Leone Melluso

The role of lower continental crust and lithospheric mantle in the genesis of Plio-Pleistocene volcanic rocks from Sardinia (Italy)

Abstract The first comprehensive chemical and Sr–Nd–Pb isotopic data set of Plio–Pleistocene tholeiitic and alkaline volcanic rocks cropping out in Sardinia (Italy) is presented here. These rocks are alkali basalts, hawaiites, basanites, tholeiitic basalts and basaltic andesites, and were divided into two groups with distinct isotopic compositions. The vast majority of lavas have relatively high 87Sr/86Sr (0.7043–0.7051), low 143Nd/144Nd (0.5124–0.5126), and are characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and European) Neogene-to-Recent mafic volcanic rocks (206Pb/204Pb=17.55–18.01) (unradiogenic Pb volcanic rocks, UPV); these rocks crop out in central and northern Sardinia. Lavas of more limited areal extent have chemical and Sr–Nd–Pb isotopic ratios indicative of a markedly different source (87Sr/86Sr=0.7031–0.7040; 143Nd/144Nd=0.5127–0.5129; 206Pb/204Pb=18.8–19.4) (radiogenic Pb volcanic rocks, RPV), and crop out only in the southern part of the island. The isotopic ratios of these latter rocks match the values found in the roughly coeval anorogenic (i.e. not related to recent subduction events in space and time) mafic volcanic rocks of Italy (i.e. Mt. Etna, Hyblean Mts., Pantelleria, Linosa), and Cenozoic European volcanic rocks. The mafic rocks of the two Sardinian rock groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Ce/Pb=8–25 and Nb/U=29–38 for the UPV; Ba/Nb

The Mt. Vulture volcanic complex (Italy): evidence for distinct parental magmas and for residual melts with melilite

