Bimodal magmatism as a consequence of the post-collisional readjustment of the thickened Variscan continental lithosphere (Aiguilles Rouges - Mont Blanc Massifs, Western Alps)

High Precision U-Pb zircon and monazite dating in the Aiguilles Rouges-Mont Blanc area allowed discrimination of three short-lived bimodal magmatic pulses: the early 332 Ma Mg-K Pormenaz monzonite and associated 331 Ma peraluminous Montees Pelissier monzogranite; the 307 Ma cordierite-bearing peraluminous Vallorcine and Fully intrusions; and the 303 Fe-K Mont Blanc syenogranite. All intruded syntectonically along major-scale transcurrent faults at a time when the substratum was experiencing tectonic exhumation, active erosion recorded in detrital basins and isothermal decompression melting dated at 327-320 Ma. Mantle activity and magma mixing are evidenced in all plutons by coeval mafic enclaves, stocks and synplutonic dykes. Both crustal and mantle sources evolve through time, pointing to an increasingly warm continental crust and juvenile asthenospheric mantle sources. This overall tectono-magmatic evolution is interpreted in a scenario of post-collisional restoration to normal size of a thickened continental lithosphere. The latter re-equilibrates through delamination and/or erosion of its mantle root and tectonic exhumation/erosion in an overall extensional regime. Extension is related to either gravitational collapse or back-are extension of a distant subduction zone.

Timing of Palaeozoic magmatism in the Maggia and Sambuco nappes and paleogeographic implications (Central Lepontine Alps)

Magmatic rocks from the pre-Mesozoic basements of the Sambuco and Maggia nappes have been dated by U-Pb zircon ages with the LA-ICPMS technique. Several magmatic events have been identified in the Sambuco nappe. The mafic banded calc-alkaline suite of Scheggia is dated at 540 Ma, an age comparable to that of mafic rocks in the Austroalpine Silvretta nappe. The Sasso Nero peraluminous augengneiss has an age of 480-470 Ma, like many other ``older orthogneisses'' in Alpine basement units. It hosts a large proportion of inherited zircons, which were dated around 630 Ma, a Panafrican age indicating the Gondwanan affiliation of the Sambuco basement. The calc-alkaline Matorello pluton yielded ages around 300 Ma, similar to numerous Late Carboniferous intrusions in other basement units of the Lower Penninic (Monte Leone, Antigorio, Verampio) and Helvetic domains (Gotthard and other External Crystalline Massifs). Associated lamprophyric dykes are slightly younger (300-290 Ma), like similar dykes sampled in gneiss blocks included in the sedimentary cover of the underlying Antigorio nappe (290-285 Ma). The Cocco granodiorite and Ruscada leucogranite, both intruding the basement of the neighbouring Maggia nappe, yielded ages of ca. 300-310 Ma, identical within errors to the age of the Matorello pluton. They are significantly older than former age determinations. This age coincidence, coupled with remarkable petrologic similarities between the Cocco and Matorello granodiorites, strongly suggests paleogeographic proximity of the Sambuco and Maggia nappes in Late Carboniferous times. In recent publications these two nappes have been interpreted as belonging to distinct Mesozoic paleogeographic domains: ``European'' for Sambuco and ``Brian double dagger onnais'' for Maggia, separated by the ``Valais'' oceanic basin. In this case, the similarity of the Matorello and Cocco intrusions would demonstrate the absence of any significant transcurrent movement between these two continental domains. Alternatively, according to a more traditional view, Sambuco and Maggia might belong to a single large Alpine tectonic unit.

Origin of Early Carboniferous pseudo-adakites in northern Brittany (France) through massive amphibole fractionation from hydrous basalt

P>The first Variscan pseudo-adakites were identified in close association with the Saint-Jean-du-Doigt (SJDD) mafic intrusion (Brittany, France) in a geodynamic context unrelated to subduction. These rocks are trondhjemites emplaced 347 +/- 4 Ma ago as 2-3 km2 bodies and dykes. Trace-element concentrations and Sr-Nd-Pb isotope ratios indicate that the SJDD pseudo-adakites probably resulted from extreme differentiation of an SJDD-type hydrous basaltic magma in a lower continental crust of normal thickness (0.8 GPa). Modelling shows that garnet is not a required phase, which was commonly believed to be the case for continental arc-derived adakite-like rocks. A massive fractionation of amphibole fits the data much better and does not require high pressures, in agreement with the inferred extensional tectonic regime at the time of pluton emplacement. Alternatively, the SJDD pseudo-adakites could have resulted from the melting of newly underplated SJDD mafic precursors, but thermal considerations lead us to believe that this was not the case.

