The youngest basic oceanic magmatism in the Alps (Late Cretaceous<Emphasis Type="Italic">;</Emphasis> Chiavenna unit,Central Alps): geochronological constraints and geodynamic significance

Cathodoluminescence-controlled radiometric dating (U–Pb SHRIMP) was carried out on zircon domains from metabasic rocks of the Chiavenna unit, a major mafic/ultramafic-bearing unit in the Central Alps. Co-magmatic zircon domains from amphibolites near Chiavenna and Prata areas yielded weighted mean ²⁰⁶Pb/²³⁸U ages at 93.0±2.0 and 93.9±1.8 Ma, respectively, interpreted as the age of crystallization of the magmatic protoliths. These ages fit well with the time of late spreading in the Valais Ocean, as suggested by previous paleogeographic reconstructions. Inherited zircon grains and/or core domains (Permo-Triassic, Carboniferous, Proterozoic) are abundant, indicating proximity of the Chiavenna unit to thinned continental crust. This is in line with the origin of this unit from subcontinental mantle sources, as suggested previously on petrological and structural grounds. Metamorphic zircon domains from one amphibolite near Chiavenna yielded a weighted mean ²⁰⁶Pb/²³⁸U age at 37.1±0.9 Ma, identical to the 38.5±0.9 Ma SHRIMP age of an amphibolitized eclogite of the Antrona ophiolites (Valais domain, Western Alps). Precise metamorphic ages were difficult to obtain from the composite (poly)metamorphic rim domains of the Prata amphibolite. This is attributed to the location of the Prata area close to the granulite-facies Gruf unit (metamorphosed at ca. 33 Ma) and to the 24–25 Ma old Novate granite, where metamorphic/fluid events probably caused multiple resetting to various degrees. The ca. 93 Ma old magmatism, identified for the first time in the Chiavenna unit, is the youngest basic oceanic magmatism reported in the Alps. The 37.1±0.9 Ma old metamorphism in the Chiavenna unit, attributed to the Valais domain, confirms the model suggesting stepwise younging of metamorphic ages from the south (Adriatic plate) to the north (European plate). It is older than metamorphism in the European margin (ca. 35–31 Ma) lying to the north of the Valais domain and younger than that in the Piemont–Ligurian Ocean (ca. 44–45 Ma) lying to the south of the Valais domain.Editorial responsibility: W. Schreyer     

The Western Mediterranean lamproitic magmatism: origin and geodynamic significance

Ultrapotassic lamproitic rocks in the Western Alps, Tuscany‐Corsica and SE Spain (c. 30 to 1 Ma) show high MgO, Ni and Cr denoting a mantle origin, but also have incompatible element and radiogenic isotope abundances that resemble upper crustal rocks, such as local metapelites and global subducting sediments. The coexistence of mantle and crustal signatures in lamproites indicates a genesis in a lithospheric mantle, which had been contaminated by crustal rocks. The occurrence of lamproitic magmatism along the Alpine collision front suggests that mantle contamination occurred during east‐verging Cretaceous‐Oligocene subduction of the European plate beneath the African margin. We suggest that crustal material originated from the overriding continental margin, which was eroded by the low‐angle subducting European slab. Mantle melting and generation of lamproites took place later, during diachronous opening of Western Mediterranean basins, contemporaneously with a new cycle of magmatism, which was genetically related to the west‐north‐dipping Apennine‐Maghrebian subduction.     

Origin and geodynamic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (⁸⁷Sr/⁸⁶Sr_i=0.7047–0.7064, ¹⁴³Nd/¹⁴⁴Nd_i=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La_N up to 400). The isotopic ratios approach crustal values (⁸⁷Sr/⁸⁶Sr_i=0.7059–0.7115 and ¹⁴³Nd/¹⁴⁴Nd_i=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).     

Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Asta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280–260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No.=66 to 70; Ni=25 to 83 ppm; Cr=248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb–Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7057 to 0.7114; ε_Nd values from −2.7 to −7.4; ∂¹⁸O values between 7.6 and 9.5‰) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite–cordierite and orthopyroxene–cordierite bearing peraluminous granites in the Cima d'Asta and Bressanone-Chiusa intrusive complexes can be interpreted as purely crustal melts (initial ⁸⁷Sr/⁸⁶Sr=0.7143–0.7167; initial ε_Nd values between −7.9 and −9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of arc-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts.     

Tertiary geodynamic evolution of the Southern Adria microplate

The southern Adria microplate is the common foreland for the Hellenide and Southern Apennine thrust belts. The Apulian Platform dominates the microplate; outcrop, well and seismic data allow us to trace the carbonate platform edge, whilst structural analysis, geophysical and palaeomagnetic data provide important clues to the geodynamic evolution of the region. The present structural fabric of Apulia is dominated by several E-W lineaments that divide the region into different blocks (Rospo Plateau, Gargano Promontory, Murge Ridge, Salento Peninsula, Apulian Plateau). One such lineament (the Pescara Dubrovnik ‘Line’, a prominent feature traversing the Adriatic Sea between central Italy and southern Croatia) has been active since the early Mesozoic, when it acted as a major transform fault controlling sedimentation along the northern margin of Southern Adria. During the Cenozoic the Pescara-Dubrovnik underwent predominantly vertical and oblique movement due to a differential flexural response of the platform and the adjacent pelagic sequences to the thrust belt loading. In Tertiary time the Southern Adria microplate was partly involved in HeIlenide collision. The Apulian platform can be considered as an area of thicker crust more resistant to underthrusting than the surrounding basins. During the orogenic events it acted as a passive rigid indentor, causing local distortion of the most external Hellenide structures. The dextral transpressive activity recorded along the south-east margin of this indentor (Kephallinia line) can be interpreted as the result of the oblique collision between the margin of the thick Apulian Platform (in this zone NNE-SSW striking) and the NW-SE striking Hellenic thrust belt. Horizontal stress generated during the collision was partly transmitted to the rigid foreland re-activating palaeo E-W faults within the south Adria microplate in a dextral strike-slip sense. The clockwise rotation recorded in the Salento Peninsula can be explained by the rotation of several NW-SE striking faulted blocks. The rotation was accompanied by the opening of small transtensional basins between blocks. This block rotation was caused by the dextral shear that is expressed along the North and the South Salento Fault Zones.     

The “andesitic” magmatism in the south-western Tyrol and its geodynamic significance

The researches carried out on the recent dyke activity in the south-western Tyrol have revealed a widespread “andesitic” magmatism in the austroalpine realm. The magmatic activity developed at least in two distinct phases; but in this paper we are only concerned with the unmetamorphosed dykes which are younger than the alpine folding. The existence of a calc-alkaline magmatism mainly of the intermediate type, and the occurrence of garnet as the first mineral on the liquidus, imply a very deep origin of such a magmatism, which agrees with the geodynamic models, providing the subduction of oceanic crust recently hypothesized for the Alpine evolution. Regarding the age of dyke activity, very recent radiometric results (K/Ar) testify for the first time in the Alps an “andesitic” magmatism from Upper Cretaceous to Tertiary and suggest both continuous or separate subductive processes from the Upper Cretaceous onwards.     

东天山中酸性侵入岩浆作用及其地球动力学意义

东天山地区华里西中-晚期中酸性侵入岩浆活动强烈,加里东期、华里西早期及印支期花岗岩类分布零星,初步可分为南、北两个岛弧花岗岩带。除哈尔里克有少量碱性花岗岩外,花岗岩类均属钙碱性岩石系列。从早期到晚期,中酸性侵入岩浆作用经历了板块碰撞前、火山弧、碰撞同期、板内伸展、陆内叠覆造山和非造山6个演化阶段,岩石成因类型可分为M、CM、C、A4种类型。南、北岛弧的碰撞与缝合从北东向南西呈现出递进演化的特点。土屋-赤湖、石英滩、维权、白山等铜、钼、金矿床与华里西中期岛弧环境产出的过铝质钙碱性系列CM型花岗岩类关系密切。     

