Geochemistry, isotope geochemistry and dating of Penninic amphibolites and meta-ultrabasites between Simplonpass and Lago di Como (Switzerland, Northern Italy)

Polymetamorphic basic rocks (amphibolites) associated with meta-ultrabasic rocks and paragneisses are widespread in many of the tectonic units of the southern-most part of the Penninic Nappes between Brig, Ticino and Lake Como. This E-W-striking suit with an extension of almost 150 km consists of seven different tectonic units: the so-called Berisal-series, Bosco-series, Isorno-series, Camughera-Moncucco-complex, Orselina-zone, Bellinzona-zone and the Cima Lunga-series. A comprehensive study of the geochemistry of major and trace elements (including REE) as well as the Sm/Nd-, Rb/Sr- and U/Pb-isotopes systems made it possible to answer fundamental questions concerning protolites, genesis, age and genetic relationship of the present amphibolites and meta-ultrabasic rocks. The characteristics of major and trace elements show that the majority of the amphibolites originate from protolites representing N- and T-MOR-basalts. These basaltic magmas have been generated from a LREE-depleted mantle through partial melting. The residual mantel-phases showed originally a lherzolitic to harzburgitic mineralogy. According to the Sm/Nd-isotope analyses the protolites of the amphibolites have been formed from MOR-basaltic magma at 985 ±21Ma. The εNd-values of these amphibolites lie between 5.2 and 9.9 and correspond to the MORB-values for 985 Ma. The interaction seawater during or shortly after the extrusion of the basalts had no influence to the Nd-isotope system. Contrary to this, the Sr-isotope composition of the amphibolites has been significantly altered as a result of seawater-contamination. It is shown that the Rb-Sr-method is unsuitable for the dating of polymetamorphic amphibolites because the Rb-Sr-isotope system may become uncontrollable under due to seawater-contamination and metamorphic processes. (author) figs., tabs., refs