Feldspathic hornblende and garnet granulites and associated anorthosite pegmatites from Doubtful Sound,Fiordland, New Zealand

Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites. The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite. The garnet forms distinctive coronas around clinopyroxene. The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work.It is thought that under conditions leading up to 750 °C, 8 kb load pressure and 5 kb H₂O pressure, partial melting occured in feldspathic hornblende granulites. The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins. The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H₂O and Na₂O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile.     

Petrographie,Stoßwellenmetamorphose und Entstehung polymikter kristalliner Breccien im Nördlinger Ries

Polymict cristalline breccias are typical impact products of the Ries crater. They occur within the Ries crater (Appetshofen, Lierheim, Leopold Meyers Keller), on its rim (Maihingen-Klostermühle) and within the immediate vicinity of the crater (Itzing). Apart from very rare admixtures of sedimentary rock fragments the polymict cristalline breccias consist almost exclusively of fragments of various cristalline rocks, namely granites, gneisses and amphibolites. The petrographical and statistical investigations have shown that breccias from different localities have different composition. This reflects a possible difference in local compositions of the cristalline basement. The rocks in the breccias have been affected to various degress by shock metamorphism. The amphibolites could thus be shown to belong predominantly to stage I (diaplectic quartz and feldspar, 100–300 kb) and stage II (diaplectic quartz and feldspar glasses, 350–500 kb) whereas the granites and geisses can be attributed mostly to stage 0 (fractured quartz and feldspar, <100 kb) and stage I. This is in part the result of the bulk shock wave impedance of the rocks in question.Deformation structures resulting from shock metamorphism have been observed for the first time in sphene as well as in various planes of apatite.A large part of breccia rocks, which contain diaplectic quartz and feldspar glasses were altered into montmorillonite at a later date. The authigenic minerals were examined by x-rays and chemically by microprobe analysis.

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Herrn Prof. Dr. W. von Engelhardt danke ich für Diskussion und Beratung. Dem Bundesministerium für Bildung und Wissenschaft sei für die finanzielle Unterstützung dieser Arbeit gedankt.     

Petrology of a leucogabbroic interval within basal layered gabbros at North Arm Mountain,Bay of Islands ophiolite

Data from detailed sample traverses in the layered gabbro unit of the North Arm Mauntain massif, Bay of Islands ophiolite, allow meter-scale resolution of magmatic processes in spreading ridge magma chambers. One suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase (An_73.7–87.5; average modal abundance=75%), clinopyroxene (Mg#=80.3–86.0; 18%), and olivine (Fo_76.6–82.1; 6%), with intergranular orthopyroxene (Mg#=78.0–83.3; 1%), and accessory Cr-Al spinel (Cr#=32.3–41.4). Ilmenite rims spinel in one sample. Whole rock Zr contents range from <6 to 15 ppm. Plots of stratigraphic height in the traverse versus petrogenetic indicators (e.g. Mg#'s of mafic phases and An in plagioclase) reveal both normal and reverse cryptic variation patterns; the patterns for all indices are generally correlated. The normal portions of the patterns formed during fractional crystallization of basalt batches. Ranges of mineral compositions in the normal trends suggest that 29–38% crystallization of each batch of basalt occurred before magmatic replenishment. The reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing. Other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends. Experiments and observations of natural assemblages indicate that 55% modal plagioclase crystallizes from basalts at the olivine+plagioclase+clinopyroxene+liquid piercing point. The average plagioclase content of this suite of leucogabbros from North Arm Mountain is too high to have formed from simple crystallization at the piercing point. Petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount (<10%) of plagioclase was resorbed during mixing; the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume. Other suites of layered gabbros from North Arm Mountain are not so plagioclase-rich as the leucogabbros described above. Crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers, or may occur episodically in the overall lifetimes of the magma chambers.     

