Triple-junction migration during Paleozoic plate convergence: the Aracena metamorphic belt, Hercynian massif, Spain

New data on the petrology and structure of the Aracena metamorphic belt shows that this is a subduction-related, low-pressure/high-temperature complex developed by plate convergence at the north margin of Gondwana during the Paleozoic. The low-pressure, inverted metamorphic gradient in MORB-derived amphibolites resulted from heating from the continental hanging wall during subduction. This implies that the previous heating of the continental rocks was related to subduction of an oceanic ridge and the creation of a slab window beneath the continental margin. This slab window brought the asthenosphere in contact with the continental margin inducing a shallow thermal anomaly and partial melting of the lithospheric mantle resulting in boninite magmatism.     

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.     

The genesis of Variscan (Hercynian) plutonic rocks: Inferences from Sr,Pb, and O studies on the Maladeta igneous complex,central Pyrenees (Spain)

The Maladeta plutonic complex formed during the latest stages of the Variscan orogeny. It was emplaced into the Paleozoic sedimentary sequence of the Pyrenees. The eastern part, investigated in the present study, consists of an early intrusion of cumulate gabbronorites followed in order of emplacement by the main biotite-hornblende granodiorite, which was itself intruded by two small stocks of two-mica cordierite granite. An ⁸⁷Rb-⁸⁷Sr isochron dates the granodiorite at 277±7 m.y. with an initial (⁸⁷Sr/⁸⁶Sr)_o ratio of 0.7117±3. Gabbroic rocks have lower strontium initial ratios, down to 0.7092, while those of granite range from that of the granodiorite up to about 0.715. The three rock types have distinctive ¹⁸O values: 8.7 to 9.6 for the gabbronorites, 9.4 to 10.4 for the granodiorites and 10.3 to 11.8 for the granites. Lead isotopic compositions of rocks and feldspars are all radiogenic. Feldspars give consistent Pb model ages around 280 m.y., with and values of about 9.7 and 4.05, respectively. No pristine, mantle-derived magma was found among the investigated samples and the rocks cannot be related to one another by any simple mechanism of fractional crystallization. Some type of mixing process involving two end members seems to be required: a high-¹⁸O, high-⁸⁷Sr material that is clearly of crustal origin, and a lower-¹⁸O, lower-⁸⁷Sr end member derived from the mantle. Examination of various mixing models does not support magma mixing nor assimilation of crustal rocks by a mafic magma. The most acceptable model involves melting at different levels of a vertically-zoned source in the continental crust; this source was formed by mixing between mantle-derived magmas and crustal metasediments. This material was apparently thickened, tectonically downwarped and partially melted. None of the Maladeta magma-types appear to have been derived at a consuming plate boundary.Contribution Number 3280, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA     

Geochemistry of granitic rocks from the Hercynian Sardinia-Corsica batholith: Implication for magma genesis

The Sardinia-Corsica batholith was structured in the late stage of the Hercynian orogenesis. The granitoids intrude mainly metamorphic complexes grading from zeolite up to amphibolite facies. The batholith is heterogeneous consisting of complexes with different affinity, chemical composition, age and degree of deformation. The present paper reports major-and trace-element data for selected samples coming only from Sardinian outcrops.

The rocks range from gabbro-diorite to tonalite, monzogranite and leucogranite. The two latter lithologies are the most abundant, gabbrodiorites and tonalites occurring in minor amounts and mainly in northern-central Sardinia. Over 75% of the granitoids contain microgranular enclaves of magmatic origin. The age of the rocks falls in the interval between 307 and 281 Ma. Sr isotope initial ratios are high, ranging between 0.7083 and 0.7107.

REE, Rb, Sr, Ba, Zr, Th, Ta, Hf, Co and Sc abundances were determined on selected samples. All elements follow three types of trends vs. CaO, which is used as differentiation index. Two trends show positive and negative correlations while the third one shows a bell-shaped pattern. LREE have different degrees of enrichment (La = 20−120× ch) and HREE show variable fractionation with prevailing (Tb/Yb)_n<1. The two peraluminous samples have very different geochemical characteristics.

