Petrology最新文献

A Permian-Triassic lamprophyric magmatism has developed as dyke and subvolcanic intrusions in the northeast of Isfahan Province, in Central Iran, where is so-called the Chahriseh-Bagherabad area. These rocks mainly consist of olivine, pyroxene, amphibole, and biotite as major minerals and apatite, biotite, muscovite, and opaque as minor minerals with porphyritic texture and with felsic xenoliths and xenocrysts. The Chahriseh-Bagherabad lamprophyres (CBL) magma has undergone significant crustal contamination and fractional crystallization. Based on whole rock geochemistry, these rocks belong to alkaline lamprophyres, derived from a low degree (less than 5%) partial melting of an amphibole-garnet lherzolite mantle and enriched by the lithospheric mantle in the source region. Their ⁸⁷Sr/⁸⁶Sr (0.70435–0.70696) and ¹⁴³Nd/¹⁴⁴Nd (0.51260–0.51276) values were supported by an enriched mantle source of the EMІI-type that has been contaminated by the continental upper crust. Thus, the CBL samples are alkaline rock formed by tensional intraplate magmatism in a Paleo-Tethyan subduction zone in the lower Paleozoic to late Permian in which metasomatism and mantle enrichment occurred. The lamprophyres magmas ascend due to tensional stress during rotation and displacement of the central-eastern Iranian microcontinent.

It is debated whether Cretaceous magmatism and mineralization in southeastern Yunnan (China) resulted from the subduction of Neo-Tethys or Paleo-Pacific lithosphere. To address this problem, we report whole-rock geochemical and Sr–Nd isotopic compositions and zircon U–Pb ages and Lu–Hf isotopic compositions from the Changlinggang syenites in the southeastern Yunnan Sn mineralization belt, western South China Block. LA–ICP–MS zircon U–Pb dating suggests that syenites were emplaced during the Late Cretaceous (79.2 ± 0.5 Ma). They contain nepheline and aegirine, and have high (K₂O + Na₂O) contents (16.0–18.6 wt %), K₂O/Na₂O ratios (0.7–1.7), FeO^T/(FeO^T + MgO) ratios (0.83–0.97), 10⁴ × Ga/Al ratios (2.3–3.7), and (Zr + Nb + Ce + Y) contents (505–2138 ppm), which are typical of A-type granitoids. The samples have slightly more enriched initial Sr–Nd isotopic compositions than the coeval Jiasha gabbros, with (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7088–0.7101 and ε_Nd(Т) values of –7.5 to –6.6. The geochemical data suggest that the Changlinggang syenites were derived by partial melting of enriched lithospheric mantle that had been metasomatized by subducted-sediment-derived melts, followed by crustal assimilation and fractional crystallization of the partial melt during ascent. These results, along with those of previous studies, indicate that Cretaceous magmatism and mineralization in southeastern Yunnan were emplaced in an extensional setting related to subduction of Neo-Tethys lithosphere. Therefore, we propose that the Neo-Tethyan slab was subducted under the western South China Block during the Late Cretaceous.

