Permian metabasalt and Triassic alkaline dykes in the northern Ivrea zone:clues to the post-Variscan geodynamic evolution of the Southern Alps

The voluminous Lower Permian metabasalts/metagabbros ("Mafic Formation") in the northern Ivrea zone belong geodynamically to the post-Variscan calc-al kaline magmatism of Central Europe. East of Finero, the mafic magmatic rock s have low mean mg#-values of 51.9 and their distinctive Ni and Cr depletio n indicates highly fractionated tholeiitic rocks. The fractionated state of the basaltic rocks requires that huge masses of olivine/clinopyroxene pare ntal rocks exist, probably at the former crust/mantle boundary. These dense cumulitic rocks at shallow crustal-level may be responsible for the observ ed gravity anomaly along the Ivrea zone in the Western Alps. The alkaline zircon syenite pegmatites, hornblende syenites and a carbonate -bearing hornblendite in the studied area are part of the Triassic magmatis m in the Southern Alps. These pegmatitic dyke rocks may represent early sig ns of the metasomatism in the lithosphere as a prelude to the "Tethyan cycl e". The magmas originated from partial melting of metasomatized upper mantl e at depths of about 50 km and temperatures between 970-1030 degreesC. Thei r ultimate source is "asthenospheric", and they derive from a mildly deplet ed upper mantle, as indicated by their isotopic composition: epsilon (Sr) = -10.18 to -10.87, epsilon (Nd) = 4.58 to 5.73 and Pb-206/Pb-204 = 18.68 to 19.18. The isotope ratios are interpreted as "asthenospheric" mixtures of depleted (DM) and enriched (HIMU) mantle components. From additional geolog ical observations in the Southern-Alpine domain, we conclude that "active" mantle upwelling during the Anisian caused uplift, while volcanism and magm atism began later in Ladinian times. It is suggested that the alkaline dyke s in the northern Ivrea zone are of Triassic age, as is the larger Monzoni intrusive complex further east, and that they are derivatives of the convec ting asthenosphere.