Stable isotopic evidence for large-scale seawater infiltration in a regional metamorphic terrane; the Trois Seigneurs Massif,Pyrenees, France

Oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a Hercynian metamorphic sequence in the Trois Seigneurs Massif, Pyrenees, France, indicate that all lithologies at higher metamorphic grades than the andalusite in isograd have relatively homogeneous ¹⁸O values. The extent of homogenization is shown by the similarity of ¹⁸O values in metacarbonates, metapelites and granitic rocks (+11 to +13), and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies. These values contrast with the ¹⁸O values of metapelites of lower metamorphic grade ( ¹⁸O about +15). Homogenization was caused by a pervasive influx of hydrous fluid. Mass-balance calculations imply that the fluid influx was so large that its source was probably high-level groundwaters or connate formation water. Hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at D=–25 to –30, suggesting a seawater origin. Many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6–12 km in the presence of this water-rich fluid, the composition of which was externally buffered. Deep penetration of surface waters in such environments has been hitherto unrecognized, and may be a key factor in promoting major anatexis of the continental crust at shallow depth. Three types of granitoid are exposed in the area. The leucogranites and the biotite granite-quartz diorite are both mainly derived from fusion of local Paleozoic pelitic metasediment, because all these rocks have similar whole-rock ¹⁸O values (+11 to +13). The post-metamorphic biotite granodiorite has a distinctly different ¹⁸O (+9.5 to +10.0) and was probably derived from a deeper level in the crust. Rare mafic xenoliths within the deeper parts of the biotite granite-quartz diorite also have different ¹⁸O (+8.0 to +8.5) and possibly represent input of mantle derived magma, which may have provided a heat source for the metamorphism.Contribution No. 4192, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

A strontium,neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif,Pyrenees, France

Nd, Sr, and O isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310–340 Ma Hercynian-age metamorphic terrane in the Pyrenees, France. Lower Paleozoic shales and phyllites have ⁸⁷Sr/⁸⁶Sr values of 0.707–0.717 at 310 Ma, but model values at 310 Ma of 0.709–0.736 (based on assumed depositional age of 450 Ma and an initial ⁸⁷Sr/⁸⁶Sr=0.707). On a regional scale, ⁸⁷Sr/⁸⁶Sr was homogenized to about 0.713 to 0.717 in the higher-grade pelitic schists during metamorphism. Much of this ⁸⁷Sr/⁸⁶Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompanied the δ ¹⁸O lowering of the phyllites (+13 to +16) during their transformation to andalusite- and sillimanite-grade schists (δ ¹⁸O=+11 to +12); all of these effects are attributed to pervasive interactions with hydrothermal fluids (Wickham and Taylor 1985). The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing of a relatively mafic magmatic end-member (⁸⁷Sr/⁸⁶Sr～ 0.7025–0.7050 and δ ¹⁸O～ +7.5 to +8.0) with two metasedimentary sources, both having ⁸⁷Sr/⁸⁶Sr～0.715 and δ ¹⁸O～ +10.0 to +12.0, but with one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels within the complex. The less homogeneous end-member that dominates at high levels is clearly derived from the local Paleozoic pelitic schists. A Rb-Sr age of 330±20 Ma was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism, as well. Although a post-metamorphic granodiorite magma body at Trois Seigneurs also displays heterogeneities in δ ¹⁸O and ⁸⁷Sr/⁸⁶Sr (and thus does not give a clear-cut Rb-Sr isochron), the data are consistent with an emplacement age between 260 and 310 Ma, similar to ages of other late granodiorites in the Pyrenees. ¹⁴³Nd/ ¹⁴⁴Nd is very uniform within the Hercynian crust, both at Trois Seigneurs (? _Nd=?3 to ?7) and elsewhere in the Pyrenees; almost all igneous lithologies have depleted-mantle, mid-Proterozoic model ages, consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 Ma or earlier. Rb-Sr mineral ages exhibit a complex cooling history reflecting late Hercynian and Mesozoic thermal events. Our results show that profound homogenization of the ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas; this may explain why many peraluminous Hercynian granitoids of Western Europe have anomalously low (⁸⁷Sr/⁸⁶Sr) initial values compared to their probable sedimentary parent rocks.     

