Early Cambrian back-arc volcanism in the western Taurides, Turkey: implications for rifting along the northern Gondwanan margin

2005 ◽  
Vol 142 (5) ◽  
pp. 617-631 ◽  
Author(s):  
S. GÜRSU ◽  
M. C. GÖNCÜOGLU
Keyword(s):  
Oceanic Lithosphere ◽  
Tholeiitic Basalt ◽  
Central Anatolia ◽  
Geochemical Data ◽  
Low Grade ◽  
Early Cambrian ◽  
Spinel Lherzolite ◽  
Mafic Rocks ◽  
Clastic Rocks ◽  
Back Arc

The Lower Cambrian (Tommotian) Gögebakan Formation in western Central Anatolia is made up of slightly metamorphosed continental to shallow marine clastic rocks with pillowed and massive spilitic lavas and dolerite dykes. Spilitic lavas, commonly amygdaloidal, are albite- and pyroxene-phyric with the metamorphic mineral paragenesis albite+calcite+sericite±epidote±tremolite±chlorite. Dolerite dykes mainly include plagioclase and pyroxene as primary minerals and tremolite±epidote±chlorite as low-grade secondary minerals. Geochemical data show that the spilitic lavas and dolerite dykes are sub-alkaline, of oceanic tholeiitic basalt character and display a tholeiitic fractional trend, characterized by an increase in FeO/MgO and Zr and TiO2 in variation diagrams. They are characterized by relatively high Zr/Y (2–4.5), relatively high Th/Yb (0.15–1.0) and La/Nb (0.5–2.5). Both show marked negative Nb and Ti anomalies relative to Th and La (Ce), implying a subduction-related chemistry. Chondrite-normalized REE patterns display slight enrichment of light REE (spilitic lavas (La/Yb)N = 0.79–1.56; dolerite dykes (La/Yb)N = 0.89–3.50) fairly comparable with MORB. The geochemical similarity of the spilitic lavas and dolerite dykes suggests a co-genetic origin. La/Nb ratios of both types are slightly higher than average MORB values and were possibly formed in the early stages of back-arc basin development. Petrogenetic modelling suggests the mafic rocks of the formation were formed by 9% batch melting of spinel lherzolite in shallower depths (c. 60 km). Taken together the data suggest that the Early Cambrian mafic rocks of the Taurus units were developed in a back-arc basin along the northern edge of Gondwana above the southward-subducting oceanic lithosphere and may represent initial rifting that resulted in separation of the peri-Gondwanan terranes.

scholarly journals Geochemistry and origin of plagiogranites from the Eldivan Ophiolite, Çankırı (Central Anatolia, Turkey)

2014 ◽  
Vol 65 (3) ◽  
pp. 197-207 ◽  
Author(s):  
Tijen Üner ◽  
Üner Çakir ◽  
Yavuz Özdemir ◽  
Irem Arat
Keyword(s):  
Partial Melting ◽  
Basaltic Magma ◽  
Oceanic Lithosphere ◽  
Central Anatolia ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
Complete Series ◽  
Tectonic Settings ◽  
Bottom To Top ◽  
Low K

Abstract The Eldivan Ophiolite, exposed around Ankara and Çankırı cities, is located at the central part of the Izmir-Ankara-Erzincan Suture Zone (IAESZ). It represents fragments of the Neotethyan Oceanic Lithosphere emplaced towards the south over the Gondwanian continent during the Albian time. It forms nearly complete series by including tectonites (harzburgites and rare dunites), cumulates (dunites, wherlites, pyroxenites, gabbro and plagiogranites) and sheeted dykes from bottom to top. Imbricated slices of volcanic-sedimentary series and discontinuous tectonic slices of ophiolitic metamorphic rocks are located at the base of tectonites. Plagiogranitic rocks of the Eldivan Ophiolite are mainly exposed at upper levels of cumulates. They are in the form of conformable layers within layered diorites and also dikes with variable thicknesses. Plagiogranites have granular texture and are mainly composed of quartz and plagioclases. The occurrences of chlorite and epidote revealed that these rocks underwent a low grade metamorphism. Eldivan plagiogranites have high SiO2 content (70-75 %) and low K2O content (0.5-1 %) and display flat patterns of REE with variable negative Eu anomalies. LREE/HREE ratio of these rocks varies between 0.2-0.99. All members of the Eldivan rocks have high LILE/HFSE ratios with depletion of Nb, Ti and P similar to subduction related tectonic settings. Geochemical modelling indicates that the Eldivan plagiogranites could have been generated by 50-90 % fractional crystallization and/or 5-25 % partial melting of a hydrous basaltic magma

