Geodynamic cycles of subcontinental lithosphere in the Central Alps and the Arami enigma

Abstract

Major masses of metamorphosed ultramafic rocks in the Central Alps, Western Alps and Liguria are former subcontinental lithospheric mantle. It occurs in two, tectonically distinct ophiolitic zones which paleogeographically correspond to the Jurassic, South Penninic, Piemontese ocean belt and to the more external, Cretaceous, North Penninic, Valais ocean belt, respectively. In both belts, the geodynamic cycle of pieces of subcontinental lithospheric mantle started with its exhumation during continental rifting and resulted in the exposure of denuded mantle in the Tethyan ocean basins. With the onset of oceanic closure, these mantle rocks were subducted and, after continental collision, integrated in the Alpine mountain chain and again exhumed. Combined evidence from two key areas of the Central Alps is presented. At Val Malenco, a stable continental crust-mantle transition (1.0–0.9 GPa; 800–600°C) welded by a 270 Ma gabbro intrusion existed into the Triassic. During Jurassic rifting, Malenco mantle with attached pieces of lower crust was rapidly exhumed and exposed in the Piemontese ocean basin near its Adriatic margin. There, an ‘Alpine type ophiolite suite’ developed, containing serpentinized ex-subcontinental mantle, rodingitized Permian gabbro, ophicarbonate rocks and MOR basalts. During Cretaceous convergence, the rifted margin of Val Malenco was only moderately subducted (∼450°C, 0.6 GPa) as it remained in a supra-subduction wedge. In the Western Alps and Liguria, rifted margins of the Piemontese belt were subducted to eclogite conditions (∼3 GPa, 650°C), but never beyond the stability of Antigorite. In the North-Penninic Adula-Cima Lunga nappe of the Central Alps, however, a suite analogous to Val Malenco was subducted to much higher metamorphic conditions (∼3 GPa, 830°C) during Eocene collision. Successive regional metamorphic prograde stages are proven by overgrowth textures in mafic rocks up to kyanite eclogite and in ultramafic rocks up to garnet peridotite. The inferred ultradeep garnet lherzolite of Alpe Arami forms part of the prograde Alpine suite of Adula-Cima Lunga. Ilmenite rods in olivine, as at Arami, invoked as ‘proof’ for an ultradeep origin, have been detected in over 10 localities. They have been shown to form from planar, Ti-bearing, (OH)-defects at conditions below 3 GPa. No direct evidence for an ultradeep origin for Alpe Arami has been presented so far.