Variscan dismembered metaophiolite suite fragments of Paleo-Tethys in Gemeric unit, Western Carpathians (original) (raw)
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Geologia Croatica
The Dinaridic segment of Neotethys was affected by a widespread shortening and related subduction-accretion-obduction processes that commenced in the middle Jurassic. In the Dinarides, the Krivaja-Konjuh Ophiolite Complex (KKOC) stands as the largest ophiolite complex with a well-exposed metamorphic sole which is the key to understanding the dynamics of intraoceanic subduction initiation in this part of Neotethys. In this contribution we present Sm-Nd geochronology on a granulite facies amphibolite from the KKOC, as well as a detailed petrological description. A five-point isochrone age calculated from clinopyroxene, plagioclase, garnet, amphibole and whole rock is 162 ± 14 Ma (MSWD = 6.2), whereas garnet and whole rock yield 160 ± 7 Ma. Ages calculated from all data points except clinopyroxene are 162 ± 5 Ma (MSWD = 1.09). Petrographic investigations suggest that these ages date granulite facies metamorphic conditions (i.e. peak metamorphism of Grt-Cpx amphibolite) rather than post-peak exhumation or obduction processes. Phase textural relationships are in line with previous research, which indicated a peak metamorphism equilibration pressure and temperature of ~1 GPa and ~800 °C, respectively. Granulite facies conditions are elucidated for an igneous precursor, which underwent a multi-stage metamorphism that gave rise to recrystallization of igneous clinopyroxene and plagioclase, epitaxial growth of amphibole, and garnet blastosis. Taking into account the age of gabbronorite from the youngest segment of the KKOC oceanic crust (Taorcian to Bathonian) and ages of radiolarian assemblages from the KKOC mélange (Bajocian to Bathonian), it may be inferred that within ~25 Ma the Dinaridic segment of Neotethys evolved rapidly from active ridge spreading through a stage of intraoceanic subduction and arc magmatism toward sub-ophiolite exhumation and further obduction along the Adria passive margin at the end of the Jurassic era.
2019
High-grade metamorphic soles in NE Bosnia and Herzegovina make part of the Krivaja-Konjuh ophiolite complex (KKOC), which is one of the most important constituents of the Jurassic ophiolite melange of the Central Dinarides. Several rock types were distinguished within the investigated metamorphic suite - sapphirine and corundum amphibolites, garnet-clinopyroxene±orthopyroxene amphibolites, clinopyroxene±garnet amphibolites, amphibolites per se and clinopyroxene+plagoclase±garnet gneisses.Peak temperature and pressure conditions calculated from different mineral pairs were estimated to range between 850 and 1100°C at 1.1 to 1.3 GPa. A prograde multi-stage metamorphic history of analyzed rocks was followed by the post-peak relaxation witnessed in decomposing porphyroblasts of garnet and ubiquitous formation of amphibole and orthopyroxene rims around clinopyroxene. The whole-rock chemical composition of magmatic protoliths largely defines it as cumulates from supra-subduction zones and...
Lithos, 2014
Orogenic garnet peridotites of diverse origins and histories in the Bohemian Massif attest to a variety of mantle processes, including partial melting, cryptic metasomatism, and modal metasomatism (refertilization), all of which are recorded by Saxothuringian garnet peridotite from the T-7 borehole in northern Bohemia. The T-7 peridotite consists of interlayered garnet lherzolite, harzburgite, and phlogopite-garnet pyroxenite lenses that yield peak temperatures and pressures of 1030-1150°C and 36.1-48.0 kbar. Olivine crystallographic preferred orientations exhibit [axial](010) slip, corresponding to a pure shear component of deformation under relatively low flow stress conditions. Some lherzolite samples are fertile, resembling primitive mantle in major and trace element composition, but other lherzolites are slightly depleted in incompatible major elements, HREE, and HFSE, and slightly enriched in LREE. Harzburgite is depleted in incompatible major elements, HREE, and HFSE, but enriched in LREE. Harzburgite adjacent to pyroxenite has been refertilized, containing phlogopite, less olivine, more orthopyroxene, and more garnet than distal harzburgite. The T-7 peridotite compositions are the result of variable degrees of partial melting in the spinel stability field, followed by cryptic metasomatism and modal metasomatism by transient basaltic melts in the garnet field. Trace elements, Sr and Nd isotopes, and occurrence of phlogopite reflect a subduction component in the metasomatising melts. Partial melting of the T-7 peridotite was a Proterozoic event, as indicated by Rhenium depletion model ages (T RD ); the age of cryptic and modal metasomatism is unconstrained, but is thought to be related to Variscan subduction and amalgamation of the Bohemian Massif.
