Understanding Perturbation on the Simplex: A Simple Method to Better Visualize and Interpret Compositional Data in Ternary Diagrams

Perturbation is an operation defined on the simplex and can be used for centering compositional data in a ternary diagram, applying objective criteria. Because a straight line in the original diagram is still astraight line in the perturbed diagram, gridlines or compositional fields defined by straight lines can easily be included in the operation. Simultaneous perturbation of data, gridlines, and/or compositional fields is shown to improve both visualization and graphical interpretation of compositions in ternary diagrams. This is illustrated by some examples using simulated as well as real data.     

Sabkha and playa environments in late Proterozoic grabens,Willouran Ranges,South Australia

We have reconstructed the depositional environment of sulphate‐dolomite‐sand‐mud sequences of the Callanna Beds of the late Proterozoic Adelaidean System in three areas of the Willouran Ranges, South Australia. We interpret the Callanna Beds which represent the earliest Adelaidean sediments as having been deposited in a series of discrete shallow cratonic basins. The sequences in all three areas consist of cyclic hypersaline sand‐shale‐carbonate sheets and wedges. Hypersalinity has been inferred from a study of evaporites and their pseudomorphs, which imply basin evolution in sabkha and playa palaeoenvironments. We interpret the Callanna Beds in the Willouran Ranges to have been formed in playa lake or prograding sabkha complexes, that formed in a series of yoked half‐grabens within the tectonic setting of the Adelaide palaeorift.     

An experimental study of Fe–Ni exchange between sulfide melt and olivine at upper mantle conditions: implications for mantle sulfide compositions and phase equilibria

The behavior of nickel in the Earth’s mantle is controlled by sulfide melt–olivine reaction. Prior to this study, experiments were carried out at low pressures with narrow range of Ni/Fe in sulfide melt. As the mantle becomes more reduced with depth, experiments at comparable conditions provide an assessment of the effect of pressure at low-oxygen fugacity conditions. In this study, we constrain the Fe–Ni composition of molten sulfide in the Earth’s upper mantle via sulfide melt–olivine reaction experiments at 2 GPa, 1200 and 1400 °C, with sulfide melt \(X_{{{\text{Ni}}}}^{{{\text{Sulfide}}}}=\frac{{{\text{Ni}}}}{{{\text{Ni}}+{\text{Fe}}}}\) (atomic ratio) ranging from 0 to 0.94. To verify the approach to equilibrium and to explore the effect of \({f_{{{\text{O}}_{\text{2}}}}}\) on Fe–Ni exchange between phases, four different suites of experiments were conducted, varying in their experimental geometry and initial composition. Effects of Ni secondary fluorescence on olivine analyses were corrected using the PENELOPE algorithm (Baró et al., Nucl Instrum Methods Phys Res B 100:31–46, 1995), “zero time” experiments, and measurements before and after dissolution of surrounding sulfides. Oxygen fugacities in the experiments, estimated from the measured O contents of sulfide melts and from the compositions of coexisting olivines, were 3.0?±?1.0 log units more reduced than the fayalite–magnetite-quartz (FMQ) buffer (suite 1, 2 and 3), and FMQ ??1 or more oxidized (suite 4). For the reduced (suites 1–3) experiments, Fe–Ni distribution coefficients \(K_{{\text{D}}}^{{}}=\frac{{(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}/X_{{{\text{Fe}}}}^{{{\text{sulfide}}}})}}{{(X_{{{\text{Ni}}}}^{{{\text{olivine}}}}/X_{{{\text{Fe}}}}^{{{\text{olivine}}}})}}\) are small, averaging 10.0?±?5.7, with little variation as a function of total Ni content. More oxidized experiments (suite 4) give larger values of K_D (21.1–25.2). Compared to previous determinations at 100 kPa, values of K_D from this study are chiefly lower, in large part owing to the more reduced conditions of the experiments. The observed difference does not seem attributable to differences in temperature and pressure between experimental studies. It may be related in part to the effects of metal/sulfur ratio in sulfide melt. Application of these results to the composition of molten sulfide in peridotite indicates that compositions are intermediate in composition (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\)?~?0.4–0.6) in the shallow mantle at 50 km, becomes more Ni rich with depth as the O content of the melt diminishes, reaching a maximum (0.6–0.7) at depths near 80–120 km, and then becomes more Fe rich in the deeper mantle where conditions are more reduced, approaching (\(X_{{{\text{Ni}}}}^{{{\text{sulfide}}}}\)?~?0.28)?>?140 km depth. Because Ni-rich sulfide in the shallow upper mantle melts at lower temperature than more Fe-rich compositions, mantle sulfide is likely molten in much of the deep continental lithosphere, including regions of diamond formation.     

Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series,Eseka area,SW Cameroon

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon. Here, we present new field data, petrography, and first comprehensible whole-rock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region. The studied rock samples are characterized by pseudomorphic textures, including mesh microstructure formed by serpentine intergrowths with cores of olivine, bastites after pyroxene. Antigorite constitutes almost the whole bulk of the rocks and is associated (to the less amount) with tremolite, talc, spinel, and magnetite. Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types. Type 1 has high MgO (> 40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values (< 40 and 82.88 wt%, respectively). Both types have low Al/Si and high Mg/Si ratios than the primitive mantle, reflecting a refractory abyssal mantle peridotite protolith. Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization. Bulk rock high-Ti content is similar to the values of subducted serpentinites (> 50 ppm). This similarity, associated with the high Cr contents, spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subduction-related environment. The U-shaped chondrite normalized-REE patterns of serpentinized peridotites, coupled with similar enrichments in LREE and HFSE, suggest the refertilized nature due to melt/rock interaction prior to serpentinization. Based on the results, we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subduction-related environment, especially in Supra Subduction Zone setting. These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the São Francisco craton in Brazil.

     

Structure of Palaeogene sediments in east Ellesmere Island: Constraints on Eurekan tectonic evolution and implications for the Nares Strait problem

The “Nares Strait problem” represents a debate about the existence and magnitude of left-lateral movements along the proposed Wegener Fault within this seaway. Study of Palaeogene Eurekan tectonics at its shorelines could shed light on the kinematics of this fault. Palaeogene (Late Paleocene to Early Eocene) sediments are exposed at the northeastern coast of Ellesmere Island in the Judge Daly Promontory. They are preserved as elongate SW–NE striking fault-bounded basins cutting folded Early Paleozoic strata. The structures of the Palaeogene exposures are characterized by broad open synclines cut and displaced by steeply dipping strike-slip faults. Their fold axes strike NE–SW at an acute angle to the border faults indicating left-lateral transpression. Weak deformation in the interior of the outliers contrasts with intense shearing and fracturing adjacent to border faults. The degree of deformation of the Palaeogene strata varies markedly between the northwestern and southeastern border faults with the first being more intense. Structural geometry, orientation of subordinate folds and faults, the kinematics of faults, and fault-slip data suggest a multiple stage structural evolution during the Palaeogene Eurekan deformation: (1) The fault pattern on Judge Daly Promontory is result of left-lateral strike-slip faulting starting in Mid to Late Paleocene times. The Palaeogene Judge Daly basin formed in transtensional segments by pull-apart mechanism. Transpression during progressive strike-slip shearing gave rise to open folding of the Palaeogene deposits. (2) The faults were reactivated during SE-directed thrust tectonics in Mid Eocene times (chron 21). A strike-slip component during thrusting on the reactivated faults depends on the steepness of the fault segments and on their obliquity to the regional stress axes.Strike-slip displacement was partitioned to a number of sub-parallel faults on-shore and off-shore. Hence, large-scale lateral movements in the sum of 80–100 km or more could have been accommodated by a set of faults, each with displacements in the order of 10–30 km. The Wegener Fault as discrete plate boundary in Nares Strait is replaced by a bundle of faults located mainly onshore on the Judge Daly Promontory.     

Structural and metamorphic evolution of the Sierras Australes (Buenos Aires Province/Argentina)

The Precambrian Basement Complex rocks as well as the Early to Late Palaeozoic cover sediments of the Sierras Australes were affected by one dominant folding and shearing event verging northeastward during Middle to Late Permian times. Strain estimates point to a minimal flattening and lateral shortening of the sedimentary cover sequence of c.20% and c.24% respectively. Continuing rotational deformation within first and second shear and thrust zones is documented by second and third folding and shearing. To the SW the fold belt changes into a fold and thrust belt where imbrication involves the basement. The deformational events were accompanied and outlasted by anchizonal to greenschist facies metamorphism proven by illite crystallinity and quartz deformation and recrystallization data. A temperature increase from ENE to WSW and also from N to S allowed a more ductile deformation of the rock sequence due to different deformation mechanisms operating. The folding and thrusting events were followed by strike-slip shearing on subvertical shear planes and shear zones under an overall sinistral transpressive regime. A model for the tectonic evolution of the Sierras Australes is proposed and some implications for its setting in the Gondwana reconstruction are given.

