Magmas Of Intermediate Composition Research Articles

Rheology of nanocrystal-bearing andesite magma and its roles in explosive volcanism

Recent petrological and experimental studies have proposed that explosive volcanism may originate from the formation of nanoscale crystals in magma and the resultant ductile–brittle transition. However, the rheology of magma with quantified volume fractions of nanoscale crystals has not been investigated before, and thus, the formation of nanoscale crystals causing magma fragmentation that explains the origin of explosive eruptions is not conclusive. Here, we investigate the rheology of andesite magma with nanoscale crystals (magnetite). For this, a glass fibre elongation experimental apparatus with a heating furnace was developed at the synchrotron radiation X-ray system (SPring-8). During melt elongation, we observed the formation of crystals using small-angle X-ray scattering and wide-angle X-ray diffraction. Our experimental data demonstrate that magma viscosity increases with the formation of nanoscale crystals, but the degree of the increase is much lower than that predicted from analogue materials. Finally, we conclude that nanocrystal formation in intermediate composition magmas cannot explain rheological transition and other mechanisms such as nanocrystal agglomeration (not observed in our experiments) and/or heterogeneous nucleation of gas bubbles on nanocrystals are required to induce mafic to intermediate explosive volcanisms.

Zircon U–Pb and titanite U–Th–Pb ages of the Ghorveh mixed granitoid pluton: Implications for the Late Jurassic supra‐subduction extension of the Sanandaj–Sirjan Zone, Iran

The Ghorveh batholith, located in the central part of the Sanandaj–Sirjan Zone (SSZ), Iran, preserves several sequences of Mesozoic plutonic and volcanic rocks. This study presents systematic analyses of the calc‐alkaline granites and monzodiorites from the southern part of the Ghorveh pluton. The granites are metaluminous to slightly peraluminous with A/CNK ratios of 0.97–1.02, indicating an A‐type granitic affinity. High Rb, Th, and K and low Sr, P, and Ti concentrations indicate an involvement of crustal materials. Whole‐rock negative εNd(t) values and relatively high TDM1 model ages support a continental crustal source. Thus, the granites are of fractionated A‐type in composition produced by partial melting of crustal materials. The monzodiorites have incompatible trace element patterns with enrichment in light rare earth elements (LREEs) and large‐ion lithophile elements (LILEs) relative to heavy rare earth elements and high‐field strength elements. This combined with the high Mg# values and the positive εNd and εHf(t) values, indicate a mantle‐derived origin. Dating results show that the granites and monzodiorites have uniform crystallization ages of ~150 Ma. Previous results and our new data suggest that the Jurassic subduction of Neo‐Tethyan Oceanic crust under the Eurasian continent was followed by slab rollback that induced back‐arc extension. Heat transfer from arc magmas increased the degree of decompressional melting in the thinned lithosphere. Melting of the presumably homogeneous crust was concurrent with emplacement of the intermediate composition magmas. Consequently, we propose that the Ghorveh area of the central SSZ developed in a supra‐subduction extensional phase during the Late Jurassic.

Chemical and isotopic characterization of the thermal fluids emerging along the North\u2013Northeastern Greece

Hydrochemical and isotopic characteristics of fluids from major geothermal fields of middle/low temperature in N/NE Greece are examined [basins: Strymon River (SR), Nestos River Delta (ND), Xanthi–Komotini (XK), Loutros–Feres–Soufli (LFS) and Rhodope Massif]. The geodynamic context is reflected to isotopic/chemical composition of fluids, heat flow values and elevated CO2 concentrations in emitted fluids. B and Li are derived from leaching of the geothermal systems hosting rocks. δ18OH2O, δ18OSO4, δ13CCO2 values and chemical compositions of Cl, B and Li of geothermal discharges suggest two distinct source fluids. Fluids in SR exhibit high B/Cl and Li/Cl ratios, suggesting these constituents are derived from associated magmas of intermediate composition (andesitic rocks). Geothermal discharges in LFS exhibit low B/Cl and Li/Cl ratios, implying acid (rhyolitic) magmatism. δ13CCO2 and CO2/(CO2 + 105He) ratios in the west part, suggest fluids affected by addition of volatiles released from subducted marine sediments. For the eastern systems, these ratios suggest gas encountered in systems issued from mixing of crustal and mantle-derived volatiles. Isotopic geothermometers reflect, for the same direction, equilibrium processes more (LFS, XK) or less (SR) pronounced and discriminate the geothermal field from low to middle [SR, ND (Erasmio)] and middle to high enthalpy [ND (Eratino), LFS, XK].

Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence, eastern India

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.

Petrogenesis of three East Fork Member rhyolites of the Jemez volcanic field, Valles caldera, New Mexico, USA

The most recent volcanism in the Valles caldera is represented, in stratigraphic order, by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), the VC-1 rhyolite (not studied herein), and Banco Bonito Flow (BBF), which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted approximately 74 to 68 ka after an approximate 460 ka lull in ring-fracture volcanism. Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides, in order of abundance. Electron probe microanalysis and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. Large (≥1 mm), typically resorbed or subhedral crystals represent one population, and small (≤0.5 mm), generally euhedral crystals represent the other. The large resorbed plagioclase crystals typically have oligoclase to andesine rim overgrowths. 40Ar/39Ar geochronology performed on euhedral biotite and groundmass glass from the BBF returned isochron ages of 478 ± 3 ka and 575 ± 2 ka and total gas ages of 125 ± 1 ka and 129 ± 1 ka, respectively. High molar Mg numbers of large euhedral biotite and 40Ar/39Ar ages older than the accepted age range indicate these crystals are xenocrystic. Radiogenic isotopes are consistent with mixing between the mantle and lower crustal reservoirs. General trends are evident between whole-rock major and trace elements with increasing SiO2 and Nb, respectively. In general, incompatible trace elements increase and compatible trace elements decrease with increasing Nb. For the major elements, MgO, P2O5, Al2O3, FeO* show decreasing trends while K2O and Na2O show increasing trends with increasing SiO2. Incompatible trace element ratios (Ta/Yb, Y/Yb, Th/Yb, Th/Nb) indicate the presence of a single magma batch. The heterogeneity in crystal morphology and chemistry can be explained by a model in which partial melting of mid- to deep continental crust occurred due to an intrusion of an intermediate composition magma. Magma mixing and an exchange of crystals took place between the partial melt and the intruding magma. The hybrid magma rose to the upper crust, where it underwent fractional crystallization prior to eruption. The geochemical and isotopic data from this study are best explained by a modified version of the rapid production and eruption model put forth by Huppert and Sparks (1988).

Temporal and spatial variations in characteristics of fluid inclusions in epizonal magmatic-hydrothermal systems: Applications in exploration for porphyry copper deposits

Abstract Temporal and spatial variations in the room temperature phase relations of fluid inclusions serve as a proxy for the PTX properties of fluids associated with emplacement and crystallization of a shallow epizonal intrusion following the “Burnham Model”. Accordingly, during magma ascent, hydrous magmas approach and achieve volatile saturation in response to decreasing pressure (“first boiling”) and/or as a result of cooling and crystallization of anhydrous mineral phases (“second boiling”). In both cases, a magmatic aqueous fluid phase will exsolve from the volatile-saturated silicate melt. Fluid inclusions that trap the exsolved magmatic fluid show predictable room temperature phase relations, compositions, and microthermometric characteristics indicative of the pressure and temperature conditions of the trapping environment. We model aqueous phase equilibria as a function of temperature, pressure, and fluid composition. The composition and phase behavior of the magmatic aqueous fluid phase is adequately approximated by the system H2O-NaCl, and the PVTX characteristics of the magmatic fluids may be determined at any temperature and pressure using phase equilibrium data for this binary system. Then, PVTX data for the system H2O-NaCl are used to predict the room temperature phase relations and homogenization temperatures of fluid inclusions that trap the magmatic fluid at various times and locations in the magmatic-hydrothermal system. The temperature at any position in the system is based on established numerical models of conductive and convective cooling of shallow plutons, and the pressure at any depth is calculated assuming that hydrostatic conditions apply at temperatures We apply the model to four different stages during the crystallization of a shallow, intermediate composition magma, beginning in the early stages of crystallization and immediately following initial volatile saturation, and ending with complete crystallization of the melt. Mapping the stable fluid phase fields onto a cross-section of the pluton allows us to predict the characteristics of fluid inclusions trapped at various times and locations in the overall system. The fluid inclusion characteristics are predicted along five depth profiles that extend from the center of the pluton to the periphery. The results show good agreement with numerous studies that describe the distribution of halite-bearing, vapor-rich, and two-phase, liquid-rich inclusions in porphyry copper deposits. The results of this study may be applied in exploration for porphyry copper mineralization, as well as other magmatic-hydrothermal porphyry deposits in which the magmatic fluids can be approximated by the system H2O-NaCl. Importantly, halite-bearing fluid inclusions occur only within the pluton and in the immediately surrounding wallrocks, which represents the part of the system that often hosts mineralization. Similarly, fluid inclusion assemblages comprised of only vapor-rich inclusions occur in the near-surface, sub-volcanic part of the system, whereas coexisting halite-bearing and vapor-rich inclusions characterize the deeper parts of the system. The distribution of fluid inclusion types constrains the vertical and lateral position within the overall magmatic-hydrothermal system and, thus, develops vectors towards the magmatic center of the system that is most likely to host economic mineralization.