Geochemical, Sr–Nd–Pb and zircon U–Pb–Hf isotopic constraints on the Late Carboniferous back-arc basin basalts from the Chengjisihanshan Formation in West Junggar, NW China

2020 ◽  
Vol 157 (11) ◽  
pp. 1781-1799
Author(s):  
Qian Zhi ◽  
Yongjun Li ◽  
Fenghao Duan ◽  
Lili Tong ◽  
Jun Chen ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Oceanic Lithosphere ◽  
Late Carboniferous ◽  
Geochemical Data ◽  
Upper Carboniferous ◽  
Basaltic Rocks ◽  
Central Asian ◽  
West Junggar ◽  
Back Arc ◽  
T Values

AbstractWest Junggar in the southwestern Central Asian Orogenic Belt is a critical area for the study of the Junggar oceanic basin and may also reveal tectonic evolutionary events before the final closure of the Palaeo-Asian Ocean. The sedimentary formations and paragenetic associations of the Upper Carboniferous Chengjisihanshan Formation in southern West Junggar jointly reveal a back-arc basin setting with zircon U–Pb ages of 313–310 Ma for the basaltic rocks. Geochemically, the basaltic rocks are tholeiitic with low SiO2 (47.76–52.06 wt %) and K2O (0.05–0.74 wt %) but high MgO (6.55–7.68 wt %) contents and Mg no. (52.9–58.9) values. They display slightly flat rare earth element patterns with weak positive Eu anomalies, and show enrichments in large ion lithophile elements relative to high field strength elements with negative Nb and Ta anomalies, exhibiting both N-MORB-like and arc-like signatures, similar to the back-arc basin basalt from the Mariana Trough. The high positive zircon εHf(t) and bulk εNd(t) values as well as high initial Pb isotopes, together with relatively high Sm/Yb and slightly low Th/Ta ratios imply a depleted spinel lherzolitic mantle source metasomatized by slab-derived fluids. The field and geochemical data jointly suggest that the volcanic rocks within the Chengjisihanshan Formation were formed in an intra-oceanic back-arc basin above the northwestward subduction of the Junggar oceanic lithosphere in southern West Junggar. The confirmation of the Late Carboniferous back-arc basin basalts, together with other geological observations, indicate that an arc-basin evolutionary system still existed in southern West Junggar at c. 310 Ma, and the Junggar Ocean closed after Late Carboniferous time.

scholarly journals A re-appraisal of the petrogenesis and tectonic setting of the Ordovician Fishguard Volcanic Group, SW Wales

2015 ◽  
Vol 153 (3) ◽  
pp. 410-425 ◽  
Author(s):  
BETHAN A. PHILLIPS ◽  
ANDREW C. KERR ◽  
RICHARD BEVINS
Keyword(s):  
Fractional Crystallization ◽  
Tectonic Setting ◽  
Source Region ◽  
Geochemical Data ◽  
Spinel Lherzolite ◽  
Magma Chamber Processes ◽  
Volcanic Group ◽  
South Wales ◽  
Iapetus Ocean ◽  
Back Arc

