Petrology最新文献

A series of Mesozoic granites related to the subduction of the ancient Pacific plate are widely developed in the eastern part of the North China Craton. These granites contain garnet as a minor phase. Garnet records important information such as magma composition, temperature and pressure, and is an important object in the study of petrological evolution of granites. We take the late Jurassic garnet–bearing monzogranite in Xingcheng, Liaoning as the research object, and the magmatic evolution process in the study area is discussed by the in–situ element geochemical variation and the Raman effect of magmatic garnet girdle. It is found that the garnet has a zonation structure in which the contents of MnO and CaO increase and then decrease, while the contents of FeO and MgO decrease and then increase from the core to the edge. Through the simulation of the crystallization sequence of garnet and biotite, it is found that the garnet has “M” type “spessartine bell–shaped profile”, Mn/(Ca + Mg + Fe) value and Mn/Fe value, which reflects that the crystallization pattern of minerals in the magma is dominated by biotite crystallization to garnet crystallization. It is considered that forward and reverse girdle can exist in the same garnet; the separation and crystallization of minerals in the late magmatic evolution is the main reason for the change in the compositional profile of the garnet. This is consistent with the late evolution of granitic magmas in the northeastern of the North China Craton after the late Jurassic.

We present new experimental data on Cl solubility in model basalt melts of eutectic compositions diopside (Di)–albite (Ab) and Di–anorthite ± quartz (Qtz). The starting glasses were equilibrated with aqueous NaCl–CaCl₂ fluid at 4 kbar in the temperature range of 900–1200°C. The experiments show that Cl solubility decreases with increasing NaCl in the fluid. Ca–Na partitioning between melts and fluid is weekly temperature dependent and resembles that of the plagioclase–fluid system. The newly obtained experimental data, along with previously published results on the model granite melting in the presence of (Na,K)Cl brines (Aranovich et al., 2013), are used to calibrate an empirical thermodynamic model for salt species (NaCl, KCl, and CaCl₂) in silicate melt. Calculations show that Cl solubility in haplogranite melt decreases with increasing K/Na ratio in the fluid (and correspondingly, melt). The data acquired on Ca and Na partitioning between melt and fluid make it possible to model the evolution of the Ca/Na ratio in the crystallization course of basalt melts. At a high pressure (10 kbar), Cl solubility in model granite increases with increasing Н₂О content. The calculated phase diagram for a simple pseudo-ternary system Ab–H₂O–NaCl demonstrates complex phase relations and, correspondingly, evolution of the Н₂О and NaCl concentrations in the melt. This complex evolution is illustrated by data on the composition of quartz-hosted melt and fluid inclusions from granites in the Verkhneurmisskii massif in the Badzhal volcano-plutonic zone.

The role of S during high-grade metamorphism is a topic that has not garnered much interest in the literature until recently. In this review, the role of S as an active component in high grade hypersaline fluids is reviewed per a series of regional studies involving orthopyroxene-bearing granulite-facies granitoids. These include the Shevaroy Block and Nilgiri Block, southern India; the Bamble Sector, southwest Norway; the Val Strona traverse of the Ivrea-Verbano Zone, northern Italy; and the Lewisian Complex, northwest Scotland. In each these terranes, S-bearing, high-grade, low H₂O activity fluids are conjectured to have been present during granulite-facies metamorphism and to have contributed to the dehydration of the rock, the oxidation state of the rock, and trace element mobility, leaving behind pyrite and/or pyrrhotite as traces of its presence. The various mineral equilibria reactions between the various oxidation states of S in these fluids and the oxide and silicate minerals encountered by the fluid are explored and a coherent framework of interdependent chemical reactions are developed, which describe both oxidation of the rock and the formation of pyrite and pyrrhotite during both peak- and post-peak metamorphism.