Morphologic variation of an evolving dome controlled by the extrusion of finite yield strength magma

Abstract Degassing-induced crystallization in volatile rich intermediate composition magmas results in material stiffening and strengthening that prior to solidification is reflected in non-Newtonian rheology. We explore the effects of a spectrum of such rheological regimes on eruptive style and morphologic evolution of lava domes, using a two-dimensional (2D) particle-dynamics model for a spreading viscoplastic (Bingham) fluid. We assume that the ductile magma core of a 2D synthetic lava dome develops finite yield strength, and that deformable frictional talus evolves from a carapace that caps the magma core. Our new model is calibrated against an existing analytical model for a spreading viscoplastic lava dome and is further compared against observational data of lava dome growth. Results indicate that a degassing-induced increase in strength of the injected magma causes a transition in the lava dome morphology from a dome with low surface relief evolving endogenously (with apparent bulk yield strength - 104 105 – 106 Pa) extruded through the dome carapace. The virtual lava dome with τ0a = 0.6 MPa shows good agreement with the observed dome heights observed at the Soufriere Hills Volcano, Montserrat during a period of endogenous growth. The calculated apparent flow viscosity (1.36 × 1011 Pa·s for τ0a = 0.6 MPa) is in the range of estimated viscosities (109 to 1012 Pa·s) for andesitic-dacitic crystal-rich lavas. Our model results indicate a strong correlation between apparent yield strength and dome morphology, with both controlled by degassing-induced crystallization and extrusion rate.

Compositional and Thermodynamic Variability in a Stratified Magma Chamber: Evidence from the Green Tuff Ignimbrite (Pantelleria, Italy)

The Green Tuff Ignimbrite, Pantelleria, is compositionally zoned from pantellerite at the base to comenditic trachyte at the top, the variation apparently representing an inverted vertical zonation in the pre-eruptive reservoir. The main phenocryst assemblages are alkali feldspar + olivine + clinopyroxene + ilmenite + apatite in the trachytes and alkali feldspar + aenigmatite + clinopyroxene ± quartz in the rhyolites. Thermodynamic modelling indicates that the temperature range was ∼900–700°C, fO2 FMQ –1·5 to FMQ –0·5 (where FMQ is fayalite–magnetite–quartz buffer) and aSiO2 (relative to quartz saturation) 0·74–1·00. Melt water contents ranged from ∼1 wt % in the trachytes to ∼4 wt % in the pantellerites. Matrix glass analyses in the more evolved rocks are highly variable, showing that compositional layers in the upper parts of the reservoir, formed by fractional crystallization, were mixed during eruption, the proportion of rhyolitic to trachytic melts increasing towards the top of the reservoir. Some areas of glass have low Al2O3 contents (5·16–5·46 wt %) and high FeO* contents (9·66–10·02 wt %), making them the most evolved melts yet reported from Pantelleria. The new glass data reveal how whole-rock analyses do not truly reflect the complete range of melt compositions in the pre-eruptive reservoir. The trachytes contain >40% modal phenocrysts, which with relatively high Ba contents and positive Eu anomalies are considered to have originated in a feldspar-accumulitic layer. Phenocrysts in the trachytes are commonly heavily resorbed, inferred to be a result of heating by influxes of intermediate composition magma, which, however, were not erupted. It is argued that magmas of intermediate composition were present in the Green Tuff reservoir but were efficiently trapped in a crystal-rich layer below the Green Tuff magmas, which was eventually erupted during a resurgent phase as the Montagna Grande Trachyte.