From the geochemical point of view all the rocks coming from the Sardinian segment of the batholith display a typical calc-alkaline chemical character showing the imprint of both “normal and mature” continental arc geodynamic environments.

Geochemical trends suggest some petrogenetic constraints. The complete sequence of differentiation can be neither the product of crystal/liquid fractionation processes starting from a single basic parent magma nor the product of an AFC process. On the contrary, a two-stage model can be proposed. In the first stage a mafic melt of subcrustal origin interacted with monzogranitic magmas derived from 25–35% degree of melting of a crustal biotite amphibolitic source. Such a mixing process acted together with a crystal/liquid fractionation process to give tonalites and granodiorites. In the second stage lesser degrees of melting of the same crustal source could give the late-stage leucogranitic masses.

A possible scenario, able to take into account field and geochemical data, can be suggested for the genesis of this suite and we propose it as a working model for future investigations.     

Experimental Constraints on Hercynian Anatexis in the Iberian Massif, Spain

We have studied experimentally the melting relationships ofthe Ollo de Sapo gneiss (OSG), an important crustal protolithfor the Iberian leucogranites, of possible volcanoclastic origin.The results of this study are compared with previously determinedPTt paths, allowing us to interpret the mechanisms of meltingand granitoid production during the Hercynian orogenic cycle.Phase relationships determined in fluid-absent experiments indicatethat the OSG is a fertile source for peraluminous leucogranites.The slope of the fluid-absent solidus is strongly controlledby the breakdown of Ms in the presence of Qtz, Pl and Kfs. Thissolidus curve has a positive slope ranging from dP/dT = 30 bar/°Cat low P (<6 kbar) to dP/dT = 70 bar/°C at higher P (6–15kbar). The relationships between the Ms vapour-absent solidusand the PTt metamorphic paths in different sectors of the Iberianmassif have two important implications: (1) melt productivityis strongly favoured at low P; (2) anatexis in the Iberian massifprobably took place by decompression associated with crustalthinning and extension. These results are in agreement withthe relationships between granite production and tectonic deformationphases observed in the Iberian massif. Our results emphasizethat anatexis is a process that is strongly controlled bothby the phase relationships of the crustal protoliths and bythe thermal structure of the continental crust. Consequently,one must be careful when assigning potential crustal protolithsto particular granite associations exclusively on the basisof geochemical comparisons. KEY WORDS: anatexis; Hercynian orogen; Iberian massif     

Considerations about Hercynian thrusting in the Marimanya massif (central Pyrenees)

We can distinguish four main units within the Marimanya massif: the Cambro-Ordovician detrital series; a carbonate unit, mainly made up of massive limestones dated as Silurian-Devonian; dark slates and limestones dated as Middle-Upper Devonian; and the late Hercynian Marimanya intrusive, mainly composed of granites and granodiorites. Mapping has shown that the carbonate units lie on different levels of the Cambro-Ordovician series and even on the dark slates and limestones. This abnormal contact is demonstrated to be a thrust. The thrusting surface is affected by Hercynian folds with vertical axial planes, a fold axis trending E-W and an associated cleavage. Moreover, it is cut by the late Hercynian intrusive. Consequently, considering paleontological and structural data as well as the regional context, the most reasonable interpretation is that this structure represents a Hercynian thrust, probably with southward displacement.     

Age, geodynamic setting, and mantle enrichment processes of a K-rich intrusion from the Meissen massif (northern Bohemian massif) and implications for related occurrences from the mid-European Hercynian

