Despite the fact that porphyry Cu deposits contain large amounts of Cu in one or more small stocks, few studies have discussed evidence for significant magma recharge in porphyry Cu deposits. A systematic elemental and Sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the Baogutu reduced porphyry copper deposit, western Junggar, NW-China. Large compositional changes in An (12–24 mol%) are observed along with strong positive correlations between An and FeO. Significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in An and FeO, and complex Sr isotope variations. Three types of crystals with different core-to-rim Sr isotope variations are recognized. Type I crystals have core-to-rim increases in (87Sr/86Sr)i that could be explained by diffusion. For example, Sr isotope variations recorded in BCK2-1-2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100–500 years with the proximate value of ∼300 years. Type II crystals with different variation trends in (87Sr/86Sr)i on opposite sides of the core could be produced either by multi-stage crystallization or by analytical uncertainty. Whereas, type III crystals with complex core-to-rim variations in (87Sr/86Sr)i, may record repeated magma recharge events. All these results suggest repeated recharging of the magma chamber by hotter, more mafic and less radiogenic Sr isotope melts. Therefore, more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber, which ultimately results in Cu mineralization at the Baogutu reduced porphyry copper deposit. 相似文献
Granodiorite from the Gęsiniec Intrusion, Strzelin Crystalline Massif, SW Poland contains complexly zoned plagioclases. Five chemically and structurally distinct zones can be correlated among crystals: ‘cores’ (25–35% An), inner mantles (40–45% An), outer mantles (40–25% An), resorption zones (35–50% An) and rims (35–30% An). Good structural and chemical (major and trace elements) correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale. Plagioclase, being the liquidus phase, records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes.
Crystallization began with the formation of inner mantles. The paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma. Normally zoned inner mantles were formed under increasing undercooling. Compositional trends in mantles suggest closed system crystallization.
The major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase. The injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast, kinetically controlled growth of complex multiple, oscillatory zonation within resorption zones. The ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles—the ‘cores’. The decompression range is estimated at a minimum of 2 kbar. Emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases. Melt inclusions crystallized completely during post-magmatic cooling.
The zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution. Zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur. Crystals probably did not travel between different regimes. Resorption occurred but as single albeit complex episodes. Good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects. 相似文献
The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe3+ and/or increase in SiO2, CaO and P2O5 may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber, indicating lateral transport of magma. Chemically distinct syenites in the upper part of the intrusion are part of the Point du Criade intrusion, a large, late composite sill. Diabase and leucogabbro components show a close link with the SIMI and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion. A series of late gabbro intrusions that cut the SIMI may represent a rejuvenation of magmatism. The Border zone is a mass of fine-grained rocks that occurs along the border of the SIMI: it may be another magmatic component, or just the lateral border series of the SIMI. 相似文献
Zoning patterns of An content and Fe, Mg and Sr concentrations in plagioclase phenocrysts in andesites from Parinacota Volcano (N. Chile) reflect alternating recharge events with two chemically distinct mafic magmas. These magmas are characterized by low and high Sr contents, similar to two recent mafic flank eruptions. One end-member basaltic andesite shows large Sr enrichment and Heavy Rare Earth depletions and thus equilibrated with lower-crustal rocks at depth where plagioclase (high Sr) is unstable, and garnet (high HREE, Y) is stable. A second end-member magma is lower in Sr, Ba contents and has REE patterns typical for parent magmas elsewhere in the Central Andes.
The number of recorded recharge events increases after a catastrophic sector collapse and during the subsequent rebuilding of the stratocone. Variations of An, Fe and Mg contents and morphology of zones suggest also changes in water pressure, including decompression under water under-saturated and water-saturated conditions. Evidence for decompression is more present in post-collapse samples, suggesting that the change in the volcano dynamics involves changes in magma chamber location. This shows the importance of the cone collapse event in the volcano's magmatic evolution. We propose that both end-members are only seen at the surface of Parinacota Volcano because of the particular dynamics of this volcanic system and that similar processes might occur in other volcanic centres of the Central Volcanic Zone. 相似文献
The effects of deformation on the kinetics of the net-transfer reaction anorthite + forsterite → cpx + opx + spinel ± gt were
studied using static and shear deformation experiments. Experiments were performed on dry anorthite-olivine (An92–Fo93) samples at 900°C and pressures between 1,000 and 1,600 MPa in a Griggs apparatus. Deformed (‘non-hydrostatic’) and undeformed
(‘static’) samples are compared in terms of phase petrology, reaction rate and reaction mechanisms. Anorthite + olivine reactions
are diffusion-controlled as seen from reaction rim structures. In undeformed samples, delayed reaction onset and low reaction
rates demonstrate sluggish nucleation of reaction products and slow rates of diffusion at dry conditions, even at 700–900 MPa
confining pressure overstepping. The reaction rate is enhanced in deformed An–Fo samples. The higher rate is mainly attributed
to a combination of high stresses and viscous deformation processes of the reactants and products, which cause an increase
in the nucleation rate of products. The results imply that viscous deformation processes alone can be responsible for the
initiation and localisation of metamorphic reactions in dry rocks in the absence of fluid infiltration.
A. A. de Ronde was supported by the Swiss National Fond grants 2100-057092.99 and 2000-065041.01. 相似文献