The genesis of felsic-mafic plutonic associations: a Sr and Nd isotopic study of the Hercynian Braga Granitoid Massif (Northern Portugal)

The late tectonic Braga plutonic complex in the “Centro Iberian Zone”, North Portugal, was emplaced during the Hercynian orogeny within a Silurian sedimentary sequence and displays an acid-basic association which consists of three well-defined intrusions: the biotite-dominant Braga granite, minor bodies of gabbro to granodiorite composition and the mildly peraluminous Gonça leucogranite. These three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a Rb---Sr age of 310±10 Ma is obtained. The distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related. However, the Rb---Sr and Sm---Nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process. The gabbros have chemical characters (high K, Ba, Sr and light rare-earth elements and low Nb, Ti and Zr contents) and isotopic compositions (Sr_i=0.70497 and ε_Nd=−2.5), which suggest an alkaline magmatic affinity of shoshonitic type. They are probably derived from an enriched mantle source. In contrast, the peraluminous Gonça leucogranite (Sr_i=0.70933 and ε_Nd=−6.8) is more likely the result of crustal anatexis. The Braga granite and the evolved members of the basic series have an intermediate isotopic composition (Sr_i=0.70532 to 0.70733 and ε_Nd=−3.0 to −6.2) which can be interpreted in terms of an hybridization process between the two previous end-members. The chemical and Sr---Nd isotopic compositions of the Braga plutonic series can be explained by an assimilation-fractional crystallization (AFC) model between a mantle-derived magma (equivalent to the gabbros) and a crust-derived magma (the Gonça granite).     

Cooling history of nested plutons from the Variscan Tichka plutonic complex (Morocco)

International Journal of Earth Sciences - Four imbricated mafic to felsic plutons of Variscan age from Morocco have been investigated for their cooling history and geochemical interactions with...     

Precambrian crustal components,plutonic associations,plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study

Chichi-jima, Bonin Islands, consists of dominant Eocene submarine volcanic rocks, comprising boninites, andesites and dacites, and subordinate sedimentary rocks. The dacites occur frequently in breccias and pillows overlying a boninite pillow lava sequence. The boninite pillows are intruded by a multiple dike, in which a core boninite is chilled against outer dacites. A density-stratified chamber may have been capped by a dacite magma. The dacites, which can be divided into quartz dacite and quartz-free dacite, are differentiates from the boninite-forming magmas, because they vary continuously in composition from boninites through andesites. The quartz dacites, corresponding to rhyolite in SiO₂, are lower in Na₂O and K₂O than most orogenic dacites. Some of the dacites are characterized by ferropigeonite (Wo_7–16En_23–39Fs_68-54) phenocrysts and are clearly ferrodacite, producing variable amounts of Fs-rich normative pyroxenes. The relation of SiO₂ to total FeO/MgO ratio indicates that many of both types of dacites, with glasses in boninites, are enriched in total FeO despite the strong calc-alkalic affinity of boninites. The crystallization temperature of ferropigeonite with Mg value 30 in a quartz dacite is estimated to be 900° C and that in a quartz-free dacite to be 1050° C, which are unusually high for differentiated silicic rocks. Some Chichi-jima rocks are fresh, having a low ratio of Fe₂O₃ to FeO. On the basis of the experimental study of magmatic ferric-ferrous equilibria at 1 bar, the oxygen fugacities are calculated as 10^–13.6 bars at 900° C for a ferropigeonite quartz dacite and 10^–8.9 bars at 1200° C for a boninite with the lowest Fe³⁺/Fe²⁺. Both values lie below the quartz-fayalite-magnetite buffer line. The boninite series volcanic rocks have preserved low oxygen fugacities as well as high temperatures until the latest differentiation stage. The ferropigeonite phenocrysts have crystallized from the dacite magmas under the conditions of moderately high temperatures, very low oxygen fugacities and high total FeO and SiO₂ concentrations.     

