Seminari di Scienze della Terra
Universita’ di Genova
30 Gennaio 2007

 Multidisciplinar Approach to Natural Triggers of Lithosphere Evolution

Genesis and evolution of mantle and crustal derived melts in the Tuscan Magmatic Province: interplay between Geodynamics and Magmatism.

Giampiero Poli

Università di Perugia

Occurrence of a large variety of rock types closely associated in space and time (8 - 1 Ma) reveals a complex magmatic setting for the Miocene-Pliocene Tuscan Magmatic Province (TMP, Central Italy). Three groups of rocks exist having different degree of evolution: i) mafic rocks with lamproitic, high-potassium calc-alkaline, and shoshonitic affinity; ii) intermediate rocks having strong field, petrographic, and geochemical evidence of magma interaction processes; iii) acid volcanics and intrusives showing petrological and geochemical characteristics of both extreme evolved and pure anatectic melts. Geochemical and thermodynamical characteristics of volcanic and plutonic rocks belonging to TMP, suggest that a process of interaction between basic and acid end-members is responsible for the evolution of the TMP magmatism. Petrologic data helps also to recognize the basic end-members as compositionally akin to three basic-intermediate magmas belonging to Capraia shoshonites, Capraia HK, and lamproites that acted together even in a single complex. Acid end-members are crustal anatectic melts derived by partial melting at ca 4-6 Kbar of Tuscan basement rocks having a metasedimentary protolith. Residual assemblages of the partial melting process, calculated by geochemical models, agree with experimental petrology data, and help to envisage levels of emplacement for intrusive complexes, and paths of extrusion for the effusive ones. The petrological model fit well with geophysical data. During Late Oligocene-Early Miocene a polyphasic compressive regime built up the "Etruscan Belt" coupling European and African lithosphere and brought upper crustal material at depth. Starting from Middle Pliocene, western Tuscany underwent a strong regional uplift caused by asthenospheric doming that thinned the crustal stack and completely restructured the crust-mantle boundary. In response to the increasing heat flow both asthenosphere and lithospheric mantle melted to give rise to basaltic magmas. These basaltic melts either intruded and mixed with anatectic melts generated in the interlayered upper crustal stacks, or extruded to give Capraia rocks and lamproites.


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