The plutonic complex of the Meissen massif (northern margin of the Bohemian massif) comprises dioritic to mainly monzonitic and granitic rocks. The diorite to monzonite intrusions show major and trace element patterns typical for shoshonitic series. The chemical signatures of less crustally contaminated diorites are similar to arc-related shoshonitic rocks derived from continental lithospheric mantle (CLM) sources previously enriched by subduction of altered oceanic crust. Laser step heating ⁴⁰Ar/³⁹Ar analyses on actinolitic to edenitic amphiboles from geographically different occurrences of the monzonitic intrusion yielded concordant plateau ages as well as total gas ages ranging from 329.1±1.4 to 330.4±1.4?Ma and from 330.4±2.1 to 330.6±1.8?Ma, respectively. These cooling ages are indistinguishable from sensitive highresolution ion microprobe (SHRIMP) ²³⁸U/²⁰⁶Pb intrusion ages measured on magmatic zircon rims from the monzonite (Nasdala et al., submitted). This shows that the monzonite intrusion is probably not related temporally to active subduction because it postdates eclogites of the adjacent Saxonian Erzgebirge by approximately 20?Ma. The shoshonitic magmas intruded during strike-slip tectonism along the Elbe valley zone. The enrichment of their mantle sources may be of Upper Devonian/Lower Carboniferous age or older. Intrusions of shoshonitic to ultra-potassic (K-rich) rocks during the Upper Visean/Namurian are widespread in the Moldanubian zone. Based on similar ages and structural relationships a similar post-collisional setting to the Meissen shoshonitic rocks can be demonstrated. Most of these occurrences cut high-grade nappe units which were subducted during the Upper Devonian/Lower Carboniferous. In contrast to the Meissen massif, at least the ultra-potassic members of the Central and the South Bohemian batholiths were derived from CLM sources enriched by fluids or melts released from subducted oceanic crust and by greater portions of crustal material. Despite the similar post-collisional geodynamic setting of the K-rich intrusions, different enrichment processes generated mid-European Hercynian CLM sources with heterogeneous major and trace element and isotopic signatures.     

Sequential, syndeformational porphyroblast growth during Hercynian low-pressure/high-temperature metamorphism in the Canigou massif, Pyrenees

The sequence of growth of garnet, staurolite and aluminosilicate in Fe-rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite-absent zone in the field) is attributed to a strong pressure-dependence for the aluminosilicate-producing reactions. Porphyroblast-matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).     

Geochemistry and Sr-Nd isotopes of amphibolite dykes of northern Victoria Land, Antarctica

Mafic dykes cutting the gneisses and migmatites in the Deep Freeze Range high-grade metamorphic complex of northern Victoria Land, Antarctica, have undergone strong recrystallization and deformation during amphibolite-facies metamorphism. Metamorphic mobility mostly affected the large-ion lithophile elements (LILE). Rare Earth (RE) and the high field-strength elements (HFSE) were essentially immobile during metamorphism. Together with the major-element geochemistry, this suggests primary characteristics of evolved tholeiitic magmas and mafic cumulates. No precise ages of intrusion are available for the dykes, but geological evidence suggest emplacement during the time interval 800 to 900 Ma. The Rb-Sr isotopic system in some of the dykes were also variably affected by a later thermal event, probably coincident with the time of amphibolite metamorphism, ca. 500–550 Ma ago. This event can be correlated with the Ross Orogeny in the Transantarctic Mountains. Nd isotopes and trace element abundances indicate that the dykes were derived by different degrees of partial melting and fractionation of heterogeneous sub-continental lithospheric mantle. The Nd isotopic compositions range from depleted to enriched signatures (ε^Nd computed back to 850 Ma = +4.5 to −11.61), and are coupled to different trace element normalized patterns characterized by a slight positive Nb anomaly in the former case to a strong negative Mb anomaly for the latter samples. On isotopic and chemical ground the depleted signature of the mantle source resembles that reported for E-type MORB. The nature of the enriched components cannot be uniquely stated; nevertheless, on the basis of isotopic and geochemical data, it could be represented by sediments recycled into the sub-continental mantle or by crustal contamination during underplating of mafic magmas, or a combination of the two processes.     