High-grade metamorphic rocks and peridotites along the Leiza Fault (Western Pyrenees,Spain)

Acid and basic granulites, migmatites, and lherzolites outcrop along the Leiza Fault (Navarra, Spain) in the western extremity of the Pyrenean Belt. The protoliths of the acid granulites have granodioritic composition. Textural and mineralogical data suggest that the acid granulites evolved from a first, syn kinematic medium-pressure granulite-facies stage [garnet-biotite I-Kfsp] to a post-kinematic granulite-facies stage of lower pressure and higher temperature [garnet-biotite II-cordierite-Kfsp-(spinel?), kinzigites]. Basic granulites were formed from protoliths with composition of tholeiitic to alkaline basalts. Basic granulites exhibit millimeter size subidiomorphic garnets dispersed through the matrix and smaller coronitic garnets between opaques or orthopyroxenes and plagioclase. Thermo-barometric estimates for the peak of the granulite-facies metamorphism are c. 800°C and 8 kbar. The migmatites presumably represent a shallower level of metamorphism (andalusite/sillimanite-Kfsp). Scapolite in the basic granulites was formed during a post-granulitic metamorphic episode. The lherzolites have been intensely brecciated and serpentinized, which makes difficult the comparison of their evolution with that observed in the other rocks associated with the Leiza Fault. Acid and basic granulites, migmatites, and lherzolites along the Leiza Fault may be correlated with similar rocks outcropping elsewhere in the North-Pyrenean Zone and along the North-Pyrenean Fault as tectonic slices and massifs. By analogy with those rocks, the granulitefacies metamorphism observed in the rocks studied must be Hercynian in age. The Leiza Fault constitutes, therefore, the western continuation of the North-Pyrenean Fault, and the rocks studied (except perhaps the lherzolites) may be considered as remnants of an Hercynian metamorphic massif, dismembered as a consequence of the activity of the fault at the end of the Hercynian cycle and during the Alpine tectonometamorphic events.     

Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California

This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.70597–0.70653; ²⁰⁶Pb/ ²⁰⁴Pb=18.862–19.018; ²⁰⁷Pb/²⁰⁴Pb=15.640–15.686; ²⁰⁸Pb/ ²⁰⁴Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (⁸⁷Sr/⁸⁶=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high ⁸⁷Sr/⁸⁶Sr (71141–0.71240) and low ²⁰⁶Pb/²⁰⁴Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H₂O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.     

Basement deformation: tertiary folding and fracturing of the Variscan Bielsa granite (Axial zone,central Pyrenees)

The Bielsa thrust sheet is a south-verging unit of the Axial zone in the central Pyrenees. The Bielsa thrust sheet consists predominantly of a Variscan granite unconformably overlain by a thin cover of Triassic and Cretaceous deposits. During the Eocene–Oligocene, Pyrenean compression, displacement of the Bielsa thrust sheet generated a large-scale south-verging monocline. Low temperature deformation of the Bielsa thrust sheet resulted in the development of: (1) E–W trending, asymmetric folds in the Triassic cover with amplitudes up to 1.5 km; these folds of the cover are related with normal and reverse faults in the granite and with rigid-body block rotations. (2) Pervasive fracturing within the Bielsa granite is also attributed to Pyrenean deformation and is consistent with a NNE to ENE shortening direction; two main, conjugate fault systems are associated with this direction of shortening, as is a subvertical strike-slip system with shallow-plunging slickenside lineations and a moderately dipping fault system with reverse movement; and (3) in addition, we recognise strike-slip and reverse shear bands, associated with sericitisation and brittle deformation of quartz and feldspar in the granite, that enclose Triassic rocks. Basement deformation within the Bielsa thrust sheet can be related to movement of faults developed to accommodate internal deformation of the hanging wall. Several models are proposed to account for this deformation during the southward displacement of the thrust.     