AbstractThe Fishguard Volcanic Group represents an excellently preserved example of a volcanic sequence linked to the closure of the Iapetus Ocean. This study re-examines the petrogenesis and proposed tectonic setting for the Llanvirn (467–458 Ma) Fishguard Volcanic Group, South Wales, UK. New major and trace element geochemical data and petrographic observations are used to re-evaluate the magma chamber processes, mantle melting and source region. The new data reveal that the Fishguard Volcanic Group represents a closely related series of basalts, basaltic andesites, dacites and rhyolites originating from a spinel lherzolite source which had been modified by subduction components. The rocks of the Fishguard Volcanic Group are co-genetic and the felsic members are related to the more primitive basalts mainly by low-pressure fractional crystallization. The geochemistry of the lavas was significantly influenced by subduction processes associated with a coeval arc, while significant amounts of assimilation of continental crust along with fractional crystallization appear to have contributed to the compositions of the most evolved lavas. The Fishguard Volcanic Group was erupted into a back-arc basin where extensive rifting but no true seafloor spreading had occurred.

scholarly journals CARACTERIZAÇÃO GEOTECTÔNICA DO SETOR SETENTRIONAL DO CINTURÃO RIBEIRA: EVIDÊNCIA DE ACRESÇÃO NEOPROTEROZÓICA NO LESTE DE MINAS GERAIS, BRASIL.

1995 ◽  
Author(s):  
Antônio Gilberto Costa ◽  
Carlos Alberto Rosiére ◽  
Luciano Melo Moreira ◽  
Daniele Piuzana
Keyword(s):  
Continental Margin ◽  
Oceanic Lithosphere ◽  
Minas Gerais ◽  
Island Arc ◽  
Geochemical Data ◽  
Magmatic Arc ◽  
High K ◽  
Alternative Hypotheses ◽  
Back Arc ◽  
Early Neoproterozoic

The early neoproterozoic evolution of eastern Minas Gerais is characterized by a history of a continental margin activity, including the accretion of suspect terranes. The Manhuaçu Terrane is one of those and is represented by a granitic continental plutonic arc and terrigeneous metasediments that reflect a continental margin. A metasedimentary gneiss belt at this margin with shallow to deep marine clastic lithologies as well as metavulcanic and metaplutonic mafic rocks was interpreted as an extensive tectonic segment with suspect development in a back-arc setting. Fragments of a volcanic arc are identified and interpreted as an evidence for a probable island-arc domain. In the studied region the Juiz de Fora and Paraiba do Sul Complexes are domains of these terranes. The distribution of magmatism in the studied region shows from west to east three different suites : 1) tholeiitic, 2) a medium to high-K cal-alkalic and 3) a high-K calc-alcalic/shoshonitic(?) magmatism which attests the evolution of early magmatism arcs (volcanic and plutonic) initially relates to ocean-plate subduction, followed by a continent-continent collision. Petrological, structural and geochemical data of mesoproterozoic/neoproterozoic metamorphic and magmatic suites of rocks are in agreement with the development of an accretionary orogeny. After a vulcanic-(island-arc) and a back-arc basin formation (by rifting of a continental margin with no spreading) in connection with eastward subduction, the island arc was accreted to a continental margin. Further subduction beneath it and a mechanism of flattening of the slab during the subduction process was responsible for the development of as granitic continental plutonic-arc (Andean-type batholith) eastwardly in a compressional setting. Continental plates became sutured and all intervening oceanic lithosphere was subducted beneath one of the converging masses. This resulted in the accretion of the Manhuaçu Microplate to the São Francisco Craton Domain. Plates continued to converge and the inversion of subduction polarity occurs resulting in a new subduction system (westward subduction) to the back of the Manhuaçu Microplate, in a easternward of the Espírito Santo state, with the establishment of a new magmatic arc of late neoproterozoic to eopaleozoic age. In this paper, only the probable early neoproterozoic volcanic (tholeiitic magmatism transitional between N-type MORB and island are basalts) and the plutonic (early continental calc-alkalic magmatism) arcs characterized by rocks with a very particular geochemistry and the back-arc basin setting will be discussed. Two alternative hypotheses to explain the evolution of these terranes may be postulated: 1) an island arc orogeny related to a westward subduction followed by a cordillerean type orogeny. With the advance of the island-arc and continent with offshore sediments, in  different plates, a collision between these domains occurred. The old subduction zone was replaced by another one, eastward directed and the development of a continental magmatic arc occurred. Continental plates became sutured and all intervening oceanic lithosphere was subducted beneath one of the converging masses. Against this model are the presence of metavulcanic basic and intermediate rocks intercalated with marine and continental margin; 2) an extensional ensialic setting (aborted marginal basin) in the continental margin formed during the opening of a rapidly subsiding basin, with moderately rapid mantle upwelling, pronounced thinning of the continental crust and slight crustal contaminanton, without formation of oceanic crust or an island-arc may explain the association of basic metavulcanic rocks with marine and continental metasediments. Here, the low potassium contents of a few studied metavulcanic basic rocks and theire oceanic tholeiitic affinity are not well in agreement with this setting where continental basalts (flood basalts), or rocks ( basalts or basaltics andesites) belong to the high-K calc-alkaline series are expected to occur. Nevertheless, our evidences are not unequivocal enough for theire disapprovals.

