Pervasive rehydration of granulites during exhumation – an example from the Pulur complex, Eastern Pontides, Turkey

Summary High-grade gneisses from the Pulur complex in NE Turkey bear evidence for biotite-dehydration melting at 820°C and 0.7–0.8GPa, melt segregation and near-isothermal decompression to 0.4–0.5GPa. During further exhumation, the rocks underwent secondary pervasive rehydration at temperatures between 400 and 230°C and fluid pressures between 0.3 and 0.1GPa. Metamorphic peak conditions are dated at 331–327Ma, while hydrothermal retrogression occurred significantly later at 315–310Ma under static conditions. During the rehydration event, primary high-grade mineral assemblages including garnet, cordierite, sillimanite, spinel, biotite, plagioclase and ilmenite were extensively replaced by muscovite, paragonite, margarite, corundum, diaspore, chlorite, kaolinite, pumpellyite, prehnite, epidote, titanite, anatase, pyrite and chalcopyrite. Secondary mineral assemblages indicate that the infiltrating fluids were characterized by low fO₂, very low X_CO2 (<0.002), variable activities of Ca²⁺, K⁺, Na⁺ and H⁺ and relatively high activities of H₂S and CH₄. Quartz veins that might have acted as pathways for the fluids are rare. Ubiquitous veinlets consisting of (i) albite, (ii) chlorite+calcite+quartz or (iii) K-feldspar+calcite+quartz were formed after the pervasive rehydraton event by precipitation from aqueous solutions that were somewhat richer in CO₂.     

Non-monotonic chemical and O,Sr, Nd,and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff,Colorado

Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO₂, fiamme group 7) had ¹⁸O= +8.5, initial ⁸⁷Sr/⁸⁶Sr=0.7099, _Nd, and ²⁰⁶Pb/²⁰⁴Pb=17.80, suggesting that the magma was produced by 50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO₂, fiamme group 1) depart from the mafic base-level composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. ¹⁸O values decrease by as much as 1.5 from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. ¹⁸O values abruptly inerease by up to 1.5 between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial ⁸⁷Sr/⁸⁶Sr ratios in phenocrysts are as high as 0.7170. values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. ²⁰⁶Pb/²⁰⁴Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.     

Sapphirine/kornenipine-bearing rocks and crustal uplift history of the Limpopo belt,Southern Africa

Sapphirine/kornerupine-bearing rocks occur within the anorthosites of the Messina layered intrusion in the Limpopo mobile belt of Zimbabwe. The X_Mg range of the major minerals is as follows: cordierite (0.98-0.93); enstatite (0.97-0.86); chlorite (0.98-0.92); phlogopite (0.98-0.90); sapphirine (0.98-0.86); kornerupine (0.94-0.88); gedrite (0.96-0.85); spinel (0.92-0.78). There are four rock types, the constituent minerals of which have different values, which decrease in the above mineral order; other minerals are corundum, sillimanite and relict kyanite. We recognise twenty reactions without phlogopite and nine reactions involving phlogopite. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al₂O₃-SiO₂-(H₂O) system are consistent with the following sequence of main reactions: (1) enstatite+corundum cordierite+sapphirine; (4) sapphirine+sillimanite cordierite+corundum; (8) kornerupine+corundum cordierite+sapphirine; (13) kornerupine cordierite+sapphirine+enstatite; (15) enstatite+spinel chlorite+sapphirine; (18) cordierite+sapphirine chlorite+corundum; (20) sapphirine chlorite+corundum+spinel. The early reactions are shown by coarse-grained reaction intergrowths, kornerupine and gedrite breakdown is shown by finer-grained symplectites, and the latest reactions by very fine-grained products in micro-fractures. These selected reactions illustrate a remarkably steep trajectory from thePT peak close to 10 kbar and 800° C to the minimum observable at 3.5–4.5 kbar and 700° C as indicated by the pure MASH system. Very rapid uplift took place under nearly isothermal conditions. The protolith of this material was possibly sedimentary, derived from altered volcanic rocks. The bulk composition is close to the composition of kornerupine or to a mixture of alunite, chlorite and pyrophyllite. These texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history.     