Inner structure,composition, and genesis of the Chineiskii anorthosite-gabbronorite massif,Northern Transbaikalia

The Chineiskii anorthosite-gabbronorite massif is the most typical layered intrusion in Russia, which is accompanied by large V and Cu deposits. This massif is first considered to be a component of the Proterozoic volcanic-plutonic system of the Kodar-Udokan district, whose largest massifs are Chineiskii and Lukturskii. This system also comprises numerous dikes (including the Main gabbronorite dike at the Udokan deposit, whose thickness reaches 200 m), which are likely the magmatic feeders of ancient volcanism. An intermediate position in the vertical section of the magmatic system is occupied by gabbroids, whose exposures occur in the peripheral part of the Lurbunskii granite massif. The intrusive rocks were proved to be genetically interrelated and show certain similar geochemical features: they bear elevated TiO₂ concentrations and have similar trace element patterns and (La/Sm)_N and (Gd/Yb)_N ratios (1.5–2.3 and 1.87–2.06, respectively). The Chineiskii Massif is thought to have been formed by the successive emplacement of genetically similar basic magmas, which produced four rock groups with fine and coarse layering and cyclicity of variable rank (microrhythms, rhythms, units, and series). The results of cluster analysis indicate that the rocks can be classified into 13 petrochemical types. The phase and chemical characteristics of the parental melts of these compositions were simulated with the use of the COMAGMAT-3.5 computer model, which was also applied to evaluate the composition of the most primitive initial magma of the whole Chineiskii Massif. Our results indicate that the primitive magma was heterogeneous (olivine + plagioclase ± titanomagnetite + melt) at a temperature of approximately 1130°C. The initial melt had a ferrobasaltic composition and was close to saturation with magnetite at ～NNO ± 0.5     

The metamorphic evolution of the Muxia-Finisterre region (Galice,NW Spain) during the Hercynian Orogenesis

Mineral chemistry and crystal growth-deformation relationships in metapelites, orthogneises and metabasic rocks from the Muxia-Finisterre region show that this area has been affected by plurifacial regional metamorphism during the Hercynian Orogenesis. Paragenetic evolution seems to be related to a change of the metamorphic gradient towards relatively lower P_s conditions in the mesozone, while this character is less evident in catazonal rocks. In both cases there was a continuous increase in T and decrease in {ie657-02}.The metamorphic climax was reached during the second metamorphic episode (M₂). This event took place during and even after the second deformation phase (P₂). The latter gave rise to the main foliation observed in studied rocks (S₂) which almost practically erased signs of previous schistosity. Relationships between metamorphic evolution and different geotectonic models are discussed.

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Zusammenfassung Mineralchemismus und die Beziehungen Kristallwachstum-Deformation in Metapeliten, Orthogneisen und metabasischen Gesteinen aus der Muxia-Finisterra-Region, zeigen eine Beeinflussung dieses Gebietes durch eine plurifazielle Regionalmetamorphose während der herzynischen Orogenese.Die paragenetische Entwicklung scheint mit einer Veränderung des metamorphen Gradienten in Richtung niedrigerer P_s-Bedingungen in der Mesozone einherzugehen, während dies in katazonalen Gesteinen weniger deutlich ist.In beiden Fällen gab es eine kontinuierliche Zunahme der Temperatur (T) bzw. Abnahme des {ie657-01}.Der Höhepunkt der Metamorphose wurde während der zweiten metamorphen Episode (M₂) erreicht. Dieses Ereignis fand statt während und sogar nach der zweiten Deformationsphase (P₂). Letzteres verursachte die Hauptschieferung (S₂) in den untersuchten Gesteinen, die die Merkmale einer älteren Schieferung nahezu ausgelöscht hat.Beziehungen zwischen metamorpher Entwicklung und verschiedener geotektonischer Modelle werden diskutiert.

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Résumé Les compositions chimiques des minéraux ainsi que leur relation croissance-déformation dans les métapélites, ortho-gneiss et métabasites de la région Muxia-Finisterre, suggèrent que cette secteur a subi un métamorphisme régional plurifacial lors de l'Orogenèse Hercynienne. L'évolution des paragenèses semble être liée à un changement du gradient métamorphique vers des conditions de P_s relativement plus basses dans la mésozone, ceci étant moins évident dans les roches catazonales. Dans les deux cas il y a eu une augmentation continue de la T et une diminution de la {ie657-03}. Le paroxisme du métamorphisme a été atteint lors du deuxième épisode métamorphique (M₂). Cet événement a lieu généralement pendant ou légèrement après la deuxième phase de déformation (P₂). Celle-ci donne la foliation principale de la roche (S₂), laquelle a presque entièrement effacé les marques d'une schistosité pré-existante. Des relations entre l'évolution métamorphique et différents modèles géotectoniques sont discutées.