The Neotethys suture in the eastern Bulgarian Rhodopes

2020 ◽  
Author(s):  
Jan Pleuger ◽  
Zlatka Cherneva ◽  
Linus Klug ◽  
Elis Hoffmann ◽  
Michael Schmidtke ◽  
...  
Keyword(s):  
Raman Spectroscopy ◽  
Late Eocene ◽  
Metamorphic Grade ◽  
Geochemical Data ◽  
Low Grade ◽  
New Findings ◽  
Thrust Sheets ◽  
Eastern Rhodopes ◽  
Back Arc ◽  
Peak Pressures

<p>Following a tectonic scheme proposed by Janák et al. (2011; Journal of Metamorphic Geology 29, 317-332) and Pleuger et al. (2011; Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 162, 171-192), the Rhodopes are composed of four nappe complexes, from bottom to top the Lower, Middle, and Upper Allochthon and the Circum-Rhodope Belt. Rocks derived from Adria and/or Pelagonia (Lower Allochthon) are separated from rocks of European origin (Upper Allochthon) by lithologically variegated thrust sheets containing sporadic occurrences of ophiolites (Middle Allochthon). These ophiolites typically yield magmatic protolith ages of c. 160 Ma and were metamorphosed under amphibolite- to eclogite-facies conditions. They represent Neotethyan lithosphere subducted below Europe in the Late Cretaceous to Palaeogene whereas the Circum-Rhodope Belt contains ophiolites of the same protolith age but with lower metamorphic grade (greenschist facies at most) and was obducted onto the former European margin in the Jurassic. We present LA-ICP-MS U-Pb zircon and additional geochemical data from the Luda Reka Unit in the Bulgarian Eastern Rhodopes. This unit consists mostly of amphibolite, metagabbro, and metadiorite that yielded two protolith ages of 163.5±2.6 Ma and 154.2±1.0 Ma. The trace element patterns resemble those of typical back-arc basalts and lower oceanic crustal cumulates. Initial epsilon Nd values of six samples calculated to 154 Ma were +10.8 ±0.8 (2σ; n = 6), in agreement with average basalts derived from depleted ambient mantle. A pegmatite crosscutting the Luda Reka Unit yielded a magmatic age of 52.04±1.1 Ma. Such pegmatites are widespread in the Luda Reka Unit (Middle Allochthon) suggesting that emplacement of this unit over the Bjala Reka Orthogneiss Unit (Lower Allochthon) where such pegmatites are lacking happened only after c. 52 Ma. The Bjala Reka Orthogneiss Unit forms the footwall of the top-to-the-SSW Bjala Reka Detachment that became active in the Late Eocene. Where the Luda Reka Unit is lacking, the Bjala Reka Orthogneiss Unit is overlain by rocks that were collectively described as “Low-grade Mesozoic Unit” (e.g. Bonev & Stampfli 2008; Lithos 100, 210-233). Based on peak temperatures determined by Raman spectroscopy of organic matter, two tectonic units can be distinguished in the “Low-grade Mesozoic Unit”. The temperature peak was at c. 530 °C in the Mandrica Unit below and at c. 285 °C in the Maglenica Unit above. For the Mandrica Unit, minimum peak pressures of c. 1.4 GPa were obtained by Raman spectroscopy of quartz inclusions in garnet, indicating that this unit underwent subduction-related metamorphism. Because of this marked difference in peak metamorphic grade, we attribute only the anchimetamorphic Maglenica Unit to the Circum-Rhodope Belt while the high-pressure Mandrica Unit probably represents the Upper Allochthon. Both units are presently separated by the top-to-the-NW Mandrica Detachment that was active before the Bjala Reka Detachment. Our new findings show that the easternmost Rhodopes expose a condensed section through all four nappe complexes, notably including the Neotethys suture.</p>