Based on the earlier obtained equations of state for the ternary systems H₂O–CO₂–CaCl₂ and H₂O–CO₂–NaCl, an equation of state for the four-component fluid system H₂O–CO₂–NaCl–CaCl₂ is derived in terms of the Gibbs excess free energy. A corresponding numerical thermodynamic model is built. The main part of the numerical parameters of the model coincides with the corresponding parameters of the ternary systems. The NaCl–CaCl₂ interaction parameter was obtained from the experimental liquidus of the salt mixture. Similar to the thermodynamic models for H₂O–CO₂–CaCl₂ and H₂O–CO₂–NaCl, the range of applicability of the model is pressure 1–20 kbar and temperature from 500 to 1400°C. The model makes it possible to predict the physicochemical properties of the fluid involved in most processes of deep petrogenesis: the phase state of the system (homogeneous or multiphase fluid, presence or absence of solid salts), chemical activities of the components, densities of the fluid phases, and concentrations of the components in the coexisting phases. The model was used for a detailed study of the phase state and activity of water on the H₂O–CO₂–salt sections when changing the ratio ({{{{x}_{{{text{NaCl}}}}}} mathord{left/ {vphantom {{{{x}_{{{text{NaCl}}}}}} {({{x}_{{{text{NaCl}}}}} + {{x}_{{{text{CaC}}{{{text{l}}}_{{text{2}}}}}}})}}} right. kern-0em} {({{x}_{{{text{NaCl}}}}} + {{x}_{{{text{CaC}}{{{text{l}}}_{{text{2}}}}}}})}}) from 1 to 0. Changes in the composition and density of coexisting fluid phases at a constant activity of water and changes in the total composition of the system are studied. A set of phase diagrams on sections H₂O–NaCl–CaCl₂ for different mole fractions of CO₂ is obtained. Pressure dependencies of the maximal activity of water in the field of coexisting unmixable fluid phases are obtained for several salt compositions of the system. Due to removal of restrictions resulting from a smaller number of components in ternary systems, the thermodynamic behavior of systems with a mixed composition of the salt significantly differs from the behavior of those with a single salt component.

Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific plate subducting beneath Kamchatka. This paper presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are magnesian (Mg# = 60–75 mol %) medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = 52–59 mol %) basaltic andesites are present among the monogenic cones of Kharchinsky Lake. The rare rock varieties include the high-K basalts–basaltic andesites of dikes in the center of Kharchinsky volcano and the magnesian andesites (Mg# = 58–61 mol %) of the extrusions of Zarechnу volcano. The distribution of trace-element contents in these samples demonstrates enrichment in large-ion lithophile elements and light REEs at depletion in high field strength elements and heavy REEs, as is typical of arc rocks. The high-K basalts and basaltic andesites show anomalous enrichment in Ba (>1000 ppm), Th (>3.8 ppm), U (>1.8 ppm), Sr (> 800 ppm, Sr/Y > 50), and light REE (La > 20 ppm), and their compositions are close to those of low-Si adakites. The basalts and basaltic andesites contain phenocrysts of high-Mg olivine (up to Fo_92.6) and clinopyroxene (Mg # up to 91 mol %). The rocks show petrographic and geochemical evidence of fractional crystallization, along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and an elevated Fe/Mn ratio (up to 80), which were interpreted as evidence of the participation of a pyroxenite source in the magma generation processes. The use of the Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivine associated with different sources: peridotite and pyroxenite, which were formed by a reaction between mantle-wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other lines of evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka Depression at the Kurile–Kamchatka and Aleutian subduction zone junction and testify to a significant heterogeneity of the mantle in this area.

The paper presents new data on the formation conditions of basalts from Menshiy Brat postcaldera volcano in the Medvezhia caldera, Iturup Island. The liquidus mineral assemblage consists of olivine (Fo 85.3–90.1 mol %) and chromium spinel (Cr# = 0.46–0.6), which crystallized at 1090–1170°C and oxygen fugacity at NNO + 0.6 (σ = 0.2) to NNO + 0.2 (σ = 0.14) in the course of the eruption. Data on melt inclusions in the liquidus olivine demonstrate that its parental melts were low-Al₂O₃ and low-K₂O, with up to 15.5 wt % MgO, and with an average H₂O content of 5.5 wt %. The newly obtained data on volatile contents in the olivine-hosted melt inclusions suggest that the mafic melts were derived by the partial melting of a peridotitic-rich source with a small admixture of an olivine-free component at 1225°C, under active influence of the slab-derived fluids. These fluids were separated from the subducting slab at 670–705°C and depths of 95–105 km beneath Iturup Island. Our results enhance our understanding of the evolution of basic magmas that serve as a heat and volatile sources during the formation of large calderas.