O Zeda

Clinopyroxene composition of ophiolite basalts as petrogenetic indicator

Abstract A statistical microprobe study has been carried out on augitic clinopyroxene in high-Ti to low-Ti metabasalts from various Phanerozoic ophiolites; a comparison has been made with clinopyroxenes from mid-ocean ridge basalts (MORB), oceanic intraplate basalts, island-arc tholeiites and boninites. Leaving aside the effect of cooling rate and of fractional crystallization as major factors influencing pyroxene chemistry, their composition appears to be essentially related to the type of host magma. Both the TiO2 and the TiO 2 Al 2 O 3 in clinopyroxenes are positively correlated with the Zr/Y ratios of the host basalts, which is a diagnostic parameter insensitive to alteration increasing regularly from low-Ti to high-Ti ophiolitic basalts. Ti also shows a positive correlation and average D Ti Cpx rock partition coefficients varying from 0.3–0.6 to 1 from low-Ti to high-Ti basalts, which compare favorably with those from Tongan (southwest Pacific) basaltic andesites (0.3–0.6) and MORB (0.75), respectively. Likewise the average K Fe Mg calculated for clinopyroxene-rock pairs varies from 0.20 in low-Ti basalts to 0.26 in high-Ti basalts up to 0.33 in the highest-Ti rocks, suggesting a significant dependence of this parameter on magma composition. The compositional correspondence of clinopyroxenes from various ophiolitic basalts with those from oceanic actualistic equivalents is shown in various diagrams. Concomitantly with increasing TiO2 and Na2O and decreasing SiO2 contents, clinopyroxenes from very low-Ti to high-Ti ophiolitic basalts are best equated with those from boninites, basaltic andesites and tholeiites from island arcs, and MORB, respectively. Accordingly, a parallel increase is shown by the substitutional molecules CaTiAl2O6 and NaTiSiAlO6. Within each basalt type, fractional crystallization processes are reflected in the NaTiSiAlO6 and FeTiAl2O6 increase and MgCrSiAlO6 decrease from early to late crystallized clinopyroxenes.

Petrology of lherzolitic rocks from the Northern Apennine ophiolites

Abstract Mineral and bulk-rock chemistry investigations on upper mantle peridotites from the Northern Apennine ophiolitic terrains indicate that significantly different evolution have been suffered by the ultramafic bodies related to different structural and palaeogeographic settings. External Ligurides (EL) lherzolites outcropping as olistoliths within sedimentary sequences of pericontinental basins, show slightly depleted characters and are mostly associated with transitional MOR-type basalts and with reworked acidic continental material. Internal Ligurides (IL) lherzolites, representing the base of the Northern Apennine ophiolite sequences, show a significant residual character and are generally associated with normal MORB. The most depleted ultramafics from the Internal Ligurides could represent residues after extraction of the associated normal MORB, starting from an already slightly depleted mantle source. Such MORB-producing events probably occurred at shallow depth (in a region of transition from spinel to plagioclase facies at about 9 kb) during adiabatic upwelling of differently depleted mantle diapirs along diverging plate margins. EL lherzolites evolved under comparably lower temperature conditions and were emplaced at shallow levels prior to the IL lherzolites, which followed higher-temperature adiabatic paths, more typical of a rising limb of a mantle convective system.

Structural setting and magmatic evolution of Montefiascone volcanic complex, vulsini district, central Italy

Abstract A comparative study concerning volcanological, petrographic and geochemical aspects of Montefiascone Volcanic Complex has been carried out. This complex, together with those of Paleo-Bolsena, Bolsena and Latera, forms the Vulsini Volcanic District, located in the northernmost part of Roman Comagmatic Province. Four groups of lavas, belonging to Montefiascone Volcanic Complex, have been recognized on petrographic and geochemical grounds: Tephritic Leucitites (TEL), Leucite-Bearing Basanites (LBSN), Evolved Tephritic Leucitites (ETEL) and Leucititic Tephrites (LT). Lavas clearly show Ti and Nb anomalies, further supporting the genesis of these magmas from a subduction-related metasomatized mantle source. The majority of the lava erupted, comparable in volume with the pyroclastic products, appears to be of leucititic affinity, having suffered only a slight amount of differentiation. Major- and trace-element compositions very similar to those of ETEL and LT have been obtained from TEL and LBSN by means of simple mass balance and Rayleigh fractional crystallization model. According to petrographic observations, main liquidus phases are clinopyroxene, leucite, olivine and magnetite, and magmatic differentiation acts essentially by fractional crystallization. Eruptive activity is spread over a large area and the volcanic centres are aligned following NNW-SSE, WNW-ESE and E-W directions. Explosive activity, on the other hand, was mainly located at Montefiascone Caldera. The great abundance of nearly primary magmas in the area leads us to think that the structural setting was not suitable for developing a great, unique magma reservoir, but rather a series of small, elongated bodies.

Harzburgite-lherzolite xenoliths and clinopyroxene megacrysts of alkaline basic lavas from Sardinia (Italy)

Abstract Spinel peridotite xenoliths commonly occur in Pliocene-Quaternary alkaline basic lavas (basanite, alkali basalt, hawaiite and trachybasalt) that crop out in various areas of the island of Sardinia (Italy). They are sometimes associated with clinopyroxene megacrysts, particularly in basanite and hawaiite lavas. A study of their mineral and whole-rock chemistry has been used to place constraints on their origin and history of ascent. Most xenoliths are mantle harzburgites or lherzolites although wehrlites, pyroxenites and gabbroic nodules are also found. The harzburgites and lherzolites are characterized by protogranular texture, large, variable grain size (0.1–1 cm), and a typical four-phase assemblage: olivine (70–85 vol.%, Fo 89.0–91.3 , unzoned), orthopyroxene (10–20 vol.%, En 87.4–90.8 , unzoned), clinopyroxene (Cr-diopside 3–12 vol.%) and chrome spinel [0.5–2 vol.%, Cr ( Cr+Al ) = 0.09–0.56 ]. Kink bands in olivine and orthopyroxene and rare narrow exsolution lamellae of clinopyroxene from orthopyroxene are present. The application of various geothermometers, based on the equilibration of clinopyroxene-orthopyroxene pairs, gives values that range from 1000° to 1190°C. The mineralogical and petrochemical characteristics of peridotite xenoliths indicate a generally more depleted nature than the inferred mantle source of the host basic magmas. Therefore, they represent refractory fragments of the uppermost subcontinental mantle accidentally incorporated by volatile-rich alkaline magmas during upwelling. Thermobarometric evaluations suggest a minimum provenance depth of harzburgite-lherzolite xenoliths of 30–40 km just below the Moho discontinuity, which is found in Sardinia at a depth of 30 km, according to geophysical data. Similar high-crystallization depths are recorded for the Al-augite megacrysts, which represent a high-pressure liquidus phase in the alkaline mafic magmas.