The Miocene volcanism of the Sulcis area (SW Sardinia, Italy): Petrology, petrogenesis and geodynamic significance

Abstract

During the Cenozoic the western Mediterranean area underwent a complex geodynamic evolution driven by the interaction between the African and Eurasian plates. The Corsica- Sardinia microplate played a central role in this evolution, which was recorded by its tectonic evolution and volcanism. During Oligo-Miocene the roll-back of the NW-wards subducting ,Apulian subduction zone caused the Corsica-Sardinia microplate to detach, drift and rotate from the European continental margin to its present position, opening the Liguro-Provençal Basin and generating abundant subduction-related calc-alkaline orogenic volcanism. After a 6 Ma-long pause in retreat, subsequent evolution of the roll-back since Pliocene left Sardinia in its current position as the trench retreated SE-ward opening the southern Tyrrhenian basin. Extensional tectonics related to this opening produced in Sardinia the generation of abundant anorogenic alkaline volcanism.

In the Sulcis area, in SW Sardinia, a special volcanic suite was produced which began sharing the same calc-alkaline characteristics as the rest of the subduction-related volcanism throughout Sardinia, forming an andesitic lava domes and flows piling, but that later migrated to different compositions, mildly alkaline, including the rarely occurring peralkaline volcanism, which formed a mainly rhyolitic ignimbritic succession. In spite of the marked differences of the Sulcis magmatism, this special volcanic suite was considered to be only one more manifestation of the widespread subduction-generated orogenic volcanism of Sardinia. The present work has, at least partially, solved this situation.

A thorough and detailed study of the Sulcis Oligo-Miocene volcanic suite has been carried out in this study involving the revision of the available data on volcanostratigraphy and cartography and the characterization of the geochemistry, petrography and mineral chemistry of the several units present in this suite, which has allowed us to obtain significant information on the petrogenesis of this suite and its geodynamic significance.

As a result of the field and laboratory work all units have been characterised from the volcanostratigraphic, petrographic and geochemical point of view.

A methodology for unit recognition based on whole rock geochemistry has been developed, which has been a fundamental tool for solving volcanostratigraphic and mapping doubts during the development of this work, and remains a powerful tool for future studies.

The study of the whole rock geochemistry data has allowed us to obtain significant information on the generation of the magmas that formed the Sulcis volcanic suite and its geodynamic significance, and combination with petrographic observations has provided valuable information on the evolution of these magmas from its formation to its eruption.

Isotope and trace element data have revealed the structure of the mantle beneath Sardinia, which is diffusely stratified with an upper part with an EMI signature and a lower more EARlike region, both of them having an OIB mantle source-like composition. Subduction during Oligo-Miocene released into the mantle wedge hydrous fluids and partial melts carrying sediment and MORB isotope and trace element compositions, which metasomatised this mantle, introducing mostly into the more EAR-like mantle a subduction signature.

The Oligo-Miocene orogenic magmas in Sardinia, including the andesitic lower sequence in the Sulcis, were produced in the subduction-modified more EAR-like mantle portion by lowering of the melting temperature due to the input of subduction fluids. Magmas from the ignimbritic sequence in the Sulcis, though, are considered to represent a transition from orogenic to anorogenic magmatism. It is suggested that the transition was due to a change in the melting mechanism from fluid- to extension-controlled, related to the cessation in the subduction influence below the Sulcis. The mechanism proposed to explain the contemporaneous occurrence of both orogenic and anorogenic magmatisms in Sardinia during the emplacement of the Sulcis ignimbritic suite is the formation of a slab break-off which started south of Sardinia and progressed northward opening a slab window.

The ignimbritic sequence in the Sulcis was formed by the arrival and differentiation of several magma batches with slightly different initial compositions, apparently at least one for each unit except for Lenzu and Seruci units, which formed by evolution of the magma that generated the immediately preceding units. Magma differentiation was dominated by crystal fractionation processes, which produced similar evolutionary trends, assimilation having a very small effect only appreciable in Pb isotopes.

Durant l’Oligo-Miocè el funcionament en roll-back de la subducció sota el marge continental europeu de l’oest Mediterrani va provocar el desenganxament de la microplaca Sard-Corsa d’aquest marge i la seva migració primer amb una translació cap al SE i posteriorment amb una rotació en sentit antihorari fins a assolir la seva posició actual. Associada a aquest desplaçament es va produir l’obertura de la conca Liguro-Provençal en posició de back-arc.

Com a resultat de la subducció es va produir abundant magmatisme calco-alcalí d’afinitat orogènica a l’illa de Sardenya. A la zona del Sulcis, al SW de Sardenya, el magmatisme va compartir inicialment les mateixes característiques que la resta del magmatisme sard, formant un apilament de doms i colades làviques de composicions andesítiques. Però cap als estadis finals del cicle magmàtic, els magmes generats van migrar de composició, donant lloc a un apilament d’unitats ignimbrítiques amb composicions majoritàriament riolítiques de transicionals a lleugerament alcalines. Malgrat la marcada diferència composicional entre la seqüència ignimbrítica del Sulcis i la resta del magmatisme Oligo-Miocè sard, aquesta va ser considerada com un exemple més d’aquest magmatisme. Per resoldre en part aquesta situació s’ha realitzat aquesta tesi.

En aquest estudi s’ha revisat la seqüència vulcanoestratigràfica descrita així com les cartografies existents per tal de realitzar un mostreig sistemàtic de la seqüència ignimbrítica. L’estudi de les mostres recollides ha permès caracteritzar totes les unitats que formen la seqüència des del punt de vista de petrogràfic i geoquímic. Les dades obtingudes han permès desenvolupar una metodologia per al reconeixement de les unitats ignimbrítiques en base a la geoquímica de roca total, així com abordar l’estudi de la petrogènesi d’aquesta associació magmàtica. S’ha obtingut informació sobre l’àrea font dels magmes, que ha pogut ser caracteritzada, sobre els mecanismes de formació dels magmes estudiats i el seu significat geodinàmic, i sobre els processos que van controlar l’evolució dels magmes des de la seva formació fins a la seva expulsió en superfície en forma de materials volcànics.