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Structural pattern and ascent model in the Central Extremadura batholith,Hercynian belt,Spain

The Central Extremadura batholith constitutes an important alignment of plutonic rocks occupying part of the internal zones of the Iberian Hercynian belt. It comprises 13 plutons and numerous minor intrusions, and covers a wide compositional range from quartz-diorites to alkali-feldspar granites. Structural study of the individual plutons reveals that their internal structural patterns may be correlated. Correlation between the different structural patterns in the plutons and interpretation of the superimposed deformation in the metasedimentary host rocks indicate that the plutons were emplaced and deformed in an E-W, dextral, intracontinental shear zone developed during the second deformation phase (302–320 Ma). A qualitative ascent model has been established as follows: (1) development of extensional fractures at 45° to the E-W, dextral, shear zone; (2) intrusion of granitic magmas along these fractures and (3) rotation of the earlier intrusions, in a dextral sense, inducing antithetic shear zones (N-S sinistral) and a transverse shortening, perpendicular to the major axis of the earlier, deformed plutons as well as the forced emplacement of the later plutons.     

Cortlandtitic enclaves associated with calc-alkaline granites from Tapia-Asturias (Hercynian Belt, northwestern Spain)

Petrographic and mineralogical characteristics of amphibole-olivine- and pyroxene-bearing ultramafic rocks from Asturias (NW Spain) are dealt with in this paper. These rocks are of cortlandtitic type and occur as small rare enclaves in basic rocks related to Hercynian calc-alkaline, post-tectonic epizonal granites, in the northwest of the Iberian Peninsula.

These particular ultramafic enclaves are characterized by poikilitic cumulate microtexture. Olivine (Fo_77–81), spinel, from chromite to pleonaste composition, enstatite, subordinated diopside and sulphides are included in large brown calcic amphibole crystals displaying an irregular zonation. Phlogopite and plagioclase are also found, in a much lower proportion, between the large amphibole crystals.

Mineral assemblage and chemical composition of minerals indicate formation conditions of 1150°C, 7–8 kbar of P_total and P_H2O<P_total. These rocks could represent the earlier products of fractional crystallization from a hydrated high-alumina basalt involved in the genesis of the calc-alkaline granites. This basic magma would start crystallizing at a relatively deep level, carrying up the first products of its crystallization during its ascent.     

Behavior of iron-group elements,oxybarometry, and genesis of unique chromite deposits in the Kempirsai massif

Ultramafic rocks and high-Cr chromite ore from the Almaz-Zhemchuzhina deposit, the largest in the Main ore field of the Kempirsai massif, have been studied. The detailed mineralogical and geochemical examination of deep structure test and exploratory boreholes allowed us to establish the rough stratification of ultramafic rocks and to demonstrate the position of unique chromite deposits in the generalized vertical section of the southeastern Kempirsai massif. From top to bottom, a barren harzburgite-lherzolite series gives way to an ore-bearing dunite-harzburgite complex with the largest chromite deposits, including the unique Almaz-Zhemchuzhina deposit, in its upper portion and then to pyroxene-free dunite densely impregnated with chromite in the upper part and containing sparsely disseminated chromite at its base. The lower unit is composed of a barren lherzolite-harzburgite series transformed into blastomylonites near the contact with dunite, suggesting a tectonically doubled section in the southeastern part of the massif. The synore asymmetric geochemical zoning developed in the course of formation of chromite deposits as a result of removal of oreforming iron-group elements from the underlying and wall ultramafic rocks into the overlying rocks. Host rocks with disturbed initial proportions of Cr, Fe, Ni, and Mn, together with orebodies, made up ore-bearing zones no less than 1 km in thickness and subdivided into supra-, inter-, and subore subzones. The subore and wall rocks are characterized by partial loss (wt %) of Cr₂O₃(0.1), NiO (0.04), FeO_tot(0.5), and MnO (0.02) and their removal into the interore and supraore (0.03 NiO) subzones. Thus, the subore ultramafic rocks served as a source of ore-forming components, while the interore zone with orebodies occurring therein served as a zone of discharge of these components. Using Mössbauer spectroscopy, the crystal chemistry of iron ions was studied in a representative selection of Cr-spinel samples from rocks and ores of the southeastern and western blocks (the Almaz-Zhemchuzhina and Geophysical XII deposits). The degree of iron oxidation in the samples varies from 8 to 33%. In most cases, a difference in degree of iron oxidation is established in stoichiometric approximation and from Mössbauer data. In other words, the integral stoichiometry of ferrous and ferric ions is disturbed. Such a disturbance may be related not only to partial inversion of the Cr-spinel structure but also to local heterogeneity of the mineral at the micro- and nanolevels with clustering of cations and formation of their associates. An empirical correction of the olivine-Cr-spinel geothermometer and oxybarometer has been performed. The inverse correlation between oxygen fugacity and degree of depletion of ultramafic rocks indicates that these rocks were formed in a closed system with participation of a water-methane fluid. Along with stratification of ultramafics, this correlation testifies to a powerful asthenospheric source of reduced fluids. The retention of low oxygen fugacity in central portions of orebodies does not rule out that after a break this source participated in the formation of unique chromite deposits in the Kempirsai massif.     

