Natale Calanchi

TEPHRA LAYERS IN LATE QUATERNARY SEDIMENTS OF THE CENTRAL ADRIATIC SEA

Abstract Magnetic susceptibility curves of ten cores, collected in the western Adriatic shelf and in the Mid-Adriatic Deep, allowed us to recognize thirty tephra layers whose age, based on physical and biostratigraphic data, encompass a time interval ranging from less than 70 ka to the Present. Chemical features of the tephra, defined by SEM–EDS analyses performed on glass shards, indicate alkaline affinity, mainly with K/Na>1, and trachytic compositions with two minor benmoreitic and phonolitic groups. These characteristics are consistent with Campanian and subordinate Etnean provenances. Because of chemical and age constraints the glass shards can be referred to six tephra layers already described in the literature, such as C20, C14 (Citara/Ischia), C10 (Campanian Ignimbrite), C2/NYT (Neapolitan Yellow Tuff), AMS/PF (Agnano Monte Spina/Phlegrean Fields) from Campanian area, and Y1 from Etna volcano, indicating an age ranging from about 70 to 4 ka. AMS/PF and C2/NYT, dated 4.4 and 12.3 ka respectively, are the most widespread tephra beds in the study cores. The occurrence of AMS/PF, never recorded in marine sequences up to now, represents a very good marker for Holocene Adriatic sediments; C2/NYT already recorded in the central Tyrrhenian Sea, in lacustrine sediments from southern Italy and in the Campanian area, is a good stratigraphic marker for inter-sea and sea–land correlations. Y1 tephra, dated 14.2 ka, also occurs in on-land sediments of central Italy and in marine sediments from the Tyrrhenian and Ionian seas: it now represents one of the most widespread stratigraphic markers in the central Mediterranean region.

Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian islands, Southern Tyrrhenian Sea)

The Pollara tuff-ring resulted from two explosive eruptions whose deposits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km3) consist of three main fall units (A, B, C) deposited from a subplinian column whose height (7–14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1–8x106 kg/s). A highly variable juvenile population characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO2=50–55%) almost exclusively form A deposits, whereas grey, mildly vesiculated, mildly porphyritic pumice (SiO2=56–67%) and white, highly vesiculated, nearly aphyric pumice (SiO2=66–71%) predominate in B and C respectively. Mafic cumulates are abundant in A, while crystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal fractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractionated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts within rhyolitic glasses. Viscosity and composition-mixing diagrams show that blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic magma rising through the metamorphic basement enterrd a mafic magma chamber whose souter portions were occupied by a highly viscous, mafic crystal mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the walls of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic and silicic melts, blended liquids rapidly formed. 4. When fractures reached the surface, the eruption began by the ejection of the mafic melts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rhyolitic magma (C).

The Strait of Sicily continental rift systems: Physiography and petrochemistry of the submarine volcanic centres

Abstract New morphological, geophysical, mineralogical and petrochemical data on the submarine volcanic centres of the Strait of Sicily continental rift systems lead to the following conclusions: Magma types both in the rift valleys (Pantelleria and Linosa) and plateau areas (Adventure Bank) suggest mantle sources and fractionation processes comparable to those of the Ethiopian Afar section of the East African Rift, despite their different tectonic regimes. However, the volume of the erupted magmas (less than 2000–3000 km 3 ) indicates the similarity with low-volcanicity (dry) rifts. This is probably related to the peculiar tectonic setting of the Pelagian Block (African Foreland) dominated by extensional transcurrent motions connected with a strong Neogene compressive dextral E W megashear along the Sicilian collisional front. In the plateau regions, the most evolved rocks are hawaiites (Graham Bank) which formed by fractionation in the uppermost mantle, probably at the base of the crust. Magma types vary from quartz tholeiites to basanites. In the rift valleys, the range of magma types is more restricted (transitional to alkali basalts) and a considerable volume of highly evolved magmas, particularly in the axial volcanic complex of the Pantelleria Rift, was produced in crustal magma chambers. Isotopic trace-element evidence suggests the involvement of transitional to enriched MORB- and OIB-type mantle sources in the genesis of the quartz tholeiites, alkali basalts and basanites throughout the Strait of Sicily continental rift system, including the Iblean Plateau.