Mafic replenishment of multiple felsic reservoirs at the Mono domes and Mono Lake islands, California

The Mono Basin has been the site of frequent volcanic activity over the past 60,000 years, including the emplacement of the Mono domes and Mono Lake islands. The Mono Basin lavas are the youngest and most poorly understood products of the Long Valley Volcanic Field. We have undertaken a study of Mono Basin volcanism encompassing whole-rock major and trace element, Sr, Nd, Pb, and O isotopic, and electron microprobe glass, plagioclase, and amphibole analyses. Variations in major and trace elements suggest that fractional crystallization of feldspar (Sr, K2O), apatite (P2O5), titanomagnetite (V), zircon (Zr), and allanite (La, Ce) has influenced the evolution of the Mono Basin lavas. Field observations, petrography, and chemistry together demonstrate that injection of more mafic magma is a common process throughout the Mono Basin. Mafic enclaves of the Mono domes are stretched and rounded, with chilled margins between enclave and host rhyolite. Thin sections reveal millimeter-scale inclusions of rhyolite in the enclaves and vice versa along the host-enclave border. Paoha Island dacite has glass with 67–72 wt% SiO2 and contains microscopic clots of more mafic glasses, with SiO2 contents as low as 64 wt%. Isotopically, the June Lake and Black Point basalts and the Mono dome enclaves represent the least evolved material in the Long Valley Volcanic Field, with 87Sr/86Sri 0.5126. The silicic Mono Lake lavas and Mono dome rhyolites display a significant crustal component, with 87Sr/86Sri >0.7058 and 143Nd/144Nd 19 and δ18O >+6.5‰. The Mono Lake lavas generally are younger and less evolved than the Mono domes, with enrichment in trace elements including Ba and Sr accompanied by lower 143Nd/144Nd and higher 206Pb/204Pb. This implies that the Mono domes and the Mono Lake lavas are derived from different magma batches, if not from separate magma chambers. There is no systematic relationship between the degree of chemical evolution and the lava ages, indicating that several magma batches have been involved in the development of the Mono domes complex. Pronounced differences in trace element composition (Nb, Y) and isotopic values between the Negit Island and Paoha Island lavas indicate that they, too, are produced by the evolution of at least two different batches of intermediate-composition magma.

Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado

The Never Summer Mountains in north-central Colorado, USA, are cored by two Oligocene, epizonal granitic plutons originally emplaced in the shallow levels of a short-lived (~1 m.y.), small-volume continental magmatic system. The younger Mt. Cumulus stock (28.015 ± 0.012 Ma) is a syenogranite equivalent compositionally to topaz rhyolites. A comparison to the chemical and isotopic composition of crustal xenoliths entrained in nearby Devonian kimberlites demonstrates that the silicic melts parental to the stock were likely derived from anatexis of local Paleoproterozoic, garnet-absent, mafic lower continental crust. In contrast, the older Mt. Richthofen stock is compositionally heterogeneous and ranges from monzodiorite to monzogranite. Major and trace element abundances and Sr, Nd and Pb isotopic ratios in this stock vary regularly with increasing whole rock wt% SiO2. These data suggest that the Mt. Richthofen stock was constructed from mixed mafic and felsic magmas, the former corresponding to lithosphere-derived basaltic magmas similar isotopically to mafic enclaves entrained in the eastern portions of the stock and the latter corresponding to less differentiated versions of the silicic melts parental to the Mt. Cumulus stock. Zircon U–Pb geochronology further reveals that the Mt. Richthofen stock was incrementally emplaced over a time interval from at least 28.975 ± 0.020 to 28.742 ± 0.053 Ma. Magma mixing could have occurred either in situ in the upper crust during basaltic underplating and remelting of an antecedent, incrementally emplaced, silicic intrusive body, or at depth in the lower crust prior to periodic magma ascent and emplacement in the shallow crust. Overall, the two stocks demonstrate that magmatism associated with the Never Summer igneous complex was fundamentally bimodal in composition. Highly silicic anatectic melts of the mafic lower crust and basaltic, mantle-derived magmas were the primary melts in the magma system, with mixing of the two producing intermediate composition magmas such as those from which Mt. Richthofen stock was constructed.

