Genesis of Ultrapotassic Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 457

magmatism. In the circum-Tyrrhenian area leucite-free (i.e., lamproite) and leucite-
bearing (i.e., kamafugite, leucitite, and plagioleucitite) ultrapotassic rocks have
been emplaced and are associated with shoshonites and high-K calc-alkaline vol-
canic rocks. Four different magmatic provinces are recognised from this area.
Eastward and then south-eastward migration of magmatism with time occurred
following roll-back of the subducting plate. Leucite-free silica-rich lamproites are
restricted to the early stages of magmatism, associated with shoshonites and high-K
calc-alkaline volcanic rocks. Present day volcanic activity is restricted to the
Neapolitan district where ultrapotassic rocks with variable geochemical and iso-
topic characteristics occur. Ultrapotassic rocks are strongly enriched in incompat-
ible trace elements with variable fractionation of Ta, Nb, and Ti with respect to Th
and large ion lithophile elements. Ma c ultrapotassic rocks are also variably enri-

ched in radiogenic Sr and Pb and unradiogenic Nd. The main geochemical and
isotopic signatures result from sediment recycling within the upper mantle via
subduction. Selected trace element ratios suggest that high temperatures are
required to generate sediment-derived melts. Recycling of carbonated pelites play
an important role in the Roman province controlling the genesis of leucite-bearing
magmas. Large volumes of metasomatic components are predicted to be accom-
modated within a vein network in the sub-continental lithospheric mantle. Partial
melting of the pure vein mineralogy is likely to generate ultrapotassic magmas of
either lamproitic or kamafugitic nature. Over time, increased interaction between
the metasomatic vein lithology and the surrounding mantle dilutes the alkaline
component producing shoshonites and high-K calc-alkaline rocks. The addition of a
further subduction-related component shortly before magma generation is required
to explain the isotopic composition of rocks from the Neapolitan district. In the last
phases of circum-Tyrrhenian evolution, a within-plate component appears within
south-eastern Italy. This component is evident at Vulture volcano, in the Lucanian
Magmatic province (SE Italy). Boari et al. (2009) supported such a view related to
cluster recycling of sedimentary materials for the genesis of K-rich magma of the
Middle Latin Valley Volcanic field of the Roman Magmatic Province.

15.10 Recycling of Nitrogen from Crust into the Mantle

Most of the ocean floor crust is returned to the mantle at subduction zone. The

process is one of the most deep cycling events in the planet (Fyfe 1997). The overall
signi cance of the subduction process is that oceanic crust including pelagic sed-

iments (Gilluly 1971) carries with it the memory of interactions with the hydro-
sphere (H 2 O, CO 2 , S, Cl, Na, U, K, Sr, O 2 and N 2 ). Fyfe considered that nitrogen
content in subduction-related volcanism may provide important clue as to the crust
mantle mixing.
Lunar surface provides information on in ux of different nitrogen isotopes due

to solar wind as it implants itself on individual soil grains to a depth of tens of
nanometer (Sanno et al. 2001). Deciphering pre-solar and planetary nitrogen