Strontium and oxygen isotope decoupling in the Hercynian Trois Seigneurs Massif,Pyrenees: evidence for fluid circulation in a brittle regime

Systematic shifts of oxygen isotopic compositions in the higher grade parts of the high temperature-low pressure Hercynian metamorphic sequence, exposed in the Trois Seigneurs Massif, have previously been explained as a result of an influx of surface-derived water during the prograde part of the metamorphic cycle. It has been suggested that this caused a regional lowering of ⁸⁷Sr/⁸⁶Sr in the metamorphic sequence. Mapping of strontium isotopic compositions across a 15 m meta-carbonate horizon in the higher grade pelite-psammite sequence shows that strontium isotopic compositions were homogenised over length scales of metres or less during the Hercynian metamorphism, which brought the carbonate and pelite-psammite to oxygen isotopic equilibrium with a common fluid. Comparison of model pre-Hercynian ⁸⁷Sr/⁸⁶Sr profiles across the carbonate (based on a depositional/diagenetic age of 450 Ma and initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7086 given by 10 m length scale averaging) with the post-Hercynian ⁸⁷Sr/⁸⁶Sr profile (calculated from analysed ⁸⁷Sr/⁸⁶Sr and Rb/Sr compositions) implies strontium isotopic diffusion distances of ca. 0.4 m in the carbonate and ca. 7 m in the pelite-psammite. The limited Sr-isotopic diffusion distance of 0.4–0.7 m within the carbonate is compatible with pervasive oxygen-isotopic exchange over distances restricted to 4–15 m if fluid strontium concentrations were between 4 and 50 ppm. The strontium isotopic transport distances are not compatible with pervasive oxygen isotopic alteration over the observed 5 km regional scale. Either the flow was perfectly layer-parallel or, more probably, the regional-scale alteration of oxygen took place by fluid circulation in the brittle regime early in, or prior to, the Hercynian metamorphic event. Flow along cracks with incomplete diffusive exchange between fluid and wall rock would allow greater decoupling of oxygen and strontium isotopic transport than pervasive advective transport with local fluid-solid equilibrium.     

云开地块印支期变质-深熔作用:混合岩、片麻岩锆石U-Pb年代学和Hf同位素证据

云开地块中生代构造演化是华南地区的研究热点之一.通过对云开地块变质基底中的混合岩、片麻岩（5个样品）和花岗岩（1个样品）开展锆石LA-ICP-MS U-Pb定年,获得440.3±3.3 Ma、230.2±2.9 Ma、230.7±1.3 Ma、459.5±2.7 Ma、431.5±4.3 Ma、229.2±5.4 Ma、229.7±2.7 Ma 7组变质（深熔）或岩浆年龄和2组（样品1432-1和ID7-3）碎屑锆石年龄,碎屑锆石年龄范围均为太古代-新元古代,且具有~1.0 Ga年龄主峰,与天堂山岩群和云开群碎屑锆石年龄谱相似.区域资料表明云开地块天堂山岩群和云开群具有相似的物质组成,均形成于早古生代-新元古代,存在变质程度和物质面貌的差异;在加里东期构造-热事件的基础上,广泛叠加了印支期区域变质（深熔）-构造-流体作用影响.4个样品中（1431-1、1432-1、D116-3和ID7-3）锆石原位Lu-Hf同位素组成显示,加里东期变质和深熔锆石Lu/Hf同位素组成基本一致,应继承了原岩的同位素组成特征.印支期变质和深熔锆石Lu/Hf同位素组成不同,可能主要由变质作用和深熔作用的差异所致.以古-中元古代为主的地壳物质参与了加里东期和印支期变质-深熔作用,在加里东期和印支期深熔作用过程中,均有少量幔源物质的加入,印支期幔源物质的贡献相对显著.      

