Experimental study of the differentiation of gabbro-syenite melt under superliquidus conditions

ABSTRACT In this work we present the results of experimental interaction of gabbro-syenite melt, corresponding to the average composition of Northern Timan rocks, with a complex hydrogen-containing fluid. The composition of the magmatic fluid was controlled to be close to natural conditions using a special cell in a high gas-pressure vessel. Under superliquidus conditions, the initial melt exsolves into melts of different composition, forming contrast, cryptic, and rhythmic melt stratifications. The experimental results agree with natural data in the petrochemical diagram. It follows from our experimental data that fluid-saturated melts in magmatic chambers are completely differentiated in the liquid state. In the absence of temperature gradients in the magma, gravitational migration of nanoclusters of different densities forms flotation, sedimentation, and rhythmic types of melt stratification. Transmission electron microscopy of the glasses formed in the cell was used to study the formation of nanoclusters in a fluid-saturated superliquidus anorthosite-granite model melt. Clusters with a size of 6 nm consist of a pseudo-crystalline anorthite core surrounded by fluid-saturated shells of the melt. The migration of fluid and fluid-enriched clusters to the upper part of the magmatic chamber results in the activation, from bottom to top, of the processes of crystallization in the magma.

Thermochronology of cordilleran metamorphic core complexes: example of Song Chay massif in Northern Vietnam

Based on the reconstruction of the thermal evolution of the Song-Chai granitoid massif (Northern Vietnam) the long-term existence of granitoid magma at deep levels of the Earths crust (H = 15-20 km, t ~ 20-50 Ma) is established. Geodynamic analysis and mathematical modeling of thermal history of the granitoid batholith cooling shows that the magmatic chamber should be considered as thermal trap on the lower level of the earths crust, preserving residual granite melts for a long time. Activation of the magmatic chamber occurs in post-collisional strike-slip tectonics zones and is associated by tectonic exhumation of large segments of the earths crust. Ultimately, this leads to the transformation of the batholith into Cordilleran type metamorphic core complexes, emplacement of residual rare-metal melts and the formation of commercial deposits.

THERMOCHRONOLOGY OF GRANITOID BATHOLITHS AND THEIR TRANSFORMATION INTO METAMORPHIC CORE COMPLEXES (EXAMPLE OF SONG‐CHAI MASSIF, NORTHERN VIETNAM)

Based on the reconstruction of the thermal evolution of granitoid batholith, represented by the Song‐Chai gneiss‐granite massif (Northern Vietnam), the long‐term existence of granitoid magma at deep levels of the Earth's crust (H≥25 km, Δt~20–50 Ma) is established. The geodynamic analysis of the granitoid batholith and mathematical modeling of its thermal history shows that the magmatic chamber should be considered as a thermal trap at the lower level of the crust, which preserved residual granite melts for a long time. Activation of the magmatic chamber occurs in post‐collisional strike‐slip fault zones and is accompanied by tectonic exhumation of large crustal segments. As a result, the batholith is transformed into a Cordilleran‐type metamorphic core complex, residual rare‐metal melts are emplaced, and, commercial deposits are thus formed.

High pressure metamorphism in the peridotitic cumulate of the Marun-Keu complex, Polar Urals

The Marun-Keu Complex of high-pressure rocks comprises granitoids, gneisses, schists, gabbroids and peridotites, which are unevenly and variably metamorphosed to the eclogite facies. A representative sample of garnet–amphibole lherzolite from the Mount Slyudyanaya area shows a cumulate texture and well preserved magmatic mafic minerals (olivine and pyroxenes) but practically no preserved plagioclase. The eclogite-facies metamorphism produced corona textures of newly formed minerals: amphibole, garnet, orthopyroxene and spinel. The metamorphic parameters of the garnet–amphibole lherzolite were estimated by geothermobarometry and by modeling phase equilibria at Р ~ 2.1 GPa and T ~ 640–740°C and are well consistent with our earlier estimate of the formation conditions of eclogites in the area. Computer simulation of the crystallization process of the gabbroic melt with the COMAGMAT program package, using literature data on the composition of the least altered plagioclase peridotites and gabbroids from the Marun-Keu Complex, shows that the mafic and ultramafic rocks are genetically interrelated: they crystallized in a single magmatic chamber. According to the modeling, the origin of the cumulate texture in the lherzolite was controlled by the peritectic reaction Ol + melt → Opx at a pressure of 0.7–0.8 GPa and a temperature of 1255–1268°C. Differences between thermodynamic parameters in the eclogites and garnet peridotites are discussed within the framework of a tectonic model for subduction and subsequent exhumation of the Baltica paleocontinent.

Chemical Composition and Petrogenetic Implications of Apatite in the Khibiny Apatite-Nepheline Deposits (Kola Peninsula)

Khibiny, one of the largest of the world’s peralkaline intrusions, hosts gigantic apatite deposits. Apatite is represented by F-apatite and it contains exceptionally high concentration of SrO. (4.5 wt % on average) and increased amounts of rare earth elements (REEs; up to 8891 ppm). Such enrichment of apatite ores in REEs defined Khibiny deposit as world-class deposit with resources reaching several millions tons REE2O3. Apatite from the Khibina alkaline complex is characterized by the significant enrichment in light REEs relative to the heavy REEs (with average Ce/Yb ratio of 682) and the absence of a negative Eu anomaly. The obtained geochemical signature of apatite suggests a residual character of the Khibiny alkaline magma and it indicates that the differentiation of the primary olivine-melanephelinitic magma developed without fractionation of plagioclase which is the main mineral-concentrator of Sr and Eu in basaltic magmatic systems. The compositional evolution of the Khibiny apatite in the vertical section of the intrusion reflects primary fractionation processes in the alkaline magma that differentiated in situ. The main mechanism for the formation of the apatite-nepheline deposits was the gravitational settling of large nepheline crystals in the lower part of the magma chamber, while very small apatite crystals were suspended in a convective magma, and, together with the melt, were concentrated in its upper part of the magmatic chamber.

