117,546 research outputs found
Modelling the petrogenesis of the Ethiopian-Yemeni picrite-basalt CFB association: inferences on mantle heterogeneities and plume processes
No full textThe Oligocene Northern Ethiopian-Yemeni LIP, represented by a CFB plateau extending ca. 700 km in diameter, is characterized by a well-defined zonal arrangement with increasing plume-related physico-chemical features of erupted magmas, such as thermal regime, incompatible element enrichment and specific Sr-Nd-Pb-He isotopic fingerprint, from
the periphery to the central plateau area (Beccaluva et al., 2009). Two CFB volcanic piles in the Lalibela district (Northern Ethiopia, ca. 2 km thick) and in the Manakhah section (Northern Yemeni plateau, ca. 1 km thick) which erupted close to the Oligocene Afar plume axis, are similarly characterized by very high-Ti transitional basalts and picrites (HT2, Beccaluva et al., 2009; 2011) that account for ca. 13% (40,000 km3) of the total Ethiopian-Yemeni CFB lavas. These magmas are characterized, in addition to the extremely high TiO2 content (3-6 wt.%) by a high MgO content (mostly between 8 and 18 wt.%), and show striking compositional analogies with those from the Karoo province and the Siberian meimechites (Ellam & Cox, 1991; Heinonen et al., 2014). Petrological modelling based on whole rock FeO-MgO and Ol composition (Herzberg et al., 2007) indicates that some of picrites (MgO 16-17 wt.%) are near-primary magmas with olivine phenoscrysts up to Fo 90.4. Calculation shows that the primary melts have picrite composition MgO 19.8-20.7 wt.% and were generated by polybaric melting in the pressure range 3-4 GPa at a potential temperature of 1570 °C. Together with high-MgO lavas from Hawaii and Gorg
ona, these are the highest temperatures of any OIB and LIP lavas. The available data suggest that HT2 magma sources necessarily require the involvement of specific high-Ti (and Fe) deep-seated sublithospheric components which were entrained and remobilized by the rising plume. References: Beccaluva L., Bianchini G., Natali C. & Siena F. 2009. Continental Flood Basalts and Mantle Plumes: a Case Study of the Northern Ethiopian Plateau. J. Petrol. 50, 1377-1403. Beccaluva L., Bianchini G., Ellam R.M., Natali C., Santato A., Siena F. & Stuart M.F. 2011. Peridotite xenoliths from Ethiopia: inferences on mantle processes from Plume to Rift settings. In: Beccaluva L., Bianchini G., Wilson M. Eds., Volcanism and evolution of the African Lithosphere. Geol. Soc. Am. Sp. Paper, 478, 77-104. Ellam R.M. & Cox K.G. 1991. An interpretation of Karoo picrite basalts in terms of interaction between asthenospheric magmas and the mantle lithosphere. Earth Planet. Sci. Lett. 105, 330-342. Heinonen J.S., Carlson R.W., Riley T.R., Luttinen A.V. & Horan M.F. 2014. Subduction-modified oceanic crust mixed
with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province. Earth Planet. Sci. Lett. 394, 229-241. Herzberg C., Asimow P.D., Arndt N., Niu Y., Lesher C.M., Fitton J.G., Cheadle M.J. & Saunders A.D. 2007. Temperatures in ambient mantle and plumes: Constraints from basalts, picrites and komatiites. Geochem. Geophys.
Geosys., 8, doi:10.1029GC001390
Chemical evolution, petrogenesis, and regional chemical correlations of the flood basalt sequence in the central Deccan Traps, India
No full textThe lava sequence of the central-western Deccan Traps (from Jalgaon towards Mumbai) is formed by basalts and basaltic andesites having a significant variation in TiO2 (from 1.2 to 3.3 wt%), Zr (from 84 to 253 ppm), Nb (from 5 to 16 ppm) and Ba (from 63 to 407 ppm), at MgO ranging from 10 to 4.2 wt%. Most of these basalts follow a liquid line of descent dominated by low pressure fractionation of clinopyroxene, plagioclase and olivine, starting from the most mafic compositions, in a temperature range from 1220degrees to 1125degreesC. These rocks resemble those belonging to the lower-most formations of the Deccan Traps in the Western Ghats (Jawhar, Igatpuri and Thakurvadi) as well as those of the Poladpur formation. Samples analyzed for Sr-87/Sr-86 give a range of initial ratios from 0.70558 to 0.70621. A group of flows of the Dhule area has low TiO2 (1.2-1.5 wt%) and Zr (84-105 ppm) at moderate MgO (5.2-6.2 wt%), matching the composition of low-Ti basalts of Gujarat, low-Ti dykes of the Tapti swarm and Toranmal basalts, just north of the study area. This allows chemical correlations between the lavas of central Deccan, the Tapti dykes and the northwestern outcrops. The mildly enriched high field strength element contents of the samples with TiO2 > 1.5 wt% make them products of mantle sources broadly similar to those which generated the Ambenali basalts, but their high La/Nb and Ba/Nb, negative Nb anomalies in the mantle normalized diagrams, and relatively high Sr-87/Sr-86, make evident a crustal input with crustally derived materials at less differentiated stages than those represented in this sample set, or even within the sub-Indian lithospheric mantle