Garnet growth in the metasediments of the Zermatt-Saas Fee zone (western alps)

The Zermatt-Saas Fee Zone (ZSZ) in the Western Alps consists of multiple slices of ultramafic, mafic and metasedimentary rocks. They represent the remnants of the Mesozoic Piemonte-Ligurian oceanic basin which was subducted to eclogite facies conditions with peak pressures and temperatures of up to 20-28 kbar and 550-630 °C, followed by a greenschist overprint during exhumation. Previous studies, emphasizing on isotopie geochronology and modeling of REE-behavior in garnets from mafic eclogites, suggest that the ZSZ is buildup of tectonic slices which underwent a protracted diachronous subduction followed by a rapid synchronous exhumation. In this study Rb/Sr geochronology is applied to phengite included in garnets from metasediments of two different slices of the ZSZ to date garnet growth. Inclusion ages for 2 metapelitic samples from the same locality from the first slice are 44.25 ± 0.48 Ma and 43.19 ± 0.32 Ma. Those are about 4 Ma older than the corresponding matrix mica ages of respectively 40.02 ± 0.13 Ma and 39.55 ± 0.25 Ma. The inclusion age for a third calcschist sample, collected from a second slice, is 40.58 ± 0.24 Ma and the matrix age is 39.8 ± 1.5 Ma. The results show that garnet effectively functioned as a shield, preventing a reset of the Rb/Sr isotopie clock in the included phengites to temperatures well above the closure of Sr in mica. The results are consistent with the results of former studies on the ZSZ using both Lu/Hf and Sm/Nd geochronology on mafic eclogites. They confirm that at least parts of the ZSZ underwent close to peak metamorphic HP conditions younger than 43 m.y. ago before being rapidly exhumed about 40 m.y. ago. Fluid infiltration in rocks of the second slice occurred likely close to the peak metamorphic conditions, resulting in rapid growth of garnets. Similar calcschists from the same slice contain two distinct types of porphyroblast garnets with indications of multiple growth pulses and resorption indicated by truncated chemical zoning patterns. In-situ oxygen isotope Sensitive High Resolution Ion Microprobe (SHRIMP) analyses along profiles on central sections of the garnets reveal variations of up to 5 %o in individual garnets. The complex compositional zoning and graphite inclusion patterns as well as the variations in oxygen isotopes correspond to growing under changing fluid composition conditions caused by external infiltrated fluids. The ultramafic and mafic rocks, which were subducted along with the sediments and form the volumetrically most important part of the ZSZ, are the likely source of those mainly aqueous fluids. - La Zone de Zermatt-Saas Fee (ZZS) est constituée de multiples écailles de roches ultramafiques, mafiques et méta-sédimentaires. Cette zone, qui affleure dans les Alpes occidentales, représente les restes du basin océanique Piémontais-Ligurien d'âge mésozoïque. Lors de la subduction de ce basin océanique à l'Eocène, les différentes roches composant le planché océanique ont atteint les conditions du faciès éclogitique avec des pressions et des températures maximales estimées entre 20 - 28 kbar et 550 - 630 °C respectivement, avant de subir une rétrogression au faciès schiste vert pendant l'exhumation. Différentes études antérieures combinant la géochronologie isotopique et la modélisation des mécanismes gouvernant l'incorporation des terres rares dans les grenats des éclogites mafiques, suggèrent que la ZZS ne correspond pas à une seule unité, mais est constituée de différentes écailles tectoniques qui ont subi une subduction prolongée et diachrone suivie d'une exhumation rapide et synchrone. Afin de tester cette hypothèse, j'ai daté, dans cette étude, des phengites incluses dans les grenats des méta-sédiments de deux différentes écailles tectoniques de la ZZS, afin de dater la croissance relative de ces grenats. Pour cela j'ai utilisé la méthode géochronologique basée sur la décroissance du Rb87 en Sr87. J'ai daté trois échantillons de deux différentes écailles. Les premiers deux échantillons proviennent de Triftji, au nord du Breithorn, d'une première écaille dont les méta-sédiments sont caractérisés par des bandes méta-pélitiques à grenat et des calcschistes. Le troisième échantillon a été collectionné au Riffelberg, dans une écaille dont les méta-sédiments sont essentiellement des calcschistes qui sont mélangés avec des roches mafiques et des serpentinites. Ce mélange se trouve au-dessus de la grande masse de serpentinites qui forment le Riffelhorn, le Trockenersteg et le Breithorn, et qui est connu sous le nom de la Zone de mélange de Riffelberg (Bearth, 1953). Les inclusions dans les grenats de deux échantillons méta-pélitiques de la première écaille sont datées à 44.25 ± 0.48 Ma et à 43.19 ± 0.32 Ma. Ces âges sont à peu près 4 Ma plus vieux que les âges obtenus sur les phengites provenant de la matrice de ces mêmes échantillons qui donnent des âges de 40.02 ± 0.13 Ma et 39.55 ± 0.25 Ma respectivement. Les inclusions de phengite dans les grenats appartenant à un calcschiste de la deuxième écaille ont un âge de 40.58 ± 0.24 Ma alors que les phengites de la matrice ont un âge de 39.8 ± 1.5 Ma. Pour expliquer ces différences d'âge entre les phengites incluses dans le grenat et les phengites provenant de la matrice, nous suggérons que la cristallisation de grenat ait permis d'isoler ces phengites et de les préserver de tous rééquilibrage lors de la suite du chemin métamorphique prograde, puis rétrograde. Ceci est particulièrement important pour expliquer l'absence de rééquilibrage des phengites dans des conditions de températures supérieures à la température de fermeture du système Rb/Sr pour les phengites. Les phengites en inclusions n'ayant pas pu être datées individuellement, nous interprétons l'âge de 44 Ma pour les inclusions de phengite comme un âge moyen pour l'incorporation de ces phengites dans le grenat. Ces résultats sont cohérents avec les résultats des études antérieures de la ZZS utilisant les systèmes isotopiques de Sm/Nd et Lu/Hf sur des eclogites mafiques. ils confirment qu'aux moins une partie de la ZZS a subi des conditions de pression et de température maximale il y a moins de 44 à 42 Ma avant d'être rapidement exhumée à des conditions métamorphiques du faciès schiste vert supérieur autour de 40 Ma. Cette étude détaillée des grenats a permis, également, de mettre en évidence le rôle des fluides durant le métamorphisme prograde. En effet, si tous les grenats montrent des puises de croissance et de résorption, on peut distinguer, dans différents calcschists provenant de la deuxième écaille, deux types distincts de porphyroblast de grenat en fonction de la présence ou non d'inclusions de graphite. Nous lions ces puises de croissances/résorptions ainsi que la présence ou l'absence de graphite en inclusion dans les grenats à l'infiltration de fluides dans le système, et ceci durant tous le chemin prograde mais plus particulièrement proche et éventuellement peu après du pic du métamorphisme comme le suggère l'âge de 40 Ma mesuré dans les inclusions de phengites de l'échantillon du Riffelberg. Des analyses in-situ d'isotopes d'oxygène réalisé à l'aide de la SHRIMP (Sensitive High Resolution Ion Microprobe) dans des coupes centrales des grenats indiquent des variations jusqu'à 5 %o au sein même d'un grenat. Les motifs de zonations chimiques et d'inclusions de graphite complexes, ainsi que les variations du δ180 correspondent à une croissance de grenat sous des conditions de fluides changeantes dues aux infiltrations de fluides externes. Nous lions l'origine de ces fluides aqueux aux unités ultramafiques et mafiques qui ont été subductés avec les méta-sédiments ; unités ultramafiques et mafiques qui forment la partie volumétrique la plus importante de la ZZS.