A New Occurrence of Terrestrial Native Iron in the Earth’s Surface: The Ilia Thermogenic Travertine Case, Northwestern Euboea, Greece

Native iron has been identified in an active thermogenic travertine deposit, located at Ilia area (Euboea Island, Greece). The deposit is forming around a hot spring, which is part of a large active metallogenetic hydrothermal system depositing ore-bearing travertines. The native iron occurs in two shapes: nodules with diameter 0.4 and 0.45 cm, and angular grains with length up to tens of μm. The travertine laminae around the spherical/ovoid nodules grow smoothly, and the angular grains are trapped inside the pores of the travertine. Their mineral-chemistry is ultra-pure, containing, other than Fe, only Mn (0.34–0.38 wt.%) and Ni (≤0.05 wt.%). After evaluating all the possible environments where native iron has been reported up until today and taking under consideration all the available data concerning the study area, we propose two possible scenarios: (i) Ilia’s native iron has a magmatic/hydrothermal origin i.e., it is a deep product near the magmatic chamber or a peripheral cooling igneous body that was transferred during the early stages of the geothermal field evolution, from high temperature, reduced gas-rich fluids and deposited along with other metals in permeable structural zones, at shallow levels. Later on, it was remobilized and mechanically transferred and precipitated at the Ilia’s thermogenic travertine by the active lower temperatures geothermal fluids; (ii) the native iron at Ilia is remobilized from deep seated ophiolitic rocks, originated initially from reduced fluids during serpentinization processes; however, its mechanical transport seems less probable. The native iron mineral-chemistry, morphology and the presence of the other mineral phases in the same thermogenic travertine support both hypotheses.

MATHEMATICAL MODEL FOR INVESTIGATE THE POSSIBILITY OF A VOLCANOS ERUPTION DUE TO THE FORMATION OF A CRACK IN THE ROOF OF THE MAGMA CHAMBER

При помощи математического моделирования рассматривается ситуация, когда в кровле магматического очага, расположенного глубоко под конусом вулкана, мгновенно возникает трещина, давление в которой первоначально близко нулю. При падении давления в магматической камере вероятен «взрывной» рост пузырьков в магме за фронтом волны разгрузки и соответствующее увеличение объема магматического расплава. С помощью моделирования быстропротекающего процесса раскрытия возникшей трещины в кровле магматического очага под действием дегазирующегося магматического расплава показано, что раскрытие трещины останавливается на расстояниях гораздо меньших, чем глубина расположения кровли магматического очага. The mathematical model is used for the case when a crack appears in the roof of the magma chamber inside a volcano, the pressure in which initially is zero. The pressure drop in the magmatic chamber leads to an increase in the size of the bubbles in the magma and an increase the volume of the magmatic melt. It is shown that the opening of the crack stops at distances much less than the depth of the location of the magma chambersroof.

New Petro-Mineralogical Results on the Magmatic Surrounding Rocks of Cu-Ag Mineralization in Tagmout (Eastern Anti -Atlas, Morocco)

Tagmout deposit is located in the Eastern Anti-Atlas, about 30 km south of the city of Qalâat Mgouna. It contains Cu-Ag mineralization which is embedded in magmatic rocks that are essentially intrusive of various facies. Detailed mapping (1/1200) and petro-mineralogical observation of these facies allow us to distinguish: 1) Olivine gabronorite, it is composed mainly of pyroxenes (CPX and OPX), olivine and biotite; 2) Quartz monzogabbro, it is characterized by the presence of pyroxenes, amphiboles, plagioclases, FK and quartz; 3) Quartz monzodiorite, which is a porphyric facies. It occupies the center of the Tagmout sector which is rich in FK, plagioclase, quartz, amphibole and pyroxene; 4) Granodiorite, it is a facies associated with quartz monzodiorite. It shows a paragenesis composed of FK, quartz and plagioclase; 5) Pink microgranite, metric dykes developing in different directions (N10° to N15°, N70° to N90°, N120° to N130 °). It is a facies rich in quartz, FK and plagioclase; 6) Dolerite, It is a dark-colored facies with a typical doleritic texture. It is rich in plagioclase and relics of pyroxenes and amphiboles. This facies occupies the eastern part of Tagmout complex; 7) Volcano-sedimentary facies with rhyolite appearance of brick red color. It is a vitreous fine matrix and conglomerate elements which is rich in quartz and FK. These are conglomerates that are affected by the terminal neoproterozoic rhyolitic lavas. They are also affected to different degrees by several phases of hydrothermal alterations of potassic, phyllitic, clayey, and propylitic types. The effect of these alterations on the rock at the surface is very variable. The granodiorite is highly affected while olivine gabbronorite is not very sensitive and retains a remarkable state of freshness. This raises an interrogation on the chronological place and the advanced age for these facies (557 ± 5 Ma, Benziane and al. 2008).With the exception of recent formations (microgranite, dolerite and volcano-sedimentary facies), the contact between the plutonic rocks of Tagmout (olivine gabbronorite, quartz monzogabbro, quartz monzodiorite, granodiorite and granite) never takes major structural discontinuity nor contact metamorphism. But the localization of some enclaves on both sides of the neighboring facies with an intense brecciation marked the contact of these facies. This configuration suggests that the plutonic formations of Tagmout massive that the setting up was done in a contemporary way and is derived from the same magma by fractional crystallization in a magmatic chamber.