Luigi Franciosi

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.

Geochronology, Geochemistry and Geodynamics of the Cabo de Gata volcanic zone, Southeastern Spain

New 40Ar/39Ar ages and major and trace element geochemistry ofthe middle-late Miocene Cabo de Gata volcanic complex, southeastSpain, indicate that the volcanic activity of the Cabo de Gata volcaniczone developed over a short period through several pulses of geochemicallyand isotopically different parental magmas. The oldestvolcanic rocks exposed in the Cabo de Gata volcanic zone are theshoshonite and high-K calc-alkaline rocks of Bujo group, which cry -stallised from a parental magma transitional from calc-alkaline toalkaline potassic generated through large degrees of partial melting,and then affected by a minor contribution from metasomatised veinsand a larger one from the surrounding mantle wedge, in comparisonto ultrapotassic melts. Subsequent partial melting of the mantlesource produced typical calc-alkaline parental magmas belonging tothe Rodalquilar and Agua Amarga groups. Sr-Nd-Pb isotope andincompatible trace element distributions of Cabo de Gata rocks arein agreement with a mantle-wedge source affected by a two-foldmetasomatism. The data suggested that mild potassic to sub-alkalinesubduction-related parental magmas (i.e., high-K calc-alkaline andcalc-alkaline) were generated in the Cabo de Gata sector within amantle wedge metasomatised by a fluid-dominated agent. In contrast,the enrichment in K2O of shoshonitic to ultrapotassic magmaswas achieved through recycling of subducted sediments throughmelts that enriched the mantle wedge in K and related elements.Such a scenario can be easily reconciled with a geodynamic settingat the edge of a destructive plate margin with the subducted slabresponsible for the recycling of sediments within the mantle wedge.

The mafic alkaline volcanism of SW Madagascar (Ankililoaka, Tulear region): 40Ar/39Ar ages, geochemistry and tectonic setting

High-precision 40Ar/39Ar ages, major and trace element, and radiogenic isotope data are presented for the basanites and alkali basalts forming the southwesternmost monogenetic volcanic field in Madagascar. The volcanic rocks were erupted along fissure zones and aligned cones in a nearly flat area covered by the Cenozoic sediments of the Morondava basin. The high-precision 40Ar/39Ar ages constrain the beginning of the magmatism in the Ankililoaka area to about 12 Ma, significantly earlier than suggested by previously published K/Ar ages. The Ankililoaka basanites include primitive compositions (MgO >10 wt%, Ni >200 ppm and Cr >400 ppm), whereas other basanites and alkali basalts experienced limited removal of olivine, chromiferous spinel and clinopyroxene. Initial Sr and Nd isotope ratios of the basanites are 0.70343–0.70445 and 0.51279–0.51282, respectively. The Pb isotope compositions are in the ranges 206Pb/204Pb = 19.08–19.38, 207Pb/204Pb = 15.61–15.64 and 208Pb/204Pb = 39.1–39.4. The alkali basalts have similar 87Sr/86Sr, 143Nd/144Nd and 207Pb/204Pb, but slightly lower 206Pb/204Pb and 208Pb/204Pb than the basanites. The isotopic composition of the Ankililoaka rocks partially overlaps with that of the Cenozoic volcanic mafic rocks of northern Madagascar, and differs significantly from that of the mafic volcanic rocks of central Madagascar, which have lower 206Pb/204Pb and 207Pb/204Pb, and higher 87Sr/86Sr. Major and trace element systematics and geochemical modelling suggest that the Ankililoaka mafic alkaline rocks are low-degree melts of an incompatible element enriched peridotite source starting from depths where garnet is stable. Crustal contamination during ascent was insignificant. We argue that the genesis of the Ankililoaka alkaline magmas was triggered by melting an enriched, volatile-rich lithospheric mantle uplifted in the Cenozoic. Supplementary material: Analytical techniques, X-ray fluorescence whole-rock data, mineral compositions, 40Ar/39Ar dataset, mantle source modelling and figures showing volcanological features and thin sections are available at https://doi.org/10.6084/m9.figshare.c.4065743