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19. Oceanic Basalts from the Tyrrhenian Basin, Dsdp Leg 42A, Hole 373A

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19. Oceanic Basalts from the Tyrrhenian Basin, Dsdp Leg 42A, Hole 373A OCEANIC BASALTS, HOLE 373 19. OCEANIC BASALTS FROM THE TYRRHENIAN BASIN, DSDP LEG 42A, HOLE 373A Volker Dietrich, Institut für Kristallographie und Petrographie, ETH Zurich, Switzerland, Rolf Emmermann and Harald Puchelt, Institut für Petrographie und Geochemie der Universitat, Karlsruhe, Germany, and Jörg Keller, Mineralogisches Institut der Universitat, Freiburg/Br., Germany ABSTRACT During Leg 42A, drilling in Hole 373A in the Central Tyrrhe- nian Sea penetrated Miocene basaltic basement for 190 meters and recovered basalt from up to 450 meters below the sea floor. Despite mineralogical and chemical alteration due to hydrothermal processes in the Tyrrhenian marginal basin, petrological analogies to high-Al abyssal basalts can be traced. Two types of interlayered basalt were recognized: (a) a lower Ti group (Core 5, Sections 1-2; Cores 7 and 10), and (b) a higher Ti group (Core 5, Section 3; Cores 11 and 12; and outer barrel). We distinguish two magma series: a more primitive magma leading to the plagioclase-bearing low-titanium basalts with affinities to high- Al abyssal tholeiites, and a more differentiated magma leading to the basalts with higher titanium content. INTRODUCTION widening basins. In this context Boccaletti and Guaz- zone (1972), Dewey et al. (1973), and Barberi et al. During DSDP Leg 42A, basalt was recovered from (1973) have compared the Tyrrhenian Basin with Hole 3 73A in the abyssal plain of the Central Tyrrhe- formerly spreading but now inactive marginal basins nian Sea (Figure 1). Basaltic breccia was first encoun- which nevertheless still have high heat flow values tered under a Quaternary and Pliocene sediment cover (Karig, 1971). about 270 meters below the sea floor. Then it was The deep-focus earthquakes which occur in the drilled 190 meters through a basaltic sequence of southeastern Tyrrhenian Sea and the calcalkaline mag- breccia and lava flows and took 11 cores at depths as matism which occurs in the Aeolian Islands are charac- indicated on Figure 2. teristic of an active island arc (Barberi et al., 1973; Site 373A is on the lower flanks of one of the large Keller, 1974). The age of the exposed island arc Tyrrhenian seamounts (Figure 1) and the basalts volcanic rocks, however, is less than 1 m.y.B.P. (Bar- belong to the acoustic basement (Site 373A report, this beri et al., 1974), whereas the Tyrrhenian Basin volume). Assuming that the age of the overlying formed about 10 m.y. ago. The high heat flow values sediments is its upper limit, the basaltic, basement is and a rather indistinct pattern of magnetic anomalies probably lower Pliocene/upper Miocene (Barberi et in the Tyrrhenian Basin suggest that it was a spreading al., 1977, Kreuzer et al., 1977). marginal basin (Ryan et al., 1970). But the pattern of the Tyrrhenian spreading centers that can be inferred GEODYNAMICS OF THE TYRRHENIAN SEA from the magnetic anomalies does not exhibit any The formation of small ocean basins in the western simple regularity. The picture is certainly complicated Mediterranean has caused considerable discussion (see by the rotation of individual crustal blocks such as Ritsema, 1970) since the discovery of their youthful Sardinia-Corsica, the Italian penisula, and smaller age and the nature (oceanic to transitional) of the crust microplates (Alvarez et al., 1974; Lowrie and Alvarez, and mantle beneath became evident (Menard, 1967). 1974). Spreading centers may have developed between Early geological work in the borderlands of the Tyrre- such rotating blocks and the existence of areas with nian Sea had revealed the existence of a former continental crust within such a domain is possible. landmass in the area presently occupied by the basin. (Honnorez and Keller, 1968; Heezen et al., 1971). Evidence of currents and clastic sedimentary compo- The main purpose of drilling in the Tyrrhenian nents in the sedimentary sequence point to the exis- basement was to test whether the chemical characteris- tence of such a landmass until Miocene time. Follow- tics of abyssal basalts, formed at oceanic spreading ing the earlier concept of the tectonic break-up of a centers or their marginal basin analogs, are present. If Tyrrhenian continent, several other concepts involving so, we could conclude that upper mantle processes complicated oceanization models were presented. similar to those occurring under mid-ocean ridges have These are reviewed by Ritsema (1970). acted as well, in the Tyrrhenian Basin. The plate tectonics concept invokes spreading Dredge samples of basalt from the Tyrrhenian processes with formation of new basaltic crust in seamounts (mainly from Mount Marsili) were availa- 515 V. DIETRICH, R. EMMERMANN, H. PUCHELT, J. KELLER 50 100 150 200 —--- +gravimetric mGal anomaly (Bouger, d = 2,67) ..