Passo di Verra area (Zwillingsjoch area), Champoluc, Ayas, Aosta Valley, Italyi

Regional Level Types
Passo di Verra area (Zwillingsjoch area)	Area
Champoluc	Village
Ayas	Commune
Aosta Valley	Autonomous Region
Italy	- not defined -

This pass (3845 m a.s.l.), located on the border between Aosta Valley (Italy) and Valais (Switzerland), separates Polluce/Pollux (4092 m) from Castore/Castor (4223 m), the two twin peaks collectively known as "I Gemelli" (Italian), "Die Zwillinge" (German) or "Les Jumeaux" (French).

From a geological point of view, the area around the pass is highly complex, because exactly at Passo di Verra/Zwillingsjoch the Penninic Monte Rosa nappe, a fragment of European continental crust, is in tectonic contact with a serpentinised mantle slice of the ophiolitic Zermatt-Saas nappe, a relict of the Mesozoic Tethys oceanic lithosphere.

The serpentinised slice, consisting of magnetite-bearing antigorite serpentinite with lenses and veins rich in reddish titanclinohumite, diopside and olivine, forms the Polluce/Pollux-Breithorn-Gobba di Rollin range (Swiss-Italian divide) and the western sector of the Verra glacial cirque.

The underlying Monte Rosa nappe forms Castore/Castor and the eastern part of the Verra glacial cirque. In spite of polyphase Alpine metamorphism (eclogitic and later greenschist facies), two groups of protoliths (a pre-granitic and a granitic complex) are identified (Bearth, 1952; Dal Piaz, 1971, 2001, and 2004). They are well preserved in a large low-strain domain which extends between Piccolo Ghiacciaio di Verra and Pian di Verra Superiore (Plan de Verra Supérieur), through the western crest of Punta Perazzi, where Alpine structural reworking was relatively weak. The pre-granitic metamorphic complex is represented by high-grade pelitic paragneiss and anatectic migmatite, including concordant-to-discordant cordierite-bearing pegmatites.
The granitic complex is represented by a granitic-granodioritic pluton, often porphyric, leucocratic granite in discordant stocks and concordant intrusions, and younger aplitic and pegmatitic dykes, locally preserving their primary features in the low-strain domain. The intrusion age is inferred from U-Pb SHRIMP dating of zircon (270 ± 4 Ma) and monazite (268 ± 2 Ma) (Lange et al., 2000).

Other Regions, Features and Areas containing this locality

References