scholarly journals Metamorphic history and geodynamic significance of the Early Cretaceous Sabzevar granulites (Sabzevar structural zone, NE Iran)

2011 ◽  
Vol 3 (1) ◽  
pp. 477-526
Author(s):  
M. Nasrabady ◽  
F. Rossetti ◽  
T. Theye ◽  
G. Vignaroli
Keyword(s):  
Subduction Zone ◽  
Early Cretaceous ◽  
Oceanic Lithosphere ◽  
Geochemical Data ◽  
Data Set ◽  
Metamorphic History ◽  
History Of ◽  
Back Arc ◽  
Temperature Path ◽  
Ne Iran

Abstract. The Iranian ophiolites are part of the vast orogenic suture zones that mark the Alpine-Himalayan convergence zone. Few petrological and geochronological data are available from these ophiolitic domains, hampering a full assessment of the timing and regimes of subduction zone metamorphism and orogenic construction in the region. This paper describes texture, geochemistry and the pressure-temperature path of the Early Cretaceous granulites that occur within the Tertiary Sabzevar suture zone of NE Iran. The geochemical data set document that the granulites are remnants of a MORB-type oceanic crust and thus of a (Early Cretaceous ?) back-arc basin formed in the upper plate of the Neotethyan subduction and thus interpreted as portions of a dismembered dynamothermal sole formed during oceanic subduction. The metamorphic history of the granulites suggests an anticlockwise pressure-temperature loop, compatible with burial in a hot subduction zone followed by cooling during exhumation. This is interpreted as the evidence of a nascent subduction zone formed at the expenses of hot and hence young oceanic lithosphere. These data point to diachronous and independent tectonic evolutions of the different ophiolitic domains of central Iran, for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure-temperature paths can be expected with further investigations.

Mafic rocks with back-arc E-MORB affinity from the Chotanagpur Granite Gneiss Complex of India: relicts of a Proterozoic Ophiolite suite

2021 ◽  
pp. 1-16
Author(s):  
Mansoor Ahmad ◽  
Abdul Qayoom Paul ◽  
Priyanka Negi ◽  
Salim Akhtar ◽  
Bibhuti Gogoi ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Granite Gneiss ◽  
Geochemical Characteristics ◽  
Spinel Lherzolite ◽  
Tectonic Zone ◽  
Plagioclase Feldspar ◽  
Mafic Rocks ◽  
Gneiss Complex ◽  
Back Arc ◽  
Opaque Minerals