Synthetic fluid inclusions XIX. Experimental determination of the vapor-saturated liquidus of the system H2O–NaCl–FeCl2

Abstract Magmatic-hydrothermal fluids associated with felsic to intermediate composition magmas are generally dominated by (Na ± K)Cl, but often the fluids also contain significant concentrations of FeCl 2 . Previously, fluid inclusions containing such fluids were interpreted using the properties of H 2 O–NaCl because the effect of FeCl 2 on the phase equilibrium and volumetric ( PVTx ) properties of aqueous fluids was essentially unknown. In this study, synthetic fluid inclusion experiments have been conducted to determine the vapor-saturated liquidus phase relations of the system H 2 O–NaCl–FeCl 2 . Microthermometric and microanalytical measurements on synthetic fluid inclusions have been combined with the limited existing data, as well as with predictions based on Pitzer’s formalism, to determine the ternary cotectic and peritectic phase boundaries and liquidus fields. The liquidus is qualitatively similar to those of other ternary systems of H 2 O–NaCl plus divalent-cation chlorides (MgCl 2 and CaCl 2 ) and has been characterized through empirical equations that represent the liquid salinity on the ice- and halite-liquidus surfaces. The ice and halite liquidi intersect at a metastable cotectic curve, which can be used to determine fluid compositions in this system if metastable behavior is observed. Furthermore, based on the experimentally determined liquidus, bulk salinities of natural fluid inclusions can be determined from the last dissolution temperatures of ice and/or halite using the new empirical equations.

Influence of extrusion rate and magma rheology on the growth of lava domes: Insights from particle-dynamics modeling

Abstract Lava domes are structures that grow by the extrusion of viscous silicic or intermediate composition magma from a central volcanic conduit. Repeated cycles of growth are punctuated by collapse, as the structure becomes oversized for the strength of the composite magma that rheologically stiffens and strengthens at its surface. Here we explore lava dome growth and failure mechanics using a two-dimensional particle-dynamics model. The model follows the evolution of fractured lava, with solidification driven by degassing induced crystallization of magma. The particle-dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh-based analyses due to mesh entanglement effects. The deformable talus evolves naturally as a frictional carapace that caps a ductile magma core. Extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure. This new model is calibrated against existing observational models of lava dome growth. Results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate. The effects of extrusion rate on magma rheology are sensitive to material stiffness, which in turn is a function of volatile content and crystallinity. Material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability. Degassing induced crystallization causes material stiffening and enhances material strength reflected in non-Newtonian magma behavior. The increase in stiffness and strength of the injected magma causes a transition in the style of dome growth, from endogenous expansion of a ductile core, to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse. Simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re-direction of magma flow, creating a new shear zone and the switching of dome growth from one side to the other. Our model shows similar dome growth dynamics as observed at Soufriere Hills Volcano, Montserrat, indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology.

Geochemistry of Upper Ordovician metabentonites and their cognate apatite microphenocrysts from Norway and Sweden

A suite of metabentonites (altered volcanic ashes or K-bentonites) from the Sandbian Stage, Upper Ordovician, from Scandinavia was subjected to a detailed geochemical study which highlighted specific correlations and magmatic processes. A bimodal distribution in elemental compositions for one suite of metabentonites suggested that it was generated by magma mixing processes. There is strong evidence to correlate three Grefsen samples from Fjäcka, Sweden, with the lowermost Grefsen samples from Sinsen, Oslo, Norway. New data for rare earth element (REE) concentrations in cognate apatite microphenocrysts support the bulk trace element scenarios and confirm the role of plagioclase feldspar and pyroxene ± hornblende fractionation in the formation of eruptive magmas of intermediate composition formed during the closure of the Iapetus Ocean and Tornquist Sea in Upper Ordovician times. New REE data from apatite microphenocrysts can assist in correlating these metabentonites with contemporary beds elsewhere in Northern Europe.