Geology of the mineralized zone of the Wanapitei complex,Grenville Front,Ontario

The Wanapitei Complex (6 km×2.5 km), lying 0.4 km southeast of the Grenville Front, consists of a northwestern zone of gabbro and folded injection breccia and a southeastern layer of intensely folded hornblendeplagioclase gneiss. Disseminated Ni-Cu sulphides are unevenly distributed in a zone between the injection breccia and the folded gneiss.Rocks of the mineralized zone occur in southeastern and northeastern areas. The former area consists of hornblende norite, the major host rock of the sulphides, and olivine norite. Steeply-dipping cross-bedded primary layers and chemical trends indicate the top faces southeast. In the latter area olivine norite, hornblende norite, and hornblende gabbro grade eastward into recrystallized rocks and breccia. The olivine norites are characterized by corona reaction rims. Reactions are: olivine+plagioclase bronzite+diopside-spinel; olivine+pyroxene bronzite; and pyroxene+plagioclase diopside-spinel. Molecular proportion ratio variation diagrams suggest that rocks evolved from a common parent magma that underwent fractionation dominated by olivine and plagioclase. Sulphide mineralization (pyrrhotite, chalcopyrite, pentlandite, pyrite) is interstitial to the silicates and appears to be of primary magmatic origin.Northeasterly-trending shear zones, felsic dikes, and matic dikes are metamorphosed to the same degree as the rocks they cut (amphibolite facies). The sequence of events for the mineralized zone are: intrusion deep in the crust; tilting; brecciation; shearing; felsic and mafic dike emplacement; metamorphism; and injection of granite pegmatite dikes.Deceased (8-16-1986)     

An experimental investigation of olivine morphology

Olivine crystals can adopt ten types of shape. Experimental crystallization of eight rock melts shows that there is a systematic change from polyhedral or granular olivines hopper olivines branching olivines randomly oriented chain olivines parallel-growth chain olivines chain+lattice olivines plate or feather olivines, with increase in cooling rate and with increase in degree of supercooling. This sequence involves changes from complete to progressively less complete crystals and from equant habit to elongate bladed habit (c>ab) to tabular habit (ac b). The sequence is not affected by the phase relations of the melt. The larger the olivine content of a melt the slower the cooling rate at which a particular olivine shape grows, whereas the lower the melt viscosity, the greater the cooling rate. Irrespective of the melt composition, comparable crystal shapes grow at the same degrees of supercooling. By comparison of the shapes of olivine crystals in experiments with those in rocks of similar composition, it is possible to deduce the cooling rate through the olivine crystallization interval and the approximate degree of supercooling at which the olivine crystals nucleated and grew in the rocks. The various shapes of skeletal olivines in many picrites, olivine-rich basalts and the Archaean spinifex rocks are not due to rapid cooling, but to rapid olivine growth caused by the high normative olivine content of the magma.     

Temperature — composition relationships of authigenic micaceous minerals in the Los Azufres geothermal system

Smectite, illite, celadonite and chlorite are the major products of alteration of rhyolites and andesites, in the upper part of the Los Azufres geothermal system. Changes in mineral assemblages and composition of phases are observed as a function of depth and host rock lithology. Two different sequences characterize the rhyolites and the andesites from the surface to a depth of about 1500 m: kaolinitesmectite (±interlayered illite/smectite)illitemuscovite (rhyolites), and kaolinitesmectite (±interlayered illite/smectite)illite+celadoniteillite+chloritechlorite (andesites).Illite, and chlorite at depth, are largely dominant. Similar substitutions and correlations among chemical constituents characterize illites from rhyolites or andesites, but their compositions in the two host rock lithologies exhibit slight but significant differences, especially in their Fe and Mg contents which are the highest in illites from andesites. Illite exhibits progressive changes in composition with depth: a strong increase in the K content in the interlayer, together with an increase of the Fe content in the octahedral site. These changes correspond to a slight increase in the molar fraction of Fe-(Mg) celadonite end-members, and mostly to a dramatic decrease of pyrophyllite with increasing temperature.Temperature of the mineralogical and compositional changes was estimated from fluid inclusions studies, combined with other geothermometric approaches (chemical geothermometers and direct measurements). Variation of X-pyrophyllite with temperature is proposed as a geothermometer for different host rock lithologies. Transitions between the stability fields of illite±interstratified illite-smectite and illite+chlorite is around 200±30° C, and between illite+chlorite and chlorite around 290±20° C.     