Geochemical and geochronological data on Triassic volcanism of the Southern Alps of Lombardy (Italy): Genetic implications

Summary A widespread volcanism mainly consisting of pyroclastic products and lava flows is found in the Southern Alps of Lombardy. The volcanics are interbedded with Anisian-Ladinian carbonatic sequences and continental terrigenous or transitional deposits of Ladinian-Carnian age. The petrographic and geochemical data for this volcanism indicate a calc-alkaline affinity with characters similar to those of convergent continental margins. In this sector of the Alps the geological evidences show that in the middle Triassic general extensional movements were the dominant deformation event, and do not support the existence of a subduction zone during this period. The apparent contrast between the tectonic environment and the type of magmas erupted in the Anisian-Carnian is tentatively explained by partial melting during the early stage of rifting of an upper mantle deeply modified during the previous Hercynian orogenesis and contaminated by crustal material. This hypothesis is in agreement with the preliminary Sr isotopic ratios (0.705 and 0.709). K/Ar and Rb/Sr ages on biotites date the beginning of the volcanism at around 225 m.y.

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Zusammenfassung In den Südalpen der Lombardei befindet sich ein weitverbreiteter Vulkanismus, der hauptsächlich aus pyroklastischen Produkten und Lavaströmen besteht. Vulkanische Produkte sind mit anisisch-ladinischen beckenartigen Karbonat-Schichtfolgen oder mit ladinisch-karnischen kontinental-terrestrischen oder Übergangsablagerungen eingebettet. Die petrographischen und geochemischen Daten für diesen Vulkanismus weisen auf eine den konvergenten Kontinentalschelfen ähnliche kalkalkaline Affinität hin. In diesem Sektor der Alpen zeigen die geologischen Beobachtungen, daß die dominanten Verformungsereignisse der mittleren Trias allgemeine Dehnungsbewegungen waren und weisen nicht auf die Existenz einer Subduktionszone zu dieser Zeit hin. Der scheinbare Widerspruch zwischen dem tektonischen Milieu und dem im Anis bis Karn ausgeworfenen Magmatyp wird versuchsweise mit einer Teilaufschmelzung eines während der vorhergehenden herzynischen Orogenese tief veränderten und mit Krustenmaterial verunreinigten oberen Mantels während der Frühphase einer Grabenbildung erklärt. Diese Hypothese ist im Einklang mit den präliminaren Sr-Isotopenverhältnissen (0.705 und 0.709). K/Ar und Rb/Sr Altersmessungen an Biotiten datieren den Anfang der vulkanischen Tätigkeit um 225 Ma.

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Résumé Un volcanisme très répandu constitué surtout par des produits pyroclastiques et par des coulées de lave se trouve dans les Alpes Méridionales en Lombardie. Ces produits volcaniques sont intercalés dans la série carbonatique du bassin d'âge Anisien-Ladinien et dans les dépôts terrigènes continentaux ou transitionnels d'âge Ladinien-Carnien. Les données pétrographiques et géochimiques de ces produits volcaniques montrent une affinité calcoalcaline avec des caractères semblables à ceux des marges continentales convergentes. Dans le Trias moyen de cette partie des Alpes, les données géologiques indiquent que le motif dominant de la déformation est un mouvement d'extension très répandu, tandis qu'il n'y a aucune évidence de l'existence d'une zone de subduction durant cette période. Le contraste entre le style tectonique et le type de magma mis en place dans l'Anisien-Carnien, peut être expliqué par une fusion partielle du manteau supérieur pendant un stade précoce d'effondrement modifié intensément pendant la précédente orogenèse hercynienne et contaminé par des matériaux d'origine crustale. Cette hypothèse est en accord avec des données préliminaires des rapports isotopiques du Sr (0.705 et 0.709). Des âges K/Ar et Rb/Sr sur des biotites donnent 225 m.a. environ pour le début de l'activité volcanique.