Copper–Gold–Sulphide Mineralization Associated with Late Precambrian Volcanic Ring Structures, Southern Sinai, Egypt

The paper considers the Precambrian ring structures in an island arc and their associated mineralization in the southern Sinai. Several ring structures in an island arcsetting have been recognized within the volcanic field in southern Sinai including the Kid Ring structure, St. Katarina Ring Structure, and the Iqna Ring Structure. All ring structures are associated with mineralization, the largest one is the Kid Ring Structure located in southeastern Sinai, and represented by a succession of Dokhan volcanic and Hammamat sediments intruded by younger granites and albitite. The mineralization includes stockwork, rhyolite bearing-sulphide, alteration zones in the El Samra area, alteration zones, gossans, quartz veins, silicified carbonate metatuffs, and skarn mineralization in the Um Zerriq area. Geochemical investigation reveals that Wadi El Samra is dominated by Au–Cu–Zn. Wadi Ghorabi–El Hatemeia is dominated by Cu–Au mineralization, while the Wadi Khashm El Fahk is dominated by Cu–Au–Zn in the El Samra area. Um Zerriq I is dominated by Au–Cu-dominated mineralization, and the Um Zerriq II and III are dominated by Zn–Pb–Cu–Au. Major mineralization of the south Sinai is closely associated with last phase of volcanism within the ring structure, and is closely related in time and space to porphyritic, intermediate composition magma emplaced within the ring complex structures.

Porphyry copper deposits: strike–slip faulting and throttling cupolas

Porphyry copper deposits sometimes form during the solidification of stocks of relatively oxidized magma of intermediate composition. Most workers have assumed ore-forming systems have special chemical attributes, but none has been found that is useful to guide exploration efforts. Stocks can form where strike–slip movements generate pull-apart pathways into which intrusions can rise from batholithic magma chambers. Upwelling of buoyant, bubble-bearing magma along the sides of a stock brings magmatic fluid to shallow depths where large bubbles can separate and pool under the cupola separating solidified igneous rock from mobile magma. Where rapid seismogenic movement on the bounding strike–slip fault ruptures the solidified, but hot and ductile carapace, downward propagating extension fractures can drain an accumulation of magmatic fluid. Decompression and cooling of fluid that jets upward into extension fractures causes mineral precipitation. Where strike–slip movements cause pull-aparts to dilate with sufficient recurrences – from decades to perhaps a century or so, throttling of the fluid accumulation acts as a safety valve that prevents explosive detonation of the system. Concurrently, the upward infiltration of magmatic fluid from the cupola is strongly focused into the pull-apart and generates the characteristic concentric alteration zones that guide exploration drilling. We conclude that porphyry copper ore deposits form where strike–slip movements are concurrent with the early stages of deep-seated bubbling (≳6 km) along the walls of a rapidly cooling stock of magma. Supergiant deposits form where the bubbling front extends into the top of a parent batholith.

The end of continental growth by TTG magmatism

Terra Nova, 25, 130–136, 2013AbstractHigh‐Al2O3 tonalite, trondhjemite and granodiorite (TTG) magmas characterise felsic Archaean crust, yet are uncommon in the post‐Archaean rock record. Consequently, understanding the petrogenesis of these rocks provides valuable insights into early Earth processes. Fluid‐absent slab melting represents the dominant hypothesis for the origin of these rocks; however, the absence of voluminous magmas of intermediate composition formed concurrently with these TTGs is incompatible with expectations of slab water loss prior to slab melting. This study demonstrates that for reasonable Archaean mantle temperatures, slab‐derived water is captured by an anatectic zone near the slab surface, which melts via reactions that consume quartz, clinopyroxene and water to produce high‐Al2O3 Archaean trondhjemite. Late in the Archaean, the mantle cooled sufficiently to prevent wet melting of the slab, allowing slab water to migrate into the wedge and produce intermediate composition magmatism, which has since been associated with subduction zones.