SummaryNew data of mineral compositions, whole-rock major and trace element contents are presented for lava flows and dykes of Mt. Vulture volcano (southeastern Roman Magmatic Province). The dominant rock series ranges from basanite, through tephrite and foidite, to phonolite. Diopsidic to Fe-salitic (hedenbergitic) clinopyroxene is the major phenocryst phase, and shows a very strong Ti- and Al-enrichment trend (up to 5 wt% TiO2 and 12 wt% Al2O3). Phonolites have Fe-rich pyroxenes low in Ti and Al, as a consequence of fractionation of Ti-magnetite and/or melanite garnet.Feldspar ranges in composition from bytownite to Ba- and Sr-rich anorthoclase (up to 7 wt % BaO and 4 wt % SrO); the latter is present in the groundmass of mafic and intermediate rocks. Phonolites are characterized by anorthoclase and sanidine with Ba-rich cores, and with Ba-depleted rims.Subordinate melilite-bearing rocks are also present, and are characterized by melilite with a very large compositional variation, Ca-rich nepheline (up to 8 wt% CaO) and lack of feldspar and amphibole. Some of them also show calcite-rich ocellar structures, possibly due to liquid immiscibility. They have also major and trace element contents different from those of the feldspar-bearing rocks. This all indicates distinct parental magmas for feldspar-bearing and melilite-bearing rocks. These latter were likely generated by differentiation processes starting from olivine- and melilite-bearing ultramafic parental magmas.Mineral compositions and suitable phase diagrams indicate that the Melfi haüynophyre, which is the most MgO-poor melilite-bearing rock of Mt. Vulture, is close to a residual melt analogous to phonolite, but in petrogenetic systems with melilite and without feldspar.The high- to very high-titanium content of most of the ferromagnesian phases, despite a relatively low TiO2 of primitive Mt. Vulture rocks, is absent in other rocks of similar degree of evolution in the Roman Province. It resembles that found in anorogenic magmas and gives further indications for a within-plate signature in this particular volcanic complex.ZusammenfassungEs werden neue Daten über Mineralzusammensetzungen sowie Haupt- und Spurenelemente des Gesamtgesteins für Lavaflüsse und Gänge des Mt. Vulture Vulkans (südöstliche Römische Magmatische Provinz) vorgestellt. Die auftretenden Gesteinsabfolgen reichen von Basaniten über Tephrite und Foidite zu Phonolithen. Diopsidischer bis Fe-salitischer (hedenbergitischer) Klinopyroxen stellt die hauptsächlichen Einsprenglinge und zeigt starke Ti- und Al-Anreicherungen (bis zu 5 Gew% TiO2 und 12 Gew% Al2O3). Phonolite weisen als Konsequenz der Fraktionierung von Ti-Magnetit und/oder Melanit-Granat Fe-reiche Pyroxene mit niedrigen Ti- und Al-Gehalten auf.Der Feldspat reicht in seiner Zusammensetzung von Bytownit zu Ba- und Si-reichem Anorthoklas (bis zu 7 Gew%/o BaO und 4 Gew% SrO), der letztgenannte findet sich in der Grundmasse mafischer und intermediärer Gesteine. Phonolithe werden durch das Auftreten von Anorthoklas und Sanidin mit Ba-angereicherten Kernen und Ba-abgereicherten Rändern charakterisiert.Untergeordnet treten auch Melilith-führende Gesteine auf. Sie sind durch Melilithe mit einer großen kompositionellen Variation, durch Ca-reichen Nephelin (bis 8Gew% CaO) und durch das Fehlen von Feldspat und Amphibol gekennzeichnet. Einige dieser Melilithe zeigen auch Calcitreiche ocellare Strukturen, die möglicherweise aufgrund der Nicht-Mischbarkeit der Schmelzen entstanden sind. Sie weisen auch Gehalte an Haupt- und Spurenelementen auf, die von jenen Feldspat-führender Gesteine abweichen. All dies zeigt verschiedene Ausgangs-Magmen für Feldspat-führende und Melilith-führende vulkanische Gesteinean. Die Melilithgesteine entstanden wahrscheinlich durch Differentiationsprozesse aus Olvin- und Melilith-führenden ultramafischen Magmen.Mineralzusammensetzung und die entsprechenden Phasendiagramme zeigen, daß der Melfi Haüynophyr, welcher das MgOärmste Melilith-führende Gestein des Mt. Vulture ist, nahe an einer Ausgangschmelze analog zu einem Phonolith liegt, allerdings in einem petrogenetischen System mit Melilith und ohne Feldspat.Der hohe bis sehr hohe Titan-Gehalt der meisten ferromagnesischen Phasen fehlt, trotz des niedrigen TiO2-Gehaltes primitiver Mte. Vulture Gesteine, in anderen Gesteinen ähnlicher Entwicklung in der Römischer Magmatischen Provinz. Dies ähnelt anorogenen alkalischen Magmen und ist ein weiterer Hinweis auf eine Intra-Platten Signatur für diesen besonderen vulkanischen Komplex.

High-Mg subduction-related Tertiary basalts in Sardinia, Italy

Abstract The Oligo-Miocene volcanics (32-15 Ma), which occur in the Oligo-Miocene Sardinian Rift, were interpreted in the literature as an intracontinental volcanic arc built upon continental crust about 30 km thick. They are characterized by a close field association of dominantly andesites and acid ignimbrites, with subordinate basalts. In this paper we deal with the origin and evolution of recently discovered high-magnesia basalts aged ca. 18 Ma occurring in the Montresta area, northern Sardinia, relevant to the petrogenesis of the Cenozoic volcanics of Sardinia. The igneous rocks of the Montresta area form a tholeiitic, subduction-related suite. Major-element variation from the high-magnesia basalts (HMB) to high-alumina basalts (HAB) are consistent with crystal/liquid fractionation dominated by olivine and clinopyroxene. Proportions of plagioclase and titanomagnetite increase from HAB to andesites. Initial 87 Sr 86 Sr ratios increase with differentiation from 0.70398 for the HMB to 0.70592 for the andesites. This suggests concomitant crustal contamination. The geochemical characteristics of the high-magnesia basalts are typical of subduction-related magmas, with negative Nb, Zr and Ti spikes in mantle-normalized diagrams. It is proposed that these high-magnesia basalts were produced by partial melting of a mantle source characterized by large-ion lithophile elements (LILE) enrichment related principally to dehydration of subducted oceanic crust. Chondrite-normalized rare earth elements (REE) patterns indicate that the lavas are somewhat enriched in light rare earth elements (LREE), with flat heavy rare earth elements (HREE) patterns. This evidence is consistent with a spinel-bearing mantle source. The sub-parallel chondrite-normalized patterns show enrichment with differentiation, with a greater increase of LREE than HREE. The occurrence of high-magnesia basalts at 18 Ma in Sardinia appears to be correlated with and favoured by pronounced extensional tectonics at that time.

