Construction and Magmatic Evolution of a Vertically Layered Mid-Crustal Gabbroic Complex (Punta Falcone, Sardinia)

Abstract:

The granite-hosted mid-crustal complex of Punta Falcone consists of transitional tholeiitic to calc-alkaline high-Al gabbros. Its pervasive vertical structure is the result of injection of at least 6 distinct pulses forming 10 to 60m wide individual cooling units. The complex was built symmetrically by injection of each new pulse in the centre of the system. Contacts between the units are well-defined and present slight chilled margins speaking against a continuous magma flux which would have had a marked thermal impact on the adjacent units.

All units have a common magma source (radiogenic isotopes), but show differences as the system evolves. An_80-90 plg is the liquidus phase in all units and crystallized in the source region. In the early units it is followed by sub-/euhedral amph of mm-size with a wide range of compositions dominated by the edenite-type substitution, evidencing relatively fast, isobaric cooling at emplacement level. The later units accumulated high-An plg and fractionated Fe-Ti oxides before ascent as indicated by whole-rock chemistry. After emplacement and due to the thermal maturation of the system, they isobarically crystallized big, cm-sized oikocrysts of px enclosing high-An plg, closely followed by amph with the same texture. This led to an An-content jump in the subsequently crystallizing plg rims, whose absence in the mafic phases indicates a strong crystallinity increase over a small T interval.

The geometry, size, number of pulses and emplacement chronology of the complex yield constraints on the thermal evolution of the system. Numerical modelling shows that the periodic injection of magma and the latent heat of crystallization of px and amph maintained the latest pulse at high T for several hundreds of years. This induced a slow and regular inward migration of the crystallization front on both sides of the central unit, which allowed the development of a spectacular vertical, cm-scale, rhythmic magmatic layering.