Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies <i>schistes lustrés</i> nappe of NE Corsica

Abstract

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle‐to‐late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies ( schistes lustrés ) nappe, was confined to a major ductile shear zone ( c . 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca‐amphibole + albite + sphene, and biotite to actinolitic Ca‐amphibole + albite + phengite + Ti‐ore + epidote. Zone 3: Hornblende to crossite + low Ti‐biotite + phengite + sphene, and biotite to crossite + low Ti‐biotite + phengite + Ti‐ore + sphene ± epidote. P‐T conditions at the base of the shear zone are estimated to have been 390‐490°C at 600‐900 M Pa (6‐9kbar) and the Corsican basement is therefore deduced to have been buried to 20‐30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.