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Mesoarchean gabbroanorthosite magmatism of the Kola region: Petrochemical, geochronological, and isotope-geochemical data

The paper presents results of petrochemical, geochemical, and isotope-geochemical study of the Patchemvarek and Severnyi gabbroanorthosite massifs of the Kola Peninsula. It was shown that the rocks of these massifs differ from the gabbroanorthosite massifs of the Neoarchean Keivy-Kolmozero Complex in the more calcic composition (70–85% An) of normative plagioclase, and low contents of TiO₂, FeO, and Fe₂O₃. In terms of chemical composition, the gabbroanorthosites of the studied massifs are close to the rocks of the Fisken?sset Complex (Southwestern Greenland) and to the anorthosites of the Vermillion Lake Complex (Canada). U-Pb zircon dating established Mesoarchean ages of 2925 ± 7 and 2935 ± 8 Ma for the gabbroan-orthosites of the Patchemvarek and Severnyi massifs, respectively. It was shown that the gabbroanorthosites of the studied massifs have fairly low REE contents (Ce _n = 2.2−4.2, Yb _n = 1.6−2.6) and distinct positive Eu anomaly. Comagmatic ultrabasic differentiates have practically unfractionated REE pattern, low total REE contents (Ce _n = 1.2, Yb _n = 1.1, La/Yb _n = 1.3), and no Eu anomaly. The studied samples of the Archean gabbroanorthosites are characterized by positive ε_Nd = +2.68 for the gabbroanorthosites of the Severnyi Massif and from + 2.77 to + 1.66 for the Patchemvarek Massif. Initial strontium isotope ratios are ⁸⁷Sr/⁸⁶Sr _i = 0.70204 ± 8 and ⁸⁷Sr/⁸⁶Sr _i = 0.70258 ± 8 for the rocks of the Severnyi and Patchemvarek massifs, respectively. Our study showed that the obtained U-Pb zircon ages for the gabbroanorthosites of the Patchemvarek and Severnyi massifs represent the oldest date for the Kola peninsula, thus marking the oldest, Mesoarchean stage in the evolution of region. The differences in the initial ¹⁴³Nd/¹⁴⁴Nd ratios between the Neoarchean gabbroanorthosites of the Keivy-Kolmozero Complex and the Mesoarchean gabbroanorthosites of the studied massifs suggest the existence of two mantle sources. One of them produced intrusions with an age of 2.67–2.66 Ga, while other was responsible for the formation of massifs with an age of 2.93–2.92 Ga. The composition and temperature of “parental” melt of the gabbroanorthosites were simulated using COMAGMAT-3.5 program. According to the calculations, the parental melt represented aluminous basalt, whose differentiation at T = 1280°C and P = 7 kbar at the crust-mantle boundary was accompanied by plagioclase floatation and formation of “crystal mesh” that produced anorthosite complexes. The gabbroanorthosies of the Patchemvarek and Severnyi massifs were presumably derived from MORB-type basalts of oceanic settings, while the Tsaga, Achinskii, and other anorthosite massifs of the Neoarchean age were generated from subalkaline magma formed in within-plate anorogenic setting. Sm-Nd isotope data suggest the existence of several mantle sources in the Kola region, which produced melts for different-age gabbroanorthosite massifs since Mesoarchean to the middle Paleoproterozoic. The Archean-Early Proterozoic anorthosite magmatism of the Kola region records a complete cycle (∼ 800 Ma) of the formation and consolidation of continental block.     

