Reverse Zoning Research Articles

Zoning of laurite (RuS2)erlichmanite (OsS2): implications for the origin of PGM in ophiolite chromitites

We have investigated several chromite deposits in the Mayari-Baracoa Ophiolite Belt (eastern Cuba) and in the Dobromirtsi metamorphosed ultramafic (ophiolitic) massif (SE Bulgaria) with regard to zoning in platinum-group minerals (PGM) of the laurite (RuS 2 )–erlichmanite (OsS 2 ) solid solution series. We found several zoned laurite–erlichmanite grains all included in unaltered chromite crystals. On the basis of internal ordering and compositional variations, three different patterns of zoning have been distinguished: (i) grains with Os-poor (laurite) core and Os-rich rim (normal zoning), (ii) grains with Os-rich core and Os-poor rim (reverse zoning) and (iii) grains made up of a complex intergrowth of Os-rich, Os-poor laurite and erlichmanite (oscillatory zoning). The origin of zoning is interpreted mainly as a result of changes in f (S 2 ), f (O 2 ) and to a lesser extent in melt temperature, before PGM trapping in chromite. A possible case of heterogeneous physicochemical environment in which such changes can take place is when chromite forms during magma mingling of silicate melts in the upper mantle. The preservation of laurite–erlichmanite zoning is attributed to the low diffusion coefficient of Ru and Os in pyrite-type structures.

Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion

The intercumulus liquid of a crystal mush fills pore spaces, and typically solidifies to form overgrowths on cumulus grains and poikilitic post-cumulus minerals. If the liquid is immobile, solidification produces zoned intercumulus minerals, as a result of progressive fractionation of the residual liquid. Convection within the mush results in buffering of the liquid composition, and thus limits mineral zonation. For fully solidified cumulates, fossil' changes in liquid composition or porosity are difficult to identify. However, detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material. Ti contents of plagioclase overgrowths, in samples from the lowermost parts of the Skaergaard Intrusion, show strong variations with anorthite content. With decreasing XAn, Ti concentrations first rise and then fall, consistent with changing TiO2 contents of the intercumulus liquid during solidification. TiO2 in plagioclase decreases sharply at An55, reflecting local saturation of Fe-Ti oxides. Ti in clinopyroxene oikocrysts also falls rimward, but zoning in faster diffusing species (Fe, Mg) is limited. Other than slight reverse zones that may occur on the plagioclase margins, XAn falls continuously during crystallization. The reverse zoning is interpreted as the result of compaction-driven dissolution and reprecipitation of plagioclase. The continual decrease in XAn is exploited, together with back-scattered electron images of the cumulates, to produce calibrated images showing regions of progressive crystallization. This allows the regions crystallizing at each stage of solidification to be visualized. These images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain-boundary melt films. This approach can provide information about the progressive reduction of porosity during cumulate solidification.

LA-ICP-MS zircon U-Pb dating and phenocryst EPMA of dikes, Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution

Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the altered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes. Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst’s mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr2O3 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206Pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GS1 and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously published data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula. These also imply that the intensive crust-mantle interaction and asthenospheric underplating had occurred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).

Geochemistry of granitic aplite-pegmatite sills and their minerals from Arcozelo da Serra area (Gouveia, central Portugal)

Granitic aplite-pegmatite sills intruded a granodiorite-granite and a biotite ≈ muscovite granite from Arcozelo da Serra (Gouveia, Portugal). A muscovite > biotite granite also crops out in the area. Variation diagrams of major and trace elements of the rocks show fractionation trends for a) granodiorite-granite and muscovite > biotite granite; b) biotite≈muscovite granite and aplite-pegmatite sills. REE patterns and δ 18 O of rocks, anorthite contents of plagioclases, Ba contents of potash feldspars, major elements and Li of biotites and muscovites confirm the two series. Least squares analysis of major elements and modelling of trace elements indicate that aplite-pegmatite sills were derived from biotite ≈ muscovite granite magma by fractional crystallization of quartz, plagioclase, potash feldspar and biotite. This mechanism is responsible for the Sn enrichment of aplite-pegmatite sills and Sn is retained in micas. Electron microprobe analyses of columbite-tantalite crystals from aplite-pegmatite sills show oscillatory, progressive and reverse zonings, which are characterized by the behaviours of eight elements and Mn/(Mn+Fe) and Ta/(Ta+Nb) ratios. Oscillatory zoning is mainly attributed to faster crystal growth than Nb, Ta, Fe and Mn can diffuse through liquid, while reverse zoning is due to nucleation and growth of evolved oxide cores and back-reaction of them with the more primitive bulk magma. Other samples of aplite-pegmatite sills show late zoned micas, consisting mainly of a Li-bearing muscovite core and a composition between zinnwaldite and trilithionite for the rim. However, alternating compositions of these two micas with relics of primary muscovite also occur. Late micas are derived from a phase melt enriched in F and Li.