Glass chemistry in volcaniclastic sediments of ODP Leg 107, Site 650, sedimentary sequence: provenance and chronological implications

Abstract A detailed chemical investigation of volcanic glass fragments from volcaniclastic strata (6 tephras, 1 volcanic debris flow, 12 volcanic turbidites) of ODP Leg 107, Site 650, sedimentary sequence, leads to a varied pattern in terms of both provenance and age constraints. The six analyzed tephra strata indicate a provenance from at least three different volcanic provinces: Aeolian, Campanian, and Sicilian Channel (Pantelleria Island). The older tephra strata (021, 018, 012) have a large amount of “orogenic” rhyodacite/rhyolite deposits that may be attributed to the Aeolian province, although no subaerial coeval volcanic activity of similar composition has so far been documented in the Aeolian Arc. Tephra 007 is related to the Pantelleria Island activity and, particularly, to an ignimbrite episode dated circa 130 ka. Tephra strata 005 and 003, have a clear Campanian provenance, and are correlated with analogous tephra layers, observed in the Tyrrhenian and Ionian seas, dated circa 107 and 60 ka respectively. In the oldest portion of the sequence (from 1.3 to 0.13 Ma), the volcaniclastic sediments were only derived from the Aeolian domain whereas in the latest 130 ka, the Campanian influx becomes much more predominant. Therefore, a general K-enrichment trend is observed in the temporal sequence of all the analyzed samples (almost 700 point analyses) which may be related both to a variation in the source area and to the specific Pleistocene magmatic evolution of the peri-Tyrrhenian volcanic provinces.

Chemostratigraphy of Lago Albano sediments (Central Italy):geochemical evidence of palaeoenvironmental changes in lateQuaternary

The results of the geochemical investigation on bulk sediment fromthree cores collected in Albano crater lake (Central Italy) are reported, andthe main markers of the palaeoenvironmental changes in the last 26ka are discussed. The sediment of Lago Albano consists of greysilt and mud, and is made of volcanogenic, calcareous, siliceous and organicmaterial. Some tephra layers provide a chronological framework for thesequence. The chemical features of the sediments are strongly impacted by theAlban Hills volcanism (Roman volcanic area), but there are also strong changesof organic/inorganic matter ratios, owing to variations in biologicalproductivity and terrigeous supply from the catchment. Six chemostratigraphiczones record the main steps of the transition from the cold and dry climate ofGlacial Maximum to the warmer and more humid Holocene climate. The most usefulgeochemical indices are: biogenic silica, CO2, Br and organiccontent (OM) for biological productivity; Al, Y, CIA (Chemical Index ofAlteration), Al/Rb, Ti/Zr and Y/Al ratios for terrigenous clasticmaterial; OM/Al ratio for organic/terrigenous ratio; S/Fe andMo/Fe ratios and Authigenic U for redox diagenetic conditions of the lakesediment. The geochemical records agree rather well with those of thelithological and paleomagnetic studies, and despite the information obtainedare less detailed than those acquired by the study of biological remains, thepalaeoenvironmental significance inferred is consistent. A comparison of thechemostratigraphic results of Lago Albano with those obtained on coevalsediments from Central and Southern Italy lakes supports the role of thegeochemical investigation as important complement to more sophisticatedtechniques in the palaeoenvironmental reconstructions.

Late-Quaternary ancient shorelines at Lipari (Aeolian Islands): stratigraphical constraints to reconstruct geological evolution and vertical movements

Indicators of three fossil shorelines, located at elevations of 43–45 m asl (I order), 23–27 m (II order) and about 12 m (III order), are recognized on the island of Lipari. Detailed evaluation of the stratigraphic relationships to the volcanic substratum allows their correlation with Late-Quaternary eustatic highstands of Tyrrhenian age, corresponding to marine oxygen isotope substages 5e (124 ka), 5c (100 ka) and 5a (81 ka). Marine deposits related to the ancient shorelines represent useful stratigraphic markers, as related unconformities constrain the geological evolution of Lipari. This is interpreted as a result of two main stages of volcanic activity (pre- and post-Tyrrhenian) spaced out by one stage characterized by prevalent marine erosional episodes (Tyrrhenian). Chronological and altimetrical data concerning the ancient shorelines have been used to estimate the vertical mobility of the volcanic edifice of Lipari, which is characterized by a continuous uplift at an average rate of 0.34 mm/a during the last 125 ka: this estimation is fully concordant with the values obtained, during the same time period, from the nearby islands of Filicudi (0.31 mm/a) and Salina (0.36 mm/a), suggesting a similar uplift trend induced by the prevalence of regional tectonic processes.

The volcanic rocks from the Mount Agnello area (Fiemme Valley, Italy): A contribution to the knowledge of the mid-Triassic magmatism of the Southern Alps

Middle Triassic volcanic rock outcrops in the Mount Agnello area (Fiemme Valley, Italy) are examined in this study. Chemical analyses based on main and trace elements (Rb, Sr, Zr, Y, Nb) allow this volcanism to be defined as “shoshonitic”. The amphiboles, which occur with thick opacitic rims, were determined as ferroan pargasites. They were often found in volcanites of calcalkaline and/or shoshonitic associations. Consequently, this volcanism can be associated with a newly orogenized area undergoing stabilization. These data support the more recent studies on the mid-Triassic volcanism in the Southern Alps and underline the particular position of this magmatism in the tectonic evolution of this area.