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The origin of celestine–quartz–calcite geodes associated with a basaltic dyke, Makhtesh Ramon, Israel
- MICHAEL ANENBURG, OR M. BIALIK, YEVGENY VAPNIK, HAZEL J. CHAPMAN, GILAD ANTLER, YARON KATZIR, MIKE J. BICKLE
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- Journal:
- Geological Magazine / Volume 151 / Issue 5 / September 2014
- Published online by Cambridge University Press:
- 29 October 2013, pp. 798-815
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Spectacular celestine geodes occur in a Jurassic peri-evaporitic sequence (Ardon Formation) exposed in Makhtesh Ramon, southern Israel. The geodes are found only in one specific location: adjacent to an intrusive contact with a Lower Cretaceous basaltic dyke. Celestine, well known in sedimentary associations worldwide and considered as a low temperature mineral, may therefore be associated with magmatic-induced hydrothermal activity. Abundant fluid inclusions in celestine provide valuable information on its origin: gas-rich inclusions in celestine interiors homogenized at T≥200°C whereas smaller liquid-rich inclusions record the growth of celestine rims at T≤200°C. Near 0°C melting temperatures of some fluid inclusions and the occurrence of hydrous Ca-sulphate solid crystals in other inclusions indicate that celestine precipitated from variably concentrated Ca-sulphate aqueous solutions of meteoric origin. Celestine crystallized from meteoric water heated by the cooling basaltic dyke at shallow levels (c. 160 m) during a Lower Cretaceous thermal perturbation recorded by regional uplift and magmatism. The 87Sr/86Sr ratio of geode celestine, 0.7074, is similar to that measured in the dolostones of the host Jurassic sequence, but differs markedly from the non-radiogenic ratio of the dyke. Strontium in celestine was derived from dolostones preserving the 87Sr/86Sr of Lower Jurassic seawater, while sulphur (δ34S = 19.9‰) was provided by in situ dissolution of precursor marine gypsum (δ34S = 16.8‰) indicated by relict anhydrite inclusions in celestine. Low-temperature meteoric fluid flow during the Campanian caused alteration of the dyke into secondary clays and alteration of geodal celestine into quartz, calcite and iron oxides.
The tectono-metamorphic evolution of a dismembered ophiolite (Tinos, Cyclades, Greece)
- Yaron Katzir, Alan Matthews, Zvi Garfunkel, Manfred Schliestedt, Dov Avigad
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- Journal:
- Geological Magazine / Volume 133 / Issue 3 / May 1996
- Published online by Cambridge University Press:
- 01 May 2009, pp. 237-254
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The six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subduction zone (SSZ) affinity for the ophiolitic suite. ρ18O values of phyllites, gabbros and serpentinites range from 6 to 15%o. Model calculations indicate that such values are consistent with low temperature (50–200°C) alteration of parent rocks by sea-water at varying water/rock ratios. This would agree with the early low temperature mineralogy of the serpentinites, but the early high temperature alteration of the gabbros would require the presence of 18O-enriched sea-water.The following overall history is suggested for Tinos ophiolitic slices. (1) Oceanic crust was generated at a supra-subduction zone spreading centre with high temperature alteration of gabbros. (2) Tectonic disturbance (its early hot stages recorded in an amphibolitic shear zone at the base of serpentinites) brought the already cooled ultramafics into direct contact with sea-water and caused low-T serpentinization. (3) Tectonism after cooling involved thrusting which caused repetition and inversion of the original order of the oceanic suite. (4) Regional metamorphism of all the ophiolite components at greenschist-facies conditions (−450°C) overprinted the early alteration mineralogy. It was probably induced by continued thrusting and piling up of nappes. The Tinos ophiolite, dated as late Cretaceous and genetically related to other low pressure rock-units of the same age in the Aegean, differs in age and degree of dismemberment and metamorphism from ophiolites in mainland Greece.
The origin, cooling and alteration of A-type granites in southern Israel (northernmost Arabian–Nubian shield): a multi-mineral oxygen isotope study
- ADAR STEINITZ, YARON KATZIR, JOHN W. VALLEY, YARON BE'ERI-SHLEVIN, MICHAEL J. SPICUZZA
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- Journal:
- Geological Magazine / Volume 146 / Issue 2 / March 2009
- Published online by Cambridge University Press:
- 27 October 2008, pp. 276-290
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A multi-mineral oxygen isotope study sheds light on the origin, cooling and alteration of Late Neoproterozoic A-type granites in the Arabian–Nubian shield of southern Israel. The oxygen isotope ratio of zircon of the Timna monzodiorite, quartz syenite and alkaline granite are within the range of mantle zircon (δ18O(Zrn) = 5.3 ± 0.6‰, 2σ), supporting the co-genetic mantle-derived origin previously suggested based on geochemical data and similar ɛNd(T) values and U–Pb ages (610 Ma). Likewise, olivine norite xenoliths within the monzodiorite (δ18O(Ol) = 5.41 ± 0.07‰) may have formed as cumulate in a parent mantle-derived magma. Within the Timna igneous complex, the latest and most evolved intrusion, an alkaline granite, has the least contaminated isotope ratio (δ18O(Zrn) = 5.50 ± 0.02‰), whereas its inferred parental monzodiorite magma has slightly higher and more variable δ18O(Zrn) values (5.60 to 5.93‰). The small isotope variation may be accounted for either by small differences in the temperature of zircon crystallization or by minor contamination of the parent magma followed by shallow emplacement and intrusion by the Timna alkaline granite. The Timna alkaline granite evolved, however, from a non-contaminated batch of mantle-derived magma. The formation of Yehoshafat granite (605 Ma; δ18O(Zrn) = 6.63 ± 0.10‰), exposed ~30 km to the south of the mineralogically comparable Timna alkaline granite, involved significant contribution from supracrustal rocks. A-type granites in southern Israel thus formed by differentiation of mantle-derived magma and upper crustal melting coevally. Fast grain boundary diffusion modelling and measured quartz-zircon fractionations demonstrate that the Timna and Yehoshafat alkaline granites cooled very rapidly below 600 °C in accordance with being epizonal. One to three orders of magnitude slower cooling is calculated for 30 Ma older calc-alkaline granites of the host batholith, indicating a transition from thick orogenic to extended crust. Significant elevation of the δ18O of feldspars occurred through water–rock interaction at moderate temperatures (100–250 °C), most probably during a thermal event in Early Carboniferous times.