The Pliocene Montiferro Volcanic Complex (Central-Western Sardinia, Italy): Geochemical Observations and Petrological Implications
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Per. Mineral. (2007), 76, 101-16 doi:10.2451/2007PM0011 SPECIAL ISSUE: From Petrogenesis to Orogenesis http://go.to/permin An International Journal of l PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage The Pliocene Montiferro volcanic complex (central-western Sardinia, Italy): geochemical observations and petrological implications LORENZO FEDELE1 *, MICHELE LUSTRINO2, 3, LEONE MELLUSO1, VINCENZO MORRA1 and FOSCO D’AMELIO1 1 Dipartimento di Scienze della Terra, Università di Napoli Federico II, via Mezzocannone 8, 80134, Napoli, Italy 2 Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, P.le Aldo Moro 5, 00185 Roma, Italy 3 CNR-Istituto di Geologia Ambientale e Geoingegneria (IGAG), P.le Aldo Moro 5, 00185 Roma, Italy Accepted, April 2007 ABSTRACT — The Montiferro volcanic complex of these products from a common mantle source, is one of the several districts of Middle Miocene- represented by a spinel-lherzolite. Primitive magmas �uaternary magmatism of Sardinia. It was active of three magmatic suites may have generated by between 3.9 and 1.6 Ma, with a climax at 3.6 Ma. different degrees of partial melting of this source, Erupted rocks have a wide spectrum of compositions, progressively increasing from strongly alkaline ranging from basic and ultrabasic to highly evolved (~1-3% partial melting) to mildly alkaline (~5-7%) lithotypes, belonging to three different magmatic and tholeiitic products (~10-12%). Geochemical suites: 1) strongly alkaline sodic series; 2) mildly modellings suggest that this source is represented by alkaline sodic series; 3) tholeiitic series. The products a DMM-like lithospheric mantle, metasomatised by of the strongly alkaline series are mainly basanites, small inputs of various crustal components. erupted both at the first and at the last stages of Montiferro activity. Strongly alkaline magmas are RIASSUNTO — Il complesso vulcanico del Montiferro likely to have risen rapidly to the surface, as testified è uno dei numerosi distretti del magmatismo medio by the common occurrence of mantle xenoliths Miocenico-�uaternario della Sardegna. L’attività and by the absence of differentiated products. The vulcanica abbraccia un intervallo di tempo compreso mildly alkaline series ranges from hawaiites to tra 3,9 e 1,6 Ma, con una fase di climax avvenuta trachytes and phonolites through very abundant intorno a 3,6 Ma. I campioni analizzati coprono mugearites and few benmoreites. Primitive magmas un ampio spettro composizionale, comprendente of this series are likely to have ponded at crustal prodotti da poco differenziati ad intermedi ed levels, experiencing both differentiation and crustal evoluti, appartenenti a tre diverse serie magmatiche: contamination processes. Finally, the tholeiitic series 1) serie sodica fortemente alcalina; 2) serie sodica is mainly represented by rare basaltic andesites and a moderatamente alcalina; 3) serie tholeiitica. few basaltic trachyandesites straddling the alkaline/ I prodotti della serie fortemente alcalina sono subalkaline boundary. rappresentati principalmente da basaniti, messe in 87 86 143 Incompatible trace elements, Sr/ Sr and Nd/ posto durante le fasi iniziali e finali di attività del 144 Nd isotopic ratios are consistent with the derivation Montiferro. La risalita in superficie di tali magmi sembra essere stata piuttosto rapida, come indicato * Corresponding author, E-mail: [email protected] dalla occasionale presenza di xenoliti di mantello e 102 L. FEDELE, M. LUSTRINO, L. MELLUSO, V. MORRA and F. D’AMELIO dall’assenza di prodotti differenziati. I prodotti della Montiferro lavas. This is unusual if compared serie moderatamente alcalina vanno da hawaiiti to the wealth of data published in the last a trachiti e fonoliti, attraverso termini intermedi decade on various Middle Miocene-�uaternary rappresentati dai mugeariti (estremamente abbondanti) Sardinian districts (e.g., Montanini et al., 1994; e benmoreiti (poco rappresentate). I magmi capostipite Lustrino et al., 1996, 2000, 2002, 2004, 2007a, di questa serie hanno potuto ristagnare nella crosta, 2007b; Gasperini et al., 2000). This paper aims andando incontro a processi di differenziazione to fill the gap of petrological knowledge within e contaminazione. Infine, i prodotti della serie the framework of the detailed characterisation subalcalina sono rappresentati principalmente da andesiti basaltiche e rare trachiandesiti basaltiche. of the most recent volcanic cycle of Sardinia, Le concentrazioni degli elementi incompatibili presenting new data for whole rock major ed i rapporti isotopici di 87Sr/86Sr e 143Nd/144Nd and trace