Garnet-bearing tonalitic porphyry from East Kunlun, Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO 17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (eNdT = −1.38 to −2.33; 87Sr/86Sri = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.

Geochemistry of mafic phenocrysts from alkaline lamprophyres of the Spanish Central System: implications on crystal fractionation, magma mixing and xenoliths entrapment within deep magma chambers

The Permian alkaline lamprophyres from the Spanish Central System (SCS) are highly porphyritic rocks which carry a heterogeneous population of clinopyroxene and kaersutite zoned phernocrysts. Clinopyroxene phenocrysts may show 1) normal zoning (Cpx-I), 2) reverse zoning with Fe-rich green cores (Cpx-II), and 3) reverse zoning with colourless Al-poor, silica-rich cores (Cpx-III). Kaersutite phenocrysts also show a slight reverse zoning. Major and trace element composition of Cpx-I suggests that their compositional variation is related to a crystal fractionation process from melts similar to the host lamprophyres. The Cpx-II cores represent crystallization from highly evolved melts (low Mg-Cr contents and incompatible element enrichment), genetically related with the SCS alkaline magmatism, and the growth or surrounding Mg-rich inner rims points to a magma mixing process. The major and trace element composition of Cpx-III cores supports derivation from a magma which has fractionated plagioclase. This characteristic, together with their similarities when compared to clinopyroxenes from charnockite xenoliths, suggests that they might be xenocrysts from deep calc-alkaline cumulates. The composition of melts in equilibrium with clinopyroxene and amphibole phenocrysts supports a model in which Cpx-II and Cpx-III cores would have been incorporated into a more primitive lamprophyric magma stagnated at lower crustal levels. The low pressure composition of all phenocryst outer rims indicates that they crystallised directly from the host alkaline magma at their subvolcanic emplacement levels.

Hybrid magma generation preceding Plinian silicic eruptions at Hekla, Iceland: evidence from mineralogy and chemistry of two zoned deposits

Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO2, with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model involves partial melting of older tholeiitic crust to produce silicic magma, which segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic andesite dyke injection, with a proportion of the dyke magma contributing to the production and eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of the hybrid magma depend on the pre-eruptive repose time.

Petrogenesis of acapulcoites and lodranites: A shock-melting model

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3–6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe–Ni and troilite, polycrystalline kamacite, fine-grained metal–troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe–Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4–14 mol %) and low-Ca pyroxene Fs (3–13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16–18, compared to 32–38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe–Ni–S melt. Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.

Effect of temperature-pressure change to the formation of reverse zoning

Effect of temperature-pressure change to the formation of reverse zoning

Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning

Phenocryst zoning patterns are used to identify open-system magmatic processes in the products of the 2001 eruption of Shiveluch Volcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclase andesites with average pre-eruptive temperatures of 840C and fO2 of 1.5–2.1 log units above nickel–nickel oxide (NNO). Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Apatite microphenocrysts display normal and reverse zoning of sulphur. The textural similarity of patchy hornblende and plagioclase, together with Ba–Sr systematics in patchy plagioclase, indicate that the cores of these crystals were derived from cumulate material. Plagioclase–liquid equilibria suggest that the patchy texture develops by resorption during H2O-undersaturated decompression. When H2O-saturated crystallization recommences at lower pressure, reduced pH2O results in lower XAn in plagioclase, causing more Al-rich hornblende to crystallize. Plagioclase cores with diffuse oscillatory zoning, and unzoned hornblende crystals, probably represent a population of crystals resident in the magma chamber for long periods of time. In contrast, oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization. Increasing Fe/Al in oscillatory zoned rims suggests oxidation as a result of degassing of H2O during decompression. A general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization. We present evidence that the host andesite has mixed with both more felsic and more mafic magmas. Olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt. Sieve-textured plagioclase resulted from mixing of a more felsic magma with the host andesite. The mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma, which implies efficient thermal quenching, and thus small volumes of intruding magma. Mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents. We suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber, producing incremental variations in whole-rock compositions.

ZONALIDADE REVERSA DOS GRANITOS DO COMPLEXO INTRUSIVO LAVRAS DO SUL, RS: PETROGRAFIA, SUSCETIBILIDADE MAGNÉTICA E ÓXIDOS DE FE-TI

We evaluate the magma redox conditions of granites from the Lavras do Sul intrusive complex - LSIC (ca. 606-586 Ma). The main goal is to explain the zoning of magnetic susceptibility-k that shows a concentric pattern revealing the reverse zoning of petrographic facies. The granite pluton includes: (a) alkali-calcic, biotite granodiorite and hybrid amphibole-biotite granites in the core (k of 4.0-7.5 10 -3 SI), and (b) alkaline, biotite-amphibole syenogranite and perthite granite that occur around the core (k of 9-10 10 -3 SI). All facies are ferromagnetic, and the differences in k-values are due to the distinct nature of parental magmas and to the variations in redox path shown by mafic phases (amphiboles, biotite and Fe-Ti oxides). Divergent Fe/Mg ratios between mafic silicates and rock imply an oxidizing environment for the crystallization of core types, in which the two Fe-Ti oxides are common (Ti-Mt:Ilm of 3:1 to 10:1). Such conditions were ascribed to the magmatic fluid phase on distinct situations, of crystal-volatile equilibrium (monzogranite) and of volatile exsolution (granodiorite). This reinforces that the core-granites represent distinct pulses of a hydrous magma (3-4 wt. % H 2 O) evolving at different pressure (4-2 kbar). The parental magma of alkaline granites had lower H 2 O content (~2 wt. %), and would be intrinsically oxidizing as suggested abundant Ti-magnetite (Ti-mt:Ilm ≥ 20:1) and primary titanite. Convergent Fe/Mg ratios for mafic phases and rock imply the mineralogical control in the redox path also marked by the decrease of pressure with differentiation (2-1 kbar). We conclude that the pluton formed during two major episodes of resurgence in a magma chamber related to the volcano-plutonic subsidence system-LSIC: a central event responsible for the alkali-calcic granites, and a circumferential one for the alkaline types. So, petrographic and magnetic zoning, strengthened by magnetic and gravity data, show such differences. Despite the low k-values in the core granites, a magmatic environment with varied fO 2 , allows the genetic link with the Au-Cu ore occurrences.

Dissolution of tonalitic enclaves in ascending hydrous granitic magmas: An experimental study

Dissolution of natural mafic magmatic enclaves in a hydrous leucogranitic synthetic melt has been tested experimentally. Results suggest that the mechanism of enclave dissolution is a potential hybridization process in granitic systems. Experiments performed in decompression, simulating ascending magmas, show interesting results: from 10 kbar to 4 kbar, for a given temperature, enhanced dissolution of the tonalitic enclave into the melt has been observed, compared with experiments at constant pressure. Furthermore, the composition of the melt changed to higher CaO, FeO and MgO contents. Dissolution textures on relict crystals from the tonalitic enclave were monitored. These results have implications for the generation of peraluminous monzogranites and granodiorites and an example for the Cabeza de Araya (Cáceres, Spain) from the “serie mixta” of the Iberian Massif is given. The tonalitic enclaves that are partially dissolved may be early or coeval intrusions into the granite magma or even into the source migmatitic area. Many mineralogical, geochemical and isotopic features of monzogranites (e.g.: reverse zoning in plagioclase, low Sr isotopic ratios) are accounted for by this mechanism of enclave dissolution during ascent and emplacement.

2001 flank eruption of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's evolution in the last three decades

Since the early 1970s enhanced eruptive activity of Mount Etna has been accompanied by selective geochemical changes in erupted lavas, among which a gradual increase of alkalis whose origin is still debated. Here we provide further insight into the origin of this recent evolution, based on a detailed study of the chemistry and dissolved volatile content of melt inclusions trapped in olivine crystals of unusual plagioclase-poor primitive basalt that was extruded during a highly explosive flank eruption in July–August 2001. Two types of lava were erupted simultaneously along a N–S fracture system. Trachybasalts from the upper vents (2950–2700 m) were simply drained out by fracturing of the central volcanic conduit. They are identical to summit crater lavas and contain Mg-poor olivines (Fo70–72) with evolved and volatile-poor melt inclusions that represent late-stage crystallisation during shallow open conduit degassing. In contrast, plagioclase-poor basalt (80% of total) extruded through the lower vents (2550–2100 m) derived from lateral dyke intrusion of a more primitive and volatile-rich magma across the sedimentary basement. This primitive melt is best preserved in rare Fo82.4–80.5 skeletal olivines present in lapilli deposits from the most powerful activities at the 2550 m vent. Its high dissolved contents of H2O (3.4 wt.%), CO2 (0.11 to 0.41 wt.%), S (0.32 wt.%), Cl (0.16 wt.%) and F (0.094 wt.%) point to its closed system ascent from ∼400 to 250 MPa (∼12 to 6.5 km depth b.s.l.). However, the predominance of euhedral olivine phenocrysts with common reverse zoning (cores Fo76–78 and rims Fo78–80) and decrepited inclusions shows that most of the erupted basalt derived from a slightly more evolved, crystallizing body of the same magma that was invaded by the uprising primitive melt prior to erupting. The few preserved inclusions in these olivines indicate pre-eruptive storage of that magma body at about 5 km depth b.s.l., in coherence with seismic data.We propose that the 2001 flank eruption resulted from gradual overpressuring of Etna's shallow plumbing system due to the influx of volatile-rich primitive basalt that may have begun several months in advance. We find that this basalt is much richer in alkalis (2.0 wt.% K2O) and has higher S/Cl (2.0) but lower Cl/K and Cl/F ratios than all pre-1970s Etnean lavas (1.4 wt.% K2O, S/Cl=1.5), as further exemplified by melt inclusions in entrained olivine xenocrysts. Combining these new observations with previously published data, we argue that the 2001 basalt represents a new alkali-rich basic end-member feeding Mt. Etna, only few amount of which had previously been extruded during a 1974 peripheral eruption and, more recently, during brief paroxysmal summit events. Over the last three decades this new magma has progressively mixed with and replaced the former K-poorer trachybasalts filling the plumbing system, leading to extrusion of gradually more primitive and alkali-richer lavas. Its geochemical singularities cannot result from shallow crustal contaminations. Instead, they suggest the involvement of an alkali-richer but Cl-poorer arc-type component during recent magma genesis beneath Etna.

Origin of reverse zoning: basic concept

Origin of reverse zoning: basic concept

Origin of Minette by Mixing of Lamproite and Dacite Magmas in Veliki Majdan, Serbia

Composite dykes consisting of leucominette and dacite as well as discrete dykes and flows of minette and lamproite composition, occur in the Veliki Majdan area, western Serbia. This area is part of the Serbian Tertiary magmatic province, which consists of numerous small occurrences of ultrapotassic igneous rocks. The composite dykes have leucominette margins (up to 150cm thick) enclosing a central part of dacite up to 100m in width. Between these two lithologies, a decimetre-sized transition zone may occur. Petrography, mineral chemistry and bulk-rock geochemistry, including Sr, Nd and Pb isotopes, provide evidence that the minettes and leucominettes formed by hybridization between a felsic magma similar in composition to dacite and a mantle-derived lamproitic magma. The leucominettes and minettes contain all phenocryst types (biotite, plagioclase, quartz) present in the dacites, but in partly resorbed and reacted form. The mica displays a great diversity of resorption textures as a result of partial dissolution, incipient melting and phlogopitization, suggesting superheating of the felsic melt during hybridization; the mineral modes and mineral compositions of the leucominettes and minettes resemble those in the lamproites. A model for the modification of lamproite melt towards minette is presented in which minette is formed by mixing of lamproite and530% felsic magma. The lack of any significant correlation between Pb isotopic ratios and some of the ‘mixing-indices’ (SiO2, Zr, Zr/Nb, 143 Nd/ 144 Ndi) recognized in the hybridization model for the Veliki Majdan dykes may be a result of similarity of the Pb-isotopic signature in the two end-members. Highly phlogopitized biotite xenocrysts in the minettes are ascribed to the retention of volatile components after magma mixing and crystallization of a new generation of phlogopite from the hybridized magma. The magma-mixing model explains the reverse zoning and resorption features of phlogopite macrocrysts commonly recognized in calcalkaline lamprophyres elsewhere. Therefore, this mixing mechanism may be globally applicable for the origin of minettes associated with calcalkaline granitic plutonism in postorogenic settings.