---

Zusammenfassung Das präkambrische Basement sowie die alt- und jungpaläozoischen Deckschichten der Sierras Australes wurden während des Mittl. bis Ob. Perm von einer NE vergenten Faltung und Scherung erfaßt. Strain-Abschätzungen deuten auf eine minimale Plättung und laterale Verkürzung der Sedimentfolge von ca. 20% bzw. 24%. Die fortlaufende relationale Deformation in den ersten und zweiten Scher- und Überschiebungszonen ist durch zweite und dritte Faltung mit Scherung dokumentiert. Der Sierras Australes-Faltengürtel geht nach SW in einen Falten-Überschiebungsgürtel über, dessen nach NE gerichtete Imbrikationen das Basement einbeziehen. Daten der Illit-Kristallinität und Quarz-Deformation/-Rekristallisation zeigen, daß die Deformationen von anchizonaler bis seicht-grünschieferfazieller Metamorphose begleitet und überdauert wurden. Mit der von ENE nach WSW bzw. N nach S zunehmenden Temperierung setzten verschiedene Deformationsmechanismen ein. Sie ermöglichten eine zunehmend duktile Verformung der Gesteinsfolgen. Auf die Faltungen und Auf-/Überschiebungen folgten Lateralverschiebungen an subvertikalen Scherflächen und -zonen unter einem sinistralen transpressiven Regime. Für die tektonische Entwicklung der Sierras Australes wird ein Modell vorgeschlagen, und die Folgerungen für die Stellung in der Gondwana-Rekonstruktion werden angeführt.

---

Résumé Les roches du socle précambrien ainsi que la couverture sédimentaire éo- à tardi-paléozoïque des Sierras Australes ont été affectées, au cours du Permien moyen à supérieur, d'un plissement et d'un cisaillement à vergence nord-est. Les déformations sont estimées à au moins 20% d'aplatissement et 24% de raccourcissement latéral. Dans la première et la deuxième zone de cisaillement et de charriage, la déformation rotationnelle continue s'exprime par les deuxième et troisième plissements. Vers le SW la ceinture plissée passe à une ceinture plissée et charriée qui comporte des imbrications du socle. Les déformations ont été accompagnées et suivies d'un métamorphisme de faciès d'anchizone à «schistes verts», ainsi qu'en attestent la cristallinité de l'illite et la déformation et recristallisation du quartz. L'accroissement de la température de l'ENE vers l'WSW ainsi que du N vers le S a donné lieu à des mécanismes de déformation de caractère de plus en plus ductile. La phase de plissement et de charriage a été suivie par un régime de cisaillements décrochants sénestres le long de plans et de shear-zones subverticaux. Les auteurs proposent un modèle de l'évolution tectonique des Sierras Australes et discutent de son insertion dans la reconstruction du Gondwana.

---

NE , . 20 24%. . , , , . : , - . ENE WSW N S, . . . .

     

Tectonic significance of Triassic mafic rocks in the June Complex,Sanandaj–Sirjan zone,Iran

This study concentrates on the petrological and geochemical investigation of mafic rocks embedded within the voluminous Triassic June Complex of the central Sanandaj–Sirjan zone (Iran), which are crucial to reconstruct the geodynamics of the Neotethyan passive margin. The Triassic mafic rocks are alkaline to sub-alkaline basalts, containing 43.36–49.09 wt% SiO₂, 5.19–20.61 wt% MgO and 0.66–4.59 wt% total alkalis. Based on MgO concentrations, the mafic rocks fall into two groups: cumulates (Mg# = 51.61–58.94) and isotropic basaltic liquids (Mg# = 24.54–42.66). In all samples, the chondrite-normalized REE patterns show enrichment of light REEs with variable (La/Yb)_N ratios ranging from 2.48 to 9.00, which confirm their amalgamated OIB-like and E-MORB-like signatures. Enrichment in large-ion lithophile elements and depletion in high field strength elements (HFSE) relative to the primitive mantle further support this interpretation. No samples point to crustal contamination, all having undergone fractionation of olivine + clinopyroxene + plagioclase. Nevertheless, elemental data suggest that the substantial variations in (La/Sm)_PM and Zr/Nb ratios can be explained by variable degrees of partial melting rather than fractional crystallization from a common parental magma. The high (Nb/Yb)_PM ratio in the alkaline mafic rocks points to the mixing of magmas from enriched and depleted mantle sources. Abundant OIB alkaline basalts and rare E-MORB appear to be linked to the drifting stage on the northern passive margin of the Neotethys Ocean.     

Über Gebirgszusammenhänge in den mitteleuropäischen Varistischen ketten (Geosynklinale und Lineament)

Zusammenfassung Die Wirksamkeit der beiden großen NNW- und NNE-streichenden Lineamente im mitteleuropäischen Bereich tritt in der Weiterentwicklung der varistischen Geosynklinale zurück. Im Unterkarbon — für die nördlichen Teile im Oberkarbon — sind sie weitgehend ausgelöscht. Mit dem Aufdringen der Granite treten sie wieder in Erscheinung, um im Perm die epirogene und orogene Entwicklung erneut zu beherrschen.