Petrogenesis of coexisting SiO2-undersaturated to SiO2-oversaturated felsic igneous rocks: The alkaline complex of Itatiaia, southeastern Brazil

Abstract The Itatiaia alkaline complex is a Late Cretaceous intrusion (72 Myr) made up of felsic differentiates, with syenitic rocks dominant throughout and with presence of both nepheline- and quartz-rich varieties. Dykes with phonolitic or trachytic composition cross-cut the coarse-grained facies. The rocks are arranged concentrically, with the core of the complex being formed by SiO 2 -oversaturated syenites (with a small outcrop of granites), and are radially displaced by faults related to regional tectonic lineaments. The minerals show gradual but significant changes in composition (salitic and augitic to aegirine-rich pyroxenes, hastingsite and actinolite to richterite and arfvedsonite amphiboles, sodic plagioclase to orthoclase feldspars and so on) and the whole-rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal. Sr-isotopic data indicate more radiogenic ratios for the SiO 2 -oversaturated rocks (0.7062-0.7067 against 0.7048-0.7054 for the SiO 2 -undersaturated syenites), consistent with small amounts of crustal input. The favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently SiO 2 -undersaturated mafic parental magmas (potassic alkali basalts to ankaratrites, present in the Late Cretaceous dyke swarms of the area), accompanied by variable crustal contamination prior to the final emplacement. The lack of carbonatite as a significant lithotype, the potassic affinity of the Itatiaia complex, and the relatively high Sr-isotopic ratios match the characteristics of the other complexes of the Rio de Janeiro-Sa˜o Paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source.

The eruption of The Breccia Museo (Campi Flegrei, Italy): Fractional crystallization processes in a shallow, zoned magma chamber and implications for the eruptive dynamics

Abstract The Breccia Museo Member (BMM) was formed by an explosive eruption that occurred in the SW sector of Campi Flegrei about 20 ka ago. The eruptive sequence consists of the Lower Pumice Flow Unit and the overlying Upper Pumice Flow Unit with its associated lithic Breccia Unit. Interlayered with the Breccia Unit is a welded deposit that mainly consists of spatter clasts (Spatter Unit). The products of this eruption range in composition from trachytic to trachyphonolitic with K 2 O decreasing from 9.5 to 7 wt.%; Na 2 O correspondingly increases from 2.6 to 7.2 wt.% with increasing differentiation (Nb from 23 to 122 ppm). The phenocrysts are mostly sanidine (Or 88-63 ) with subordinate plagioclase (An 33-27 ), clinopyroxene (Ca 47 Mg 44 Fe 9 to Ca 46 Mg 35 Fe 19 ), biotite, titanomagnetite, and apatite. The observed major- and trace-element variations are fully consistent with about 80% fractional crystallization of a sanidine-dominated assemblage starting from the least differentiated trachytes. The compositions of the erupted products are compatible with the progressive tapping of a shallow magma chamber that was thermally and chemically zoned. The incompatible trace elements indicate a slightly different magma composition with respect to trachytes of the Campi Flegrei mainland. The geochemical stratigraphy suggests that after an early eruptive phase during which the upper, most differentiated level of the magma chamber was tapped, the sudden collapse of the roof of the reservoir triggered drainage of the remaining magma, which ranged in composition from trachyte to trachyphonolite, and formed the Breccia Unit and the Upper Pumice Flow Unit. The strongly differentiated trachyphonolite composition of the spatter clasts also suggests that they likely originated from the uppermost part of the reservoir soon after the eruption of Lower Pumice Flow Unit and the collapse of the chamber roof. This is in agreement with the eruptive model proposed by Perrotta and Scarpati (1994).

