Petrology最新文献

Polycomponent condensate glasses found in nature provide an insight into condensation mechanisms, which are still understood inadequately poorly. Condensate glasses found in the impactites of the Lonar crater contain nanosized inclusions of metallic Fe, Cr, Cu, Zn, Ag, In, Te, Au, Pt, and Bi, along with Fe, Cu, and Zn sulfides. This combination may be indicative either of a brief condensation window for the almost simultaneous condensation of components with so different fugacity or of a possible mechanism of cluster condensation, provided that the condensation temperatures of such clusters are close.

Similar to the other areas of the Late Cenozoic volcanic province of Central Asia, the Udokan volcanic plateau (UVP) was formed in the time span between the Middle Miocene and the Pleistocene. Its rocks are highly alkaline and vary from alkaline picrobasalts and basanites to alkaline trachytes. The compositional variations of the rocks were controlled by two differentiation trends, which corresponded to different generation conditions of the parental magmas. The rocks with low SiO₂ contents (<45 wt %) were formed by melts of low degrees of melting, whose melts were derived under elevated pressures and temperatures. The formation of the rocks with 45–61 wt % SiO₂ was associated with the differentiation of basalt melts, which were derived at shallower depths and at lower temperatures. The geochemical characteristics of the UVP basaltoids make them similar to OIB-type basalts. They are also close in Sr, Nd, and Pb isotopic composition, corresponding to the parameters of the moderately depleted mantle, which is close to the composition of oceanic basalt sources corresponding to the mantle of deep mantle plumes. The corresponding mantle component is present in the sources of other volcanic regions of the Late Cenozoic intraplate volcanic province in Central Asia, which indicates that the material of a lower mantle plume was involved in the formation of these regions.

Experiments aimed at the synthesis of Cr- and Ti-bearing phlogopite in the silicate-carbonate systems peridotite—K₂CO₃ + H₂O and basalt—K₂CO₃ + H₂O at 7 GPa and 900–1200°С were carried out. It is shown that the crystallization of titanium-bearing phlogopite requires subducted crustal material at mantle depths. However, the mantle peridotite should predominate over basalt for Ti-phlogopite crystallization; otherwise, dioctahedral mica (aluminoceladonite) with (Mg + Fe)/^VIAl > 1 is formed via the scheme 2^VIAl = ^VITi⁴⁺ + ^VI(Mg + Fe). The competitive behavior of Ti and Cr upon incorporation into phlogopite is considered. It is shown that the presence of >1.3 wt % TiO₂ introduces a limitation on the high concentrations of Cr₂O₃ via the scheme ^VI(Mg²⁺) + ^IV(Si⁴⁺) = ^VI(Cr³⁺) + ^IV(Al³⁺). This can explain the compositional patterns of phlogopite from inclusions in natural diamonds, in which the Ti content is much higher than that of Cr. The results obtained support the original idea that the composition of phlogopite may be applied to distinguish the paragenetic associations of diamond.

Fe–Mg staurolite is a typical and widespread mineral of medium-temperature high-alumina metapelites, whereas magnesian staurolite is only relatively rarely found in metamorphosed mafic rocks (metabasites). The most significant factors controlling staurolite stability in metabasites were identified by thermodynamic modeling and analysis of the common features of the mineral-forming processes. In contrast to staurolite in low- and medium-pressure metapelites, staurolite in metabasites is stable at medium- and high-pressure metamorphism. An increase in the proportion of carbon dioxide in the water–carbon dioxide fluid shifts the staurolite-forming mineral reactions to lower temperatures and higher pressures. Al, Fe, Mg, and Ca are the major components of rocks that are critically important for the formation of magnesian staurolite in these rocks, and the contents and ratios of these components are of crucial importance for the stability of staurolite in metabasites. To understand the processes forming the mineral in metabasites, it is instrumental to subdivide metabasites into subgroups of predominantly magnesian, ferruginous–magnesian, and ferruginous protoliths. With regard to this subdivision, three petrochemical modules are proposed in the form of ratios of major components: MgO/CaO, CaO/FM, and Al₂O₃/FM, based on which it is possible to predict the stability of staurolite in mafic rocks at appropriate P–T parameters of metamorphism.

Understanding the tectonic evolution of the Yenisei Range offers important clues not only for the tectonic evolution of orogenic belts at margins of ancient cratons but also for solving the problem of the incorporation of the Siberian craton into the Rodinia supercontinent. Results of mineralogical−petrological, geochemical, and isotope–geochemical studies provide an insight into the petrogenesis, geotectonic settings, thermodynamic parameters of formation, and the ages of the metamorphism and protoliths for the contrastingly compositionally different rocks of the Garevka metamorphic complex. The paper discusses the possible models for the origin of the rock complexes and the geodynamic settings in which they were formed. The western margin of the Siberian craton was determined to have been affected by two pulses of Neoproterozoic endogenic activity, which were related to the origin of the Rodinia supercontinent (930–900 and 880–845 Ma), and which correlated with Grenville and post-Grenville processes responsible for Valhalla folding. The regional geodynamic history is correlated with the coeval sequence and similar style of tectono−thermal events in the peripheries of the large Precambrian cratons Laurentia and Baltica, which is consistent with the proposed Neoproterozoic paleogeographic reconstructions of close spatiotemporal relationships between these cratons and their incorporation into Rodinia configuration.

Eocene volcanic rocks with basaltic-trachyandesite and trachybasalt composition which cross-cut the Cretaceous sedimentary rocks, are exposed in the northwestern part of the Central-East Iranian Microcontient (CEIM) (SE of Khur, Isfahan Province, Iran). The rock-forming minerals of these volcanic rocks are olivine (chrysolite and hyalosiderite, Mg# = 0.69–0.71), clinopyroxene (augite with Mg# = 0.74–0.84), orthopyroxene (enstatite with Mg# = 0.61–0.62) and plagioclase (andesine and labradorite with An_48.3-65.1). Phenocrysts set in a fine-grained matrix of the same minerals plus sanidine (Or_59.1Ab_36.6An_4.3) with minor amounts of opaque minerals (magnetite and ilmenite). Secondary minerals are chlorite and calcite. The main textures of these volcanic rocks are porphyritic, microlitic porphyritic, poikilitic, and glomeroporphyritic. The Eocene volcanic rocks of the Khur area are characterized by SiO₂ content of 51.8 to 54.9 wt %, Al₂O₃ amounts of 14.35 to 16.47 wt %, and TiO₂ values of 0.88 to 0.92 wt %. They exhibit strong enrichment in light rare earth elements (LREE) relative to heavy REE (HREE) (La/Lu ratio up to 102.35), enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSE), and present negative anomaly in Eu (Eu/Eu* = 0.72–0.87). Chemical characteristics and homogeneity of these volcanic rocks reveal their calc-alkaline nature and suggest that they were derived from a same parental magma and underwent a similar melt extraction. Major and trace elements geochemical features of the analyzed samples indicate that the parental magma was possibly derived from relatively low degrees of partial melting of a mantle wedge spinel lherzolite which was previously enriched by fluids/melts released from the Neo-Tethyan subducted slab.