Lamprophyre dykes have been recently discovered in blocks of gneiss
embedded in a calcschist formation of wildflysch type that forms the top
of the Mesozoic-Tertiary metasedimentary cover of the Antigorio nappe
(the Teggiolo zone) in the Val Bavona (Lower Penninic, NW Ticino,
Switzerland). The presence of the lamprophyres gives a clue to the
possible source of these blocks. Similar dykes occur in the N part of
the Maggia nappe where they are intruded into the Matorello granite and
the surrounding gneisses.
We studied these lamprophyres at two localities in the Teggiolo zone
(Tamierpass and Lago del Zott) and at one locality in the Maggia nappe
(Laghetti). Detailed mineralogical and geochemical investigations
confirm their great similarity, particularly between the Tamier and
Laghetti dykes. They all recrystallized during Alpine metamorphism under
amphibolite facies conditions and lost their primary mineral assemblages
and textures. The chemistry reveals a calc-alkaline affinity, a limited
differentiation range, features of mineral accumulation and intense
remobilization in some cases. The lamprophyres are characterized by a
high mg# and relatively low contents in REE and other incompatible
elements. In situ SHRIMP and LA-ICPMS U-Pb zircon dating yielded ages of
284.8 +/- 1.7 Ma (Tamier), 290.0 +/- 1.3 Ma (Zott) and 290.5 +/- 3.7 Ma
(Laghetti). These ages are compatible with the general late- to
post-Variscan magmatic evolution of the Helvetic and Lower Penninic
domains. The lamprophyres are considered as melts derived from the
lithospheric Variscan mantle, variously hybridized and differentiated at
the contact with crustal material during late- to post-orogenic
extension. These lamprophyres are chemically distinct from earlier
lamprophyres of Visean age, emplaced together with their associated
granites in transcurrent fault zones during the Variscan orogenic
compression.
The similarity of these different dykes suggests that the front of the
Maggia nappe is a likely source of the gneissic blocks embedded in the
calcschists at the top of the Teggiolo zone. They would have been
provided by the advancing Maggia nappe during its thrusting over the
Antigorio nappe and simultaneous closure of the Teggiolo sedimentary
basin.