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

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 co rrespond 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 lithospher ic mantle started with its exhumation during continental rifting and result ed in the exposure of denuded mantle in the Tethyan ocean basins. With the onset of oceanic closure, these mantle rocks were subducted and, after cont inental collision, integrated in the Alpine mountain chain and again exhume d. Combined evidence from two key areas of the Central Alps is presented. A t Val Malenco, a stable continental crust-mantle transition (1.0-0.9 GPa; 8 00-600 degrees C) welded by a 270 Ma gabbro intrusion existed into the Tria ssic. During Jurassic rifting, Malenco mantle with attached pieces of lower crust was rapidly exhumed and exposed in the Piemontese ocean basin near i ts Adriatic margin. There, an 'Alpine type ophiolite suite' developed, cont aining serpentinized ex-subcontinental mantle, rodingitized Permian gabbro, ophicarbonate rocks and MOR basalts. During Cretaceous convergence, the ri fted margin of Val Malenco was only moderately subducted (similar to 450 de grees C, 0.6 Cpa) as it remained in a supra-subduction wedge. In the Wester n Alps and Liguria, rifted margins of the Piemontese belt were subducted to eclogite conditions (similar to 3 GPa, 650 degrees C), but never beyond th e stability of Antigorite. In the North-Penninic Adula-Cima Lunga nappe of the Central Alps, however, a suite analogous to Val Malenco was subducted t o much higher metamorphic conditions (similar to 3 GPa, 830 degrees C) duri ng Eocene collision. Successive regional metamorphic prograde stages are pr oven by overgrowth textures in mafic rocks up to kyanite eclogite and in ul tramafic rocks up to garnet peridotite. The inferred ultradeep garnet Iherz olite of Alpe Arami forms part of the prograde Alpine suite of Adula-Cima L unga. Ilmenite rods in olivine, as at Arami, invoked as 'proof for an ultra deep orinin 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. (C) 2000 Elsevier Science Ltd. All rights reserved.