Geologica Carpathica
The complex of Ky-Grt paragneisses, granitic to tonalitic orthogneisses, migmatitic gneisses and homogeneous to layered amphibolites was intruded by dioritic-gabbroic more or less concordant dykes and sills during the synmetamorphic extension in the host regional metamorphic rocks. They now have the character of dioritic orthogneisses. The whole lithological sequence was then structurally unified during the late-metamorphic cooling and exhumation within the deep-crustal shear zone. The dioritic orthogneisses bear the features of pre-metamorphic cornpositional layering into cuntulate-like hornblendite(tPx), gabbro-diorite, tonalite to trondhjemite. Magmatic layering has been transfbrmed to subsolidus high-temperature layering along the extensional meso-shear bands filled with leucotonalitic melt seggregates crosscutting fbrmer magmatic structures at acute to medium angles. A new ductile strain relocalization enhanced mechanical diff-erentiation of dark and f'elsic minerals into often isoclinally folded layers, with the crystallographic preferred orientation fabrics of amphibole, plagioclase and quartz indicating a layer parallel shear. The U-Pb zircon age of porphyric metadiorites 346 t I Ma (Early Carboniferous) is related to their magmatic emplacement, immediately fbllowed by the high-temperature mylonitization. We also dated zircon of metatrondhjemitic orthogneiss from the layered amphibolite. An upper intercept age 514t24 Ma (Late Cambrian) is interpreted as dating the magmatic compositional differentiation of a gabbro-dioritic complex into cumulate-like hornblenditic, gabbrodioritic, tonalitic to trondhjemitic layers, which is an inherited feature in layered amphibolites (with blastic textures). The lower intercept age 348131 Ma (Early Carboniferous) obviously reflects the time of regional-metamorphic event and fbnnation of layered amphibolites. The whole composite (VZP-CB) structural complex is crosscut by plagiogranite-aplitic veins dated 233t4 Ma (Early Triassic).
Geologica Carpathica
The paper presents whole rock chemical composition and Sr-Nd isotope data from selected metabasic rocks from the Mississippian and Pennsylvanian sequences of the Northern Gemeric Unit. The analysed metabasic rocks belong to the subalkaline magmatic series with Nb/Y ratios ranging from 0.03 to 0.21. They fit into the low-Ti tholeiitic series, characterized by TiO 2 contents of less than 2.5 wt. % and Ti/Y ratios below 500. Petrological and chemical signatures show the separation of the Group I (Pennsylvanian) from the Group II (Mississippian) metabasalts, which is supported by biostratigraphical data of the surrounding metasediments. The Group I metabasalts display higher contents of Zr, Th, Rb and U, Pb, Zn, Ni compared to the metabasalts of Group II and conversely lower contents of Nb, Ta and V. The chondrite normalized rare earth elements curves show a uniform pattern, with rare earth element enrichment and no or weak positive or negative Eu anomalies (0.88-1.23 vs. 0.89-1.17). The (Tb/Yb) N ratios from 1.36-1.62 in the Group I or 0.92-1.55 in Group II are symptomatic of spinel-bearing peridotite mantle source. Based on trace and rare earth element distribution patterns, the Group I metabasites correspond to the N-MORB/E-MORB field and the Group II metabasites shift significantly towards the BABB and CAB fields. The Sr/Nd isotope systematics confirmed depleted mantle isotopic signatures, with minor influences from crustal sources and affected by fluid-related subduction metasomatism. All the studied samples have positive εNd (0) ranging from 7.92 to 8.68 for Group I and from 4.59 to 10.52 for Group II metabasalts. The 87 Sr/ 86 Sr (0) values vary between 0.7053-0.7081 and between 0.7052-0.7076, respectively, and 0.7109 for basaltic andesite.
