1,720,993 research outputs found
Tetra-Plot: A Microsoft Excel spreadsheet to perform tetrahedral diagrams
No full textTetra-plot is a Microsoft Excel spreadsheet developed for the visualization of mineralogical, petrological and
geochemical data in three dimensions. This program allows to normalize and plot a number of data on tetrahedral
diagrams. The tetrahedron can be freely rotated in space. Tetra-Plot includes a set of functionalities that help users to
manipulate data for 3D visualization
Tetra-Plot: A Microsoft Excel spreadsheet to perform tetrahedral diagrams short note
No full textTetra-plot is a Microsoft Excel spreadsheet developed for the visualization of mineralogical, petrological and geochemical data in three dimensions. This program allows to normalize and plot a number of data on tetrahedral diagrams. The tetrahedron can be freely rotated in space. Tetra-Plot includes a set of functionalities that help users to manipulate data for 3D visualization
The geochemistry of primitive volcanic rocks of the Ankaratra volcanic complex, and source enrichment processes in the genesis of the Cenozoic magmatism in Madagascar
No full textThe Ankaratra volcanic complex in central Madagascar consists of lava flows, domes, scoria cones, tuff rings and maars of Cenozoic age that are scattered over 3800 km2. The mafic rocks include olivine-leucite-nephelinites, basanites, alkali basalts and hawaiites, and tholeiitic basalts. Primitive samples have high Mg# (>60), high Cr and Ni concentrations; their mantle-normalized patterns peak at Nb and Ba, have troughs at K, and smoothly decrease towards the least incompatible elements. The Ankaratra mafic rocks show small variation in Sr–Nd–Pb isotopic compositions (e.g., 87Sr/86Sr = 0.70377–0.70446, 143Nd/144Nd = 0.51273–0.51280, 206Pb/204Pb = 18.25–18.87). These isotopic values differ markedly from those of Cenozoic mafic lavas of northern Madagascar and the Comoro archipelago, typical Indian Ocean MORB and oceanic basalt end-members. The patterns of olivine nephelinitic magmas can be obtained through 3–10% partial melting of a mantle source that was enriched by a Ca-rich alkaline melt, and that contained garnet, carbonates and phlogopite. The patterns of tholeiitic basalts can be obtained after 10–12% partial melting of a source enriched with lower amounts of the same alkaline melt, in the spinel- (and possibly amphibole-) facies mantle, hence in volumes where carbonate is not a factor. The significant isotopic change from the northernmost volcanic rocks of Madagascar and those in the central part of the island implicates a distinct source heterogeneity, and ultimately assess the role of the continental lithospheric mantle as source region. The source of at least some volcanic rocks of the still active Comoro archipelago may have suffered the same time-integrated geochemical and isotopic evolution as that of the northern Madagascar volcanic rocks
Geochemistry of the Palitana flood basalt sequence and the Eastern Saurashtra dykes, Deccan Traps: clues to petrogenesis, dyke???flow relationships, and regional lava stratigraphy
No full textRecent studies of large mafic dyke swarms in the Deccan Traps flood basalt province, India, indicate that some of the correlative lava flows reached several hundred kilometers in length. Here we present field, petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd iso- topic) data on the Palitana lava sequence and nearby dykes in the Saurashtra region of the northwestern Deccan Traps. These rocks are moderately evolved, many with low-Ti-Nb characteristics. We infer that most dykes are notably (and systematically) less contaminated by ancient continental crust than the Palitana flows, but four dykes are equally or signif- icantly more contaminated, with some of the most extreme Sr-Nd isotopic compositions seen in the entire Deccan Traps (initial ??Nd is as low as ???18.0). A Bhimashankar-type and a Poladpur-type dyke are present several hundred kilometersfrom the type section of these magma types in the Western Ghats escarpment. We find no geochemical correlations between the Palitana sequence and three subsurface sequences in NE Saurashtra containing abundant picritic rocks, surface lavas previously studied from Saurashtra, or the Western Ghats sequence. Intriguingly, the Eastern Saurashtra dykes cannot have been feeders to any of these lava sequences. Feeder dykes of these sequences may be located in southwestern or central Saurashtra, or in the Dhule-Nandurbar Dediapada areas across the Gulf of Cambay, 200???300 km east of Palitana. Our results indicate polycentric flood basalt erup- tions not only on the scale of the Deccan Traps province, but also within the Saurashtra region itself
U-Pb ages, Pb-Os isotopes and PGE composition of Mailaka picritic basalt to rhyodacite volcanic sequence, West-Central Madagascar LIP
No full textU-Pb ages, Pb-Os isotopes, and latinum group element (PGE) composition of the west-central Madagascar flood basalt province
No full textThe Mailaka lava succession (central-western Madagascar) forms part of the Madagascar large igneous province and is characterized by basaltic to picritic basalt lava flows and minor evolved flows. In situ U-Pb dating of zircon in rhyodacites yields concordant ages of 89.7 +/- 1.4 and 90.7 +/- 1.1 Ma. Therefore, the capping rhyodacitic unit of the Mailaka lava succession was emplaced just after the underlying basalt sequence (dated paleontologically at Coniacian-Turonian). Two geochemically different lava series are present. A transitional series ranging from picritic basalts to basalts has incompatible element abundances and Pb, Os, and Nd isotope ratios within the range of mid-ocean ridge basalts. In addition, the concentrations of platinum-group elements (Ir < 0.35 ng/g, Ru < 0.17 ng/g, Pd = 1.0-1.6 ng/g) in the transitional basalts are generally lower than in basaltic lavas from oceanic plateaus (e.g., Ontong Java and Kerguelen) and other continental flood basalt provinces (e.g., Deccan and Etendeka). A tholeiitic series ranges from picritic basalts to rhyodacites and has relatively high concentrations of trace elements (e.g., Rb, Ba, Th, and light lanthanides) and the Pb-Sr-Nd and Os isotopic characteristic of magmas that have assimilated continental crust. The Pb isotope ratios of tholeiitic andesites indicate the involvement of a component highly depleted in radiogenic Pb, very likely old lower crust. Energy-constrained-assimilation-fractional-crystallization modeling indicates that the rhyodacites may be the result of similar to 25% assimilation of upper continental crust, with a ratio between assimilated mass and subtracted solid of similar to 0.35. An andesite with low Pb isotope ratios may be the result of similar to 8% assimilation of lower continental crust with a mass assimilated/mass accumulated ratio of similar to 0.1. Interaction of mantle-derived magmas with crustal lithologies of different age and evolutionary history thus occurred in this sector of the flood basalt province. Contamination of mantle-derived rocks by material of different crustal domains is a process also observed in other large igneous provinces, such as the Deccan Traps
Petrology of Karoo volcanic province magmatism in the southern Lebombo Monocline, Mozambique
No full textMINERALOGY, GEOCHEMISTRY AND PETROGENESIS OF THE KHOPOLI MAFIC INTRUSION, DECCAN TRAPS, INDIA
No full textThe Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50???55 % modal olivine, 20???25 % plagioclase, 10???15 % clinopyroxene, 5???10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5???26.9 wt.%), Ni (733???883 ppm) and Cr (1,432???1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The com- positions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49??? 58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (??Ndt = ???9.0 to ???12.7; 87Sr/86Sr=0.7088???0.7285) indicate crustal contamina- tion. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic process- es associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm3, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophys- ical observations such as high gravity anomalies and high seismic velocity crustal horizons
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