Pan-African magmatism in the Menderes Massif: geochronological data from leucocratic tourmaline orthogneisses in western Turkey

The Menderes Massif, exposed in western Anatolia, is a metamorphic complex cropping out in the Alpine orogenic belt. The metamorphic rock succession of the Massif is made up of a Precambrian basement and overlying Paleozoic-early Tertiary cover series. The Pan-African basement is composed of late Proterozoic metasedimentary rocks consisting of partially migmatized paragneisses and conformably overlying medium- to high-grade mica schists, intruded by orthogneisses and metagabbros. Along the southern flank of the southern submassif, we recognized well-preserved primary contact relationship between biotite and leucocratic tourmaline orthogneisses and country rocks as the orthogneisses represent numerous large plutons, stocks and vein rocks intruded into a basement of garnet mica schists. Based on the radiometric data, the primary deposition age of the precursors of the country rocks, garnet mica schist, can be constrained between 600 and 550?Ma (latest Neoproterozoic). The North Africa–Arabian-Nubian Shield in the Mozambique Belt can be suggested as the possible provenance of these metaclastics. The intrusion ages of the leucocratic tourmaline orthogneisses and biotite orthogneisses were dated at 550–540?Ma (latest Neoproterozoic–earliest Cambrian) by zircon U/Pb and Pb/Pb geochronology. These granitoids represent the products of the widespread Pan-African acidic magmatic activity, which can be attributed to the closure of the Mozambique Ocean during the final collision of East and West Gondwana. Detrital zircon ages at about 550?Ma in the Paleozoic muscovite-quartz schists show that these Pan-African granitoids in the basement form the source rocks of the cover series of the Menderes Massif.     

Neoarchean sanukitoid magmatism in the Kola region: Geological, petrochemical, geochronological, and isotopic-geochemical data

New geological, isotopic-geochronological, petrochemical, and isotopic geochemical data were obtained on the Porosozero and Kolmozero sanukitoid intrusions in the Kola region. The Porosozero differentiated intrusion was formed in four phases successively emplaced during approximately 60 Ma. Phase 1 consists of a gabbrodiorite-quartz monzodiorite-granodiorite-granite series. The zircon ages of granodiorite and quartz monzodiorite from the Porosozero are 2733 ± 6 and 2734 ± 4 Ma, respectively. Phase 2 of the intrusion comprises biotite leucogranites and aplite and leucoplagiogranite veins. The zircon age of the leucogranite is 2712 ± 6 Ma. Phase 3 consists of lamprophyre dikes of odinite-spessartite-vogesite composition. The emplacement age of the lamprophyres is constrained by the age of magmatic zircon from an odinite dike: 2680 ± 10 Ma. The age of the metasomatic zircon is 2629 ± 8 Ma. Phase 4 is composed of the youngest pegmatite veins. The Rb-Sr isochron age of the phase-1 rocks is 2724 ± 74 Ma. The zircon age of granitoids from the Kolmozero is 2736 ± 4 Ma. The rocks of the sanukitoid intrusions affiliate with the calc-alkaline series, have Mg# = 0.45?0.60, are enriched in Ba, Sr, K, P, and LREE, and contain elevated concentrations of Cr and Ni. Sm-Nd isotopic data on sanukitoids from both intrusions suggest that they were derived from a mantle source enriched in LILE and LREE and having ?_Nd(2740) from +1.02 to +0.36. It was melted approximately 140 Ma after its origin [T(DM) = 2.9?2.8 Ga]. The rocks of the Porosozero and Kolmozero are proved to be similar to magmatic sanukitoid series of Archean and Phanerozoic age whose genesis was controlled by mantle-crustal interaction in suprasubduction environments at active continental margins. Elevated concentrations of Ag and Au in rocks from the Porosozero make it metallogenically promising in terms of precious metals.     

Phanerozoic mafic magmatism in the southern Siberian craton: geodynamic implications

The Phanerozoic history of mafic magmatism in the southern Siberian craton included three major events. The earliest event (～500 Ma) recorded in dolerite dikes occurred during accretion and collision at the early stage of the Central Asian orogen. Injection of mafic melts into the upper crust was possible in zones of diffuse extension within the southern Siberian craton which acted as an indenter. The Late Paleozoic event (～275 Ma) produced dikes that intruded in a setting of subduction-related extension at the back of the active continental margin of Siberia during closure of the Mongolia–Okhotsk ocean, as well as slightly older volcanics (290 Ma) in the Transbaikalian segment of the Central Asian orogen. Early Mesozoic magmatism in the southern Siberian craton resulted in numerous 240–250 Ma mafic intrusions in the Angara–Taseeva basin. The intrusions (Siberian traps) appeared as the subducting slab of the Mongolia–Okhotsk ocean interacted with a lower mantle plume. The post-Late Paleozoic ages of flood basalts (290–275 Ma) correspond to progressive northwestward (in present coordinates) motion of the slab beneath the southern craton margin which likely ceased after the slab had reached the zone of the Siberian superplume. Since its consolidation after the Early Mesozoic activity, the crust in the area has no longer experienced extension favorable for intrusion of basaltic magma.     

