinguish between these two endmember scenarios as the resulting palaeogeographic and geodynamic reconstructions may significantly differ. In this contribution, we investigated eclogite relics from Lac Cornu in the Aiguilles‐Rouges massif, one of the External Crystalline Massifs of the Western Alps. Phase equilibrium modelling suggests that these mafic rocks were buried along a prograde path (M1) from ~600°C/1.2 to 1.6 GPa to peak conditions of ~630–775°C and >1.6 GPa. Zircon rims, with a rare earth element signature typical of eclogite facies zircon (no Eu anomalies, flat HREE spectrum), and rutile were dated by U–Th–Pb laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) at c . 335–330 Ma. Prograde deformation has not been identified in the field and is only recognized thanks to the crystallographic preferred orientation (CPO) of inclusions of omphacite and rutile in garnet. Peak pressure conditions were followed by a decompression stage (M2) from ~760°C/1.4 GPa to ~600–650°C/0.9 GPa supported by the breakdown of omphacite into plagioclase–clinopyroxene symplectite and the crystallization of plagioclase–amphibole corona around garnet. The M2 retrogression stage is associated with the development of a main sub‐horizontal planar fabric and the CPO of minerals composing symplectite. This deformation stage is interpreted as the result of horizontal lower crustal flow. The final stage of exhumation (M3) is characterized by the replacement of symplectite and garnet by plagioclase and large euhedral amphibole and by the breakdown of rutile and ilmenite into titanite dated at c . 300 Ma. The CPO of titanite and amphibole are consistent with the development of vertical dextral shear zone in a transpressive regime. The combination of field observations and petrological, microtextural and geochemical analyses suggests that the mafic eclogites preserved in migmatitic rocks of the Aiguilles‐Rouges massif are remnants of a continental lower crust exhumed and juxtaposed with lower‐grade migmatites in crustal‐scale vertical transpressive shear zones.