New field, textural, petrologic and geochemical researches on the Lanzo
South ophiolitic peridotite constrain the mantle processes which
accompanied the geodynamic evolution during rifting and opening of the
Ligurian Tethys ocean basin. They reveal the presence of different
types of peridotites with variable structural-geochemical
characteristics and mutual relationships: (1) ``lithospheric'' spinel
peridotites preserve records of a long residence in the thermal
lithosphere, (2) ``reactive'' spinel peridotites record effects of
melt/rock interaction; (3) ``impregnated'' plagioclase-rich
peridotites show significant enrichment of basaltic components
(''refertilization'') in the form of mm-size
plagioclase-pyroxene-rich veins and pockets.
Our present results indicate that the Lanzo South peridotites were
accreted to the thermal lithosphere, probably from
garnet-peridotite-facies depths, where progressively cooled and
completely recrystallized under spinel-peridotite-facies conditions.
Subsequently, in response to the pre-oceanic rifting related to the
opening of the Ligurian Tethys, lithospheric mantle sections of the
Lanzo massif were progressively exhumed to shallow lithospheric levels,
whereas the underlying asthenosphere rose and underwent near-adiabatic
decompression melting. The resulting fractional melts migrated through
and reacted with the overlying extending mantle lithosphere.
During initial melting stages of the ascending asthenosphere,
fractional melt increments migrated upwards in the lithospheric mantle
column via diffuse and reactive porous flow, and caused depletion of
the lithospheric mantle by melt/rock interaction (pyroxene dissolution
and olivine precipitation), being olivine-saturated but
pyroxene-undersaturated. Large areas of pyroxene-depleted,
olivine-enriched ``reactive'' peridotites were thus formed.
Subsequently, progressively pyroxene-saturated melts migrated
pervasively in the ``lithospheric'' and ``reactive'' peridotites;
at shallower levels, the competing effects of heating by melt
percolation and cooling by ongoing exhumation led to interstitial
crystallization of percolating melts, and to progressive clogging of
melt channels. This process formed an upper zone of refertilized,
``impregnated'' plagioclase peridotites, and forced the ascending
melts to percolate along focused channels where high melt/peridotite
ratios caused the complete dissolution of pyroxenes and the formation
of ``replacive'' spinel dunites. These high-porosity dunite channels
allowed ``rapid'' migration of the first aggregated MORB melts, which
were produced in the underlying asthenosphere and escaped melt/rock
interaction during upwelling.
The theology of the lithospheric mantle was modified largely by
lithosphere-asthenosphere interaction; the lithospheric mantle attained
asthenospheric characteristics during erosion by melt percolation.
Following continuous upwelling in the thermal lithosphere and
increasing conductive heat loss, the thermochemically modified
lithospheric mantle returned to more cold and brittle conditions.
Later, the Lanzo South peridotites were intruded along fractures by
variably fractionated, Mg-rich to Fe-rich magmas deriving from MORB
primary melts, most probably aggregated at asthenospheric levels and
differentiated in shallow magma chambers.
The above evidence reveals a composite magmatic stage recorded in the
mantle; percolating melts were trapped in the lithospheric mantle and
never reached the surface. This magmatic stage preceded later shallow
emplacement of MORB magmas, which formed gabbroic and basaltic rocks of
the Jurassic oceanic crust.
Records of melt percolation, impregnation and melt/peridotite reaction
are common in ophiolitic peridotites and present-day oceanic mantle
lithosphere. Relationships between the results obtained for the Lanzo
ophiolitic peridotites and those determined for others Alpine-Apennine
peridotites, provide a mechanism to explain non-volcanic and volcanic
stages during rift evolution of the Ligurian Tethys, and might be
equally applicable to modem slow spreading ridges, which are
characterized by variable magmatic (volcanic) and amagmatic
(non-volcanic) stages.