Low-pressure Granulites of the Lisov Massif, Southern Bohemia: Visean Metamorphism of Late Devonian Plutonic Arc Rocks

The Liov Granulite Massif differs from neighbouring granulitebodies in the Moldanubian Zone of southern Bohemia (Czech Republic)in including a higher proportion of intermediate–maficand orthopyroxene-bearing rocks, associated with spinel peridotitesbut lacking eclogites. In addition to dominantly felsic garnetgranulites, other major rock types include quartz dioritic two-pyroxenegranulites, tonalitic granulites and charnockites. Minor bodiesof high-pressure layered gabbroic garnet granulites and spinelperidotites represent tectonically incorporated foreign elements.The protoliths of the mafic–intermediate granulites (quartz-dioriticand tonalitic) crystallized 360–370 Ma ago, as indicatedby laser ablation inductively coupled plasma mass spectrometryU–Pb ages of abundant zircons with well-preserved magmaticzoning. Strongly metamorphically recrystallized zircons giveages of 330–340 Ma, similar to those of other Moldanubiangranulites. For the overwhelming majority of the Liov granulitespeak metamorphic conditions probably did not exceed 800–900°Cat 4–5 kbar; the equilibration temperature of the pyroxenegranulites was 670–770°C. This is in sharp contrastto conditions of adjacent contemporaneous Moldanubian granulites,which are characterized by a distinct HP–HT signature.The mafic–intermediate Liov granulites are thought tohave originated during Viséan metamorphic overprintingof metaluminous, medium-K calc-alkaline plutonic rocks thatformed the mid-crustal root of a Late Devonian magmatic arc.The protolith resembled contemporaneous calc-alkaline intrusionsin the European Variscan Belt. KEY WORDS: low-pressure granulites; geothermobarometry; laser-ablation ICP-MS zircon dating; whole-rock geochemistry; Sr–Nd isotopes; Moldanubian Zone     

The Ghingee Granite, Tamil Nadu, South India: Geochemistry and Petrogenesis

The Palaeo-Proterozoic Ghingee granite is an anatectic granite formed in high grade granulite terrain by ultrametamorphism. The compositional variations both in major and trace elements observed in this granite (SiO₂ : 64.16-73.81; Fe₂O₃ : 0.12-2.19; FeO : 0.12-2.80; MgO : 0.10-2.19; CaO : 1.66-4.71; K₂O : 1.09-5.09; Ba: 223-1883 ppm; Cr : 4-60 ppm) are attributed to a) source rock heterogeneity and b) the tectonic disturbances that might have abruptly ended the anatectic melting process. The granite is compositionally similar to Perur, Closepet and Hyderabad granites and is formed during Archaean-Proterozoic transition by anatectic and crustal remelting processes.     

Crustal recycling and juvenile addition during lithospheric wrenching: The Pontivy-Rostrenen magmatic complex,Armorican Massif (France), Variscan belt

The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithospheric wrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustal magmatism, and a weakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315 Ma (between 316.7 ± 2.5 Ma and 310.3 ± 4.7 Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7 ± 2.7 Ma. The leucogranites (− 4.8 < εNd (T) < 2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK > 1.1) and formed by partial melting of metasediments and peraluminous orthogneisses. The monzogranite (− 4.0 < εNd (T) < − 3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1 < A/CNK < 1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (− 3.2 < εNd (T) < − 2.2; no inherited zircon) is metaluminous (0.7 < A/CNK < 1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.     

Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France

Petrological and geochemical variations are used to investigatethe formation of granite magma from diatexite migmatites derivedfrom metasedimentary rocks of pelitic to greywacke compositionat St. Malo, France. Anatexis occurred at relatively low temperaturesand pressures (<800°C, 4–7 kbar), principally throughmuscovite dehydration melting. Biotite remained stable and servesas a tracer for the solid fraction during melt segregation.The degree of partial melting, calculated from modal mineralogyand reaction stoichiometry, was <40 vol. %. There is a continuousvariation in texture, mineralogy and chemical composition inthe diatexite migmatites. Mesocratic diatexite formed when metasedimentaryrocks melted sufficiently to undergo bulk flow or magma flow,but did not experience significant melt–residuum separation.Mesocratic diatexite that underwent melt segregation duringflow generated (1) melanocratic diatexites at the places wherethe melt fraction was removed, leaving behind a biotite andplagioclase residuum (enriched in TiO₂, FeO_T, MgO, CaO, Sc,Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocraticdiatexite (enriched in SiO₂, K₂O and Rb) where the melt fractionaccumulated. Leucocratic diatexite still contained 5–15vol. % residual biotite (mg-number 40–44) and 10–20vol. % residual plagioclase (An₂₂). Anatectic granite magmadeveloped from the leucodiatexite, first by further melt–residuumseparation, then through fractional crystallization. Most biotitein the anatectic granite is magmatic (mg-number 18–22). KEY WORDS: anatexis; diatexite; granite magma; melt segregation; migmatite     