Middle Paleozoic and Early Mesozoic anorogenic magmatism of the South Yenisei Ridge: first geochemical and geochronological data

In this paper we present complex geological, petrographic and geochronological data of the study of intermediate and acid composition intrusive and volcanogenic rocks from the Porozhnaya massif of the South Yenisei Ridge. For the first time in the Yenisei Ridge Devonian and Triassic U-Pb age values (SHRIMP method) have been obtained for leucogranites—387 ± 5 Ma and alkaline trachytes—240 ± 3 Ma, which allows us to attribute them to two different complexes, despite the fact that these rocks were formed within the same Severnaya riftogenic structure. Geochronological Ar-Ar data (392–387 Ma) for micas from paragneisses and leucogranitic dikes of the Yenisei suture zone on whose extension the Severnaya riftogenic structure is located are also given in this study. These data on Devonian tectonic-magmatic events in the South Yenisei Ridge agree well with coeval events of continental rifting—the formation of intrusive and volcanogenic rocks of the Agul graben in the Prisayan region and the Minusa basin in the Altai-Sayan folded area. The forming of alkaline trachytes and alkaline syenites of the Severnaya riftogenic structure, for which an age of 240 ± 3 Ma has been established, is related to the trap magmatism of the Siberian platform.     

Early Miocene post-collisional magmatism in NW Turkey: geochemical and geochronological constraints

The Alaçam region of NW Turkey lies within the Alpine collision zone between the Sakarya continent and the Menderes platform. Four different tectonic zones of these two continents form imbricated nappe packages (including the Afyon zone), intruded by the Alaçam granite. Newly determined U-Pb zircon ages of this granite are 20.0 ± 1.4 and 20.3 ± 3.3 Ma, indicating early Miocene emplacement. Rb-Sr biotite ages of the granite are 20.01 ± 0.20 and 20.17 ± 0.20 Ma, suggesting fast cooling at a shallow crustal level. Geochemical characteristics show that the Alaçam granite is similar to numerous EW-trending plutons in NW Anatolia.

Gneissic granites of the Afyon tectonic zone were intruded by the Miocene Alaçam granite and have been interpreted in earlier studies as sheared parts of the Alaçam granite, which formed along a crustal-scale detachment zone under an extensional regime. We determined a U-Pb zircon age of 314.9 ± 2.7 Ma for a gneissic granite sample of the Afyon zone, demonstrating that these rocks are unrelated to the Miocene Alaçam granite. The early Miocene granitic plutons bear post-collisional geochemical features and are interpreted as products of Alpine-type magmatism along the Izmir–Ankara suture zone in NW Turkey, and seem to have no genetic relation to the detachment zone.     

Cenozoic magmatism in Kalimantan and its related geodynamic evolution

The NE–SW Tertiary magmatic belt of central Kalimantan is related to two separate periods of subduction; during the Eocene–Oligocene and Late Oligocene–Miocene. The younger magmatic belt is superimposed upon the earlier belt. This magmatic belt is characterized chiefly by Late Oligocene–Miocene volcanic products, among which limited exposures of the Eocene volcanics have also been mapped by previous investigators. This calc-alkaline magmatic belt has become known as the ‘gold belt’ of Central West Kalimantan on account of a number of discoveries of Neogene epithermal gold mineralization. This mineralization is found in central to proximal volcanic settings and occurred at relatively shallow depths. The earliest known subduction-related magmatism took place in the Eocene–Early Oligocene with the emplacement of calc-alkaline silicic pyroclastics, followed by a period of continental collision. Subsequent subduction-related magmatism continued from Late Oligocene–Pleistocene, during which time the magma evolved from calc-alkaline to potassic calc-alkaline. Plio-Pleistocene magmatism resulted in the formation of basalt flows. The present available K–Ar ages of the Cenozoic volcanics range from 51 to 1 Ma.     