Abstract The Proterozoic Chotanagpur Granite Gneiss Complex (CGGC) at the northern boundary of the Central Indian Tectonic Zone (CITZ) of the eastern Indian shield preserves relics of fossilized oceanic back-arc crust. We describe the field, petrographical and geochemical characteristics of the mafic rocks comprising pillow basalts and dolerites from the Bathani area of the northern fringe of the CGGC, eastern India. The basalts consist of plagioclase feldspar, hornblende, opaque minerals (Fe–Ti oxide) and chlorite, and the dolerite consists of plagioclase, hornblende and opaque minerals. Our data indicate that the Bathani mafic rocks have tholeiitic to transitional composition and are overprinted by greenschist facies metamorphic conditions; however, REE and fluid immobile elements preserve their primary geochemical signatures. The (La/Sm)N ratios (1.38–2.15) and chondrite-normalized REE patterns point to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Geochemical characteristics indicate a mixed signature of MORB and arc tholeiite with enrichment of Ba, Th, Eu and Sr, similar to that of back-arc supra-subduction zone ophiolites. These mafic rocks are the product of MORB-like magma derived from a depleted mantle corresponding to < 2% partial melting of spinel lherzolite, enriched by subduction-induced slab metasomatism and melting. The Bathani mafic rocks are representative of the upper part of a supra-subduction zone columnar ophiolite section, which was emplaced onto the present-day northern margin of the CGGC during suturing of the northern and southern Indian block at c. 1.9 Ga during the Nuna amalgamation.

scholarly journals Precambrian Basement and Late Paleoproterozoic to Mesoproterozoic Tectonic Evolution of the SW Yangtze Block, South China: Constraints from Zircon U–Pb Dating and Hf Isotopes

Minerals ◽  
2018 ◽  
Vol 8 (8) ◽  
pp. 333 ◽  
Author(s):  
Wei Liu ◽  
Xiaoyong Yang ◽  
Shengyuan Shu ◽  
Lei Liu ◽  
Sihua Yuan
Keyword(s):  
South China ◽  
Tectonic Evolution ◽  
Detrital Zircons ◽  
Geochemical Data ◽  
Precambrian Basement ◽  
Yangtze Block ◽  
Two Stage ◽  
Clastic Rocks ◽  
Columbia Supercontinent ◽  
Isotopic Analyses

Zircon U–Pb dating and Hf isotopic analyses are performed on clastic rocks, sedimentary tuff of the Dongchuan Group (DCG), and a diabase, which is an intrusive body from the base of DCG in the SW Yangtze Block. The results provide new constraints on the Precambrian basement and the Late Paleoproterozoic to Mesoproterozoic tectonic evolution of the SW Yangtze Block, South China. DCG has been divided into four formations from the bottom to the top: Yinmin, Luoxue, Heishan, and Qinglongshan. The Yinmin Formation, which represents the oldest rock unit of DCG, was intruded by a diabase dyke. The oldest zircon age of the clastic rocks from the Yinmin Formation is 3654 Ma, with εHf(t) of −3.1 and a two-stage modeled age of 4081 Ma. Another zircon exhibits an age of 2406 Ma, with εHf(t) of −20.1 and a two-stage modeled age of 4152 Ma. These data provide indirect evidence for the residues of the Hadean crustal nuclei in the Yangtze Block. In combination with the published data, the ages of detrital zircons from the Yinmin Formation yielded three peak ages: 1.84, 2.30 and 2.71 Ga. The peaks of 1.84 and 2.71 Ga are global in distribution, and they are best correlated to the collisional accretion of cratons in North America. Moreover, the peak of 1.84 Ga coincides with the convergence of the global Columbia supercontinent. The youngest age of the detrital zircon from the Yinmin Formation was 1710 Ma; the age of the intrusive diabase was 1689 ± 34 Ma, whereas the weighted average age of the sedimentary tuff from the Heishan Formation was 1414 ± 25 Ma. It was presumed that the depositional age for DCG was 1.71–1.41 Ga, which was in accordance with the timing of the breakup of the Columbia supercontinent. At ~1.7 Ga, the geochemical data of the diabase were characterized by E-MORB and the region developed the same period A-type granites. Thus, 1.7 Ga should represent the time of the initial breakup of the Yangtze Block. Furthermore, the Yangtze Block continues to stretch and breakup until ~1.4 Ga, which is characterized by the emergence of oceanic island, deep-sea siliceous rock and flysch, representing the final breakup. In brief, the tectonic evolution of the Yangtze Block during the Late Paleoproterozoic to Mesoproterozoic coincided with the events caused by the convergence and breakup of the Columbia supercontinent, because of which, the Yangtze Block experienced extensive magmatic activity and sedimentary basin development during this period.

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