The Vedrette di Ries (rieserferner) plutonic complex: Petrological and geochemical data bearing on its genesis

Data on Co, Sc, Rb, Sr, Th, Zr, Ta, Hf and REE contents are reported for tonalites, granodiorites, granites and one diorite from the Alpine plutonic complex of Vedrette di Ries in the Eastern Alps. Co and Sc range respectively between 19.6-1.2 ppm and 32-3.3 ppm showing a good negative correlation with the Differentiation Index (D.I.). Rb values range between 61–197 ppm displaying a positive correlation with D.I. All the samples have variably fractionated light and heavy REE with La_N/Sm_N=1.7–5.55 and Tb_N/Yb_N=0.58–1.65 and no important europium anomalies. On the basis of the HREE fractionation two groups of rocks can be distinguished: one showing higher Tb_N/Yb_N values which range between 1.27–1.65 and are positively correlated with the D.I. and a second group with Tb_N/Yb_N values less than unity which decrease with the differentiation. The rocks with high Tb_N/Yb_N ratio also display a positive Sr versus D.I. variation whereas the samples with flat or upward concave HREE patterns (i.e. with Tb_N/Yb_N<1) define negative Sr versus D.I. trends. Th, Ta, Hf, Zr and LREE show an overall tendecy to increase and then to decrease with the differentiation, with a large scattering of values.The data obtained fit the hypothesis that the entire rock series under study is the product of a two-stage crystal/liquid fractionation process starting from one parent magma of tonalitic or dioritic composition. During the first stage, which occurred at high pressure, the separation of hornblende+garnet produced the liquids displaying the positve Tb_N/Yb_N and Sr versus D.I. correlation. These liquids during their rise through the crust would have undergone a second stage fractionation with separation of hornblende and plagioclase evolving toward low-Tb_N/Yb_N and Sr-poor composition.     

The influence of enclosing rock type on barite deposits, eastern Pyrenees, Spain: fluid inclusion and isotope (Sr, S, O, C) data

The Rocabruna and Coll de Pal barite deposits, located in the eastern Pyrenees of Spain, fill karstic cavities within carbonate rocks of Cambrian and Devonian age, respectively. The deposits contain barite, chalcopyrite, tetrahedrite, pyrite and minor sphalerite and galena with saddle dolomite and quartz as gangue. Fluid inclusion data from Coll de Pal quartz and dolomite indicate that the mineralizing fluid was a polysaline CaCl₂-rich brine, with temperatures between 125 and 150 °C. C and O isotopic compositions of carbonates in both deposits are consistent with a progressive increase in temperature during deposition. The ³⁴S values of barite, which range from 14.2 to 15.9‰ in Coll de Pal, and from 13.9 to 19.3‰ in Rocabruna, together with ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7118 to 0.7168 in Rocabruna, and from 0.7115 to 0.7136 in Coll de Pal, indicate two different fluid sources. We propose that these deposits formed as a result of mixing between a Ba-rich, sulfate-poor hot fluid, and sulfate-rich solutions of surficial origin. The different Sr isotope ratios in the deposits indicate that the hot Ba-rich fluids involved in each deposit were equilibrated with different rock types (carbonates and shales), in agreement with the geology of the two areas. Received: 22 October 1997 / Accepted: 24 March 1998     

Geochemical and Sr–Nd isotopic constraints on the genesis of the Soheyle-PaKuh granitoid rocks (central Urumieh-Dokhtar magmatic belt,Iran)

ABSTRACT

Soheyle-Pakuh granitoid rocks, with a variety of quartz diorite, quartz monzodiorite, granodiorite, tonalite, and granite, have been emplaced into the Tertiary volcanic rocks in the Urumieh-Dokhtar magmatic arc in central Iran. Zircon U–Pb dating yields an age of 39.63 ± 0.93 Ma for the crystallization of this body. Whole-rock compositions show that SiO₂ changes from 52.31 to 65.78 wt.% and Al₂O₃ varies from 15.54 to 18.24 wt.%, as well as high concentrations of large-ion lithophile elements (LILE, e.g. Cs, Rb, Ba, and K) and quite low contents of high field strength elements (HFSE, e.g. Nb, Ti, P), as expected in I-type arc granitoids formed in an active continental margin setting. Initial ratios of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd exhibit ranges 0.7043–0.7047 and 0.51284 to 0.51287, respectively, with positive εNd_(t) from +4.9 to +5.5 with a young TDM1 age (483–674 Ma); this tracer isotopic data suggesting that the SPG originated from juvenile basaltic crust derived from depleted mantle (~90%) with variable contributions from undepleted mantle and approximately 10% old lower crust, despite diverse processes (e.g. magma mixing and fractional crystallization) during their evolution and emplacement into a local extensional setting within the continental margin arc. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly mantle-derived juvenile crustal protoliths and indicating extensive addition of new continental crust, during Cambrian-Neoproterozoic time, in the suprasubduction zone beneath the central Urumieh-Dokhtar magmatic arc. Generation of these types of granitoids favours a model whereby rollback and (or) break-off of a subducted slab with subsequent lithospheric extension triggered by mantle upwelling, heat advection, and underplating resulting in melting of the central UDMA mantle-derived juvenile lower continental crust in the Late Eocene.     