琼南晚白垩世基性岩墙群的年代学、元素地球化学和Sr-Nd同位素研究

系统的年代学、元素地球化学和Sr—Nd同位素地球化学研究表明,琼南晚白垩世基性岩墙群形成于81Ma,富K(K20约为1．95％～3．63％,K20／Na20约为0．61～1．12)、大离子亲石元素、轻稀土元素和Pb,亏损高场强元素,具高的^87Sr／86Sr初始值(0．7078～0．7084)和负的εNd(t)值(—3．2～—2．3),为典型的钾玄质系列岩石,类似于与俯冲环境相关的高K／Ti—低Ti钾质岩石。该岩墙群形成于“后碰撞弧”的板内拉张环境,源自与俯冲作用有关的富钾和大离子亲石元素的交代地慢,并经历了明显的以单斜辉石为主的结晶分异作用。此外,琼南基性岩墙群和粤北及福建沿海地区晚白垩世基性岩脉的对比表明,晚白垩世中国东南部岩石圈地慢组成存在区域性的差异,这种差异很可能与区域构造演化历史的不同有关。     

湘东北金井地区花岗岩成因及地球动力学暗示：岩石学、地球化学和Sr-Nd同位素制约

出露于湘东北地区的燕山期花岗质岩岩体,呈岩基或岩株状广泛侵位于中元古界冷家溪群浅变质碎屑沉积岩中, 部分岩体为白垩纪红层覆盖。文章详细研究了金井岩体的岩石学、地球化学及Sr-Nd同位素特征,并与其围岩冷家溪群进行了对比。该岩体具有较高的SiO2(71．44-74．31wt．％)、K2O(3．86-4．98wt．％)和Na2O(3．24-3．84wt．％),以及较低的 FeO Fe2O3 MgO TiO2(均小于3％),结合高含量的Al2O3(13．25-14．89wt．％)及高ASI值(普遍大于1．1),为典型的强过铝质(SP)高钾钙碱性花岗岩。金井花岗岩富硅、碱和高Sr、Ba丰度、高LILE／HSFE和LREE／HREE比值,低镁铁质及REE 含量,表明金井岩体为高度分异的花岗岩,其源区的残留物中黑云母含量高。REE配分图和原始地幔多元素标准化图谱上 Rb、Th、U、K和LREE相对富集,而Ba、Sr和高强场元素(Nb、Ta、P和Ti)相对亏损,Eu可变的负异常(δEu:0．53-0．78),表明存在角闪石、黑云母、堇青石、磷灰石和钛铁矿等分异演化的影响,类似于同碰撞型花岗岩地球化学特征。高Rb／Sr(3-6)、低 Sr／Ba(0．2-0．7)、高P2O5含量(0．09-0．15wt．％)、低Al2O3／TiO2比值(40-700．4和<0．2两组)以及Nd同位素的不同(高Nd和低Nd两组),类似于围岩冷家溪群砂质和泥质沉积岩特征,以及高Sr[(87Sr／86Sr)i=0．72204-0．72540]、低Nd[εNd(145 Ma)=-10--12]等均表明花岗质岩浆主要起源于中元古代冷家溪群变质砂岩(高Ca)和变质泥岩(低Ca)的部分熔融或中下地壳物质的熔融,但有少量地幔物质的加入。结合印支期以来华南及湘东北地区地球动力学演化史,作者认为金井燕山期花岗岩形成于陆内碰撞造山晚期,是160-140Ma 陆内俯冲(A型俯冲)碰撞造山作用增温减压体制达到鼎盛阶段的产物,标志湘东北地区由此全面转入陆内伸展阶段。     