Geologische Rundschau, 1996
Orthoamphibolites within and marginal to the Orlica-Ś nieżnik dome in the eastern Bohemian massif are associated with a series of Proterozoic-Lower Palaeozoic supracrustal meta-sediments of the Stronie, Nové Mě sto and Staré Mě sto formations. Massive and variably foliated amphibolites range from common epidote-plagioclase-blue-green/green hornblende varieties to assemblages with diopside, garnet and brown amphibole. All the amphibolites have a basaltic composition and are divided into three main chemical groups which are intimately associated in the field: main series tholeiites, low-Ti tholeiites and alkali basalts. All groups exhibit a common enrichment in LIL ele-mentscNbcTa (relative to high-field-strength elements) which reflects contamination by upper continental crust and/or pelagic sediment compositions. The gross chemistry of the tholeiites approaches that for MORB, although the geological environment and nature of crustal contamination suggests that the basaltic precursors were probably emplaced in a rifted ensialic basinal environment. If the amphibolites are representative of an early phase of the Lower Palaeozoic fragmentation of Gondwanaland, then rifting did not fully develop ocean crust. The wider significance of the chemical interpretation implies that some of the variation from Variscan Proterozoic-Palaeozoic metabasite suites that is ascribed to a complex of different eruptive settings could be accounted for by variable contamination and/or source composition.
Bulletin De La Societe Geologique De France, 2009
The paper reviews the main West-Carpathian Early Paleozoic metamorphosed originally sedimentary-magmatic complexes, dated by SHRIMP on zircons, as indicators of crustal extension and shortening events. Igneous precursors of a Layered Amphibolite Complex (LAC) -fractionated upper mantle gabbros to diorites, dated at 503 ± 4 and 492 ± 4 Ma from the North-Veporic, or 480 ± 5 and 450 ± 6 Ma from the Tatric basement are contemporaneous with subaluminous to metaluminous I-type (507 ± 4 Ma, the South-Veporic basement), peraluminous S-type (497 ± 4 Ma, the South-Veporic basement; 516 ± 7, 485 ± 6 and 462 ± 6 Ma, the North-Veporic basement; 497 ± 6, 472 ± 6 and 450 ± 6 Ma, the Tatric basement), alkaline A-type (511 ± 6 Ma, South-Veporic basement) granitic orthogneisses and calcalkaline rhyolitic (482 ± 6 Ma) and dacitic (476 ± 7 Ma) metavolcanics (Gemeric basement), indicating a magmatic immature back arc setting. The ages point to Middle/Late Cambrian, Early and Late Ordovician magmatic phases, coeval with the extension in the northern Gondwana margin. Separation of an inferred Avalonian and/or Galatian terranes distal continental ribbon corresponds with the opening of a Medio-European Basin. A 430-390 Ma dated MP/HP metamorphic event, recorded in the LAC and associated orthogneisses, occurred in the area of thinned immature back arc basin crust due to closure of the Medio-European Basin. Thus a distal Gondwana continental ribbon north of this basin could be an eastward lateral pendant of Armorica, derived from Galatian terrane. Metaophiolites of the Pernek Group (a metagabbrodolerite dated at 371 ± 4 Ma) in the Tatric basement, analogous to island-arc tholeiites and back-arc basin basalts, indicate a back-arc basin setting north of a 430-390 Ma old northward dipping subduction/collision zone, dividing the northward drifting western Galatian terrane microplate from the Gondwana margin. Some metabasites of the Gemeric basement might indicate Late Devonian to Mississippian opening of a peri-Gondwanan Paleotethyan oceanic basin: a 383 ± 3 Ma old remelted metagabbro (482 ± 9 Ma) from the Klátov gneiss-amphibolite complex, ca. 385 Ma old porphyritic metabasite of the Zlatník ophiolite complex, as well as a 350 ± 5 Ma old HP metabasite as tectonic fragment within the Rakovec Group. The closure of Devonian-Mississippian basins, accompanied by medium-pressure (the Pernek Group) to high-pressure (blueschist to eclogitic tectonic fragments in greenschist facies rocks of the Rakovec Group) metamorphism, occurred in late Carboniferous to early Permian, when Paleotethyan realm complexes accreted to a Galatian terrane microplate, the latter represented by the older and the higher-grade Tatric and Veporic basement complexes.
Saccani et al 2008b THE JURASSIC ASSOCIATION OF BACKARC BASIN OPHIOLITES
Saccani et al., 2008 - THE JURASSIC ASSOCIATION OF BACKARC BASIN OPHIOLITES AND CALC-ALKALINE VOLCANICS IN THE GUEVGUELI COMPLEX (NORTHERN GREECE): IMPLICATION FOR THE EVOLUTION OF THE VARDAR ZONE
subdivided into two distinct sub-units, that are the East and West Guevgueli, both including intrusive and volcanic sequences crosscut by several dykes. The Guevgueli Complex is intruded by the Fanos Granite and, together with this, is sandwiched, through a north-south striking thrust zone, between the Serbo- Macedonian Massif, to the east, and the Paikon Unit, to the west. Intrusive rocks are represented mainly by gabbros showing both cumulitic and isotropic textures and very subordinate ultramafic cumulates, Fe-gabbros, and diorites. Mineralogically and chemically, they are very similar to high-Ti, mid-ocean ridgetype gabbros, having high contents of Ti and V, and very low contents of incompatible elements. However, many isotropic gabbros show some subduction-related imprinting testified by Nb, Zr, and Ti relative depletion and light rare element (LREE) enrichment with respect to heavy rare earth elements (HREE). Incompatible elements and rare earth elements analyses carried out on volcanic, and subvolcanic rocks from both East and West Guevgueli indicate that two different rock-types can be identified within the volcanic sequences. They are: (1) calc-alkaline (CAB) rocks ranging from basalt to basaltic andesite, andesite, dacite, and rhyolite represented by lavas and dykes; (2) back-arc basin basalts (BABB) occurring as pillow lavas, as well as dykes crosscutting High-Ti gabbros and CAB volcanic series. CAB rocks show marked depletion in Nb, Ta Ti and enrichment in LREE and Th, which point out for their generation in a volcanic arc setting that can be most likely correlated with the development of the Paikon volcanic arc onto the Serbo-Macedonian continental realm. BABB rocks show many similarities with mid-ocean ridge basalts (MORB), such as marked Fe, Ti, V enrichment from less fractionated to fairly fractionated rocks, relatively high Ti, P, Y contents, and significant Nb depletion. However, they show higher Th/Ta and LREE/HREE ratios compared with MORBs. Such characteristics are commonly interpreted as a typical supra-subduction zone chemical imprinting, which can be correlated with the injection of a MORB-type mantle source into the sub-arc mantle wedge. In addition, BABB volcanic and subvolcanic rocks display co-magmatic relationships with the associated gabbros. So, the Guevgueli ophiolites can be regarded as representative of an ensialic back-arc basin located between the Paikon Volcanic arc and the continental margin of the Serbo-Macedonian Massif. In this framework, the close association in the Guevgueli Complex of coeval oceanic and continental arc magmatic rocks can be explained as the result of the opening of the back-arc basin mainly controlled by transtension leading to complex geometry of boundaries between the volcanic arc and the oceanic basin.