Mineralogy, geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain)

Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif, western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks, which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2 tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning. Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying heavier δ¹⁸O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines show intermediate values of 11.3 to 14.6‰ for δ¹⁸O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales (≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids, and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism. Received: 18 November 1996 / Accepted: 25 April 1997     

Sr isotopes and K,Rb, Sr balance in sediments and igneous rocks from the subducted plate of the Vanuatu (New Hebrides) active margin

To evaluate the possible contribution of ocean floor sediments during the genesis of the volcanism of Vanuatu (New Hebrides) active margin, we have determined the balance of Sr isotopes and K, Rb and Sr contents for the stratigraphic column of site 286 (leg 30, DSDP). This site is located on the oceanic plate that will be subducted. Analyses have been performed on sedimentary and igneous rocks, before and after acid leaching. The Sr isotopic data do not support the occurrence of some continental component in arc magmas of this active margin which is really intraoceanic. It is demonstrated that the d'Entrecasteaux fracture zone results from the intense fracturing of typical oceanic crust. The analyses of the volcanogenic components of the sediments show a change in the source of volcanoclastic detritus from the Loyalty islands in the Eocene to the volcanic arc of Vanuatu (New Hebrides) during Pliocene and Quaternary times. The determined balance of Sr isotopes and of K, Rb, Sr contents, may be used for calculation of multicomponent melting mixing models for the origin of Vanuatu arc magmas, but we emphasize that in these models the Sr isotopes cannot be considered as an appropriated tracer of sediment contribution.     

The Variscan Millares granite (central Pyrenees): Pluton emplacement in a T fracture of a dextral shear zone

This work deals with the magnetic susceptibility and its anisotropy (AMS) in the Variscan Millares pluton in the Central Pyrenees. The zonation of low-field magnetic susceptibility is consistent with the concentric arrangement of rock-types, with more basic compositions at the external areas. Magnetic foliations defined from AMS strike NE-SW and dip gently towards the NW. Magnetic foliations are mainly perpendicular and oblique to the elongation of the pluton in map view (NW-SE) and show a concentric pattern at the central part, where the more acid rocks crop out. Magnetic lineations are scattered between NW-SE and NE-SW and plunge shallowly to the N. In map view magnetic lineations are distributed in domains normal to the elongation of the pluton. The contours of P' (degree of magnetic anisotropy) are oriented NE-SW and bands of oblate and prolate ellipsoids alternate perpendicular to the elongation of the pluton in map view. P' is between 1.009 and 1.055 in 93% of the specimens. Such low values are currently recorded in granites having magmatic fabrics and for which the anisotropy is mainly carried by biotite. The attitude of the magnetic foliation and the magnetic lineation, the geometry of the pluton, and their relationship with the host-rock structure suggest an intrusion contemporary with a transpressional regime, syntectonic with the late stages of the Variscan orogeny.     

The role of hybridization in the genesis of Hercynian granitodis in the gredos massif,Spain: inferences from Sr-Nd isotopes