Fluid absent melting of a layered crustal protolith: implications for the generation of anatectic granites

We report the result of H₂O-undersaturated melting experiments on charges consisting of a layer of powdered sillimanite-bearing metapelite (HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments were conducted at 10 kbar at 900°, 925° and 950°C. When run alone, the pelite yielded 40 vol% strongly peraluminous granitic melt at 900°C while the tonalite produced only 5 vol% weakly peraluminous granitic melt. At 950°C, the pelite and the tonalite yielded 50 vol% and 7 vol% granitic melt, respectively. When run side by side, the abundance of melt in the tonalite was 10 times higher at all temperatures than when it was run alone. In the pelite, the melt abundance increased by 25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al₂O₃ in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelite, the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite-layer to the pelite-layer. This has the effect of changing the bulk composition of this layer in the direction of minimum-temperature granitic liquids. Our results show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration-melting. Because of this effect, the continental crust may have an even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions.     

Suevite of the ries crater,Germany: Source rocks and implications for cratering mechanics

Suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamorphic rocks. From 138 analyses of crystalline rock samples average compositions of the major rock types were calculated. Comparison of these averages with the average glass composition leads to the conclusion that suevite glasses were formed by shock melting of gneisses in deeper levels of the basement. Suevite matrices consist in most cases of 80 to 90 % montmorillonite, in special cases of celadonite. Chemical analyses are given of some matrices and montmorillonite formulas calculated. It is supposed that montmorillonite was formed by early hydrothermal alteration of rock flour or fine glass particles. In the latter case the original glass content of suevites was higher than at present. Of all ejecta from the Ries crater only crystalline rocks contained in suevites occur in all stages of shock metamorphism up to complete fusion. The overwhelming majority of the ejecta from the sedimentary sequence (about 580 m) show no indications of shock pressures above 10 GPa. The same holds true for crystalline megablocks and breccias around the crater which consist mainly of granites from upper levels of the basement. We assume that the Ries impact can be approximated by a deep-burst model: The projectile penetrated through the sedimentary cover into the basement in such a way that the highest pressures and temperatures developed within the gneiss complex below the upper, predominately granitic layer and that rocks of the sedimentary sequence experienced weak shock compression. Numerical data are given for such a model of the Ries impact on transient crater geometry and volumes of vaporized, melted, shocked and excavated rocks. Fall-out suevites are supposed to have been lifted from the central zone by an expanding plume of vaporized rocks and deposited as fluidized turbulent masses outside the crater whereas the main mass of crater suevite was not removed from the crater cavity.     

Microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta-anorthosite

The very fine-grained (1 m) polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the Harris meta-anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested. The intrusive nature of the veins is shown by their geometry and internal structures. Clasts, which are almost always only plagioclase, are generally concentrated towards the centres of veins. Flow of tens of millimetres can account for this concentration in millimetre-thick veins as a result of the Bagnold effect. The veins are generally thin (5 m to 5 mm or more), are frequently zoned and always contain transparent granules of high relief (probably Al-rich pyroxene) and opaque granules of magnetite up to a few micrometres in size. The granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures. In some veins, spherulitic or bow-tie textures occur. The coarsest textures are found in the centres of the thickest veins. The groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 m in thin veins or in the margins of thicker veins. This fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0.2–1.5 m, which show little sign of deformation. The local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins, the variation being within the compositional space defined by the minerals of the host rock. This shows that homogenization of the pseudotachylite has occurred. In cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock. This was produced by segregation of the granules, pyroxene being absent from the host rock. The plagioclase in the pseudotachylite is more disordered than that in the host rock. All the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine-grained and intensely strained rock powder. The physical state on and after intrusion was that of a melt and injection was followed by crystallization. The melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation. The fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys. It thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow-tie textures.     

Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites: variation with crystal fractionation and interaction with sea water

Lavas and pyroclastics on Ascension Island contain plutonic blocks that include fluid-inclusion-bearing peralkaline-granite. ¹⁸O/¹⁶O ratios, F and Cl have been analysed on whole rocks and/or minerals for lavas and granites, and D/H ratios and H₂O⁺ for comenditic obsidians and granites. Whole rock ¹⁸O/¹⁶O ratios of fresh alkali-basalt, hawaiite, trachyandesite, trachyte and comendite range from 6.0 to 6.9 with ¹⁸O tending to increase with increase in SiO₂. The ¹⁸O values of the granites are from 0.0 to 0.3 depleted in ¹⁸O relative to the comendites. Comenditic obsidians have D= –80±4 and H₂O⁺ 0.3 wt.% while amphiboles from the granites have D= –56±2 The O-isotope trend of the lavas is consistent with a crystal fractionation model. Fresh igneous rocks with ¹⁸O values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma. The D/H ratios and Cl contents ( 3,000 ppm) of the H₂O-poor comenditic obsidians represent undegassed primary magmatic values. The H-isotope compositions and low H₂O and Cl (167 ppm) contents of the granites are consistent with the major degassing (loss of >90% of initial H₂O) of an H₂Osaturated magma derived from the interaction of sea (or possibly meteoric) water with the H₂O-undersaturated comenditic melt. It is proposed that, associated with caldera subsidence and stoping, water was sucked in around the residual magma before the system had time to be sealed up. The H₂O-undersaturated magma consumed this H₂O with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks, at a pressure of about 1.5 kb. The granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites. Granites from oceanic islands may, in general, be a result of generating an H₂O-saturated acid melt by such direct or indirect crustal water-magma interaction processes.Contribution No. 603 Centre de Recherches Pétrographiques et Géochimiques     

Glass inclusions and melt compositions of the Toba Tuffs,northern Sumatra

Glass (melt) inclusions in quartz, plagioclase and K-feldspar phenocrysts in Toba Tuff ignimbrites all exhibit highly evolved, rhyolitic compositions, identical to glass forming the matrix of the rocks. About 4% H₂O is present, dissolved in the glass, suggesting a water saturation pressure ( \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) ) of about 1 kbar. Melt compositions are consistent with phase relations for the condition \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) =P _total = 1 kbar. The residual rhyolitic melt formed as the result of fractional crystallisation from a more basic, possibly rhyodacitic melt, leading to the development of zoned feldspars. Water saturation in the melt probably arose as a result of this process. Melt temperatures prior to eruption and quenching were probably less than 800° C. However, hot-stage homogenisation experiments yield entrapment temperatures significantly higher (>900° C). This discrepancy is not clearly understood but indicates care must be taken in the interpretation of such experiments. Ignimbritic magmas at Toba, from pressure estimates, appear to have been erupted from about 3–4 kms depth and represent the silicic cap to a batholithic body consolidating beneath the Toba caldera.     

An example of post-collisional mafic magmatism: the gabbro–anorthosite layered complex from the Tin Zebane area (western Hoggar, Algeria)

The Tin Zebane gabbro–anorthosite layered mafic intrusion represented by plagioclase-rich cumulates forms a set of small lenticular to round-shaped mainly undeformed bodies intruding the Pan-African high-pressure metamorphic rocks from western Hoggar (Tuareg shield, southwest Algeria). The coarse-grained anorthosites are mainly made of slightly zoned bytownite (An_86–74) with the higher anorthite content at the cores. Anorthosites are interlayered with leucogabbros and gabbros that show preserved magmatic structures and with olivine gabbros characterised by coronitic textures. The primary assemblage in gabbros includes plagioclase (An_93–70), olivine (Fo_77–70), zoned clinopyroxene (En_43–48Fs_05–13Wo_41–49 with Al₂O₃ up to 4.3 wt.%) and rare orthopyroxene (En_73–78). Pyroxenes and olivine are commonly surrounded by Ca-amphibole. The olivine–plagioclase contact is usually marked by a fine orthopyroxene–Cr-spinel–amphibole symplectite. A magnesian pigeonite (En_70–75Fs_19–20Wo_6–10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P–T–a_H2O conditions of 797 ± 42 °C for a_H2O=0.5 and 808 ± 44 °C for a_H2O=0.6 at 6.2 ± 1.4 kbar. The Tin Zebane gabbroic rocks are depleted in REE with a positive Eu anomaly, high Sr (>10 * chondrite) and Al₂O₃ concentrations (17–33%) that support plagioclase accumulation with the extreme case represented by the anorthosites. The REE patterns can be modelised using plagioclase, clinopyroxene and orthopyroxene REE signature, without any role played by accessory minerals. High MgO content points to olivine as a major cumulate phase. Anorthositic gabbros Sr and Nd isotopic initial ratios are typical of a depleted mantle source (Sr_i=0.70257–0.70278; _Nd=+5.9 to +7.8). This isotopic signature is identical to that of the 10-km wide 592 Ma old dyke complex composed of alkaline to peralkaline granites and tholeiitic gabbros and one single bimodal complex can be inferred. The source of the Tin Zebane basic rocks corresponds to the prevalent mantle (PREMA). The Tin Zebane complex was emplaced along the mega-shear zone bounding to the west the Archaean In Ouzzal metacraton. The model proposed suggests a linear lithospheric delamination along this rigid and cold terrane due to post-collisional transtensional movements. This allowed the asthenosphere to rise rapidly and to melt by adiabatic pressure release. Transtension along a rigid body allowed these mantle melts to reach the surface rapidly without any crustal contamination.     

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW-ESE to ENE-WSW or NE-SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite-pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near-surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite. Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline-type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km. The bulk chemical composition of the pseudotachylite veins is characterized by low SiO₂ and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high-Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two-pyroxene geothermometer. Using published data relating water solubilities in high-temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites. Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.     

Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental,Chihuahua, Mexico

The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains 1% K₂O at 60% SiO₂. Trace element abundances of a typical ignimbrite with 73% SiO₂ are Sr 225 ppm, Rb 130 ppm, Y 32 ppm, Th 12 ppm, Zr 200 ppm, and Nb 15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.     

Genesis of Peraluminous Granites II. Mineralogy and Chemistry of the Tourem Complex (North Portugal). Sequential Melting vs. Restite Unmixing

The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO₂ and K₂O (72–74 wt.% and5?4–6?4 wt.%, respectively), moderate P₂O₅ and REE (0?22–0?24%and 85?0–95?7 ppm), and low Fe₂O₃^* and Zr contents (1?3–1?5%and 80–90 ppm). These features are consistent with thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite contents (up to 30%) in addition to biotite (5–25%),complexly zoned plagioclase, and high Fe₂O₃ (2?72–6?99%),CaO (0?56–1?95%), Zr (101–213 ppm), and Ce (39?8–98?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (30–60% melting) with probably a largeramount of available water compared with the felsic granites. The major and trace element chemical trends of the granites,which do not define single arrays on two-element variation diagrams,and experimental data show that the generation of the Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention.     

Mid-Tertiary magmatism of the Toquima caldera complex and vicinity,Nevada: development of explosive high-K,calc-alkaline magmas in the central Great Basin,USA

The Toquima caldera complex (TCC) lies near the middle of a west-northwest-trending belt of Oligocene to early Miocene volcanic rocks that stretches from southwestern Utah to west-central Nevada. Three overlapping to eccentrically nested calderas, called Moores Creek, Mt. Jefferson, and Trail Canyon, comprise the TCC. The calderas formed due to eruption of the tuffs of Moores Creek, Mt. Jefferson, and Trail Canyon at 27.2 Ma, 26.4 Ma, and 23.6 Ma, respectively. In total, 900+ km³ of magma was erupted from the complex. The high-silica rhyolite tuff of Moores Creek is the least strongly zoned in silica (78.0–76.8 wt% SiO₂), and the tuff of Mt. Jefferson is the most strongly zoned (77.5–65.3 wt% SiO₂); the tuff of Trail Canyon is moderately zoned (75.9–70.4 wt% SiO₂). All eruptive products contain plagioclase, sanidine, quartz, biotite, Fe–Ti oxides, and accessory zircon, allanite, and apatite. Amphibole and clinopyroxene join the assemblage where compositions of bulk tuff are 74 wt% SiO₂ and 70 wt% SiO₂ respectively. Proportions and compositions of phenocrysts vary systematically with composition of the host tuff. Compositional zoning trends of sanidine and biotite suggest the presence of a high Ba-bearing magmatic component at depth or its introduction into the Mt. Jefferson and Trail Canyon magma chambers at a late stage of magmatic evolution. Rocks of the complex constitute a high-K, calc-alkaline series.Empirical data from other systems and results of published phase-equilibria and thermo-chemical studies suggest that magma erupted from the TCC was oxidized ( 1.5 to 2.0 log units above NNO), thermally zoned ( 700–730° C for high-silica rhyolite to 800–840° C for dacite) and water-rich (5.0–5.5. wt% H₂O for highsilica rhyolite to 4.0 wt% H₂O for dacite). Geologic relations and amphibole compositional data are consistent with total pressures of 1.5 to 2 kbars.Onset of mid-Tertiary magmatism in vicinity of the TCC began with intrusion of a small granodioritic stock and a northeast-trending dike swarm at 37–34 Ma. The dikes are broadly bimodal assemblage of silicic andesite and rhyolite. Voluminous ash-flow-tuff magmatism commenced at 32.3 Ma and persisted for 9 m.y. without eruption of intermediate to mafic magmas (<62 wt% SiO₂). As such, the TCC is probably a remnant of a more extensive complex of calderas whose identities are obscured by recurrent volcanism and by late Tertiary basin-range block faulting. The change from small-volume, broadly bimodal volcanism to voluminous outpourings of silicic magma is similar to that which occurred in east-central Nevada, where magmatism and rapid crustal extension overlapped in space and time. Although supracrustal extension at the time of formation of the TCC apears limited, the comparable magmatic histories and compositional characteristics of rocks erupted from east-central Nevada and the TCC suggest that fundamentally similar magmatic processes acted at depth and that extension may have been more pronounced in the lower and middle crust below the TCC and vicinity. Because strain is partitioned heterogeneously in the upper crust, the magmatic record, rather than surface structural features, may reflect better the actual state of crustal stress during volcanism.Mid-Tertiary magmatism in the TCC and vicinity probably began with intrusion of mantle-derived basalt into the lower crust, which led to crustal heating, local partial melting of crustal rocks, and intrusion of rhyolitic melts and contaminated basaltic differentiates (alkalirich andesite) into the upper crust. With time, intrusion to extrusion ratios increased as silicic melts retarded the rise of mafic magmas and mixing between mafic magmas and crustal partial melts occurred. The oxidized, water-rich, and low-temperature nature of these magmas reflects protracted crustal residence and interaction prior to eruption. The resulting hybridized and differentiated magmas ultimately erupted to form extensive deposits of silicic ash-flow tuff. By contrast, silicic lavas are scarce possibly because of coherent roof rocks that limited volatile degassing between major pyroclastic eruptions.     

Origin of mantle (rapakivi) feldspars: experimental evidence of a dissolution- and diffusion-controlled mechanism

Experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks. In hydrous dacite melt (69% SiO₂) at 0.8 GPa, 850°C, orthoclase (Or₉₃) and sanidine (Or₃₀) partially dissolved and were mantled by sodic plagioclase (An_25–30). Although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption, plagioclase subsequently grew inward —mostly in the form of parallel blades — toaard the receding dissolution surface. Orthoclase dissolved at a rate approximately proportional to the square root of run duration, indicating diffusional control. Plagioclase grew inward within a static boundary zone of melt that formed between the original crystal-dacite interface and the dissolution surface. During orthoclase dissolution, this boundary zone rapidly and simultancously gained Na (by diffusion from dacite) and lost K (by diffusion into dacite); Ca diffused more slowly into this zone, from which non-feldspar species were mostly excluded. Plagioclase was stable where sufficient Ca had diffused in that the boundary zone melt intersected the plagioclase-saturation liquidus. Plagioclase subsequently grew toward the receding dissolution surface as the Ca compositional gradient (and hence the site of plagioclase saturation) stepped inward. Crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt. Bladed growth also served to maintain (at blade tips) proximity of plagioclase to the dissolution surface, thereby apparently preserving (locally) a thin zone of low-variance melt. In natural systems, mantling of alkali feldspar by plagioclase will occur in a similar manner when (a) P, T, or X are changed to induce alkali feldspar dissolution, (b) sufficient Ca is available in the host melt to drive (by diffusion) boundary zone melt compositions to plagioclase saturation, and (c) temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone. These reqjirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of dacitic composition. Mantled feldspars in some intrusive systems (i.e., rapakivi granites) may form by a similar dissolution- and diffusion-controlled mechanism. Textural evidence of a similar origin may be obscurred in intrusive rocks, however, by products of late-stage magmatic and subsolidus processes.     