elements, mineral chemistry, Sr-Nd 40 39 suggeriscono una comune origine per i prodotti delle isotopic ratios and Ar/ Ar geochronology. tre serie, la cui sorgente sarebbe rappresentata da una lherzolite a spinello. A partire da tale sorgente, i magmi primitivi si sarebbero originati per diversi GEOLOGICAL BACKGROUND gradi di fusione, progressivamente maggiori muovendo dai prodotti di serie fortemente alcalina During the last 30 Ma the entire circum- (per una fusione parziale del ~1-3%) a quelli di serie Mediterranean area has been characterised by a moderatamente alcalina (~5-7%) fino a quelli di serie complex geotectonic evolution, which strongly tholeiitica (~10-12%). Le modellizzazioni effettuate influenced the magmatic activity of the area sulla base dei rapporti tra elementi incompatibili e (e.g., Beccaluva et al., 2007 and references dei rapporti isotopici indicano che la sorgente dei therein; Lustrino and Wilson, 2007 and magmi del Montiferro è costituita da un mantello references therein). The large-scale tectonic litosferico di tipo DMM, metasomatizzato attraverso l’introduzione di piccoli quantitativi di materiale di evolution was mainly driven by the convergent provenienza crostale. movement of African and European plates in the Alpine orogeny framework (e.g., Ricou, 1994; Carminati et al., 1998; Gueguen et al., KEY WORDS: Sardinia, geochemistry, petrology, alkaline, tholeiitic 1998; Mattei et al., 2004). Up to lower Oligocene times, Sardinia was part of the southern European continental INTRODUCTION margin (in spatial continuity with southern France). During Burdigalian the continental The Montiferro volcanic complex is one block of Sardinia and Corsica rotated of the largest Middle Miocene-�uaternary counter-clockwise (Speranza et al., 2002) and volcanic districts of Sardinia, covering an area successively moved eastwards, determining the of ~400 km2 in the central-western sector of opening of the Ligurian-Provençal basin. These the island (Fig. 1). The stratigraphic sequences processes were related to the roll-back of an of the Montiferro products have been studied oceanic lithosphere slab, subducting towards in detail by several authors during the ‘70s NNW since late Oligocene times, whose hinge (e.g., Beccaluva et al., 1973; Deriu et al., progressively drifted towards SE (Gueguen et 1974a, 1974b; Gallo et al., 1974; Assorgia et al., 1998; Gelabert et al., 2002; Rosenbaum et al., 1976, 1978) up to the early ‘80s, when a al., 2002; Speranza et al., 2002; Mattei et al., geopetrographic map was published by Assorgia 2004; Langone et al., 2006). Consequentely, a et al. (1981). Since then, after the work of magmatic cycle including calcalkaline and arc- Di Battistini et al. (1990), no more papers tholeiitic products developed in the central- focusing on the Montiferro complex have been western part of the island approximately from so far published, with the exception of a couple 32 to 15 Ma (Lecca et al., 1997; Lustrino et al., of papers (Montanini et al., 1992; Montanini 2004, and references therein). and Harlov, 2006) specifically dealing with After Langhian, the ongoing migration of the the petrology of lower crust xenoliths within subduction hinge determined the opening of The Pliocene Montiferro volcanic complex (central-western Sardinia, Italy):geochemicalobservations... volcaniccomplex(central-westernSardinia, The PlioceneMontiferro 103 Fig. 1 – Geological sketch map of the Montiferro volcanic complex (modified from Assorgia et al., 1981) and its localisation within Sardinia [modified from Lustrino et al. (2007b)]. 1 = Oligo-Miocene volcanic rocks; 2 = Lower Analcite Basanites (LAB) unit; 3 = Trachytes and Phonolites (TP) unit; 4 = Basalts and interlayered Differentiated rocks (BD) unit; 5 = Upper Analcite Basanites (UAB) unit; 6 = �uaternary sedimentary cover. 104 L. FEDELE, M. LUSTRINO, L. MELLUSO, V. MORRA and F. D’AMELIO the Tyrrhenian back-arc basin (Gueguen et al., shallow sea deposits (mainly limestones and 1998; Gelabert et al., 2002; Rosenbaum et al., sandstones). From bottom to top these eruptive 2002). The global extensional tectonic regime sequences are: was active also in Sardinia where, during Lower analcite basanites (LAB): this thin Middle Miocene-�uaternary, a new igneous unit (maximum thickness 30 m; Gallo et al., episode developed throughout the island (e.g. 1974) is represented by very small and scattered Lustrino et al., 2000, 2004, 2007a, 2007b). outcrops of few thin lava flows. The lower Middle Miocene-�uaternary volcanic rocks of analcite basanites commonly host ultramafic Sardinia are represented by both alkaline and (Beccaluva et al., 2001) and gabbroic xenoliths tholeiitic products, with geochemical features (Assorgia et al., 1981; Montanini and Harlov, resembling magmas emplaced