CHRONOLOGY AND COMPOSITION OF VOLCANICLASTIC ASH LAYERS IN THE CENTRAL TYRRHENIAN BASIN (SITE 974) 1 Paul van den Bogaard, 2 Beate Mocek, 2 and Marianne Stavesan d 2 ABSTRACT Petrographic descriptions, major-element glass compositions, and laser 40Ar/39Ar ages are presented for Pliocene Pleistocene tephra layers and volcaniclastic sediments of cores from Hole 974B, drilled during Ocean Drilling Program Leg 161. Individual fallout ash layers and volcaniclastic turbidites show highly diagnostic glass particle morphologies and a wide spectrum of silicic to intermediate major-element compositions, ranging from low-K calc-alkaline, to high- K calc-alkaline, high-K shoshonitic, and to alkaline magma suites. High-K shoshonitic components and individual ash layers are derived mainly from explosive eruptions on Ponza (Pontian Islands) between 3 Ma and 0.8 Ma. Single- crystal and step-heating 40Ar/39Ar analyses of tephra layers from meters below seafloor (mbsf) (Sample B-4H-6, 6-7 cm) and mbsf (4H-2, cm) yield eruption ages of 0.51 ± 0.02 and 0.42 ± 0.04 Ma, respectively. Step-heating analyses of biotite crystals from volcaniclastic crystal-lithic sands yield ages from 74 to 291 Ma, which indicate a detrital origin from pre-alpine granitoids (Sardinia?). Six tephrostratigraphic markers are defined on the basis of glass shard compositions and morphologi e s.

Figure 2. A: Total alkali vs. silica classification diagram for Italian Plio- Quaternary magmatic rocks (Peccerillo, 2002). B: delta-Q vs. K 2 O/Na 2 O classification diagram for Plio- Quaternary mafic volcanic rocks (MgO > 4%) from Italy. Delta-Q is the algebraic sum of normative quartz (q), minus leucite (lc), nepheline (ne), kalsilite (kal) and olivine (ol). Silica oversaturated rocks have delta-Q > 0, whereas silica undersaturated rocks have delta-Q < 0. Click on figure for enlargement.

Figure 1: Distribution of Recent magmatism in Italy. Open symbols indicate seamounts. Ages (in Ma) are given in parentheses. Different colours denote various magmatic provinces. Inset: schematic distribution of orogenic and anorogenic volcanism: red arrows show migration of orogenic magmatism with time. Click on figure for enlargement.

Esercizio in aula (TAS Total Alkali vs Silica) Eolian samples Riempire, effettuando gli opportuni calcoli, le ultime tre colonne mancanti nella tabella fornita; Proiettare i dati relativi alle analisi chimiche sul diagramma binario Na 2 O+K 2 O vs SiO 2 fornito; Confrontare i diagrammi Si riscontra un possibile trend evolutivo?