The occurrence and distribution of Mo and molybdenite in unaltered peralkaline rhyolites from Pantelleria,Italy

Small hexagonal and triangular platelets of molybdenite (MoS₂), 5 to 25 m in diameter, were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from Pantelleria, Italy. The MoS₂ occurs commonly in pantellerites (peralkaline rhyolites), rarely in pantelleritic trachytes, and never in trachytes. The occurrence of euhedral MoS₂ platelets in all phenocryst phases, in matrix glass, and even in some melt inclusions indicates that MoS₂ precipitated directly from the peralkaline melt. Despite MoS₂ saturation, the melt (glass) contains greater than 95% of the Mo in Pantellerian rocks: X-ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less Mo than corresponding matrix glasses, the differences being in proportion to phenocryst abundances. The Mo contents increase with differentiation from trachytes (2–12 ppm) to pantellerites (15–25 ppm) and correlate positively with incompatible elements such as Th, Y, and Nb. The Mo concentrations, as determined by secondary ion mass spectrometry, are essentially the same in matrix glasses and melt inclusions, showing that Mo did not partition strongly into a volatile fluid phase during outgassing. The high Mo contents of the pantellerites (relative to metaluminous magmas with 1–5 ppm) may be due to several factors: (1) the enhanced stability of highly charged cations (such as Mo⁶⁺, U⁴⁺, and Zr⁴⁺) in peralkaline melts; (2) the rarity of Fe-Ti oxides and litanite into which Mo might normally partition; (3) reduced volatility of Mo in low fO₂, H₂O-poor (1–2 wt%) peralkaline magmas. Geochemical modeling indicates that the precipitation of MoS₂ can be explained simply by the drop in temperature during magmatic differentiation. The occurrence of MoS₂ in pantellerites may result from their high Mo concentrations and low redox state (Ni/NiO=-2.5) relative to metaluminous magmas, causing them to reach MoS₂ saturation at magmatic temperatures. The apparent absence of MoS₂ microphenocrysts in more oxidized, metaluminous rhyolites may indicate that Mo is dissolved primarily as a hexavalent ion in those magmas.     

Petrogenesis of the magmatic complex at Mount Ascutney,Vermont, USA

The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had ⁸⁷Sr/⁸⁶Sr=0.7039, ¹⁴³Nd/¹⁴⁴Nd=0.512678 ( _Nd=+3.8) and ¹⁸O=6.1, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in ¹⁸O, to about 7.8, is the result of fractional crystallization. Mafic rocks show a large range in initial ¹⁴³Nd/¹⁴⁴Nd ratio, from about 0.51267 to 0.51236 ( _Nd= +3.7 to –2.5), which is correlated with elevated ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.     

A HFSE- and REE-enriched ferrodiorite suite from the Bolangir Anorthosite Complex, Eastern Ghats Belt, India

The massif-type anorthosite complex at Bolangir in the northern part of the Eastern Ghats belt occurs in a milieu of predominantly supracrustal granulite-grade rocks. The massif is separated from the host gneisses by coarse-grained garnetiferous granitoid gneisses which are interpreted as coeval crustallyderived melts. Melanocratic ferrodiorite rocks occur at the immediate contact with the anorthosite massif which they intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir and the associated igneous suites is deemed to be pre-D₂. Recrystallization of the igneous assemblages of the ferrodiorite suite (750–800°C, 7–8kbar, ) during a period of near-isobaric cooling from the igneous crystallization stage to the regionalP-T regime led to extensive development of coronitic garnet at the interface of plagioclase phenocrysts with the mafic matrix assemblage (opx + fay + cpx + ilm ± amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic phases have extremely ferrous compositions (X_Fe gar: 0.93-0.87, fay: 0.90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the rocks. Compared to worldwide occurrences of similar rocks, the Bolangir ferrodiorites (SiO₂ 36–58 wt.%, FeO*: 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO₂: 4.8-1.0 wt.%, P₂O₅: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well defined linear variation trends for major and trace elements reflect progressive plagioclase accumulation towards the felsic members of the suite. The ferrodiorites are interpreted to represent residual liquids of anorthosite crystallization which after segregation and extraction from the ascending diapir became enriched in HFSE and REE through selective assimilation of accessory phases (zircon, monazite, apatite) from crustal felsic melts. Ferromonzodioritic rock presumably formed through hybridization between the ferrodiorite and overlying felsic melts.     

SHRIMP U–Pb zircon geochronology of the Strzelin gneiss,SW Poland: evidence for a Neoproterozoic thermal event in the Fore-Sudetic Block,Central European Variscides

Zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central European Variscides. A SHRIMP U–Pb zircon geochronological study of the Strzelin gneiss in the Fore-Sudetic Block (SW Poland) indicates the presence of: (1) inherited zircon cores of Palaeo- to Mesoproterozoic ²⁰⁶Pb-²³⁸U ages (between ca. 2,000 and 1,240 Ma), and (2) zoned rims of Neoproterozoic age with two distinct means of 600±7 and 568±7 Ma. The Proterozoic age range of the cores suggests that different Precambrian crustal elements were the source for the protolith of the gneiss. A likely scenario is the erosion of various Proterozoic granites and gneisses, sedimentation (after 1,240 Ma), and partial resistance of the original components to subsequent metamorphic dissolution and/or anatectic resorption (in Neoproterozoic times). The zoned zircon rims of both of the younger Neoproterozoic ages are indistinguishable in the cathodoluminescence images. The data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca. 600 and 568 Ma. In general, the new results confirm earlier assumptions of the Proterozoic age of the gneiss protoliths, and indicate their similarity to orthogneisses in the East Sudetes tectonic domain (e.g. the Velké Vrbno and Desná gneisses). The Neoproterozoic dates are different from the age of 504±3 reported earlier for the Gocicice gneiss from a neighbouring locality in the Strzelin Massif. The new data strongly indicate a Moravo-Silesian (Bruno-Vistulian) affinity for the Strzelin gneiss and support the hypothesis that the Strzelin Massif lies within the tectonic boundary zone between the West- and East Sudetes domains, which represents the northern continuation of the Moldanubian Thrust.     

Origin and differentiation of picritic arc magmas,Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg 80 to 93.4) and clinopyroxene (mg 80 to 92), and include accessory Cr-rich spinel (cr 50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to 20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with 10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo₂ FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has 15 wt% MgO and an mg ^Fe2 value of 81. It requires deep (3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T _p 1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.     

Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H2O at 15 kbar

We report the T-X(H₂O) phase relations for the trondhjemitic Nûk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthåb region of southwestern Greenland. A pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths. Under H₂O-saturated conditions, a melting interval of 135° C separates the solidus at 610° C from the liquidus at 745° C. H₂O-saturation at 15 kbar occurs at approximately 15.5 wt % H₂O. The H₂O-undersaturated liquidus extends along a curved path from 745° C at 15.5 wt % H₂O to 1100° C at 2% H₂O. Lower H₂O contents were not investigated. At low H₂O contents (<6%) sodic plagioclase (Pl, An₃₂) is the liquidus phase followed at lower but still near-liquidus temperatures by quartz (Qz) and then garnet (Ga). At 6% H₂O, Ga replaces Pl on the liquidus and is joined at slightly lower temperatures by Pl and hornblende (Hb). The field for liquidus Ga extends to only 7.5% H₂O where it is replaced by Hb which is the liquidus phase up to 13% H₂O. At all higher H₂O contents, epidote (Ep) is the first phase to crystallize, followed by biotite (Bi) at slightly lower temperatures. Following the standard inverse approach, the near-liquidus phase assemblages are interpreted as potential residues from which trondhjemitic melts could be extracted. At high melt H₂O contents (>7%), mafic residues consisting of some combination of Hb, Ga, Ep, and Bi are possible and could correspond to amphibolitic source rocks. At lower melt H₂O contents (< 5%), possible residues consist of Na-Pl+Qz±Ga and could correspond to an earlier generation of tonalitic-trondhjemitic rocks. However, such residues would not impart the highly fractionated REE patterns characteristic of Archean trondhjemites. If a first generation of tonalitic-trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth, it would crystallize to an assemblage consisting almost entirely of Na-Pl+Qz with highly fractionated REE patterns. These rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated REE patterns because neigher Pl nor Qz is capable of significantly fractionating HREE from LREE.