In this paper we present new whole rock analyses of amphibolites from
the ophiolitic and adjacent continental tectonic units in the Monte
Rosa region. Mg numbers and Ni contents indicate that these
amphibolites were derived from fractionated magmas with compositions
ranging from E- to N-MORB. Based on their Ni, Ti, REE and Nb
systematics. the metabasalts from the ophiolitic Zermatt-Saas and
Antrona units and from the continental units of the Furgg zone and the
Portjengrat unit are ascribed to a common origin. They represent a
coherent suite ranging from T- to N-MORB. In contrast, amphibolites
from the continental Siviez-Mischabel and Monte Rosa nappes were
derived from enriched MORB and/or gabbroic precursors, which are not
related to the metabasalts from the ophiolites, the Furgg zone or the
Portjengrat unit.
The geochemical differences between the basalts of the ophiolitic
Zermatt-Saas and Antrona units and the adjacent continental Furgg zone
and the Portiengrat unit are very subtle. Most mafic rocks were derived
from low to moderate degrees of melting of an N-MORB type mantle
source. Some compositional parameters such as (Ce/Sm)(n), Zr* and
(Nb/Zr)(n) indicate a transition from T-MORB compositions in the
continental units towards less enriched compositions in the ophiolitic
units. Y, Ti, V and Zr concentrations are highly correlated in the
metabasalts from the Furgg zone, whereas such inter-element
correlations are less well defined in the metabasalts from the
ophiolitic units. This renders the previously proposed interpretation
of the Furgg zone amphibolites as tectonically incorporated ophiolitic
fragments unlikely. Our data rather suggest that the distal continental
units (Portjengrat unit and Furgg zone) and the nearby ophiolitic units
were intruded by similar magmas. Portjengrat unit and Furgg zone are
interpreted as a formerly continuous tectonic unit which, based on
structural grounds, represents the ocean-continent transition zone of
the Brianconnais to the immediately adjacent oceanic Antrona unit.
However, the ambiguity in the paleogeographic provenance of the Antrona
unit (Valais vs. Piemont-Liguria ocean) cannot be resolved with the
existing geochemical data.