The Distribution of H2O between Cordierite and Granitic Melt: H2O Incorporation in Cordierite and its Application to High-grade Metamorphism and Crustal Anatexis

Experiments defining the distribution of H₂O [D_w = wt % H₂O(melt)/wt% H₂O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H₂O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H₂O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H₂O vapour-saturatedconditions D_w ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedD_w decreases to minimum values of 2·6–5·0at moderate to low cordierite H₂O contents (0·6–1·1wt %). At very low aH₂O, cordierite contains less than 0·2–0·3wt % H₂O and D_w increases sharply. The D_w results are consistentwith melt H₂O solubility models in which aH₂O is proportionalto X_w² (where X_w is the mole fraction of H₂O in eight-oxygenunit melt) at X_w     

Temperature and Bulk Composition Control on the Growth of Monazite and Zircon During Low-pressure Anatexis (Mount Stafford, Central Australia)

The formation, age and trace element composition of zircon andmonazite were investigated across the prograde, low-pressuremetamorphic sequence at Mount Stafford (central Australia).Three pairs of inter-layered metapelites and metapsammites weresampled in migmatites from amphibolite-facies (T 600°C)to granulite-facies conditions (T 800°C). Sensitive high-resolutionion microprobe U–Pb dating on metamorphic zircon rimsand on monazite indicates that granulite-facies metamorphismoccurred between 1795 and 1805 Ma. The intrusion of an associatedgranite was coeval with metamorphism at 1802 ± 3 Ma andis unlikely to be the heat source for the prograde metamorphism.Metamorphic growth of zircon started at T 750°C, well abovethe pelite solidus. Zircon is more abundant in the metapelites,which experienced higher degrees of partial melting comparedwith the associated metapsammites. In contrast, monazite growthinitiated under sub-solidus prograde conditions. At granulite-faciesconditions two distinct metamorphic domains were observed inmonazite. Textural observations, petrology and the trace elementcomposition of monazite and garnet provide evidence that thefirst metamorphic monazite domain grew prior to garnet duringprograde conditions and the second in equilibrium with garnetand zircon close to the metamorphic peak. Ages from sub-solidus,prograde and peak metamorphic monazite and zircon are not distinguishablewithin error, indicating that heating took place in less than20 Myr. KEY WORDS: accessory phases; anatexis; trace element partitioning; U–Pb dating     

Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

Emplacement of mantle-derived magma (magmatic accretion) isoften presumed or inferred to be an important cause of regionalgranulite facies metamorphism and crustal anatexis. The juxtapositionof mafic cumulates and regionally distributed granulite faciesrocks has led some to consider the Ivrea zone (northern Italy,Southern Alps) as an important exposure that demonstrates thiscausal relationship. However, regional PTt paths indicated bymetamorphic reaction textures and PT conditions inferred fromgeothermobarometry indicate that the emplacement of mafic plutonicrocks (Mafic Complex) at the Ivrea zone occurred during decompressionfrom ambient pressures at the regional thermal maximum. Fieldand petrographic observations, supported by PT estimates, indicatethat regional retrograde decompression and emplacement of theupper parts of the Mafic Complex probably accompanied extensionduring the Late Carboniferous–Early Permian. A spatiallyrestricted decompression-melting event accompanied final emplacement,depleting supracrustal rocks enclosed by an     

Palaeozoic Intraplate Crustal Anatexis in the Mount Painter Province, South Australia: Timing, Thermal Budgets and the Role of Crustal Heat Production

The effect of radiogenic heat production within the crust onthermal processes such as crustal anatexis is generally disregardedas bulk geochemical models suggest that crustal heat generationrates are too low to effect significant heating. However, theMount Painter Province in northern South Australia is characterizedby a total crustal contribution to surface heat flow of morethan twice the global average. The province is composed dominantlyof Proterozoic granites and granite gneisses with an area averageheat production of 16·1 µW/m³; individual lithologieshave heat production >60 µW/m³. These Proterozoic rocksare intruded by the British Empire Granite, a younger intrusivewhose origin has remained enigmatic. Isotope geochemistry suggestscrustal sources for the melt and it has a crystallization ageof 440–450 Ma, which places the setting >750 km inboardof the nearest active plate boundary zone at this time. Phaseequilibria calculations suggest that temperatures of at least720–750°C are required to produce the granite butthe intensity of crustal thickening during Palaeozoic deformation(12%) cannot account for these conditions. Here we describea model for the generation of the British Empire Granite inwhich the primary thermal perturbation for mid-crustal anatexiswas provided by the burial of the high heat-producing MountPainter basement rocks beneath the known thickness of Neoproterozoiccover sediments. The high heat-producing rocks at Mount Painterimply that the natural range and variability of crustal heatproduction is much greater than previously believed, with importantconsequences for our understanding of temperature-dependentcrustal processes including the exploitation of geothermal energyresources. KEY WORDS: geothermal energy; low-pressure anatexis; thermal conductivity; thermal regime     