Origin of chemically zoned and unzoned cordierites from the South Mountain and Musquodoboit Batholiths, Nova Scotia

ABSTRACTTextural relations and chemical zoning of cordierites in granites act as sensitive recorders of the conditions of their crystallisation history and underlying magma chamber processes. In this contribution, we present new data on texturally distinct and variably zoned cordierites from the late-Devonian, granitic South Mountain and Musquodoboit Batholiths, and infer the conditions of their formation. Using a combined textural (grain size, grain shape and inclusion relationships) and chemical (major element composition and compositional zoning) classification, we recognise the following six cordierite types: CG1/TT1, anhedral to subhedral macrocrysts with random inclusions and patchy normal zoning; CG2a/TT2, euhedral to subhedral macrocrysts with random inclusions and normal zoning; CG2b/TT2, euhedral to subhedral macrocrysts with random or oriented inclusions, and oscillatory zoning; CG3a/TT3, subhedral to euhedral microcrysts with no inclusions and reverse zoning; CG3b/TT4, euhedral macrocrysts with no inclusions and no zoning; and CG4/TT5, anhedral macrocrysts with random inclusions and normal zoning. The textural criteria suggest that these cordierites formed as a product of cotectic crystallisation from a melt, or as the result of a peritectic reaction involving country-rock material. The combined chemical and textural criteria suggest that: (1) normal zoning results from cotectic crystallisation during cooling, cotectic overgrowths on grains formed in a peritectic reaction with country-rock material, or cation exchange with a fluid; (2) oscillatory zoning results from cotectic crystallisation during variations in XMg of the silicate melt following magma replenishment; (3) reverse zoning results from crystallisation during pressure quenching; and (4) the unzoned cordierite results from cotectic crystallisation under fluid-rich conditions.

A new paradigm for granite generation

ABSTRACTIdeas about granite generation have evolved considerably during the past two decades. The present paper lists the ideas which were accepted and later modified concerning the processes acting during the four stages of granite generation: melting, melt segregation and ascent, and emplacement. The active role of the mantle constitutes a fifth stage.Fluid-assisted melting, deduced from metamorphic observations, was used to explain granite and granulite formation. Water seepage into meta-sedimentary rocks can produce granitic melt by decreasing melting temperature. CO2 released by the mantle helps to transform rocks into granulites. However, dehydration melting is now considered to be the origin of most granitic melts, as confirmed by experimental melting. Hydrous minerals are involved, beginning with muscovites, followed by biotite at higher temperatures. At even deeper conditions, hornblende dehydration melting leads to calc-alkaline magmas.Melt segregation was first attributed to compaction and gravity forces caused by the density contrast between melt and its matrix. This was found insufficient for magma segregation in the continental crust because magmas were transposed from mantle conditions (decompression melting) to crustal conditions (dehydration melting). Rheology of two-phase materials requires that melt segregation is discontinuous in time, occurring in successive bursts. Analogue and numerical models confirm the discontinuous melt segregation. Compaction and shear localisation interact non-linearly, so that melt segregates into tiny conduits. Melt segregation occurs at a low degree of melting.Global diapiric ascent and fractional crystallisation in large convective batholiths have also been shown to be inadequate and at least partly erroneous. Diapiric ascent cannot overcome the crustal brittle-ductile transition. Fracture-induced ascent influences the neutral buoyancy level at which ascent should stop but does not. Non-random orientation of magma feeders within the ambient stress field indicates that deformation controls magma ascent.Detailed gravity and structural analyses indicate that granite plutons are built from several magma injections, each of small size and with evolving chemical composition. Detailed mapping of the contact between successive magma batches documents either continuous feeding, leading to normal petrographic zoning, or over periods separated in time, commonly leading to reverse zoning. The local deformation field controls magma emplacement and influences the shape of plutons.A typical source for granite magmas involves three components from the mantle, lower and intermediate crusts. The role of the mantle in driving and controlling essential crustal processes appears necessary in providing stress and heat, as well as specific episodes of time for granite generation. These mechanisms constitute a new paradigm for granite generation.