The mafic–silicic layered intrusions of Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands), Armorican Massif: Gabbro–diorite layering and mafic cumulate–pegmatoid association

Abstract The Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands) Intrusive Complexes (hereafter referred to as SJIC and NGIC, respectively) are examples of mafic–silicic layered intrusions in the Armorican Massif. Both are characterized by the occurrence of (1) a basal/peripheral gabbroic unit interlayered with sheets (generally dioritic in composition, occasionally gabbroic) and crossed by leucocratic diapirs and pipes (from monzodioritic to Q-monzonitic in composition), (2) peripheral pegmatoids associated with mafic cumulates and (3) coeval granitoids. Beside these main similarities, some contrasted features lead us to propose two distinct models of formation. The Variscan SJIC includes tholeiitic mafic rocks (monzogabbro) that locally mingle and mix with leucocratic components (monzonite or Q-monzonite). The Cadomian NGIC is calc-alkaline. The SJIC sheet-bearing gabbro is homogeneous from a petrologic point of view, whereas the NGIC exhibits gabbroic macrorhythmic sequences with mineral layering. The Sr–Nd isotopic compositions of the SJIC gabbros are significantly different from those of the associated dioritic layers. This is not the case in the NGIC where the magmas could be cogenetic. We argue that the SJIC gabbro was a liquid that crystallized in situ without significant crystal settling. By contrast, the rhythmic sequences of the NGIC are consistent with crystal accumulation. Subsequently, both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition. We interpret the sheets in the SJIC as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below, at levels of neutral buoyancy. Injections and convection in the central part of the reservoir possibly resulted in spectacular mixing/mingling structures. In the NGIC, the emplacement of the dioritic sheets was rather controlled by pre-existing rhythmic cumulative structures. In both intrusions, late differentiated diapirs were extracted from the dioritic sheets. Associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase. This extraction might have been enhanced by the disruption of the peripheral cumulate stack, perhaps following pressure drops.

Intermediate composition magma production in an intracontinental setting: Unusual andesites and dacites of the mid-Miocene Santa Rosa–Calico volcanic field, Northern Nevada

Abstract The mid-Miocene Santa Rosa–Calico volcanic field (SC) of northern Nevada provides an outstanding example of the role open-system magmatic processes play in producing calc-alkaline and tholeiitic andesite–dacite magmas in an intracontinental setting. SC volcanism commenced at ∼ 16.7 Ma and is associated with the initial manifestations of the Yellowstone hotspot, the Columbia River–Steens flood basalt event(s), and the formation of the Northern Nevada rift. Locally a diverse package of magmatic products ranging from tholeiitic basalt to high-Si rhyolite was produced during an ∼ 2 myr duration. Within this package are the products of at least four distinct intermediate composition magmatic systems that may represent as much as 40% of the SC volcanic pile. These help differentiate the SC from contemporaneous Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee, Northwest Nevada) that are dominated by bimodal basalt–rhyolite assemblages. All SC intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide. SC intermediate units are dominantly tholeiitic, but include lava flows with transitional to calc-alkaline affinities. Relative to locally erupted Steens Basalt, SC intermediate lava flows have similar elemental enrichments and depletions, but dissimilar Sr and Nd isotopic compositions. These isotopic differences, coupled with the abundant disequilibrium features and variable incompatible element ratios, indicate that open system magmatic processes played a major role in the genesis of the intermediate units. SC silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous Cretaceous granitoid. Interaction between fractionating mafic Steens flood basalt magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC tholeiitic and calc-alkaline intermediate magmas. Between system heterogeneities primarily reflect magma- versus assimilation-dominated mixing. These complex processes and the resulting production and eruption of SC intermediate composition melts were stimulated by incipient, focused lithospheric extension and driven by the virtually continuous local availability of upwelling Steens Basalt, over at least a 2 myr duration. These results demonstrate that at least some of the mid-Miocene and younger calc-alkaline magmas that are found across the Pacific Northwest and are unrelated to subduction processes, were formed as a direct result of open-system processes affecting regional flood basalt magmas, rather than just melting of previously enriched (via subduction) lithospheric mantle as suggested by previous workers.