Origin of Triassic magmatism of the Southern Alps (Italy): Constraints from geochemistry and Sr-Nd-Pb isotopic ratios

During Middle-early Late Triassic (∼243–235?Ma) a diffuse igneous activity developed in the Southern Alps (Italy). Sparse lava flow and pyroclastic succession remnants of such Southern Alps Triassic Igneous Rocks (SATIR) crop out in the Brescian Prealps, the Vicentinian Alps (Recoaro-Schio-Posina), Non Valley, Dolomites and Julian Alps. Plutonic rocks are found in two main plutonic complexes (Monzoni and Predazzo) and a small stock (Cima di Pape). Coeval igneous products can be traced eastward to Austria and Dinarides, for a total length of ∼450?km.The coeval formation of major late Anisian-late Ladinian carbonate platforms in the subsiding eastern Dolomites, and significant uplift and subaerial erosion in the western Dolomites suggest, for these areas, the occurrence of large-scale strike-slip and extensional tectonics with the development of horst-and-graben structures.This study reports the first complete review of the SATIR activity, including new mineral chemical data on 14 samples, 61 major and trace element whole rock analyses and 7 Sr-Nd-Pb isotopic ratios for volcanic and plutonic samples from Dolomites (lavas plus Monzoni and Predazzo plutonic rocks) and Vicentinian Alps lavas. Despite the variable post-magmatic modifications, the large areal distribution of the products and their wide spectrum of chemical compositions, these samples show rather common geochemical and mineralogical characteristics and define major and trace element trends that can be associated with nearly close-system magmatic evolution. Minor upper crustal contaminations can be observed in specific cases, mostly in the most differentiated products (SiO₂ >70?wt%).A specific characteristic of SATIR is their calcalkaline to shoshonitic affinity, resembling the derivation from subduction-modified mantle sources, a feature at odds with the coeval strike-slip and extensional tectonics. Geochemical modelling, petrological considerations and geological constraints allow us to propose a model in which the SATIR mantle sources reflect previous subduction metasomatism (likely occurred during Variscan times). These metasomatised mantle sources were reactivated ∼90–100 Myr after the end of the subduction, when continental rifting caused a raise of the geotherms and passive upwelling of asthenospheric mantle.     

Jurassic granitoid magmatism in the Dinaride Neotethys: geochronological constraints from detrital minerals

Three independent single‐grain geochronometers applied to detrital minerals from Central Dinaride sediments constrain the timing of felsic magmatism that associated the Jurassic evolution of the Neotethys. The Lower Cretaceous clastic wedge of the Bosnian Flysch, sourced from the Dinaride ophiolitic thrust complex, yields magmatic monazite and zircon grains with dominant age components of 164 ± 3 and 152 ± 10 Ma respectively. A unique tephra horizon within the Adriatic Carbonate Platform was dated at 148 ± 11 Ma by apatite fission track analysis. These consistent results suggest that leucocractic melt generation in the Central Dinaride segment of the Neotethys culminated in Middle to Late Jurassic times, coeval with and slightly post‐dating subophiolitic sole metamorphism. Growth of magmatic monazite and explosive volcanism call for supra‐subduction‐zone processes at the convergent Neotethyan margin. New compilation of geochronological data demonstrates that such Jurassic felsic rocks are widespread in the entire Dinaride–Hellenide orogen.     

Petrology and geodynamic significance of the post-collisional Pan-African magmatism in the Eastern Saghro area (Anti-Atlas, Morocco)

The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show T_DM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption.Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580–560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr–Nd isotopic compositions and Nd T_DM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts.Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas.