Showing papers in "Ofioliti in 2006"

Geochemistry, tectono-magmatic discrimination and radiolarian ages of basic extrusives within the izmir-ankara suture belt (nw turkey): time constraints for the neotethyan evolution

Abstract: The Dagkuplu Melange in the Central Sakarya Valley represents the northernmost outcrops of the Izmir-Ankara Suture Belt in northwest Anatolia. In addition to blocks and slivers of serpentinite, gabbro, blueschist, neritic and pelagic limestones, it includes blocks of basic volcanic rocks associated with radiolarian cherts, pelagic carbonates and mudstones. The preliminary geochemical data revealed the existence of a variety of basaltic rocks with magma types ranging in composition of MORB, IAT, OIB and CAB, in the melange. The age of the radiolarian assemblage from a tectonic block of chert-mudstone alternation associated with OIB-type basalts within the melange is assigned to early Berriasian - early Hauterivian, based on the co-occurrence of radiolarian taxa as Angulobracchia sp. cf. A. (?) portmanni, Godia nodocentrum, Pantanellium masirahense, Thanarla brouweri, Pseudoeucyrtis hanni, Svinitzium mizutanii, Mirifusus dianae s.l., Tethysetta boesii. Another block of chert-mudstone alternation associated with MORB-type basalts includes the following Cenomanian radiolarian fauna: Thanarla pulchra, Novixitus mclaughlini, Pseudodictyomitra pseudomacrocephala, Pseudodictyomitra tiara, Stichomitra communis. New findings from the Central Sakarya area combined with previous data of the authors reveal that the Izmir-Ankara Ocean started to open already in the Late Triassic. The formation of OIB-type intra-plate seamounts within the Izmir-Ankara Ocean began in late Bathonian and persisted until early Aptian. The the intra-oceanic subduction and the generation of supra-subduction-type volcanism started in early Santonian and the spreading-ridge of the Izmir-Ankara Ocean plate was not subducted until the Cenomanian.

Occurrence and significance of serpentinite-hosted, talc- and amphibole-rich fault rocks in modern oceanic settings and ophiolite complexes: an overview

Abstract: This paper reviews the occurrence and significance of talc- and amphibole-rich fault rocks developed in mafic-ultramafic sequences and evaluates their role in deformation and alteration of the oceanic lithosphere from different tectonic settings (from spreading mid-ocean ridges up to orogenic belts) Recently, talc and amphibole-rich fault rocks have been sampled and studied from detachment fault surfaces along slow and ultra-slow spreading mid-ocean ridges, and constraining the conditions of deformation and strain localization during the evolution of oceanic core complexes These rocks are documented not only in oceanic core complexes, but also in other oceanic fracture zones where ultramafic rocks are exposed on the seafloor, while only few occurrences have been reported in ophiolite sequences Samples recovered in situ in oceanic settings record heterogeneous deformation (crystal-plastic to cataclastic) under greenschist- facies conditions and are commonly restricted to localized shear zones (< 200 m) and are associated with intense talc-amphibole metasomatism The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of such fault zones and may enhance unroofing of upper mantle peridotites and lower crustal gabbroic rocks during seafloor spreading Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation along such faults The rheology of these rocks, and its evolution during dehydration reactions could play an important role also in subduction-zone processes and during the formation of ultramafic orogenic belts Here, we review the occurrence and significance of talc- and amphibole-rich fault rocks in different tectonic settings on the seafloor and evaluate their role in deformation and alteration of the oceanic lithosphere

Triassic morb magmatism in the southern mirdita zone (albania)

Abstract: In southern Albania the Mirdita Ophiolitic nappe is characterized by subophiolitic complexes in which remnants of volcanic ophiolite sequences of Triassic age have been identified, either as rare blocks of variable dimension in the Rubik Complex, or as a thin tectonic unit (the Porava Unit), sited immediately under the main ophiolitic masses of the Eastern Ophiolite Belt. In this paper the results of petrological investigations on basalts and biostratigraphical studies on associated radiolarian cherts included in these subophiolitic complexes units are presented. Biostratigraphical investigations indicate that cherts have ages ranging from Middle to Late Triassic. The associated basalts are represented by both high- Ti mid-ocean ridge basalts (MORB) and alkaline ocean island basalts (OIB). MORB rocks mainly consist of basalts and ferrobasalts with a mild enrichment in low field strength elements and flat rare earth element patterns and, subordinately, by basalts strongly depleted in incompatible elements and light rare earth elements. The chemistry of slightly enriched MORB is consistent with a generation in a mid-ocean ridge setting, from somewhat enriched sub-oceanic mantle source(s), whereas depleted MORB generated from a primitive MOR-type mantle source. The OIB rocks imply a generation in a within-plate oceanic setting from a mantle source enriched by plume chemical components. Basalts and associated cherts from southern Albania subophiolitic melanges represent remnants of a Triassic oceanic lithosphere, which testify for the existence, from northern Albania to southern Greece, of a Middle to Late Triassic oceanic basin located between the Adria and Eurasia plates. The occurrence in the Rubik Complex and Porava Unit of MOR basalts generated from differently enriched sources, as well as of alkaline OIBs, suggests that the early stage of oceanic spreading was variably associated with a plume activity.

Petrogenesis of mantle peridotites from the izu-bonin-mariana (ibm) forearc

Abstract: Serpentinised spinel harzburgites to orthopyroxene-rich spinel dunites recovered during the Ocean Drilling Program (ODP) Leg 195 on top of the South Chamorro Seamount (southern sector of the Mariana forearc, West Pacific Ocean), along with additional spinel harzburgites from Conical and Torishima Seamounts (northern Mariana and Izu-Bonin forearc, respectively), previously collected during the ODP Leg 125, have been investigated to shed light on the nature and evolution of forearc mantle in the intra-oceanic supra-subduction environment. All the samples show a marked heterogeneity in terms of petrographic, mineralogical and geochemical features that suggests a complex, multistage evolution involving, at variable extent, partial melting, reactive porous flow melt migration and subsolidus metamorphic re-equilibration under decreasing T and open system conditions. Geochemical evidence of the interaction between peridotites and various melts/fluids is the ubiquitous enrichment in highly incompatible elements, such as Large Ion Lithophile Elements (LILE). As for the high-T evolution of these peridotites, a three-stages-model is proposed, involving: 1) a former depletion event, during which the IBM forearc peridotites experienced 20-25% polybaric fractional melting during adiabatic upwelling; 2) a second depletion event characterised by a marked impoverishment in modal orthopyroxene, related to the upraise migration of ultra-depleted melts; 3) a late interaction between a relatively small volume of residual melts and the refractory mantle sequence. Oxidation state of the mantle minerals meanly decreases from north (Torishima Seamount) to south (South Chamorro), according to significant different contributions coming from the subducted Pacific Plate. In particular, the absence of a marked oxidation in South Chamorro peridotites suggests that the percolating melts during Stage 2 had not significant slab-derived component. This observation lead us to conclude that a thermal anomaly in the western Pacific mantle rather than the injection of hydrous components must be the “engine” determining the extreme depletion of the oceanic forearc peridotites and the arc formation. In this frame, it is proposed that IBM peridotites during Stage 1 underwent decompression partial melting and contributed to arc volcanism as actual mantle source. Successively, they were emplaced at relatively shallow levels (Stage 2), constituting the top of a strongly refractory mantle column and being percolated by melts produced by plumbing sources of the arc volcanism.

Chlorite/smectite - alkali feldspar metasomatic xenoliths from hyblean miocenic diatremes (sicily, italy): evidence for early interaction between hydrothermal brines and ultramafic/mafic rocks at crustal levels

Abstract: Upper-Miocene tuff-breccia pipes from the Hyblean area (south-eastern Sicily) bear a number of ultramafic and mafic xenoliths (spinel-peridotites, pyroxenites, gabbroic rocks) whose study has led to conclude that the Meso-Caenozoic carbonatic and volcanic succession stands upon a fossil oceanic core-complex, probably connected with the adjacent Ionian crust. Evidence for hydrothermal metasomatism is common in the Hyblean ultramafic and mafic xenoliths. In some cases, extreme metasomatic transformation and shearing have deleted original mineralogy and textures yielding unusual rocks, hereafter called “metasomatites”. These rocks generally exhibit cataclastic texture, their hydrothermal mineral assemblage consisting of Na-rich alkali feldspar, chlorite/smectite (C/S) and/or smectite/illite (S/I) mixed layers, Fe-Ti oxide/ hydroxide ± sieve-textured aegirine-augite ± titanite ± zircon. In addition, relics of Fo91 olivine, probably related to peridotite protoliths, rarely occur in these rocks. Comparing metasomatite whole chemistry to typical Hyblean oxide-gabbro, overall gain in H2O, alkalis, HFSE, U, Th, HREE and significant loss in Ca and Mg are remarkable. The studied xenoliths probably represent fragments of deep-seated, mafic/ultramafic fault-breccia, recording the effects of longlasting hypersaline hydrous fluids circulation in the Hyblean crustal basement.

Petrology and tectonic significance of greenschists from the medvednica mts. (sava unit, nw croatia)

Abstract: The Lower Cretaceous (123-115 Ma) low-grade metamorphic complex of the Medvednica Mts. is located within the southwestern edge of the Mid-Trans- Danubian shear zone. The SW-NE striking complex consists of metabasites overlain by metasediments, both affected by a single metamorphic event. Metamorphic parageneses and mineral compositions of the metabasites indicate greenschist facies conditions in the SE part and upper greenschist facies conditions in the NW one. Temperature estimates based on chlorite thermometry are consistent with temperatures derived from petrogenetic grids. Chemical compositions of the metabasites are likely controlled by pre- and syn-metamorphic modifications. Nevertheless, “immobile” element characteristics (HFSE, REE) indicate a subalkaline nature of the igneous precursors. Most probably, the protoliths were subduction-related low-K tholeiitic basalts. Structural analysis suggests that the greenschists of the Medvednica Mts. were formed during southwest-vergent obduction of Jurassic to Lower Cretaceous ophiolites onto the Dinaridic continental margin composed of Ordovician to Upper Triassic sedimentary and volcano-sedimentary successions. The petrological and geochemical investigations thus indicate that an island arc was involved in the obduction that formed the Medvednica Mts. low-grade metamorphic complex.

Microstructural features of a subaqueous lava from basaltic crust off the east pacific rise (odp site 1256 , cocos plate)

Abstract: This work examines a massive basaltic lava emplaced in a subaqueous environment and drilled at ODP Site 1256. Site 1256 was drilled on the eastern flank of the East Pacific Rise during ODP Leg 206 (6°44N, 91°56W; Guatemala Basin), located in 15 Ma old oceanic crust created by superfast seafloor spreading (ca. 220 mm/yr). The massive lava lies between thin sheet flows and caps pillow lavas, sheet flows with minor hyaloclastite, breccia and dikes. The massive basalt was encountered in two holes, 1256C and 1256D, which are 30 m apart, and has a thickness of 35 m in Hole 1256C and 75 m in Hole 1256D. It can be interpreted as a ponded lava, originated by rapidly erupted lava accumulated in a off axis >3-5 km depression of a steep paleotopography (Teagle et al., 2004). The Hole 1256 lava pond has been divided into distinct petrographic units and structural units. Five main structural units with different key flow-related textures and syn-magmatic or late magmatic structures were recognized. Ductile and brittle-ductile structures attributed to the flow gives constraints about the emplacement mechanism of the lava and possibly seafloor topography. Unusual textural features related to the flow kinematics were recognized mainly in the top and in the bottom parts of the lava pond, whereas brittle-ductile and brittle deformations occur throughout the whole ponded body. Microstructures in the base may be interpreted as flow-related deformation of hot ductile coalesced spatter clasts erupted during the first stages of emplacement. Alternatively, they may have been formed during lava drain-back, in the final emplacement stages.

Petrology of mantle xenoliths from megado and dilo; kenya rift; southern ethiopia

Abstract: A study has been made of ultramafic xenoliths embedded in the Quaternary basanitic lavas of Dilo and Megado areas; southern Ethiopia. This is the first time that xenoliths have been found at Dilo. In both areas; the xenoliths are spinel lherzolites and pyroxenites; garnet- and/or spinel-bearing pyroxenites occasionally containing amphibole. Temperature/pressure estimates indicate that the lherzolites and most of the pyroxenites last equilibrated in the upper mantle in similar P-T conditions (T ~ 950-1050°C at P ~ 1.3-1.7 GPa); whereas some spinel pyroxenites equilibrated at higher temperatures (~ 1150-1250°C). Based on mineral assemblage; four groups of pyroxenites were distinguished; two of which exhibit spectacular garnet coronas around spinels. Whole-rock and mineral composition (major and trace elements) indicates that garnet-free pyroxenites segregated from OIB-type melt. Successively; melt evolution caused garnet crystallisation (so that garnet bearing pyroxenites formed) and; eventually; pargasitic amphibole caused the generation of garnet- and amphibole- bearing pyroxenites with the consequent LILE-enrichment of residual melts. Melt evolution resembles that hypothesised by Bedini et al. (1997) to explain metasomatism of lherzolites based on reactive porous flow simulations. Thus; pyroxenites may represent the “counter-part” of the melts responsible of the metasomatism in this area. Furthermore; the OIB nature of parental permeating melts suggests that metasomatism (and pyroxenite genesis) was related to Quaternary magmatic activity responsible for the xenoliths ascent to the surface.

Radiolarian biostratigraphic evidence for a late jurassic age of the el tambor group ophiolites (guatemala)

Abstract: We present a radiolarian biostratigraphic study of the metacherts of the El Tambor Group ophiolites (South Motagua Unit), Guatemala. The ophiolite sequence comprises MOR pillow metabasalts, massive metabasalts, metacherts and micaschists. The age of the studied metacherts is referable to the Late Jurassic (Oxfordian - Kimmeridgian). The radiolarian assemblage described in this paper is the first Jurassic finding in the ophiolitic MOR succession of the Motagua zone and represents a valuable tool to constrain the geodynamic evolution of the Caribbean area. A review of the ages of Jurassic rocks associated with the ophiolites from the Caribbean area is also reported.

Geochemistry and tectono-magmatic significance of hp/lt metaophiolites of the attic-cycladic zone in the lavrion area (attica, greece)

Abstract: The Lavrion area corresponds to the northwestern end of the Attic-Cycladic Complex and mainly consists of metamorphic rocks formed during the Eocene high-pressure/low-temperature (HP/LT) event and the Upper Oligocene-Lower Miocene medium-pressure metamorphic event. These metamorphic rocks are found in two superimposed tectonic units: the Kamariza Unit, which includes metavolcanic rocks and the overlying Lavrion blueschist Unit, which is largely represented by metaophiolites. Protoliths of metavolcanic rocks in the Kamariza Unit are calc-alkaline basalts displaying a marked enrichment in Th, U, and LREE and depletion in Ta, Nb, Hf and Ti, which point to a genesis from a depleted mantle source further enriched by subduction components. The Lavrion blueschist Unit mainly includes metavolcanic rocks with tholeiitic composition, as well as subordinate metagranites and metavolcanic rocks with calc-alkaline affinity. The tholeiitic metavolcanic rocks are mainly represented by enriched-type (E-) mid-ocean ridge basalts (MORB) and subordinately by normal-type (N-) MORB. E-MORB chemistry implies a genesis from a depleted asthenospheric source modified by an OIB component, whereas the N-MORB has chemical features typical for rocks generated in a mid-ocean ridge setting from a primitive asthenospheric source. Previous works suggested that the magmatic protoliths of similar HP/LT metamorphic rocks from elsewhere in the Cyclades reflect an arc-back-arc tectonic setting, which developed during the Cretaceous closure of the Pindos oceanic basin. However, recent geological studies have shown that the Lavrion metamorphic Units, unlike similar units from the Cycladic zone, represent Triassic Pelagonian sequences metamorphosed under HP/LT conditions typical of the Cycladic zone. The geochemical and petrological characteristics of the Lavrion metamorphic rocks support this conclusion. In particular, calc-alkaline protoliths display many similarities with the Triassic calc-alkaline rocks associated with the rift of Gondwana, whereas MORB-type protoliths are similar to the Triassic MORB found in the Subpelagonian ophiolitic melanges. The magmatic protoliths of the Lavrion HP/LT metamorphic rocks are thus compatible with a paleotectonic evolution which encompasses the Triassic continental rift, followed by the early oceanization stage of the Pindos ocean, and emplaced on the border between the Pelagonian continental margin and the Pindos basin. These rocks were probably included into melanges during the Jurassic closure of the Pindos basin, and finally they were involved in the Eocene and Upper Oligocene-Lower Miocene metamorphic events that affected the Cycladic zone.

Petrology of gabbroic to plagiogranitic rocks from southern tuscany (italy): evidence for magmatic differentation in an ophiolitic sequence

Abstract: This work deals with the petrogenesis of a composite ophiolite body composed of gabbroic to plagiogranitic rocks. In particular; the sequence consists of evolved gabbros; Fe-gabbros and Fe-diorites crosscut by ferrobasaltic dikes and intruded by dikelets and lenses of plagiogranites; mainly represented by diorites and trondhjemites. Complex relationships among the different lithotypes indicate multiple melt injections into both crystal-mush and solidified rocks. Whole-rock compositions show continuously increasing FeO/(FeO + MgO) and increasing REE; Th; Nb; Y; LaN/SmN and Eu depletion from gabbros to plagiogranites. Major and trace element variations and modelling show that the whole intrusive sequence originated from the differentiation of a ferrobasaltic magma reservoir. Ferrobasaltic liquid compositions are compatible with fractional crystallization of N-MOR type basalts through olivine + plagioclase + clinopyroxene removal; whereas the evolved gabbros represent clinopyroxene + plagioclase cumulates from ferrobasaltic liquids with small amounts of trapped melt. Fegabbroic to Fe-dioritic rocks; characterized by abundant Fe-Ti oxides + apatite as liquidus phases; likely represent cumulates from evolved liquids; possibly segregated from the gabbroic crystal-mush. The attainment of Fe-Ti oxide saturation promoted silica enrichment yielding residual liquids of dioritic composition which were segregated and injected into fractures of solidified gabbroic rocks or infiltrated into the Fe-gabbroic crystal-mush. Locally; silica-rich and slightly LREE-enriched liquids of trondhjemite composition were produced through fractionation of plagioclase + clinopyroxene + amphibole + Fe-Ti oxides (+ apatite + zircon + allanite).

Odp-iodp site 1256 (east pacific rise): an in-situ section of upper ocean crust formed at a superfast spreading rate

Abstract: The igneous rocks cored in Holes 1256C and 1256D during Ocean Drilling Program (ODP) Leg 206 and Integrated Ocean Drilling Project (IODP) Expedition 309 are dominated by thin (10’s of cm to 3 m) basaltic sheet flows separated by chilled margins, with several massive flows (> 3 m thick), minor pillow basalt and hyaloclastites, and rare small dikes. In Hole 1256D, the percentage of massive flows decreases downhole. One notable feature of both holes cored is the presence of a very thick massive lava (~35 m thick in Hole 1256C and ~75 m thick in Hole 1256D) near the top of each hole. The structural aspect of this thick massive unit are here described. It has been interpreted as a lava pond emplaced in a topographic depression. Although the ocean crust drilled at Hole 1256C and Hole 1256D partly fits the “Penrose” model for showing the superposition of volcanics on sheeted dikes and intrusives, the relative thickness of the lava-dike sequence could reflect a combination of two or more of the following processes: local (spatial) heterogeneity along ridge-axis, (temporal) variability, essentially linked to a more or less intense activity of the magma chamber, and off-axis eruptions.

Petrogenesis of eocene oceanic basalts from the west philippine basin and oligocene arc volcanics from the palau-kyushu ridge drilled at 20°n, 135°e (western pacific ocean)

Abstract: The West Philippine Basin (WPB) is a back-arc basin that opened within the Philippine Sea Plate (PSP) between the current position of the Palau-Kyushu Ridge (PKR) and the margin of East Asia. Spreading occurred at the Central Basin Fault (CBF) mainly from 54 until 30 Ma. The PKR was active since ~48 to 35 Ma constituting a single volcanic arc with the Izu-Bonin-Mariana (IBM) Arc. At ~42 Ma ago spreading rate and direction changed from NE-SW to N-S, stopping at ~30 Ma. A late phase of spreading and volcanism took place between 30 and 26 Ma. ODP Leg 195 Site 1201 is located in the WPB, ~100 km west of the PKR, on 49 Ma crust formed by NE-SW spreading at the CBF. From ~35 to 30 Ma, pelagic sedimentation at Site 1201 was followed by turbidite sedimentation, fed mostly by arc-derived volcanic clasts. The geochemical and isotopic features of Site 1201 basement rocks, which represent Eocene WPB oceanic crust, compared with those of Site 1201 volcanics from the turbidite sequence, representing products of the early Mariana Arc (PKR), provide some insights into the early history of the IBM subduction factory. The WPB basement is made up of aphyric to porphyritic basalts with altered olivine, and preserved plagioclase, clinopyroxene and opaques. The PKR volcanics are porphyritic basalts and andesites with plagioclase, clino- and orthopyroxene, hornblende, alkali feldspar and opaques. Variable textures, and degree of alteration suggesting zeolite facies metamorphic grade, characterize both groups of rocks. The mineralogical and geochemical characteristics of the investigated Site 1201 PKR volcanics highlight their calc-alkaline affinity. This feature is at variance with both other PKR rocks, having mostly boninitic and arc tholeiitic affinity, and WPB basement basalt, having tholeiitic affinity, with some characters transitional to arc-like, as expected for a back-arc basin. New Sr and Nd isotope data, coupled with published Sr, Nd, Pb and Hf isotope data (Savov et al., 2006), highlight the Indian Ocean MORB-like character of Site 1201 basement basalts. This suggests that WPB volcanism tapped an upper mantle domain distinct from that underlying the Pacific Plate. The isotopic features of Site 1201 PKR volcanics are more enriched relative to those of basement basalts reflecting higher amounts of subduction-derived component(s) in the source of arc magmas. Th-Nb relationships and isotope geochemistry of the WPB basement and overlaying arc volcanics suggest addition of subducted sediment mostly as siliceous melts, to the mantle source of the arc volcanics. In that respect, Site 1201 PKR volcanics resemble calc-alkaline volcanics of the currently active Mariana Arc. In addition, the calc-alkaline affinity, unradiogenic neodymium, and inferred Middle Oligocene age of PKR volcanics, suggest they might represent an evolved stage of arc volcanism at Palau-Kyushu Ridge, perhaps shortly before the end of its activity.

Shallow geological structures of the south shetland trench, antarctic peninsula

Abstract: In order to study the differential passive subduction among the three oceanic segments of the Phoenix Plate remnant, the R/V OGS-Explora acquired new geophysical data in the South Shetland Trench (offshore to the Antarctic Peninsula). An integrated geophysical dataset was acquired in the trench across the incoming oceanic plate segmented by the “D” and “E” oceanic Fracture Zones, and along the frontal part of the Antarctic Peninsula accretionary prism. We present here a high resolution multibeam bathymetry map, Chirp sub-bottom profiles and marine magnetic data to study changes in the deformational style along strike and to better understand features and processes of passive subduction. The new multibeam image and Chirp data display in detail shallow structures affecting the seafloor as a response to crustal deformation. Normal faults, bounding horst and graben structures cutting the incoming plate, are detected and characterized by variable orientation with respect to the inferred “D” and “E” Fracture Zones. Normal faults, width and depth of the trench and morphology of the frontal prism are related to bending and roll-back of the Phoenix Plate, and to inherited structural discontinuities. Some of these oblique faults offset the seafloor, with variable throws and with local dip changes. The orientation of the oblique faults, the different depth of the three oceanic segments and narrowing and deepening of the trench indicate that differential coupling between the Hero, Shackleton, “E” and “D” Fracture Zones may locally modify the regional stress field orientation and the sinking of each oceanic segment, during bending and roll-back.

The antrona nappe: lithostratigraphy and metamorphic evolution of ophiolites in the antrona valley (pennine alps)

Abstract: The Antrona ophiolite (western Central Alps) represents a tectonic fragment of the oceanic lithosphere of the Upper Jurassic - Lower Cretaceous Ligurian- Piedmont basin, a section of the Western Alpine Tethyan Ocean. The Antrona ophiolite occurs at lower structural levels in the Alpine nappe stack and is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The Monte Rosa Nappe is overlain by the Zermatt Saas ophiolitic Unit. Despite the tectono-metamorphic reworking, the Antrona ophiolite exhibits all typical lithologies of oceanic lithosphere: ultramafic and mafic plutonic rocks, mafic volcanic rocks and deep-sea sediments can be recognized. Several rock types were distinguished among mafic rocks. New findings and inferences are: i) the occurrence of probable relics of magmatic structures in some amphibolites, inferred to be pillow lavas or pillow breccia; ii) the occurrence of lawsonite pseudomorphs-bearing amphibolites, not described so far in the study area as precursor of the origin of epidote-amphibolites the Antrona Valley. A qualitative P-T diagram deduced from the stability conditions of mineral parageneses is presented and compared with published P-T paths for the Antrona and Zermatt-Saas ophiolites. The metamorphic evolution of the studied rocks is characterized by blueschist prograde path followed by high pressure (eclogitic) metamorphic peak. P-T estimates for the metamorphic peak were calculated by the Na-clinopyroxene garnet equilibria and the jadeite content in omphacite. T = 372°C for a nominal pressure of P = 1 GPa and T = 386°C for a nominal pressure of P = 1.5 Gpa were obtained. Jd30 as maximum Jadeite content suggests P > 1 Gpa. Retrograde path, although not well constrained, is dominated by epidote-amphibolite/ amphibolite facies conditions, in accord with published data, differing from those inferred so far for the overlying Zermatt-Saas ophiolite.

Age of the ottone unit in the zignago area (external ligurian units, northern apennines): constraints from calcareous nannofossils

Abstract: Quantitative analyses of the calcareous nannofossils collected by closely spaced sampling of the Ottone Unit in the Zignago area allow to refine the age of this succession and the timing of the main steps during its tectono-sedimentary evolution The Casanova Complex/Ottone Flysch boundary lies within the Ceratolithoides aculeus Zone (uppermost early Campanian) The sampled Ottone Flysch spans from the Ceratolithoides aculeus (uppermost early Campanian) to Quadrum gothicum Zone (early/late Campanian boundary) Sedimentation of the Mt Veri Complex, represented by mass-gravity deposits intercalated in the Ottone Flysch, ends within the Quadrum gothicum Zone (early/late Campanian boundary) Furthermore, based on our results and on the previous ages found in the Ottone Unit successions, the lower Campanian inception of the Helminthoid Flysch sedimentation is inferred to be isochronous for the western sector of the External Ligurian Domain The same data suggest that the end of syntectonic sedimentation of the ophiolite-bearing mass-gravity deposits occurred close the early/late Campanian boundary