Petrologic Reconstruction of Magmatic System Variables and Processes

Explosive volcanic eruptions constitute a major class of natural hazard with potentially profound economic and societal consequences. Although such eruptions cannot be prevented and only rarely may be anticipated with any degree of accuracy, better understanding of how explosive volcanoes work will lead to improved volcano monitoring and disaster mitigation. A major goal of modern volcanology is linking of surface-monitored signals from active volcanoes, such as seismicity, ground deformation and gas chemistry, to the subterranean processes that generate them. Because sub-volcanic systems cannot be accessed directly, most of what we know about these systems comes from studies of erupted products. Such studies shed light on what happens underground prior to and during eruptions, thereby providing an interpretative framework for post hoc evaluation of monitoring data. The aim of this review is to present some of the current petrological techniques that can be used for studying eruptive products and for constraining key magmatic variables such as pressure, temperature, and volatile content. We first review analytical techniques, paying particular attention to pitfalls and strategies for analyzing volcanic samples. We then examine commonly used geothermometry schemes, evaluating each by comparison with experimental data not used in the original geothermometer calibrations. As there are few mineral-based geobarometers applicable to magma storage regions, we review other methods used to determine pre-eruptive magma equilibration pressures. We then demonstrate how petrologically-constrained parameters can be compared to the contemporaneous monitoring record. These examples are drawn largely from Mount St. Helens volcano, for which there are abundant petrological and monitoring data. However, we emphasize that our approaches can be applied to any number of active volcanoes worldwide. Finally, we illustrate the application of these techniques to two different types of magmatic systems—large silicic magma chambers and small intermediate-composition magma storage regions—with particular focus on the combined evolution of melt …

Geochemistry and petrology of Mesozoic-Cenozoic magmatic complexes of the southern framing of the Aldan shield: Geodynamic problems

This paper summarizes the results of long-term geological, petrological, and geochemical investigations of the Mesozoic-Cenozoic complexes of the Stanovoy Range in order to determine the main reasons for their generation and evolution. The analysis of this material showed that the compositionally variable Late Mesozoic igneous complexes of the Stanovoy Range were formed in various depth facies, from abyssal to surficial. The majority of their salic complexes show minor compositional variations, whereas the mafic complexes are more variable, especially in the southeast of the region. The southeastern Stanovoy Range comprises comparable amounts of both subalkaline and low-alkali igneous rocks, whereas the central part is dominated by subalkaline rocks, and the northwestern part contains rocks only of the shoshonite-latite series. This zoning is fundamentally different from that of typical island arcs, which are characterized by the occurrence of volcanic rocks of similar alkalinity in each zone. Extrusive and intrusive rocks with similar alkali and silica contents (and schlieren-like inclusions in the granitoids of the region) were formed from common magmas of corresponding chemical compositions. In addition, the mafic and most of the salic magmas were formed as independent melting products, whereas the magmas of intermediate composition were formed mainly by mixing of chemically contrasting liquids (i.e., salic and basic). It was shown that the available information on the magmatism of the region is best interpreted in terms of the model of mantle diapirism. In particular, mantle diapirs ascended rather slowly during the Mesozoic and occurred over the whole territory of the Stanovoy Range during the Jurassic-Cretaceous stage (J3-K1), when alkaline and subalkaline basalts were formed. During the Early-Late Cretaceous stage, mantle diapirs produced alkali-poor basalts in the central and eastern parts. During the Cenozoic, the diapir ascended rather rapidly but only in a small area in the eastern part of the region forming alkali basalts. In contrast to the Cenozoic, the Earth’s crust was strongly affected by mantle diapirs and related mafic magmas in the Mesozoic. As a result, crustal sequences were reworked by fluids and subsequently yielded tremendous volumes of compositionally corresponding salic magmas, which interacted and mixed with mafic magmas producing the corresponding chemical zoning. The maximum generation of crustal magmas was confined to the axial zones of ascending diapirs, where the highest energy effects took place, whereas the role of autochthonous gneissic granites increased away from the axis at the expense of typical intrusive complexes.