Jurassic sedimentary record and tectonic evolution of the northwestern corner of Africa

The history of the opening seaway from the westernmost Tethys to the Central Atlantic is traced by the analysis of the sedimentary facies development in the external Rif basin of Northern Morocco and the geological and seismic data from the Moroccan Atlantic continental margin. In the Rif basin, after the early Sinemurian, sedimentary facies dated by ammonites, foraminifers and brachiopods, indicate a progression of rapid subsidence resulting from extensional tectonic (tilted blocks, escarpment fault breccias, neptunian dykes etc.) from the N and NE to the S and SW. From the Toarcian to the Bajocian, deltas progress from the W and SW into the `'Rides sud-rifaines'' realm. From the late Bathonian to the Oxfordian, deep-sea fans develop in the external Rif. During the same period, deltaic sediments fill in the Middle Atlas basin of Eastern Morocco and progress into the external Rif. The top of the Jurassic is characterised by carbonate deposits. At the northwestern corner of Africa, the subsidence of the sedimentary basins by rifting is initiated in the late Triassic; however, at the Mazagan transect of the Atlantic continental margin, the tectonic pattern characteristic of a passive continental margin appears clearly only in the early Jurassic. At the foot of the Mazagan escarpment, the sedimentary record shows a foundering of the first bloc during early to middle Lias. A thermal uplift phase is indicated by emersion of the African margin shoulder in late Liassic, and thermal relaxation starts in the middle Jurassic. The morphology of this transect, compared with the conjugate side of the American continent is most easily explained by the uniform sense simple shear model.

Late Labradorian metamorphism and anorthosite-granitoid intrusion, Cape-Caribou River allochthon, Grenville Province, Labrador - Evidence from U-Pb geochronology

In the Cape Caribou River allochthon (CCRA), metaigneous and gneissic units occur as a shallowly plunging synform in the hanging wall of the Grand Lake thrust system (GLTS), a Grenvillian structure that forms the boundary between the Mealy Mountains and Groswater Bay terranes. The layered rocks of the CCRA are cut by a stockwork of monzonite dykes related to the Dome Mountain suite and by metadiabase-amphibolite dykes that probably form part of the ca. 1380 Ma Mealy swarm. The mafic dykes appear to postdate much of the development of subhorizontal metamorphic layering within the lower parts of the CCRA. The uppermost (least metamorphosed) units of the CCRA, the North West River anorthosite-metagabbro and the Dome Mountain monzonite suite, have been dated at 1625 +/- 6 and 1626 +/- 2 Ma, respectively. An amphibolite unit that concordantly underlies the anorthosite-metagabbro and is intruded discordantly by monzonite dykes has given metamorphic ages of 1660 +/- 3 and 1631 +/- 2 Ma. Granitoid gneisses that form the lowest level of the CCRA have given a migmatization age of 1622 +/- 6 Ma. The effects of Grenvillian metamorphism become apparent in the lower levels of the allochthon where gneisses, amphibolite, and mafic dykes have given new generation zircon ages of 1008 +/- 2, 1012 +/- 3, and 1011 +/- 3 Ma, respectively. A posttectonic pegmatite has also given zircon and monazite ages of 1016(-3)(+7) and 1013 +/- 3 Ma, respectively. Although these results indicate new growth of Grenvillian zircon, this process was generally not accompanied by penetrative deformation or melting. Thus, the formation of gneissic fabrics and the overall layered nature of the lower CCRA are a result primarily of Labradorian (1660-1620 Ma) tectonism and intrusion, and probably reflect early movement on an ancestral GLTS. Grenvillian heating and metamorphism (up to granulite facies) was strongly concentrated towards the base of the CCRA and probably occurred during northwestward thrusting of the allochthon over the Groswater Bay terrane.

Evidence for mantle exhumation along the Arabian margin in the Zagros (Kermanshah area, Iran)

The Kermanshah Crush Zone (Zagros, Iran) comprises elements from the Tethys Ocean and the former ocean-continent transition. Serpentinites and gabbros exposed in this area were formerly interpreted as originated from Tethys ocean and other residual Tethys oceanic domains all situated northeast of the Bisotoun platform. However, the structural relationships between these ultramafic units remained unclear. New field work in the Kermanshah-Harsin area led to the description of detachment faults over serpentinised mantle. ``Mid-Cretaceous'' carbonate ``extensional allochthons'' (pre-rift) and pelagic sediments (syn- to post-rift) dated from the Liassic are exposed above these detachments. Such an age mismatch can be explained by a polyphased mantle exhumation in a narrow basin along the Arabian margin with the assumption that no radiolarite nappe has been thrusted over the Bisotoun. Another detachment has been identified further to the NE on Eocene gabbro. So far, this one is considered as an intra-oceanic detachment from the residual Tethys. A first evolution model is proposed from early Jurassic to late Cretaceous obduction along with how this interpretation may be improved by future field work.

Variscan lamprophyres in the Lower Penninic domain (Central Alps): age and tectonic significance

Lamprophyre dykes have been recently discovered in blocks of gneiss embedded in a calcschist formation of wildflysch type that forms the top of the Mesozoic-Tertiary metasedimentary cover of the Antigorio nappe (the Teggiolo zone) in the Val Bavona (Lower Penninic, NW Ticino, Switzerland). The presence of the lamprophyres gives a clue to the possible source of these blocks. Similar dykes occur in the N part of the Maggia nappe where they are intruded into the Matorello granite and the surrounding gneisses. We studied these lamprophyres at two localities in the Teggiolo zone (Tamierpass and Lago del Zott) and at one locality in the Maggia nappe (Laghetti). Detailed mineralogical and geochemical investigations confirm their great similarity, particularly between the Tamier and Laghetti dykes. They all recrystallized during Alpine metamorphism under amphibolite facies conditions and lost their primary mineral assemblages and textures. The chemistry reveals a calc-alkaline affinity, a limited differentiation range, features of mineral accumulation and intense remobilization in some cases. The lamprophyres are characterized by a high mg# and relatively low contents in REE and other incompatible elements. In situ SHRIMP and LA-ICPMS U-Pb zircon dating yielded ages of 284.8 +/- 1.7 Ma (Tamier), 290.0 +/- 1.3 Ma (Zott) and 290.5 +/- 3.7 Ma (Laghetti). These ages are compatible with the general late- to post-Variscan magmatic evolution of the Helvetic and Lower Penninic domains. The lamprophyres are considered as melts derived from the lithospheric Variscan mantle, variously hybridized and differentiated at the contact with crustal material during late- to post-orogenic extension. These lamprophyres are chemically distinct from earlier lamprophyres of Visean age, emplaced together with their associated granites in transcurrent fault zones during the Variscan orogenic compression. The similarity of these different dykes suggests that the front of the Maggia nappe is a likely source of the gneissic blocks embedded in the calcschists at the top of the Teggiolo zone. They would have been provided by the advancing Maggia nappe during its thrusting over the Antigorio nappe and simultaneous closure of the Teggiolo sedimentary basin.

Petrography and geochemistry of accreted oceanic fragments below the Western Cordillera of Ecuador

The Western Cordillera of Ecuador consists of Cretaceous crustal fragments of oceanic plateaux and superimposed insular arcs, which were accreted to the northwestern South American margin during the Late Cretaceous and Paleocene. Slices of high-grade metabasites, ultramafic rocks, gabbros and basalts, unmetamorphosed radiolarian cherts and scarce garnet-bearing metasediments were randomly exhumed along Miocene to Recent transcurrent faults crosscutting the Western Cordillera. The basalts show geochemical characteristics of oceanic plateau basalts (flat REE patterns, La/Nb = 0.85). The gabbros differ from the basalts in having lower REE levels, positive Eu anomalies, and negative Nb and Ta anomalies; they are interpreted as resulting from arc magmatism. The amphibolites and banded amphibolites have major and trace element chemistry similar to that of oceanic plateau basalts (flat REE patterns, La/Nb = 0.86) or to cumulate gabbros. The granulite shares with oceanic plateaus similar trace element chemistry (flat REE patterns, La/Nb < 1) and epsilon(Ndi) values (+7.6). Continent-derived metasediments are depleted in heavy REE (La/Y = 4.8) and have a negative Eu anomaly. Foliated Iherzolites, melagabbronorites and pyroxenites consist of serpentinized olivine + cpx + opx +/- Ca-plagioclase. Lherzolites, melagabbronorites and pyroxenites are LREE depleted with positive Eu anomalies, while the harzburgite displays a U-shaped REE pattern. The trace element abundances of the ultramafic rocks are very low (0.1 to 1 times the chonctritic and primitive mantle values). The ultramafic rocks represent fragments of depleted mantle, deformed cpx-rich cumulate, and continental lithospheric mantle or mantle contaminated by subduction-fluid. Except the scarce quartz-rich metasediments, all these rocks likely represent remnants of accreted oceanic crustal fragments and associated depleted mantle. Since these samples were randomly sampled at depth by the fault, we propose that the Western Cordillera and its crustal root are mainly of oceanic nature.

High-precision U-Pb and 40Ar/39Ar dating of an Alpine ophiolite (Gets nappe, French Alps)

Coarse-grained gabbros from two different localities in the Gets nappe (Upper Prealps) have been dated by U-Pb and Ar-40/Ar-39 isotopic analyses. Zircons from both gabbros gave identical concordant U-Pb ages of 166 +/- 1 Ma (Fig. 4). Amphibole from one of them gave an Ar-40/Ar-39 plateau age of 165.9 +/- 2.2 Ma (Fig. 5). This concordance implies that 166 +/- 1 Ma is the age of magmatic crystallization of these gabbros. The Gets wildflysch with its mafic and ultramafic lenses is an ophiolitic melange, that we infer to come from a proximal part of the accretionary prism at the foot of the active SE margin of the Piemont ocean. In this position we can expect to find remnants of the oldest parts of the Piemont oceanic crust. These are the first high-precision dates using modern techniques from an Alpine ophiolite and are in excellent agreement with the following: 1) The few, somewhat younger, reliable ages on ophiolites from the probable continuation of the Piemont basin into the Apennines and Corsica; 2) Recent data on the age of the first supra-ophiolitic sediments (Late Bathonian to Early Callovian radiolarites); 3) The structural and stratigraphic evolution of the Brianconnais (s.s.) domain, the future NW margin of the Piemont ocean. We note a remarkable coincidence, in Late Bajocian time, between: (A) the end of tensile fracturing in the Brianconnais continental crust; (B) the beginning of its subsidence; (C) the age of the Gets ophiolites. This coincidence is consistent with an ocean opening mechanism based on a combination of subhorizontal extension and thermally driven vertical movements of the lithosphere.

Upper Triassic to Cretaceous radiolaria from Nicaragua and northern Costa Rica - The Mesquito composite oceanic terrane

We propose a new terrane subdivision of Nicaragua and Northern Costa Rica, based on Upper Triassic to Upper Cretaceous radiolarian biochronology of ribbon radiolarites, the newly studied Siuna Serpentinite Mélange, and published 40Ar/39Ar dating and geochemistry of mafic and ultramafic igneous rock units of the area. The new Mesquito Composite Oceanic Terrane (MCOT) comprises the southern half of the Chortis Block, that was assumed to be a continental fragment of N-America. The MCOT is defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks and radiolarites of Late Triassic, Jurassic and Early Cretaceous age: 1. The Siuna Serpentinite Mélange (NE-Nicaragua), 2. The El Castillo Mélange (Nicaragua/Costa Rica border), 3.The Santa Elena Ultramafics (N-Costa Rica) and, 4. DSDP Legs 67/84. 1. The Siuna Serpentinite Mélange contains, high pressure metamorphic mafics and Middle Jurassic (Bajocian-Bathonian) radiolarites in original, sedimentary contact with arc-metandesites. The Siuna Mélange also contains Upper Jurassic black detrital chert formed in a marginal (fore-arc?) basin shortly before subduction. A phengite 40Ar/39Ar -cooling age dates the exhumation of the high pressure rocks as 139 Ma (earliest Cretaceous). 2. The El Castillo Mélange comprises a radiolarite block tectonically embedded in serpentinite that yielded a diverse Rhaetian (latest Triassic) radiolarian assemblage, the oldest fossils recovered so far from S-Central America. 3. The Santa Elena Ultramafics of N-Costa Rica together with the serpentinite outcrops near El Castillo (2) in Southern Nicaragua, are the southernmost outcrops of the MCOT. The Santa Elena Unit (3) itself is still undated, but it is thrust onto the middle Cretaceous Santa Rosa Accretionary Complex (SRAC), that contains Lower to Upper Jurassic, highly deformed radiolarite blocks, probably reworked from the MCOT, which was the upper plate with respect to the SRAC. 4. Serpentinites, metagabbros and basalts have long been known from DSDP Leg 67/84 (3), drilled off Guatemala in the Nicaragua-Guatemala forearc basement. They have been restudied and reveal 40Ar/39Ar dated Upper Triassic to middle Cretaceous enriched Ocean Island Basalts and Jurassic to Lower Cretaceous depleted Island arc rocks of probable Pacific origin. The area between localities 1-4 is largely covered by Tertiary to Recent arcs, but we suspect that its basement is made of oceanic/accreted terranes. Earthquake seismic studies indicate an ill-defined, shallow Moho in this area. The MCOT covers most of Nicaragua and could extend to Guatemala to the W and form the Lower (southern) Nicaragua Rise to the NE. Some basement complexes of Jamaica, Hispaniola and Puerto Rico may also belong to the MCOT. The Nicoya Complex s. str. has been regarded as an example of Caribbean crust and the Caribbean Large Igneous Province (CLIP). However, 40Ar/39Ar - dates on basalts and intrusives indicate ages as old as Early Cretaceous. Highly deformed Jurassic and Lower Cretaceous radiolarites occur as blocks within younger intrusives and basalts. Our interpretation is that radiolarites became first accreted to the MCOT, then became reworked into the Nicoya Plateau in Late Cretaceous times. This implies that the Nicoya Plateau formed along the Pacific edge of the MCOT, independent form the CLIP and most probably unrelated with he Galapagos hotspot. No Jurassic radiolarite, no older sediment age than Coniacian-Santonian, and no older 40Ar/39Ar age than 95 Ma is known from S-Central America between SE of Nicoya and Colombia. For us this area represents the trailing edge of the CLIP s. str.

Post-Plutonic Magmatism: from the Source to the Emplacement of an Upper Crustal Dyke Swarm, Adamello Massif, Italy

Magmas of the arc-tholeiitic and calc-alkaline differentiation suites contribute substantially to the formation of continental crust in subduction zones. Different geochemical-petrological models have been put forward to achieve evolved magmas forming large volumes of tonalitic to granitic plutons, building an important part of the continental crust. Primary magmas produced in the mantle wedge overlying the subducted slab migrate through the mantle and the crust. During the transfer, magma can accumulate in intermediate reservoirs at different levels where crystallization leads to differentiation and the heat transfer from the magma, together with gained heat from solidification, lead to partial melting of the crust. Partial melts can be assimilated and mix with more primitive magma. Moreover, already formed crystal cumulates or crystal mushes can be recycled and reactivated to transfer to higher crustal levels. Magma transport in the crust involves fow through fractures within a brittle elastic rock. The solidified magma filled crack, a dyke, can crosscut previously formed geological structures and thus serves as a relative or absolute time marker. The study area is situated in the Adamello massif. The Adamello massif is a composite of plutons that were emplaced between 42 and 29 million years. A later dyke swarm intruded into the southern part of the Adamello Batholith. A fractionation model covering dyke compositions from picrobasalts to dacites results in the cummulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to achieve an andesitic composition out of a hydrous primitive picrobasalt. These rocks show a similar geochemical evolution as experimental data simulating fractional crystallization and associated magma differentiation at lower crustal depth (7-10 kbar). The peraluminous, corundum normative composition is one characteristic of more evolved dacitic magmas, which has been explained in a long lasting debate with two di_erent models. Melting of mafic crust or politic material provides one model, whereas an alternative is fractionation from primary mantle derived melts. Amphibole occurring in basaltic-andesitic and andesitic dyke rocks as fractionating cumulate phase extracted from lower crustal depth (6-7.5 kbar) is driving the magmas to peraluminous, corundum normative compositions, which are represented by tonalites forming most of the Adamello Batholith. Most primitive picrobasaltic dykes have a slightly steepened chondrite normalized rare earth elements (REE) pattern and the increased enrichment of light-REE (LREE) for andesites and dacites can be explained by the fractional crystallization model originating from a picrobasalt, taking the changing fractionating phase assemblage and temperature into account. The injection of hot basaltic magma (~1050°C) in a closely spaced dyke swarm increases the surface of the contact to the mainly tonalitic wallrock. Such a setting induces partial melting of the wall rock and selective assimilation. Partial melting of the tonalite host is further expressed through intrusion breccias from basaltic dykes. Heat conduction models with instantaneous magma injection for such a dyke swarm geometry can explain features of partial melting observed in the field. Geochemical data of minerals and bulk rock further underline the selective or bulk assimilation of the tonalite host rock at upper crustal levels (~2-3 kbar), in particular with regard to light ion lithophile elements (LILE) such as Sr, Ba and Rb. Primitive picrobasalts carry an immiscible felsic assimilant as enclaves that bring along refractory rutile and zircon with textures typically found in oceanic plagiogranites or high pressure/low-temperature metamorphic rocks in general. U-Pb data implies a lower Cretaceous age for zircon not yet described as assimilant in Eocene to Oligocene magmatic rocks of the Central Southern Alps. The distribution of post-plutonic dykes in large batholiths such as the Adamello is one of the key features for understanding the regional stress field during the post-batholith emplacement cooling history. The emplacement of the regional dyke swarm covering the southern part of the Adamello massif was associated with consistent left lateral strike-slip movement along magma dilatation planes, leading to en echelon segmentation of dykes. Through the dilation by magma of pre-existing weaknesses and cracks in an otherwise uniform host rock, the dyke propagation and according orientation in the horizontal plane adjusted continuously perpendicular to least compressive remote stress σ3, resulting in an inferred rotation of the remote principal stress field. Les magmas issus des zones de subduction contribuent substantiellement à la formation de la croûte continentale. Les plutons tonalitiques et granitiques représentent, en effet, une partie importante de la croûte continentale. Des magmas primaires produits dans