The Ivrea^Verbano Zone in the western Italian Alps contains one
of the world’s classic examples of ponding of mantle-derived, mafic
magma in the deep crust. Within it, a voluminous, composite
mafic pluton, the Mafic Complex, intruded lower-crustal,
high-grade paragneiss of the Kinzigite Formation during
Permian^Carboniferous time, and is now exposed in cross-section as
a result of Alpine uplift. The age of the intrusion is still debated
because the results of geochronological studies in the last three decades
on different rock types and with various dating techniques range from
250 to about 300Ma. Sensitive high-resolution ion microprobe
(SHRIMP) U^Pb zircon age determinations on 12 samples from
several locations within the Mafic Complex were performed to
better constrain the age of the igneous event.The results indicate a
long history of magma emplacement and cooling, which reconciles
the spread in previously published ages. The main intrusive phase
took place at 2884Ma, causing a perturbation of the deep-crustal
geotherm, which relaxed to the Sm^Nd closure temperature in
garnet-free mafic rocks after about 15^20Myr of sub-solidus cooling
at c. 270Ma.These results suggest that large, deep crustal plutons,
such as those identified geophysically at depths of 10^20 km within
extended continental crust (e.g. Yellowstone, Rio Grande Rift,
Basin and Range) may have formed rapidly but induced a prolonged
thermal perturbation. In addition, the data indicate that a significant
thermal event affected the country rock of the Mafic Complex
at about 310Ma. The occurrence of an upper amphibolite- to
granulite-facies thermal event in the Kinzigite Formation prior to
the main intrusive phase of the Mafic Complex has been postulated
by several workers, and is corroborated by other geochronological
investigations. However, it remains uncertain whether this event (1)
was part of a prolonged perturbation of the deep-crustal geotherm,
which started long before the onset of intrusion of the Mafic
Complex, or (2) corresponded to the intrusion of the first sills of
the Mafic Complex, or (3) was related to an earlier, independent
thermal pulse.