The genesis of calcite in carbonate rocks of the sedimentary basins: Evidence from the stable carbon and oxygen isotopes composition

The distributions of stable carbon and oxygen isotopes in modern and ancient limestones of various types were studied. Carbonate samples from modern sediments were collected in the Black and Barents Seas. Ancient carbonates were represented by Upper Jurassic (Kimmeridgian-Tithonian) limestones from the central part of the West Siberian basin. Carbonate samples include remains of modern and Upper Jurassic fauna, carbonate crust from sediments of the Black Sea, carbonate tube from sediments of the Barents Sea, and Upper Jurassic limestone from the carbonate layer found at top of Abalak, bottom of Bazhenov deposits in the central part of the West Siberian basin. According to the results of the isotope analysis and comparison with modern carbonates, Upper Jurassic limestones of the West Siberian basin belong to the group of methane-derived carbonates and precipitated as a result of anaerobic oxidation of methane (AOM). Fractures in limestones are filled with secondary calcite.     

Geology, mineralogy, and genesis of PGE mineralization in the South Sopcha massif, Monchegorsk complex, Russia

New data are reported on the localization and genesis of PGE mineralization at the South Sopcha deposit situated in the southern framework of the Monchegorsk pluton. Disseminated PGE-Cu-Ni mineralization, the thickness of which in particular boreholes exceeds 100 m, is hosted in the zone of alternating peridotite, pyroxenite, norite, and gabbronorite. The PGE grade does not exceed 1?C2 gpt with Pd/Pt = 3?C4 at Ni and Cu contents from 0.2 to 1.5 wt %. The PGE contents up to 4?C6 gpt and Pd/Pt = 4?C8 are noted at local sites of hydrothermally altered rocks. Another type of PGE mineralization is established in the outcrops of the southeastern marginal group of the massif. Pyroxenite, norite, and gabbronorite fragments are incorporated here in the gabbroic matrix, making up a complex zone of magmatic breccia complicated by mylonites and late injections. Elevated PGE contents (1.0?C6.5 gpt) are detected in all types of rocks in the zone of brecciation, mainly in the matrix. Platinum-group minerals (PGM) occur in association with magmatic and late sulfides, amphibole, mica, and chlorite. PGM vary in composition depending on the petrographic features of rocks. In rocks of the layered series and in pegmatoid pyroxenite PGM are extremely diverse comprising PGE compounds with As, Sb, Bi, Te, Se, and S. In the brecciated rocks of the marginal group, Pd bismuthotellurides (mainly merenskyite), sperrylite, hollingworthite, and Pd- and Rh-bearing cobaltite and gersdorffite are predominant. The PGE mineralization in rocks of the layered series and pegmatoid pyroxenite was formed from the magmatic melt enriched in volatiles and with subsequent transformation of PGE assemblages under the influence of hydrothermal fluids at a lower temperature. In gabbroic rocks of the marginal group, PGM are associated with the latest sulfides (chalcopyrite, bornite, chalcocite), forming separate grains and thin veinlets in hydrothermally altered rocks. The gabbroic melt affected incompletely crystallized rocks of the layered series by formation of contact-type PGE mineralization, deposition and redeposition of ore matter.