le 'mantle wedge ', partie du manteau se trouvant au-dessus de la plaque plongeante dans des zones de subduction, migrent à travers le manteau puis la croûte. Pendant ce transfert, le magma peut s'accumuler dans des réservoirs intermédiaires à différentes profondeurs. Le stockage de magma dans ces réservoirs engendre, d'une part, la différentiation des magmas par cristallisation fractionnée et, d'autre part, une fusion partielle la croûte continentale préexistante associée au transfert de la chaleur des magmas vers l'encaissant. Ces liquides magmatiques issus de la croûte peuvent, ensuite, se mélanger avec des magmas primaires. Le transport du magma dans la croûte implique notamment un flux de magma à travers différentes fractures recoupant les roches encaissantes élastiques. Au cours de ce processus de migration, des cumulats de cristaux ou des agrégats de cristaux encore non-solidifiés, peuvent être recyclés et réactivés pour être transportés à des niveaux supérieures de la croûte. Le terrain d'étude est situé dans le massif d'Adamello. Celui-ci est composé de plusieurs plutons mis en place entre 42 et 29 millions d'années. Dans une phase tardive de l'activité magmatique liée à ce batholite, une série de filons de composition variable allant de picrobasalte à des compositions dacitiques s'est mise en place la partie sud du massif. Deux modèles sont proposés dans la littérature, pour expliquer la formation des magmas dacitiques caractérisés par des compositions peralumineux (i.e. à corindon normatif). Le premier modèle propose que ces magmas soient issus de la fusion de matériel mafique et pélitique présent dans la partie inférieur de la croûte, alors que le deuxième modèle suggère une évolution par cristallisation fractionnée à partir de liquides primaires issus du manteau. Un modèle de cristallisation fractionnée a pu être développé pour expliquer l'évolution des filons de l'Adamello. Ce modèle explique la formation des filons dacitiques par la cristallisation fractionnée de 17% olivine, 2% spinelle riche en Cr, 18% clinopyroxène, 41% amphibole, 4% plagioclase et 0.1% magnetite à partir de liquide de compositions picrobasaltiques. Ce modèle prend en considération les contraintes pétrologiques déduites de l'observation des différents filons ainsi que du champ de stabilité des différentes phases en fonction de la température. Ces roches montrent une évolution géochimique similaire aux données expérimentales simulant la cristallisation fractionnée de magmas évoluant à des niveaux inférieurs de la croûte (7-10 kbar). Le modèle montre, en particulier, le rôle prépondérant de l'amphibole, une phase qui contrôle en particulier le caractère peralumineux des magmas différentiés ainsi que leurs compositions en éléments en traces. Des phénomènes de fusion partielle de l'encaissant tonalitique lors de la mise en place de _lons mafiques sont observée sur le terrain. L'injection du magma basaltique chaud (~1050°C) sous forme de filons rapprochés augmente la surface du contact avec l'encaissante tonalitique. Une telle situation produit la fusion partielle des roches encaissantes nécessaire à l'incorporation d'enclaves mafiques observés au sein des tonalites. Pour comprendre les conditions nécessaires pour la fusion partielle des roches encaissantes, des modèles de conduction thermique pour une injection simultanée d'une série de filons ont été développées. Des données géochimiques sur les minéraux et sur les roches totales soulignent qu'au niveau supérieur de la croûte, l'assimilation sélective ou totale de l'encaissante tonalitique modifie la composition du liquide primaire pour les éléments lithophiles tel que le Sr, Ba et Rb. Un autre aspect important concernant la pétrologie des filons de l'Adamello est la présence d'enclaves felsiques dans les filons les plus primitifs. Ces enclaves montrent, en particulier, des textures proches de celles rencontrées dans des plagiogranites océaniques ou dans des roches métamorphiques de haute pression/basse température. Ces enclaves contiennent du zircon et du rutile. La datations de ces zircons à l'aide du géochronomètre U-Pb indique un âge Crétacé inférieur. Cet âge est important, car aucune roche de cet âge n'a été considérée comme un assimilant potentiel pour des roches magmatiques d'âge Eocène à Oligocène dans les Alpes Sud Centrales. La réparation spatiale des filons post-plutoniques dans des grands batholites tel que l'Adamello, est une caractéristique clé pour la compréhension des champs de contraintes lors du refroidissement du batholite. L'orientation des filons va, en particulier, indiqué la contrainte minimal au sein des roches encaissante. La mise en place de la série de filon recoupant la partie Sud du massif de l'Adamello est associée à un décrochement senestre, un décrochement que l'on peut lié aux contraintes tectoniques régionales auxquelles s'ajoutent l'effet de la dilatation produite par la mise en place du batholite lui-même. Ce décrochement senestre produit une segmentation en échelon des filons.

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder ('melt layer') is overlain by a 'peridotite layer' and a layer of vitreous carbon spheres ('melt trap'). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170-1,290 degrees C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T (s) similar to 1,260 degrees C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290A degrees C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr(2)O(3) at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260 degrees C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si 'low-pressure' signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.

Juxtaposition of melt impregnation and high-temperature shear zones in the upper mantle; field and petrological constraints from the Lanzo peridotite (Northern Italy)

Results of a field and microstructural study between the northern and the central bodies of the Lanzo plagioclase peridotite massif (NW Italy) indicate that the spatial distribution of deformation is asymmetric across kilometre-scale mantle shear zones. The southwestern part of the shear zone (footwall) shows a gradually increasing degree of deformation from porphyroclastic peridotites to mylonite, whereas the northeastern part (hanging wall) quickly grades into weakly deformed peridotites. Discordant gabbroic and basaltic dykes are asymmetrically distributed and far more abundant in the footwall of the shear zone. The porphyroclastic peridotite displays porphyroclastic zones and domains of igneous crystallization whereas mylonites are characterized by elongated porphyroclasts, embedded between fine-grained, polycrystalline bands of olivine, plagioclase, clinopyroxene, orthopyroxene, spinel, rare titanian pargasite, and domains of recrystallized olivine. Two types of melt impregnation textures have been found: (1) clinopyroxene porphyroclasts incongruently reacted with migrating melt to form orthopyroxene plagioclase; (2) olivine porphyroclasts are partially replaced by interstitial orthopyroxene. The meltrock reaction textures tend to disappear in the mylonites, indicating that deformation in the mylonite continued under subsolidus conditions. The pyroxene chemistry is correlated with grain size. High-Al pyroxene cores indicate high temperatures (11001030C), whereas low-Al neoblasts display lower final equilibration temperatures (860C). The spinel Cr-number [molar Cr/(Cr Al)] and TiO2 concentrations show extreme variability covering almost the entire range known from abyssal peridotites. The spinel compositions of porphyroclastic peridotites from the central body are more variable than spinel from mylonite, mylonite with ultra-mylonite bands, and porphyroclastic rocks of the northern body. The spinel compositions probably indicate disequilibrium and would favour rapid cooling, and a faster exhumation of the central peridotite body, relative to the northern one. Our results indicate that melt migration and high-temperature deformation are juxtaposed both in time and space. Meltrock reaction may have caused grain-size reduction, which in turn led to localization of deformation. It is likely that melt-lubricated, actively deforming peridotites acted as melt focusing zones, with permeabilities higher than the surrounding, less deformed peridotites. Later, under subsolidus conditions, pinning in polycrystalline bands in the mylonites inhibited substantial grain growth and led to permanent weak zones in the upper mantle peridotite, with a permeability that is lower than in the weakly deformed peridotites. Such an inversion in permeability might explain why actively deforming, fine-grained peridotite mylonite acted as a permeability barrier and why ascending mafic melts might terminate and crystallize as gabbros along actively deforming shear zones. Melt-lubricated mantle shear zones provide a mechanism for explaining the discontinuous distribution of gabbros in oceancontinent transition zones, oceanic core complexes and ultraslow-spreading ridges.

Evidence for middle Cretaceous accretion at Santa Elena Peninsula (Santa Rosa Accretionary Complex), Costa Rica

An oceanic assemblage of alkaline basalts, radiolarites and polymictic breccias forms the tectonic substratum of the Santa Elena Nappe, which is constituted by extensive outcrops of ultramafic and mafic rocks of the Santa Elena Peninsula (NW Costa Rica). The undulating basal contact of this nappe defines several half-windows along the south shores of the Santa Elena Peninsula. Lithologically it is constituted by vesicular pillowed and massive alkaline basaltic flows, alkaline sills, ribbon-bedded and knobby radiolarites, muddy tuffaceous and detrital turbidites, debris flows and polymictic breccias and megabreccias. Sediments and basalt flows show predominant subvertical dips and occur in packages separated by roughly bed-parallel thrust planes. Individual packages reveal a coherent internal stratigraphy that records younging to the east in all packages and shows rapid coarsening upwards of the detrital facies. Alkaline basalt flows, pillow breccias and sills within radiolarite successions are genetically related to a mid-Cretaceous submarine seamount. Detrital sedimentary facies range form distal turbidites to proximal debris flows and culminate in megabreccias related to collapse and mass wasting in an accretionary prism. According to radiolarian dating, bedded radiolarites and soft-sediment- deformed clasts in the megabreccias formed in a short, late Aptian to Cenomanian time interval. Middle Jurassic to Lower Cretaceous radiolarian ages are found in clasts and blocks reworked from an older oceanic basement. We conclude that the oceanic assemblage beneath the Santa Elena Nappe does not represent a continuous stratigraphic succession. It is a pile of individual thrust sheets constituting an accretionary sequence, where intrusion and extrusion of alkaline basalts, sedimentation of radiolarites, turbidites and trench fill chaotic sediments occurred during the Aptian-Cenomanian. These thrust sheets formed shortly before the off-scraping and accretion of the complex. Here we define the Santa Rosa Accretionary Complex and propose a new hypothesis not considered in former interpretations. This hypothesis would be the basis for further research.