The Gredos massif is one the better exposed granitoid complexes of the Iberian massif. It is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes. The massif also contains mega-enclaves of migmatites with which the granitoids show transitional contacts. Two major magmatic associations have been distinguished in this study: (1) One comprises the granitoids with microgranular enclaves, the enclaves, and basic rocks; (2) the other is formed by leucogranites, intrusive into the former series and free of microgranular enclaves. Field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process. The Sr-Nd isotopic study reveals that this process is complex, relating different end-members of mantle and crustal affinities, and occurred around 295 Ma ago, late with respect to the main deformation phases of the Hercynian orogeny. The granitoids with microgranular enclaves (GME) are part of an overall mixing trend involving Palaeozoic mantle-derived magma and melts of older crustal material. Amphibole-bearing GME, in general, contain greater proportions of the mantle-derived component than the cordierite-bearing GME. The actual mixing processes took place on a variety of scales, sometimes between melts which were themselves hybrids. On a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke. The isotopic signatures of the basic rocks are probably, to a large degree, the result of interaction with crustal melts, though additionally the presence of an enriched mantle source cannot be elmininated. Microgranular enclaves and their immediate hosts have differing initial Sr and Nd isotopic signatures, indicating that isotopic equilibrium was not attained. This suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system. The enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees. Late-stage peraluminous leucogranites have similar initial Nd isotopic compositions to the evolved GME; a crustal source with a radically different Nd isotopic composition or age does not need to be invoked in their petrogenesis.     

Nature and evolution of the lower crust under central Spain: Inferences from granulite xenoliths (Calatrava Volcanic Field-Spanish central system)

So far, the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System (SCS). In recent years, ultramafic volcanics from the Calatrava Volcanic Field (CVF) have also provided deep-seated crustal xenoliths which have not been studied in detail. Our data, combining mineral, whole-rock and isotopic geochemistry with U–Pb–Hf isotope ratios in zircons from the CVF and SCS xenoliths, highlight the felsic composition of the lower crust under central Iberia. A number of the Calatrava xenoliths represents Variscan igneous protoliths, which are a minor population in the SCS, and were likely formed by crystallisation of intermediate and felsic melts in the lower crust during the Variscan orogeny (leucodiorite protolith age of 314 ± 3 Ma and leucogranite protolith age of 308 ± 2.5 Ma). U–Pb data of metamorphic zircons show that granulite-facies metamorphism mainly occurred from 299 to 285 Ma in both areas. These ages are slightly younger than those of granitic intrusions that could be genetically related to the granulitic residue, which points to a main role of U–Pb isotope resetting in lower crustal zircons during HT or UHT conditions. The zircon U–Pb–Hf isotopic ratios support the idea that the lower crust in central Iberia consists mainly of Ordovician–Neoproterozoic metaigneous and metasedimentary rocks associated with the Cadomian continental arc of northern Gondwana. These rocks provide evidence of mixing between juvenile magmas and an enriched crustal component, ultimately extracted from an Eburnean crust. Other more evolved components present in detrital zircons are likely related to recycling of Archean crust derived from North Africa cratonic terranes.     

Multistage crystallization of tonalitic enclaves in granitoid rocks (Hercynian belt,Spain): implications for magma mixing

The crystallization sequence, nucleation density and crystal index (n = nucleation density/mode) of tonalitic enclaves from different granitoids of the Hercynian orogeny indicate that they are pieces of magma partially crystallized in the interior of synplutonic bodies of basic magma, which was injected into a silicic magma chamber. A rapid cooling stage can be identified from the high nucleation density and high n-values of minerals. A final stage of slow cooling, is identified by a low nucleation density and low n-values. At this stage a residual melt crystallized in thermal equilibrium to the cooling rate of the whole magma chamber. Thermal equilibrium and convection are necessary conditions for mixing. Enclaves can be interpreted as the remaining, non-disaggregated portions of an early injected mafic magma, which in turn underwent hybridization during injection. Their presence indicates the existence of magma mixing processes and the possibility that the host granitoid was a hybrid rock.

---

Zusammenfassung Die Kristallisationsabfolge, Keimdichte und der Kristallindex (n = Keimdichte/Modus) von tonalitischen Einschlüssen verschiedener Granitoide des variszischen Orogens zeigen, daß sie Teile eines Magmas sind, das teilweise im Inneren von basischen synplutonischen Magmenkörpern auskristallisierte und in eine silikatische Magmenkammer injiziert wurde. Eine Zeit schnellen Abkühlens kann an Hand der hohen Keimdichte und der niedrigen n-Werte festgestellt werden. Zu diesem Zeitpunkt kristallisierte eine Restschmelze unter thermischen Gleichgewichtsbedingungen und der Abkühlungsgeschwindigkeit der ganzen Magmenkammer aus. Thermisches Gleichgewicht und Konvektion sind für das Mischen notwendig. Einschlüsse können als nichtzerfallene Reste eines früh injizierten mafischen Magmas angesehen werden, welches während der Injizierung eine Hybridisierung erfuhr. Ihre Gegenwart zeigen das Vorhandensein von Magmenmischungsvorgängen und die Möglichkeit, daß der Muttergranitoid ein Hybridgestein war, an.

---

Resumen La secuencia de cristalización, densidad de nucleación e índice cristalino (n = densidad de nucleación/moda) de enclaves tonalíticos de diferentes granitoides hercínicos, indican que dichos enclaves representan fragmentas de magma parcialmente cristalizados en el interior de cuerpos simplutónicos de magma básico, que originalmente fue inyectado en una cámara magmática félsica. Se puede identificar una etapa de enfriamiento rápido a partir de los altos valores de n y alta densidad de nucleación de los minerales. Una etapa final de enfriamiento lento es detectada por una baja densidad de nucleación y bajos valores de n de determinados minerales. Un líquido residual cristalizó en esta etapa lenta en equilibrio térmico y a la misma tasa de enfriamiento que toda la cámara magmática en conjunto. Equilibrio térmico y convección son condiciones necesarias para que se produzca mezcla. Los enclaves pueden ser interpretados como porciones no hibridadas de magma que han sido homogéneamente distribuidos dentro del granitoide encajante. Su presencia indicaría la existencia de procesos de mezcla de magmas y la posibilidad de que el granitoide encajante sea una roca híbrida.

---

, (n = /) , , , . . . . , , . , , , , .

     

He–Ar and Nd–Sr isotopic compositions of late Pleistocene felsic plutonic back arc basin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesis of young plutonic rocks in a back arc basin

We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. ⁴⁰Ar/³⁹Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K₂O + Na₂O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (⁸⁷Sr/⁸⁶Sr = 0.70454–0.71264, ¹⁴³Nd/¹⁴⁴Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (⁸⁷Sr/⁸⁶Sr = 0.70466–0.70892, ¹⁴³Nd/¹⁴⁴Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial ⁸⁷Sr/⁸⁶Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (³He/⁴He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.     

Trace elements behavior in granite genesis: A case study The calc-alkaline plutonic association from the Querigut complex (Pyrénées,France)

The different granitoids of the zoned Querigut complex (Hercynian Pyrenees) are associated with a series of basic to intermediate rocks ranging from hornblende-bearing peridotites to quartz-diorites. The whole complex appears as a calc-alkaline plutonic suite typical of orogenic zones. The distribution of lanthanides and other trace elements amongst coexisting minerals indicate they are essentially held by accessory phases, particularly in granitoids. This restricts the use of those elements in the calculation of petrogenetic models for acidic plutonic rocks. Magmatic differentiation, mainly by hornblende + plagioclase fractionation, can produce the basic series. This differentiation cannot directly produce the different granitoids, which require a preponderant contribution of crustal melts. The sequence of different granitoids can be explained either by an heterogeneity in the source region, or by magmatic differentiation. The most plausible interpretation of the whole complex calls for the emplacement of a mantle-derived magma into a wet, anatectic continental crust, with interactions between basic rocks and the soproduced acidic melts.     

Synkinematic emplacement of the Pan-African Ngondo igneous complex (west Cameroon,central Africa)

The Ngondo Complex is one of the Pan-African plutons intruded in the West Cameroon Pan-African Orogenic Belt. The complex consists of three major groups of rocks: basic to intermediate rocks (diorites, granodiorites and minor gabbros), fined-grained granites and coarsed-grained granites successively emplaced in a metamorphic country rock of amphibolite-facies. Synkinematic emplacement of the complex, in relation with a ductile mega shear zone, is documented by a study of microstructures and foliation patterns which indicate a continuous transition from magmatic to high temperature solid-state deformation. The geometry of the internal foliation trajectories and the joint orientation in the complex suggest that the emplacement of the three groups of rocks was totally controlled by a N30° sinistral shear zone. Emplacement mechanisms, which are related in time and space to a continuum of deformation, may indicate a relative rheological change of the crust from ductile to brittle behaviour.