Oxygen isotope geochemistry of hydrothermally-altered synmetamorphic granitic rocks from South-Central Maine,USA

Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock ¹⁸O (SMOW) values 10.7 to 13.8. Constituent quartz, feldspar, and muscovite have ¹⁸O in the range 12.4 to 15.2, 10.0 to 13.2, and 11.1 to 12.0, respectively. Mean values of _Q–F ( ¹⁸O_quartz– ¹⁸O_feldspar)=2.4 and _Q–M ( ¹⁸O_quartz– ¹⁸O_muscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured _Q–F and _Q–M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic ¹⁸O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have D (SMOW) values in the range –65 to –82.Feldspars in the granitic rocks are mineralogically altered to either (a) muscovite+calcite, (b) muscovite+calcite+epidote, (c) muscovite+epidote, or (d) muscovite only. A consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks, quartz, and feldspar. Rocks with assemblage (a) have whole-rock ¹⁸O>12.1 and contain quartz and feldspar with ¹⁸O>13.8 and >11.4, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock ¹⁸O<11.4 and contain quartz and feldspar with ¹⁸O< 13.1 and <11.0, respectively. The correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration.Three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400°–150° C, 100°–150° C below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons. Calculations of mineral-fluid equilibria indicate that samples with assemblage (a) coexisted during the event with CO₂-H₂O fluids of and ¹⁸O=10.8 to 12.2 while samples with assemblages (b), (c), or (d) coexisted with fluids of and ¹⁸O=9.4 to 10.1. Compositional variations of the hydrothermal fluids were highly correlated: fluids enriched in CO₂ were also enriched in ¹⁸O. Because CO₂ was added to the granites during hydrothermal alteration and because fluids enriched in CO₂ were enriched in ¹⁸O, some or all of the variation in ¹⁸O of altered granites may have been caused by addition of ¹⁸O to the rocks during the hydrothermal event. The source of both the CO₂ and ¹⁸O could have been high-¹⁸O metasedimentary country rocks. The inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in ¹⁸O close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism.A close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from Maine is in marked contrast to hydrothermally-altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium. The difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics. Both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration. Deeper hydrothermal events occur at higher temperature and are longer-lived. Minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium. Shallower hydrothermal events occur at lower temperatures and are shorter-lived. Generally there is insufficient time for fluid to closely approach equilibrium with all minerals.     

Olivine-normative dolerite dikes from western South Carolina: Mineralogy,chemical composition and petrogenesis

This investigation describes five Mesozoic dolerite dikes which intrude Paleozoic metamorphic and igneous rocks of the Inner Piedmont of western South Carolina. The dikes are vertical or nearly so and strike approximately N40° W. Three major northeast-trending faults also occur in the study area. Left lateral displacement of one dolerite is documented at a locality near Cleveland, South Carolina. Elsewhere, several of the dolerite dikes appear to terminate at or near the faults. — The dolerite dikes have subophitic to microporphyritic textures and consist principally of plagioclase (generally An_70–80), olivine (dominantly Fo_80–90) and augite with subordinate pigeonite, titanomagnetite, chromite and brown, partly glassy mesostasis. In one dike pyroxene compositions trend from augite to ferroaugite in contrast to an augitesubcalcic augitepigeonite trend characteristic of the other dolerites. The contrasting trends primarily result from differences in SiO₂ abundance in the dolerite magmas. — Major and trace element analyses indicate the presence of two different olivine-normative dolerite magma types. The two magma types are not related by near surface crystal fractionation. Models for genesis of the olivine-normative dolerite magmas by partial melting of a plagioclase peridotite upper mantle source region are presented. The models require that the source region be enriched in LREE and incompatible elements such as Rb, Ba, Hf and Th relative to Cl chondritic abundances. One magma type appears to represent a primary dolerite magma that ascended from the source region with little subsequent compositional change. The second magma type most likely experienced assimilation of clinopyroxene-garnet (eclogite) during ascent, thereby acquiring a REE pattern with a less steep negative slope for the LREE and a slight positive slope in the HREE.     

Thermochemistry of carbonate-pyroxene equilibria

The enthalpies of drop solution of calcite, magnesite, dolomite, wollastonite and diopside have been measured in a lead borate solvent at 977 K in a Calvettype microcalorimeter. The carbonate calorimetry was done under flowing gas atmosphere. Both natural and synthetic samples were used. From these calorimetric data, the enthalpies of several reactions of carbonate with quartz were calculated. The enthalpies of these reactions (kJ/mol) at 298 K are: calcite+quartzwollastonite+CO₂, 92.3±1.0; magnesite+quartzenstatite+CO₂, 82.9±2.8; dolomite+quartzdiopside+CO₂, 163.0±1.9. These values generally are in agreement with those calculated from Robie et al., Helgeson et al., Berman and Holland and Powell. The enthalpy of dolomite-quartz reaction overlaps marginally with those from Berman and Holland and Powell. The enthalpy of formation of dolomite from magnesite and calcite (-11.1±2.5 kJ/mol) was also derived from the measured enthalpies, and this value is consistent with that from acid solution calorimetric measurements as shown by Navrotsky and Capobianco, but different from values in the earlier literature. These results support the premise that drop-solution of carbonates into molten lead borate results in a well-defined final state consisting of dissolved oxide and evolved CO₂. This was also confirmed by weight change experiments. Thus, oxide melt calorimetry is applicable to carbonates.     

The origin of fumarolitic andradite at Menoyre,France and Fant'Ale,Ethiopia

Recent discoveries of fumarolitic andradite in alkaline or peralkaline trachytes (Menoyre, Cantal, France, and Fant'Ale, Ethiopia) are interpreted genetically.The andradites were formed from the constituents of pyroxene, always calcic and iron-rich in these rocks. The elements Ca and Fe were fixed on the surface of fractures by late fumaroles of high oxidizing capacity.The following reaction can be established at Menoyre: hedenbergite+O₂andradite+hematite+cristobalite.In a hyperalkaline medium, which is the case for the trachyte of Fant'Ale, the reaction is: Hedenbergite+sodium metasilicate+O₂andradite+aegirinic pyroxene+cristobalite.     

Graphical estimations of magma densities and compositional expansion coefficients

Assuming that the partial molar volume of each chemical component in a magma is constant, the magma density, _m , is expressed as 1/ _m =C _i / _fi , whereC _i is the weight fraction, and _fi is the fractionation density of thei ^th component. Using this linear relationship between 1/ and weight fraction, the density change due to addition or subtraction of any component can be graphically estimated on 1/ vs oxide wt% diagrams. The compositional expansion coefficient of thei ^th component, _fi , is expressed as _i = _m / _fi –1. The compositional expansion coefficient of H₂O has a much larger absolute value than those of any other oxide or mineral components, showing that addition of a small amount of H₂O can significantly decrease magma density. These simple expressions facilitate the estimation of magma densities during fractionation.     

High-density CO2−N2 inclusions in eclogite-facies metasediments of the Münchberg gneiss complex,SE Germany

The eclogite-facies metasedimentary rocks in the Münchberg gneiss complex (T=630±30° C/P17–24 kbar) locally contain CO₂–N₂-rich fluid inclusions of extremely low molar volumes (32 cm³/mol) in quartz. These fluid compositions are mainly found in rocks intercalated with calcsilicate bands. Densities were determined from low-temperature phase transitions like stable or metastable homogenization (L+VL), partial homogenization (S+L+VS+L) and the transition S+LL (L = liquid, V = vapour, S = solid). The high fluid densities are in agreement with eclogite-facies pressure and temperature and subsequent amphibolite facies. CO₂–N₂ inclusions were not observed in adjacent eclogites nor in non-calcareous metasediments. These rock types contain predominantly H₂O-rich inclusions correlating with amphibolite-facies conditions. The variation of fluid composition with lithological differences indicates local fluid gradients and speaks against a pervasive fluid flow during eclogite-facies metamorphism.     

Two-brine model of the genesis of strata-bound Zechstein deposits (Kupferschiefer type), Poland

These Kupferschiefer deposits were probably formed as a result of a mixing of two brines. The upper cold brine (UCB) is an unmineralized brine rich in Na, Ca, Cl and SO₄, with a pH>7 and originating from evaporites overlying the metal-bearing Zechstein rocks. The lower hot brine (LHB) rich in Mg, K, Cl, SO₄ and CO₃ with a pH<=7 formed in sediments in the central part of the Zechstein basin at a depth of 7,000 m. This brine was subjected to heating and upward convection toward the Fore-Sudetic monocline along the bottom of the Z₁ carbonates. During its migration, it caused albitization, serpentinization and leaching of the primary metal deposits in rocks underlying the Zechstein becoming enriched in heavy metals. The mineralization process, being a result of the mixing of the two brines (UCB and LHB), and catalytic oxidation of the organic matter of the black shale were initiated at shallow depths in the area of the Fore-Sudetic monocline. The boundary of the two brines generally overlapped the strike of the black shale.Parts of the deposit with shale-free host rock suggest that the action of two brines alone was capable of producing economic concentrations of Cu, Pb and Zn. Where the boundary of the two brines overlaps the autooxidation zone (the black shale bottom) and also coincides with radiation of thucholite, concentrations of noble metals result.The characteristic vertical distribution of the triplet CuPbZn from the bottom upward is universal in the Kupferschiefer environment.     

Untersuchungen über die Petrographie kulmischer Kieselschiefer

Zusammenfassung Profile von kulmischen Kieselschiefern des hessischen Hinterlandes wurden optisch, röntgenographisch, chemisch, spektralanalytisch und differential-thermoanalytisch untersucht. Die nicht-lyditischen Bestandteile wurden besonders berlicksichtigt.Als Vergleichsmaterial dienten kulmische Kieselschiefer aus dem Harz, sowie tertiäre (Monterey), silurische und algonkische Kieselschiefer. Es wurde festgestellt, daß in der Kulmstufe II–III (dunkle und bunte Lydite) im hessischen Hinterland und im Harz zahlreiche, gering mächtige Tuffhorizonte auftreten; ihre Häufigkeit nimmt nach obenhin zu. Die bunten Lydite führen tuffitische Bänke, deren Grundmasse ursprünglich kalkig war. Die bisher nur aus dem Harz bekannten, kieseligen Tuffe des Kulms (Typ Lerbacher Adinole) Bind auch im hessischen Hinterland verbreitet.Met Röntgen- und DTA-Aufnahmen wurde zu zeigen versucht, daß die Kieselsäure seit der Entstehung des Sediments folgende Phasenumwandlungen erfahren hat:Opal Cristobalit mikrokristalliner Quarz (Chalzedon, Quarzin), und daß in paläozoischen Kieselsedimenten noch amorphe oder stark fehlgeordnete Kieselsäure vorhanden sein kann.Die Entstehung der kulmischen Kieselschiefer wird diskutiert.     

Osumilite-sapphirine-quartz granulites from Enderby Land Antarctica — Mineral assemblages and reactions

The pre-Cambrian granulites of Enderby Land Antarctica, contain coexisting spinel-quartz, sapphirine-quartz, hypersthene-sillimanite-quartz and osumilite on a regional extent. Osumilite is present in a variety of mineral assemblages, most of which are documented in granulites for the first time. The mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism. The development of many of the mineral coronas can be explained by continuous rather than discontinuous reactions, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature. The highest P-T conditions of metamorphism (8–10 kb, 900 °–980 ° C, Ellis, in preparation) were beyond the stability limit of coexisting garnet-cordierite. Secondary cordierite has developed through a large number of mineral reactions in response to cooling of these granulites.A theoretical analysis of the phase relations involving osumilite in the chemical systems K₂O-MgO-Al₂O₃-SiO₂ and K₂O-MgO-FeO-Al₂O₃-SiO₂ is presented. In the pure Mg-system the lower temperature stability limit of Mg-osumilite is inferred to be defined with increasing pressure by the reactions OsCd+En+Kfeld+Qtz, OsSa+En+Kfeld+Qtz, OsSill+En+Kfeld+Qtz. In iron-bearing systems an important reaction involving osumilite is Os+GtCd+Hy+Kfeld+Qtz.At moderate temperatures and pressures, osumilite is limited to rocks which lie on the Mg-rich side of the Cd-Hy stable tie line on an AFM diagram. At higher pressures and temperatures osumilite occurs in a widerrange of rock compositions because of the stability of coexisting garnet and osumilite. Petrographic data, as well as chemographic relations indicate that for many common rock compositions, garnet, cordierite, hypersthene, sapphirine and sillimanite cannot coexist with both osumilite and K-feldspar.Published with the permission of the Director, Bureau of Mineral Resources     

Corundum,Cr-muscovite rocks at O'Briens,Zimbabwe: the conjunction of hydrothermal desilicification and LIL-element enrichment — geochemical and isotopic evidence

Monomineralic domains of chlorite, corundum and Cr muscovite coexist over a kilometer scale within ultramafic schists of the Harare greenstone belt (2.73 Ga). This exotic lithological association includes the conjunction of some of the most aluminous (Al₂O₃88 wt%) and potassic (K₂O10 wt%) rocks known. The paragenetic sequence developed from chloritecorundumcorundum+ diaspore: Cr muscovite variably overprinted both the corundum and chloritite domains. Terminal stages were marked by sporadic production of andalusite+quartz, and finally margarite.Chlorite (Cr₂O₃=0.31–2.65 wt%), corundum (0.79–2.66 wt%), and diaspore are all Cr-rich varieties. The chromian (Cr₂O₃3.86 wt%) paragonitic muscovite incorporates up to 17% of the paragonite molecule, and significant Mg and Fe substitutions.The suite of rocks are characterized by chondritic Ti/Zr ratios (–x=107), systematically enhanced Cr (up to 14000 ppm) and Ni (up to 1200 ppm) abundances, low levels of the alteration-insensitive incompatible elements Th, Ta, Nb. Chlorite, corundum and Cr muscovite represent progressive stages in the incremental metasomatic alteration of a komatiite precursor. Mass balance calculations, constrained by the isochemical behaviour of Ti, Zr and Hf reveal that the komatiite chloritite transformation involved volumetric contractions of 60% by hydrothermal leaching of Si, Fe, Mn, Ca and Na. Reaction of chloritite to corundum involved further volumetric reductions of 50% due to essentially quantitative loss of Si, Fe, Mn, Mg, K and Ca. Conversion of corundum to muscovite required additions of Si, K, Fe, Mn, Mg, Rb and Ba at 50–200% dilation. K, Rb, Ba, Li and Cs are enriched by up to 2×10³ over background abundances in ultramafic rocks, and the suite is also enriched in B, Se, Te, Bi, As, Sb and Au. REE were extensively leached during chloritite-corundum stages, whereas LREE additions accompany development of muscovite. Ti, Zr, Hf and Al were all concentrated by selective leaching of mobile components, but absolute additions of Al accompanied development of the corundum domains due to Al precipitation in response to depressurization.Corundum ( ¹⁸O=3.5–4.8), muscovite ( ¹⁸O=6.7–7.5) and chlorite (4.5–5.6) are isotopically uniform and formed at 380–520° C from a fluid where ¹⁸O=5.6–6.9. The corundum is ¹⁸O depleted relative to either igneous or anatectic counterparts (O_cor=7.6–8.2), or to gibbsitic laterites ( ¹⁸O=12–17).Previous genetic schemes involving metamorphism of exhalites or bauxite, or Si-undersaturation of magmas, can all be ruled out from the data. The chloritite, corundum, Cr-muscovite association represents sequential alteration products of ultramafic rocks by high temperature, low pH hydrothermal solutions carrying LIL-elements, and in which excursions of pH and/or degree of quartz undersaturation account for the mineralogical transitions. A deep level acid epithermal system, or fluid advection across steep inverted thermal gradients in a thrust regime could account for required hydrothermal conditions.     

The Mari Rosa late Hercynian Sb-Au deposit, western Spain

The central Iberian zone of the Hesperian Massif hosts a series of late Hercynian vein-type Sb deposits. One of them is the Mari Rosa mineralization, hosted by metagreywackes and slates of the Schist-Greywacke Complex (Upper Precambrian). The mineralization is characterized by a complex paragenesis comprising three hydrothermal stages: stage H1 arsenopyrite-(pyrite); stage H2 stibnite-gold; and stage H3 pyrite-pyrrhotite-galena-sphalerite-chalcopyrite-tetrahedrite-boul-angerite-stibnite. Of these only the second episode was of importance and gave rise to the main mineralized bodies of the deposit. Hydrothermal alteration consists of a mild sericitization, chloritization and carbonatization of the metasedimentary rocks around the veins. Chemical changes in the hydrothermal halos include a remarkable increase in the ratio K₂O/Na₂O, and a decrease in the ratio SiO₂/volatiles, together with a sharp increase in Sb, Mo, Au and N. Fluids associated with ore deposition lie in the H₂O-NaCl-CO₂-CH₄-N₂ compositional system. These fluids evolved, progressively cooling, from initial circulaion temperatures close to 400 °C in the early stage (H1) to temperatures of approximately 150 °C in the late one (H3). Fluid composition evolution was characterized by a progressive increase in the bulk water content of the fluids and with an increase in the relative proportion of N₂ with respect to CH₄ and CO₂ in the volatile fraction. Massive stibnite deposition resulted from a boiling process developed at 300 °C and 0.9–1 Kb at a depth of 4–5 km. Geological, geochemical and fluid inclusion evidence suggest that the intrusion of the Alburquerque batholith (late Hercynian S-type granitoids) triggered hydrothermal activity leading to the transport and deposition of Sb and Au in Mari Rosa.     

Isotopic studies of processes in mafic magma chambers: II. The Skaergaard Intrusion,East Greenland

Isotopic ratios of Nd and Sr have been measured in a suite of samples spanning most of the exposed stratigraphy of the Skaergaard intrusion in order to detect and quantify input (such as assimilated wallrock and fresh magma) into the magma chamber during crystallization. Unlike ¹⁸O and D, Nd and Sr isotope ratios do not appear to have been significantly affected by circulation of meteoric waters in the upper part of the intrusion. Variations in initial ⁸⁷Sr/⁸⁶Sr and _Nd suggest that the Skaergaard magma chamber was affected during its crystallization by a small amount (2%–4%) of assimilation of Precambrian gneiss wallrock (high ⁸⁷Sr/⁸⁶Sr, low _Nd) and possibly recharge of uncontaminated magma. Decreases in _Nd and increases in ⁸⁷Sr/⁸⁶Sr during the early stages (0%–30%) of crystallization give way to approximately unchanging isotopic ratios through crystallization of the latest-deposited cumulates. Modelling of assimilation-fractional crystallization-recharge processes using these data as constraints shows that the assimilation rate must have been decreasing throughout crystallization. In addition, the isotope data allow replenishment by an amount of uncontaminated magma equal to 20%–30% of the total intrusion mass, occurring either continuously or in pulses over the first 75% of crystallization. Comparison of the recharge models with published Mg/(Mg+Fe²⁺) data from Skaergaard cumulates shows that the modelled replenishment rates are not inconsistent with available major element data, although significant recharge during the final 25% of crystallization can be ruled out. The isotope data show that the Skaergaard magma could have incorporated only a small amount of the gneiss that it displaced from the floor of the chamber; assimilation appears to have taken place primarily across a partially molten zone that formed at the roof from the wallrock that was dislodged during emplacement. In the latest stages of crystallization (>75% crystallized), the Skaergaard magma may have become stratified into two separately-convecting layers, effectively insulating Layered Series cumulates from further contamination.     

A staurolite-talc assemblage in tourmaline-phlogopite-chlorite schist from northern Victoria Land,Antarctica, and its petrogenetic significance