The Petrogenesis of A-type Magmas from the Amram Massif, Southern Israel

The (     

吉东南新太古代晚期片麻岩类的时代、成因及其对早期地壳形成演化的制约

吉林省东南部通化地区广泛出露早前寒武纪花岗质片麻岩类,其岩石组成、形成时代和成因对深化认识华北克拉通东北缘早期地壳形成演化历史具有重要意义。本文系统的岩石学、锆石U-Pb年代学、元素和Lu-Hf同位素地球化学等研究结果表明,这些花岗质片麻岩类按照SiO₂质量分数总体可划分为高硅和低硅两组：前者主要由二长花岗质片麻岩、奥长花岗质片麻岩及英云闪长质片麻岩组成,其原岩形成于2 549~2 557 Ma;而后者由石英二长闪长质片麻岩及花岗闪长岩质片麻岩组成,其原岩形成于2 534~2 552 Ma;并且两组岩石都含有约2 600 Ma的捕获锆石,共同遭受了约2 500 Ma变质作用的影响。地球化学分析结果显示,低硅岩组具有较高的MgO、CaO、Na₂O质量分数,属于高钾—中钾钙碱性系列,并且富集LREE、亏损HREE和HFSE;与之相比,高硅岩组则具有较低的MgO和CaO质量分数,显示更强烈的轻、重稀土元素分馏以及Nb、Ta、P、Ti等亏损的特征;但两者均具有较弱的正或负Eu异常。结合区域最新研究成果,认为研究区低硅和高硅两组岩石应具有相同的源区,其形成可能与大洋板片俯冲、岩浆底侵引起的地壳部分熔融作用有关。此外,两组岩石具有相似的ε_Hf（t）值（2.72~7.95）和模式年龄（2.86~2.55 Ga）,暗示区域主要存在新太古代晚期地壳生长事件;结合区域内变质火山岩的研究进展,认为吉林省东南部通化地区花岗质片麻岩类可能形成于活动大陆边缘的构造背景。     

Metamorphism of sedimentary rocks recognised by 19th century French naturalists: A case study from the Chavanon sequence marbles,Massif Central,France

The Chavanon metamorphic sequence in the Variscan French Massif Central contains marble lenses that have been exploited since at least the 18th century to produce lime. They provided an opportunity to some major pioneers among nineteenth century French geologists to understand how metamorphic rocks are formed. The intimate association between marbles and gneisses led them to propose that the initial pile of rocks was deposited as a single unit under water. They thoroughly described the attitude, folding and mineralogy of the marbles, making their works of great historical value. Originally written in French, their translation into English and the reproduction of original figures from the works they published will be very useful to the international community of geologists interested in the history of their science.     

Granulite-Facies Metamorphism and Crustal Magmatism in the Ashuanipi Complex, Quebec--Labrador, Canada