ABSTRACT The Late Ordovician–Devonian intrusive, volcaa nic, and volcanoosedimentary complexes of the Tagil paleooisland arc system constitute the synonymous meridional megablock extending for over 700 km along the eastern slope of the Middle and North Urals. In some recent publications, their authors mention preePaleozoic igneous and metamorphic rocks in diff ferent parts of the megablock, which likely constituted the basement of the Paleozoic island arc [2, 5, 7–9]. The formation of complexes constituting the Tagil structure on the preePaleozoic basement was also assumed, based on the geochemical properties of voll canics, in earlier works [3, 4, 6, 10, 11, and others]. Three groups of preePaleozoic rocks forming tectonic blocks and slices in the western and eastern parts of the Tagil megablock are definable in the Middle Urals: (1) dunite–clinopyroxenite–tylaite association [2, 9] and olivine gabbro [5, 7], which represent constituents of polyformation mafic–ultramafic massifs in the Platii nummbearing belt (PBB) of the Urals; (2) metamorr phosed gabbro of the ophiolite type [8]; (3) Belaya Gora gneiss–amphibolite complex defined in strucc tures surrounding the Kytlym pluton [7]. The available Sm–Nd isochron dates indicate that all these rocks are Vendian in age (542–574 Ma). Our investigations revealed the same three Vendian complexes that were studied and dated in the Tagil megablock of the North Urals. The aboveementioned rocks constitute isolated tectonic blocks or large xenoliths among Silurian gabb broids. Taking into consideration the data available in [7, 8], it may be concluded that preePaleozoic comm plexes are widespread in the basement of the Ordovii cian–Devonian Tagil paleooisland arc. When carrying out investigations in the Northern Urals, we received new data on the composition and age of rock associations constituting the basement of the Tagil paleooisland arc. In this large region, we have defined and dated all three aboveementioned Paleoo zoic rock associations: (1) olivine gabbro in the Yalpp ingn'er massif; (2) amphibolites and gneisses of the Belaya Gora Complex; and (3) amphibole–sossurite metagabbro of the ophiolite type that constitute screens in the complex of parallel dolerite dikes. Olivine gabbro forms an irregularly shaped block 1.8 × 3.5 km in size in structures surrounding amphibb ole gabbro in the northern part of the Yalpingn'er mass sif (Khuntyn'ya and Arbyn'ya river interfluve). Their contacts with host rocks are unobservable. Olivine gabbro are mediumm to coarseegrained locally banded gray–green rocks composed of anorthite (up to 50%), clinopyroxene (20–30%), olivine (up to 10–15%), orthopyroxene (up to 15–20%), pargasite, green spinel, VVbearing (1.1–1.2%) magnetite with ingrowths of Mn ilmenite (2.0–2.8%), saussurite, zoisite, epidote, and serpentine. Gneisses and amphibolites of the Belaya Gora Complex occur in tectonic slices in the zone of the Main Uralian Fault west of the Yalpingn'er massif (Fig. 1). The complex includes garnettbearing biotite–amphibole and bimicaceous gneisses, garnet– muscovite–quartz crystalline schists, and plagioo clase–hornblende amphibolites. The latter represent dark gray to gray finee to mediummgrained rocks comm posed of Mg hornblende and plagioclase. Gneisses are composed of almandine garnet, plagioclase (albite– oligoclase), amphibole (ferrochermakite, ferroparr gasite), biotites, phengite (Na 2 O, 1.9–2.1%), and quartz. Mineral parageneses imply that the temperaa tures and pressures of the formation of these metaa morphic rocks were 450–500°C and up to 8–9 kbar, respectively. Metamorphosed gabbro of the ophiolite type occurs in the form of lenticular blocks (screens) and small residual massifs among the complex of parallel dolerite dikes most widespread in the Vizhai and Ivdel