Experimental petrology of basement lavas from Ocean Drilling Program Leg 192: implications for differentiation processes in Ontong Java Plateau magmas

Abstract Melting relations of the basement lavas drilled from the Ontong Java Plateau during ODP Leg 192 were experimentally determined at 1150–1250°C and 0.1–190 MPa under the oxygen fugacity along the fayalite-magnetite-quartz (FMQ) and cobalt-cobalt oxide (CCO) buffers. The basement lavas were classified into two types according to phenocryst assemblage and whole-rock composition: one type is low in MgO (&lt;8 wt%) and olivine + plagioclase + augite-phyric (Kwaimbaita type); and the other is rich in MgO (&gt;8 wt%) and olivine-phyric (Kroenke type). One sample was chosen from each type as a starting material of the melting experiments. The experimental results demonstrate that the variations in phenocryst assemblage and whole-rock composition in the basement lavas can be modelled adequately by fractional crystallization processes in a shallow magma chamber (&lt;6 km in depth). The experimentally determined mineral-melt equilibria, in combination with detailed petrographical investigation, revealed that the vast majority of phenocrysts are in equilibrium with their host magma composition, but some are not. The latter include unusually An-rich parts of plagioclase phenocrysts in the Kwaimbaita-type lavas. These An-rich parts probably crystallized in a mushy boundary layer along the wall of the magma chamber where the melt was relatively rich in H 2 O. Some olivine phenocrysts in the Kroenke-type lavas show reverse zoning, with core compositions that can be in equilibrium with the Kwaimbaita-type magmas. The cores of these olivine phenocrysts were most probably assimilated from a solidified pile of the Kwaimbaita-type lavas when the Kroenke-type magmas ascended through it.

Anomalous Reverse Zoning of Saponite and Corrensite Caused by Contact Metamorphism And Hydrothermal Alteration of Marly Rocks Associated with Subvolcanic Bodies

Anomalous Reverse Zoning of Saponite and Corrensite Caused by Contact Metamorphism And Hydrothermal Alteration of Marly Rocks Associated with Subvolcanic Bodies

Anomalous Reverse Zoning of Saponite and Corrensite Caused by Contact Metamorphism and Hydrothermal Alteration of Marly Rocks Associated with Subvolcanic Bodies

AbstractThe effects of a subvolcanic intrusion on its host rocks in the Priego de Córdoba area (SE Spain) was studied by optical microscopy, X-ray diffraction and electron microscopy. The intrusion of a laccolith of stratiform dolerite in partially consolidated marly sediments at quite shallow depths below the ocean floor took place during the intracontinental rifting phase of the Subbetic zone of the Betic Cordilleras. In the first stage, contact metamorphism caused the crystallization of calc-silicate consisting of grossular andradite garnet, diopside, vesuvianite and titanite for which the upper temperature limit was 500°C. Phyllosilicates are found in a network of veins cross-cutting the carbonate and calc-silicate matrix, indicating their formation during a process of hydrothermal alteration superimposed on the contact aureole. In the area closest to the subvolcanic rocks, saponite is the main phyllosilicate although some dispersed chlorite is found. In the zone furthest from the contact, corrensite together with saponite and beidellitic smectites were observed. The presence of low-temperature phases (saponite) in the area closest to the laccolith reveals the evolution of the hydrothermal system toward cooler conditions. In the early stage, the circulation of a hot hydrothermal fluid caused the crystallization of chlorite in the area surrounding the subvolcanic body and corrensite in the more distal area, which might have begun even during the contact metamorphism stage. A cooling phase followed, resulting in the crystallization of saponite in the host rocks, and the crystallization of dioctahedral and trioctahedral smectites inside the subvolcanic body.