The high-grade Archean Ashuanipi complex contains an older sequenceof granulite-facies migmatitic paragneiss and tonalite cut byabundant orthopyroxene-bearing, enclave-laden granitoid bodies(diatexite) of strongly peraluminous (garnet-bearing) and mildlyperaluminous (garnet—absent) granodioritic composition,inferred to be magmatic in origin. Temperature estimates forgarnet–orthopyroxene–biotite–plagioclase–quartzassemblages in both metamorphic and igneous rock types are mainlyin the range 700– 835 ?C, but apparent pressures are higher(0?6–0?65 GPa) in a wide belt of paragneiss and associatedtonalite than in the enclosing diatexites (0?35–0?55 GPa),possibly owing to fluid-enhanced retrograde re-equilibrationwithin the crystallizing igneous assemblages. Paragneiss has bulk compositions typical of Archean greywacke(58–68 wt. % SiO₂), including high Cr (110–250 ppm),Ni (20–100 ppm), and LREE [(70–100) ?chondrites].Garnet-bearing diatexites have compositions virtually identicalto paragneiss whereas garnet-absent diatexites are characterizedby marked HREE depletion. High degrees of fusion of a sourcesuch as paragneiss, with entrainment of crystalline phases suchas garnet and orthopyroxene, are required to explain the compositionof garnet-bearing diatexites, whereas lower amounts of melting,leaving residual garnet, may account for the origin of the garnet-absentvarieties. CO2 may have been a melt component in diatexite, based on severalobservations: (l)the high degrees of fusion implied in the genesisof diatexite require either extreme temperatures (> 1000?C)for which there is no mineralogical evidence, or some fluxingagent other than H₂O (cf. Peterson & Newton, 1990); (2)some xenoliths have orthopyroxene-rich (dehydration) margins,implying relatively anhydrous melt conditions; and (3) orthopyroxeneis unaltered, suggesting that low aH₂O conditions persistedduring crystallization. U–Pb zircon geochronology constrains the time for heatingand magma production to <18 Ma (2700 Ma for detrital zirconin paragneiss; 2682 Ma for crystallization of igneous zirconin diatexite). Combined with the evidence for high crustal temperaturesand possible CO₂ involvement, the rapid heating implies thatunderplated basaltic magmas played a key role as heat and fluidsources driving high-grade metamorphism and granitoid melt production.     

The petrology and petrogenesis of some muscovite granite sills from the Barousse Massif,Central Pyrenees

The mineralogy, petrology and Rb-Sr isotope compositions of muscovite granite sills and associated pegmatites from the Barousse Massif, Central Pyrenees, are described. The petrogenesis of the granites is discussed with respect to their structure, mineral assemblages, mineral compositions and initial ⁸⁷Sr/⁸⁶Sr ratio. It is concluded that the muscovite granite sills have been intruded as magma, formed from the partial melting of continental crust at 276 million years. Progressive crystallisation of the siliceous magma has concentrated the aqueous phase in the remaining melt, and the associated pegmatites are the final expression of the crystallising intrusion.     

The Weichselian Late-glacial in southwestern Europe (Iberian Peninsula,Pyrenees, Massif Central,northern Apennines)

This paper presents a summary of Late-glacial environmental changes in southwestern Europe (lberian Peninsula, Pyrenees, Massif Central and the northern Apennines). The emphasis is on palaeoclimatic interpretations inferred from key sites in the region from which the most detailed records are available and which have been radiocarbon dated. The earliest evidence for climatic improvement following the end of the last glacial stage is dated to ca. 15 ka BP and is found at a few sites only. By 13 ka BP, a more widespread and marked climatic improvement is evident, although it is difficult to be precise in the timing and magnitude of the event. There are significant variations in detail between the Late-glacial records, but evidence for a significant cooling correlated with the Younger Dryas event is widespread throughout the region. Just two sites in the region provide evidence for an earlier, less emphatic phase of climatic cooling, which is tentatively equated with the ‘Older Dryas’ of continental northern Europe. Dry conditions appear to have predominated throughout the region in the later part of the Younger Dryas and the early Holocene.     

大别山地区天堂寨花岗岩的侵位时代及地质意义

天堂寨片麻状花岩，多数文献将其归属前寒武纪混合花岗岩。笔者认为是岩浆冷凝结晶产物，属Iu型花岗岩。其侵位时代为早白垩世。锆石一致曲线年龄为124．7Ma。岩体侵位以后，大别山地区的抬升速率不很快，平均每年抬升0．1mm。     

Petrology and geothermometry of retrograded Hercynian charnockites and host gneisses, Agly Massif, French Pyrenees

Abstract A Hercynian charnockite occurs within high-grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe-Mg distribution coefticient ( K _D) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H₂O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H₂O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns'width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti⁴⁺+ Fe²⁺) and (Mg²⁺+ Al³⁺_VI); Ti and Fe both increase, whereas Mg and Al_VI decrease. There is an inverse linear correlation between Fe²⁺ and Mg²⁺ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and K ^Ga-Bi_DFe-Mg is less clear: it seems that K _D slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga-Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600-650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.