Crystal chemistry of clinopyroxene from alkaline undersaturated rocks of the Monte Vulture Volcano, Italy

Abstract The Monte Vulture is a Late Pleistocene stratovolcano, composed of highly undersaturated alkaline potassic to ultrapotassic rocks belonging to the Roman Magmatic Province. These rocks are notably richer in Na2O if compared to similar rocks of the Roman Province. Two distinct magmatic Series have been recognized: (1) feldspar-bearing series, ranging from basanite to phonolite, and a volumetrically subordinate (2) feldspar-free series, consisting of melilitite, melafoidite, and hauynophyre. The clinopyroxene compositions of the feldspar-bearing series ranges from diopside to ferro-salite (hedenbergite), and shows, from basanite to phonotephrite, increasing FeOtot, Al2O3, and TiO2 and decreasing of MgO contents. Clinopyroxene in basanites and tephrites has generally high Fe3+ contents, which is typical for clinopyroxene from the Roman Province. Clinopyroxene of the feldspar-free series shows a more restricted variation in MgO, and has often very high Al2O3, FeOtot, and TiO2 contents. In all the crystals examined the Al3+ content is high and is present mostly on the T site and for a minor part on the M1 site. The M1–O2 distance shows a good correlation with the RM13+ content; clinopyroxene from basanites (feldspar-bearing series) has the lowest RM13+ and that from melilitite and hauynophyre (feldspar-free series) the highest RM13+ contents. Clinopyroxene crystals from feldspar-free rocks have smaller M1 and larger T polyhedral volumes when compared to those in olivine–melilitites and melilitites (kamafugites) from Umbria, but they show similar polyhedral volumes as clinopyroxene crystals from leucite-bearing rocks. Although clinopyroxene from feldspar-free rocks has small M1 volumes due to the high R3+ contents, M1 volumes of clinopyroxene from melilitites are larger than expected because of the higher (Fe3+/Al3+)M1 values. This larger M1 volume of clinopyroxene in melilitites causes a shortening of and a lengthening of the M2–O3 distance. Strong compositional and structural similarities between clinopyroxene of Monte Vulture and Leucite-bearing rocks of the Roman Province (plagioclase-bearing High Potassium Series=HKS) indicate a common petrogenetic affinity. On the other hand, differences between clinopyroxene in feldspar-free rocks from Monte Vulture and that in kamafugites (i.e, olivine melilitites, kalsilitites) from Central Italy, suggest significant magma dissimilarities between these two groups.

The transition from alkaline to tholeiitic magmas: a case study from the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy)

Abstract During the Pliocene, simultaneously with the opening of the Tyrrhenian Sea, mafic magmas were erupted in NE Sardinia (Orosei-Dorgali area). These range from mildly alkaline with sodic affinity (about 80% of exposure) to tholeiitic (about 20%). The tholeiitic rocks (basaltic andesite) are slightly more evolved than the alkaline ones and show geochemical features (e.g., Mg# 20). Similar incompatible element ratios for both alkaline and tholeiitic rocks suggest different degrees of melting of a single mantle source. Mathematical modeling indicates ∼4–6% and ∼10–15% partial melting for alkaline and tholeiitic lavas, respectively. Trace element abundances of the Orosei-Dorgali volcanic rocks are typical of Plio-Pleistocene volcanic rocks of Sardinia but differ strongly from other Cenozoic anorogenic volcanic rocks of Europe. Similarly, Sr ( 87 Sr/ 86 Sr=0.70442–0.70455), Nd ( 143 Nd/ 144 Nd=0.512465–0.512558) and Pb ( 206 Pb/ 204 Pb=17.74–17.86; 207 Pb/ 204 Pb=15.53–15.60; 208 Pb/ 204 Pb=37.89–38.02) isotopic ratios are very unusual when compared with other Cenozoic European volcanic rocks. Trace element abundances and isotopic composition of the Orosei-Dorgali volcanic rocks suggest a lithospheric mantle origin.