Staurolite and corundum are found as inclusions in tourmaline in a talc-phlogopite-chlorite-albite chist near Mount Bernstein (71°37S, 163°07E), northern Victoria Land, Antarctica. These inclusions are interpreted as relics of a staurolite-talc-corundum-chlorite assemblage that was stable during an early stage in the metamorphic cycle and subsequently armored by tourmaline, probably during the middle stage. Pressures and temperatures during the middle stage are estimated to be 650–700°C and 5.5–6.4 kbar. The transition from the early to the middle stage represents a roughly isothermal decrease in pressure of 2–3 kbar. During a late retrograde stage (T=300–370°C, P=3–5 kbar), staurolite was partly replaced by a muscovitic aggregate containing clinozoisite, pumpellyite, and margarite.The staurolite is unusually Si-poor (26.77, 25.85 weight % SiO₂ or 7.275, 7.091 Si per formula unit for 46 oxygens anhydrous), Al-rich (58.00, 57.85% Al₂O₃, 18.579, 18.702 Al), low in divalent cations (Fe+Mg+Mn+Zn=3.301, 3.560) and magnesian (atomic Mg/(Mg+Fe)=0.42, 0.40). Ion microprobe analysis of the first grain indicates about 0.2% Li₂O (0.219 Li) is present. The following substitutions are proposed to explain the unusual chemistry of this staurolite (crystallographic site notation of Smith 1968, in bold letters): Al(Si)+Al(Al(3A,B))Si(Si)+Fe(Fe), Li(Fe)+Al(Al(3A,B))2 Fe(Fe), and 2 Al(Al(3A,B)) 3 Fe(Fe).According to a pressure-temperature diagram constructed by the method of Schreinemakers for the model system FeO-MgO-Al₂O₃-SiO₂ (H₂O in excess), the talc-staurolite assemblage should be stable only in quartz-free rocks at temperatures near 700° C and pressures of 8 kbar or more. The rarity of the staurolite-talc assemblage even in Mg-Al-rich rocks metamorphosed at the appropriate pressure-temperature conditions is attributed to the appearance of anthophyllite or, in Na₂O-bearing rocks, gedrite. Orthoamphibole-cordierite and orthoamphibolekyanite assemblages with chlorite or corundum are incompatible with staurolite-talc±albite. In rocks lacking corundum and formed at pressures above the stability limit of cordierite, staurolite-talc may be metastable relative to orthoamphibole-kyanite, while in corundum-bearing rocks, staurolite-talc may appear under certain conditions, possibly at higher water activities than the orthoamphibole-kyanite assemblage.     

Petrogenesis of the Zoned Laacher See Tephra

The late Quaternary Laacher See phonolitic tephra deposit (EastEifel, W. Germany) is mineral-ogically and chemically zonedfrom highly evolved, volatile-rich and crystal-poor at its basetowards a mafic, crystal-rich phonolite at the top (Wörner& Schmincke, 1984). This zonation is interpreted as theresult of a continuous eruption from a zoned magma column. Majorand trace element evidence shows that the last erupted maficULST (Upper Laacher See Tephra) phonolite can be derived froma basanite parent magma via fractional crystallization of 30per cent clinopyroxene, 24 per cent amphibole, 4 per cent phlogopite,3.8 per cent magnetite, 2.5–3.0 per cent olivine and 1per cent apatite, leaving a derivative of 30 per cent evolvedmagma. Starting from the mafic (ULST) phonolite as a parent, the zonedsequence is postulated to have been formed by progressive fractionalcrystallization of the observed phenocryst phases. This modelwas tested by a series of 7 step-by-step mass balance fractionationcalculations. Abundance, modal composition and relative variationsof calculated fractionated phases agree well with the observedphenocryst abundances: sanidine followed by plagioclase andminor amounts of mafic phases are to be fractionated to givethe observed zoned sequence. The most evolved phonolite, however, cannot be generated bysubtraction of phenocrysts from the underlying phonolite. Processessuch as liquid-state differentiation may therefore have chemicallymodified the upper part (cupola) of the Laacher See magma columnsubsequent to crystal fractionation. The erupted phonolite magma (5.3 km³) was calculated to havestarted with a volume of 56 km³ of parental basanite magma whichfractionated to form 16.6 km³ of mafic phonolite. This magmafurther differentiated to give a 5.3 km³ zoned (erupted) phonolitecolumn. The non-erupted volume of 50 km³ is postulated to forma cooling cumulate body below the present day Laacher See volcano. The Laacher See magma system represents a complex end-membertype of a highly evolved small volume composition ally zonedmagma chamber with steep major and trace element gradients,the uppermost volatile rich magma layer resembling the stableroof part of rhyolitic chambers.