-•• \. Cenozoic and Quarternary •...: volcanics • DSDP Leg 42 A, Drill site 373 A Figure 1. The Tyrrhenian Basin. ble for study (Maccarone, 1970; Del Monte, 1972; chloritic minerals, or both. This matrix constitutes up to Keller and Leiber, 1974) (Figure 1). These basalt 30% of the rock volumes. X-ray analyses, with use of samples are mostly alkaline. No data were published to ethylene glycol treatment, on three samples showed the support a more detailed characterization (for example, typical shift of the 100 montmorillonite peak from 14 to designation as "abyssal tholeiite") for there were few 18Å. A chlorite + montmorillonite mixture is abundant samples with tholeiitic affinity (e.g., No. 29M of Del in two of the samples (7-2, 136-140 cm, and 7-3, 111- Monte, 1972). Nevertheless, such designation was wide- 112.5 cm). Some smectite patches, clearly pseudo- ly quoted to support the existence of spreading precesses morphs after olivine and very rarely occurring olivine within the Tyrrhenian Sea. Moreover, seamount relics are preserved (e.g., 7-4, 135-140 cm, and 7-5, 10- volcanism apparently occurred much later than basin 14 cm). However, the extent of original modal olivine formation and lasted until the upper Quaternary (Keller remains uncertain. and Leiber, 1974). Two petrographic types of basalt are recognized; (1) plagioclase phyric and (2) aphyric to sparsely micro- PETROGRAPHIC DESCRIPTION OF ANALYZED phyric. This distinction corresponds to a chemical SAMPLES grouping explained below (Table 4). Most basalt samples are well crystallized and have Plagioclase phenocrysts are calcic with compositions intergranular textures of fresh plagioclase, clinopyrox- ranging up to An84. Groundmass plagioclase ranges ene, and titanomagnetite in a matrix of smectite, mainly from An50.60 (Figure 3, Table 1). Along with 516 OCEANIC BASALTS, HOLE 373 Tyrrhenian Sea 27-29 April 1975 LITHOLOGIC mineralogy as described for oceanic basalts by Mi- UNITS 39°43.68'N, 12°59.56'E e yashiro et al. (1971) is apparent. ^744.5 m Pliocene marls Cor Rec<>very In general, however, groundmass clinopyroxenes are 2 basaltic breccia 3 A A A A fresh. Their composition (Table 2, Figure 4) is salitic III ?pillow breccia with SAMPLES ROCK TYPE calcareous matrix 4 A A A A and similar to the clinopyroxenes reported by Ridley et 300 5-1,68-77 low Ti basalt al. (1974). Iron enrichment is present but not pro- 5-1,90-93 low Ti basalt basaltic breccia and 5 AÀJ•• 5-2, 35-38 low Ti basalt nounced. No Ca-poor pyroxene has been detected. pillow breccia with 5-2, 53.5-57 low Ti basalt 5-3, 74-75.5 high Ti basalt interlayered 1 Calcite occurs as veinlets, vesicle fillings, and amygdalar and 5-3, 129-131 high Ti basalt porphyritic basalts 6-1,72-75 breccia patches within the groundmass and plagioclase pheno- 6 6-1,96-100 high Ti basalt crysts. All samples analyzed contain only minor . 6, CC high Ti basalt '7-1, 139-144 low Ti basalt amounts of calcite. Zeolites replace plagioclase and fill 7-2, 136-140 low Ti basalt vesicles in the most altered samples, but these are not 7-3, 111-112.5 low Ti basalt 7 7-4, 19-23 low Ti basalt suitable for chemical analysis (5-1, 68-71 cm; 5-2, 35- 7-4, 135-140 low Ti basalt '7-5,10-14 low Ti basalt 38 cm; and 6-1, 133-136 cm). 8 GEOCHEMICAL METHODS AND DISCUSSION basaltic flows, porphyritic OF RESULTS 9 — 10-1, 123-125 10 r-11-1, 114-117 high Ti basalt Twenty samples were analyzed by X-ray fluores- 11 —11-1, 130-135 high Ti basalt 450 —12, CC high Ti basalt cence, atomic absorption, ionic neutron activation, and T. D. = 3964.5 m 12 •OB 1 . high Ti basalt wet chemical methods to determine their major and trace .OcBJ outer barrel hi^n Ti basalt element contents. All major element determinations Figure 2. Graphic lithology of Site 373A. The drilling were made at least three times by each of the different started at a depth of 96.5 meters sub-bottom within laboratories involved (Freiburg i.Br., Karlsruhe, Zu- Quaternary Nannofossil marls (lithologic Unit I; Site rich, and Canberra). The results given in Table 3 are in 373A Report, this volume). At 270 meters, Glomar good agreement. Determinations were carried out by: Challenger drilled into basaltic breccias (containing pil- a) XRF for SiO2, TiO2, A12O3, Fe2O3 (total), low fragments) associated with lava flows. The drilling MnO, MgO, CaO, and K2O; was terminated at 457.5 meters in basaltic lava flows. b) AAS for Na2O, K2O, CaO, MgO, Fe2O3 (total), Twenty basaltic samples from six cores out of Units IV and A12O3; and V were analyzed. c) INAA for Na2O, K2O, CaO, MnO, and Fe2O3 (total); these groundmass labradorites, almost pure albite d) Wet chemical methods for SiO2, TiO2, FeO, (with an increased orthoclase component) has been P2O5, H2O+, and CO2; found by microprobe analysis. This suggests the begin- e) Electron microprobe analysis (by J.
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