Basites of Vilyui-Markhinsky dike belt (Vilyui paleorift) and their relations with Nakyn field kimberlites

The Vilyui-Markhinsky dike belt (VMDB), which was formed as a result of Devonian rifting on the eastern margin of the Siberian Platform, is the marginal part (area of scattered rifting) of the Vilyui paleorift structure. The Nakyn field is located in the central part of the belt, but is controlled by an independent system of NNE-trending tectonic faults. The belt dyke intrudes the Nyurbinskaya kimberlite pipe. On their contact, specific breccias were formed resulting from the interaction between degassing products of basic magma with kimberlites. The typical zonality of the dyke endocontact indicates a later dyke introduction. Dolerite dikes thermally metamorphosed breccias in which high-temperature neoplasms of andradite, Al-lizardite, and clinochrysotile were generated. VMDB basites represent a single association, in which two series of rocks are distinguished: moderate-titanium (TiO2 ~ 2.5 wt. %) with normal alkalinity and low P2O5 content, and high-titanium (TiO2 ~ 4.4 wt. %), occasionally with moderate alkalinity. The differences in the dike composition are insignificant and are the result of natural variations in the composition of individual bodies. 40Ar/39Ar dating of the VMDB basites, the method characterized by the best results convergence, shows that they formed in a narrow timeframe corresponding to the Upper Frasnian – Famenian stage of the Upper Devonian (368.5 to 376.3 Ma). The location of the Nakyn field basites and kimberlites is controlled by faults of various types, orientation and age. Kimberlites formed first, and VMDB intrusions followed.

Zirconolite, baddeleite, zircon and thorite of island arc quartz gabbro-norite-dolerites of Ayu-Dag intrusion (Mountain Crimea)

Early Bajocian island arc quartz gabbro-norite-dolerites of Ayu-Dag contain syngenetic zirconolite and baddeleite. Zirconolite presents as Ca-dominant and uncommon Y-dominant (Y, Ca, Th, REE)2FeZr2Ti3O14. Two genetic types of zircon and thorite are developed. Predominant xenogenic zircon-I with thorite inclusions and a mass of lacunes — traces of reflow, is enriched with Hf, Th, Y, P (up to 6 wt.% HfO2, 5% ThO2, 6% Y2O3, 3% Р2О5). Thorite contains 7–9% UO2. Zircon and thorite of such composition are typical minerals of highly radioactive granites. Isotope age of zircon (xenogenic) is >2 billion years. Rare syngenetic zircon-II is sharply poor of Th, U, Y and Hf. So, basic magma of Ayu-Dag was contaminated with substance of Early Precambrian highly radioactive granites, which bodies are probabely located in the Mountain Crimea substructure.

Spatial and temporal distributions of the late Mesozoic volcanic successions and their controlling effects on the Changling fault depression of the Songliao Basin, northeast China

Volcanic successions, including their spatial, temporal, tectonic, and geochemical attributes, were identified based on well data, 2D and 3D seismic data, U–Pb isotopic ages, and major and trace element data from the Changling fault depression of the Songliao Basin in northeastern China. Three eruption cycles developed in the Changling fault depression: K1h (Huoshiling Formation) (124–118 Ma), K1yc1 (first member of the Yingcheng Formation) (115–106 Ma), and K1yc3 (third member of the Yingcheng Formation) (106–102 Ma). These three eruption cycles comprised seven eruption stages. The volcanic successions in every eruption cycle were bimodal and evolved from basic to acidic. The magma of the basic and intermediate rocks in these cycles was derived from partial melting of the asthenosphere. In K1yc1 and K1yc3, partial melting of the newly formed basic rocks in the lower crust formed the high-SiO2 acidic magma. In addition, fractional crystallization of basic magma formed the small-scale intermediate–acidic magma. In K1h, the most likely origin of the acidic magma was fractional crystallization of basic magma. During the early stage of the fault depression, the volcanic rocks of K1h were controlled primarily by the boundary fault activity. In K1yc1 and K1yc3, basic–intermediate rocks were distributed widely along syngenetic faults, and acidic rocks were concentrated in areas where the syngenetic faults had their largest amplitudes. Multiple cycles and stages of the volcanic successions were controlled by magmatic evolution, and the spatial distribution was controlled by basin tectonics.

Petrogenesis of plagioclase phenocrysts of Mount Etna, Sicily, with particular reference to the 1983 eruption: contribution from cathodoluminescence petrography

The cathodoluminescence (CL) characteristics of plagioclase phenocrysts in water-quenched lavas from the 1983 eruption of Etna have been investigated to examine the application of plagioclase CL to the study of magmatic processes. The phenocrysts have a green luminescent inner zone that is sharply bounded by a blue luminescent outer zone, with the boundary often coinciding with a concentric zone of glass inclusions.Strong compositional differences between the green (An70–An75) and blue (An50–An60) luminescent areas are interpreted as the result of two phases of growth under differing conditions. The green luminescent cores are considered to be anorthite-rich cumulate crystals from a basic magma which have been disrupted by the injection of a more evolved melt, resulting in heterogeneous nucleation and crystallization of the outer blue luminescent zone. The relationship between the CL and the trace element contents of the plagioclase crystals is discussed.