Xenoliths in kimberlites are the most promising material for studying the composition and structure of the lower levels of the continental crust. This study is aimed at the estimation of P–T parameters and fluid regime of metamorphism for garnet–biotite–feldspar and orthopyroxene–garnet–biotite–feldspar rocks found as xenoliths in kimberlites of the Yubileynaya and Sytykanskaya pipes, Yakutian kimberlite province. The seven studied samples show inverse dependences of relative contents of garnet and orthopyroxene, orthopyroxene and biotite, garnet and plagioclase, plagioclase and potassium feldspar. This indicates a consistent series of transformations of the assemblage garnet + plagioclase + orthopyroxene ± quartz to the assemblage garnet + biotite + potassium feldspar. In this process, the replacement of plagioclase by potassium feldspar was the leading reaction. Evidence of this reaction is specific reaction textures in the rocks, negative correlations between the contents of the minerals, and petrochemical characteristics of the rocks. Modeling of the mineral assemblages of the xenoliths using the pseudosection approach (PERPLE_X) revealed two groups of rocks corresponding to different depth levels of the Siberian cratonic crust. For rocks without orthopyroxene or with this mineral as single relics, the pressure was estimated at 9.5–10 kbar, and it is 6–7 kbar for orthopyroxene-bearing samples. The xenolith rocks have close metamorphic peak temperatures of 750–800°C. They experienced 200–250°C cooling and 3–4 kbar decompression, regardless of the level of the crust at which they had initially occurred. This indicates that the metamorphic evolution of the rocks during their exhumation was probably associated with collisional processes during the amalgamation of individual terrains of the Siberian craton. Xenoliths enriched in K-feldspar might have been products of metamorphic reactions with participation of aqueous–(carbonic)–salt fluids, which were sourced from basaltic magmas in the lower crust. The most strongly metasomatized rocks were located closest to the place of accumulation of crystallizing magmas.

The possibility of changing the ratio of the concentrations of NaCl and CaCl₂ salts in fluid phases formed as a result of heterogenization of the H₂O–CO₂–NaCl–CaCl₂ fluid with a decrease in P-T parameters has been studied. A well-known experimental fact regarding the ternary systems H₂O–CO₂–NaCl and H₂O–CO₂–CaCl₂ is the greater tendency of the H₂O–CO₂–CaCl₂ system to separate into coexisting predominantly aqueous-salt and aqueous-carbon dioxide phases compared to the similar system H₂O–CO₂–NaCl. This experimental fact can be interpreted as a greater affinity of NaCl for CO₂ compared to CaCl₂. Using a recently developed numerical thermodynamic model of the H₂O–CO₂–NaCl–CaCl₂ quaternary fluid system, it was possible to identify geologically significant consequences of this difference in the interaction of NaCl and CaCl₂ with CO₂. Multistage heterogenization of the H₂O–CO₂–NaCl–CaCl₂ fluid with a significant decrease in P-T parameters ultimately leads to the formation of aqueous-carbon dioxide fluid phase f2, the salt component of which is significantly enriched in NaCl and depleted in CaCl₂ compared to the initial fluid. The fluid phase f1 formed at each stage of heterogenization has a predominantly water-salt composition with the ratio of the mole fractions of NaCl and CaCl₂ salts, differing little from that in the initial fluid. However, the total mole fraction of salt in the f1 phase, as a rule, significantly exceeds that in the original fluid. The density of phase f1 significantly exceeds the density of phase f2. During the process of multistage heterogenization of the fluid phase f1, there is no formation of a fluid with a significant enrichment of CaCl₂ compared to the initial ratio of the mole fractions of NaCl and CaCl₂. At the same time, successive multiple separation of the f2 phase leads to the enrichment of its salt component in NaCl. Under favorable conditions, this process can lead to the formation of a fluid with almost pure NaCl salt. Changes in the salt composition of the fluid H₂O–CO₂–NaCl–CaCl₂ are considered in application to the evolution of fluid composition along the regressive branch of the P-T trend of HP metamorphism and syngranulite metasomatism in the Lapland granulite belt.

The temperature and compositional parameters of the parental magma of ore-bearing apophysis DV10 of the Yoko-Dovyren massif are estimated by the method of geochemical thermometry based on results of thermodynamic modeling of the equilibrium crystallization of the melts of 24 rocks. The thermometric calculations were carried out using the COMAGMAT-5.3 program with increments of 0.5 mol % to a maximum degree of crystallization 75–85%, under oxygen fugacity controlled by the QFM buffer. The model crystallization sequence of minerals was as follows: olivine (Ol) + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate−sulfide immiscibility was calculated to occur mostly before the onset of plagioclase crystallization, which is consistent with initial sulfide saturation of the parental magma. The calculation results demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185^oC. When applied to the average composition of apophysis DV10, this temperature indicates the existence of suspension of the original crystals, including 52.1 wt % cumulus olivine (Fo_83.6), 2.3 wt % plagioclase (An_79.7), 0.24 wt % clinopyroxene (Mg# 88.8), 1 wt % aluminochromite (Cr# 0.62), and about 0.2% sulfide liquid in a moderately magnesian melt (53.6 wt % SiO₂, 7.4 wt % MgO). Therewith the sulfur concentration at sulfide saturation (SCSS) was estimated at 0.083 wt %. This heterogeneous system had a viscosity of 4.71 log units (Pa s) and integral density of 2929 kg/m³. Such rheological properties do not contradict the possibility of the migration and emplacement of the protocumulus mush from the main Dovyren chamber. However, a more probable scenario is the localized accumulation of olivine in the trough-shaped part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.