Petrology and mineralogy of wollastonite- and melilite-bearing paralavas from the Central Apennines, Italy

Abstract Small outcrops of wollastonite- and melilite-bearing pyrometamorphic rocks (paralavas) are found along the Apennine chain in Central Italy at the localities of Colle Fabbri and Ricetto. These rocks have coarse- to fine-grained crystalline, spotted, and glassy textures. The Colle Fabbri rocks have abundant and ubiquitous wollastonite, with plagioclase ± clinopyroxene. Melilite is found only in the most Ca-rich and silica-poor samples. Garnet, perovskite, quartz, leucite, silica-rich glassy mesostasis, and ocelli filled by calcite are present locally. The Ricetto rock is made up of wollastonite and melilite with minor clinopyroxene set in a glassy matrix with a few crystals of restitic quartz. These rocks have variable contents of SiO2 (43 to 64 wt%) and CaO (37 to 3.7 wt%). The latter decreases with increasing MgO, Al2O3, Fe2O3tot, TiO2, K2O, SiO2, and most trace elements, excluding Sr. Peculiar parageneses (e.g., coexisting melilite and anorthite in the most silica-poor samples, abundance of wollastonite), mineral compositions (gehlenite-rich melilite; Al-, Ti-, and Fe3+-rich clinopyroxene; Al-, Ti-rich, but Fe-poor garnet), and major- and trace-element geochemistry suggest that the Colle Fabbri and Ricetto rocks are not ultimately mantle-derived, but are the result of melting, devolatilization, and recrystallization of marly sediments (50-60 to 5-10% calcite, with clay minerals ± quartz), likely due to coal fires. The 87Sr/86Sr and 143Nd/144Nd range from 0.70772 to 0.71190 and from 0.51223 to 0.51219, respectively, again suggesting sedimentary protoliths of a mixed nature. Substantial chemical and isotopic differences between these samples and those from the Roman Magmatic Province are highlighted

Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar: implications for Late Cretaceous India–Madagascar reconstructions

Abstract: The St Marys Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites) emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88–90 Ma ago. The St Marys Islands rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible elements (e.g. Zr = 580–720 ppm, Nb = 43–53 ppm, La/Ybn = 6.9–7.2), relatively low initial 87Sr/86Sr (0.7052–0.7055) and near-chondritic initial 143Nd/144Nd (0.51248–0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites exposed between Vatomandry–Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other sectors of the Madagascan province. We therefore postulate that the St Marys and the Vatomandry–Ilaka–Mananjary silicic rock outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable tool to check and aid traditional Cretaceous India–Madagascar reconstructions based on palaeomagnetism, matching Precambrian geological features, and geometric fitting of continental shelves. Supplementary material: Mineral analyses, mass-balance calculations and locality information are available at http://www.geolsoc.org.uk/SUP18332.

Petrogenesis of Late Cenozoic mafic alkaline rocks of the Nosy Be archipelago (northern Madagascar): relationships with the Comorean magmatism

Abstract Results of a geochemical and isotopic (Sr-Nd-Pb) study of mafic volcanic rocks outcropping in the Nosy Be archipelago are reported. This volcanic area (10–7 Ma) is part of the igneous province extending throughout northern Madagascar. Lava flows and spatter cones are present in the northern and western sides of the Nosy Be island, while the central part is characterized by the presence of several tuff rings. The rocks range in composition from basanite to phonotephrite, and have phenocrysts of olivine, Cr-rich spinel and Ti-rich clinopyroxene. Kaersutite phenocrysts are present in the most differentiated rocks. Small ultramafic spinel-bearing xenoliths occur in the primitive lavas. The mafic rocks have Zr/Nb=3.1±0.3, (La/Yb)n=20.8, 87Sr/86Sr=0.70332–0.70366, 143Nd/144Nd=0.51280–0.51284, 206Pb/204Pb=19.39−19.41, 207Pb/204Pb=15.6; 208Pb/204Pb=39.3–39.4. The range of Zr/Hf (44–45) and Nb/U (47–49) is similar to the values of normal MORB and OIB worldwide. The geochemical and isotopic composition of the Nosy Be rocks is similar, but not identical, to that observed in the Comore Islands. The preferred genetic hypothesis for the Nosy Be basanites is low degree (4%–5%) melting of enriched subcontinental lithospheric mantle, likely within the spinel peridotite field. Late Pan-African Nd model ages obtained (≈550–610 Ma) could indicate the timing of the enrichment event. The mantle source of the Nosy Be volcanism was remobilized during rifting events which have occurred throughout Madagascar in recent times. These rifting events could have also triggered the formation of the large alkaline volcanic fields of Itasy and Ankaratra in central Madagascar.