Constraints and current problems for the Caribbean Plate margins evolution: the main results of the Italian-Caribbean research group (IGCP 433)
No full textCaribbean Plate margin evolution: constraints and current problems
No full textOceanic crust was generated at multiple spreading centres during the Jurassic and Early Cretaceous, forming a “proto-Caribbean” oceanic domain. During the Cretaceous, part of that crustal domain thickened into an oceanic plateau, of petrologic Mid-Ocean Ridge (MOR) to Ocean Island Basalt (OIB) affinity. Simultaneously, the South and North American continental plates developed rifting and tholeiitic magmatism in the Middle America region (Venezuela and Cuba). The rifting created space for the proto-Caribbean oceanic domain. Petrological and regional correlations suggest that, beginning in the Cretaceous, the proto-Caribbean domain was involved into two main stages of subduction, referred to as first and second “eo-Caribbean” phases. Each phase is characterized by oblique convergence. The older (mid-Cretaceous) stage, involved in subduction (probably eastward dipping) of thin proto-Caribbean lithosphere, with generation of Island Arc Tholeiitic (IAT) and Calc-Alkaline (CA) magmatism, accompanied by high pressure - low temperature (HP - LT) metamorphic effects, and formation of arc units and ophiolitic melanges (Guatemala, Cuba, Hispaniola and Puerto Rico, in the northern margin; Venezuela in the southern). The Late Cretaceous second stage consisted of westward dipping intra-oceanic subduction; it is recorded by tonalitic arc magmatism related to the onset of the Aves - Lesser Antilles arc system. Since the Late Cretaceous, the inner undeformed portions of the Caribbean oceanic plateau (i.e. the Colombian and Venezuelan Basins) were trapped east of the Pacific subduction of the Chortis, Chorotega and Choco blocks, ultimately building the Central American Isthmus. From Tertiary to Present, continuous eastward movement of the Caribbean Plate with respect to the Americas, gave rise to transpression along both the northern and southern margins, marked by scattered and dismembered ophiolitic terranes
A reappraisal of ultra-alkaline Intra-Apennine volcanism in central-southern Italy: evidence for subduction-modified mantle sources
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Helium and argon isotopic compositions of mantle xenoliths from Tallante and Calatrava, Spain.
No full textWe have analyzed by single-step crushing helium and argon isotopes in olivine and orthopyroxene from mantle xenoliths of Calatrava (CLV) in central Spain and Tallante (TL) in southeast Spain. The investigation
focused on carefully selected samples previously characterized in terms of major and trace elements on both bulk rock and constituent minerals, and Sr and Nd isotopes on clinopyroxene separates. Six analyses were
performed on protogranular spinel lherzolites from CLV, and 17 were performed on spinel harzburgites, lherzolites, and orthopyroxenites from TL. The 40Ar/36Ar ratio was between 296 and 622, indicating
atmospheric contamination, which probably occurred during exposure to the surface. The helium-isotope
ratio (3He/4He) ranged between 3.6 and 6.5 Ra in CLV samples and between 1.4 and 5.7 Ra in TL samples.
There was a positive correlation between the 3He/4He and 4He/40Ar* ratios, possibly reflecting diffusive
fractionation between 3He, 4He, and 40Ar within mantle sections interacting with ascending melts. However,
the difference between the maximum 3He/4He ratios measured in CLV and TL appears to be related to
significant differences in the metasomatic melts that affected the two sectors of the lithospheric mantle.
In agreement with the findings of previous studies, the helium isotopes at CLV are compatible with
metasomatism due to ascending HIMU-type asthenospheric melts. In contrast, the lower 3He/4He values
recorded at TL suggest subduction-related metasomatic components that are possibly related to the Cenozoic
subduction of the Betic system. Such event plausibly introduced crust-derived fluids that metasomatized the
mantle wedge, slightly decreasing its 3He/4He value. Noble gases appear decoupled from other elements
during these mantle processes, since comparatively low 3He/4He values have been recorded also in samples
that are relatively unmetasomatized in terms of incompatible lithophile elements. We hypothesize a role for
volatile-dominated, CO2-rich fluids progressively decoupling from the ascending metasomatic melts and
migrating in the surrounding peridotite matrix to form a diffuse aureola enriched in noble gases.Published18-263.5. Geologia e storia dei vulcani ed evoluzione dei magmiJCR Journalreserve
Garnet-spinel subsolidus re-equilibration and K-metasomatism in Cape Verde lithospheric mantle.
No full textMesozoic to Present tectono-magmatic evolution of the Central-Western Mediterranean area
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Petrogenesis and regional chemical correlations of the flood basalt sequence in the Central Deccan Traps, India
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