Radiolarites Research Articles

Geochemistry of radiolarian cherts from a Late Devonian continental margin basin, Loei fold belt, Indo-China terrane

More than 42 species of well-preserved Upper Devonian radiolarians have been obtained from cherts and siliceous shales in four sections (Chom Noi, Phu Kham Phe (P), Phu Kham Phe (C) and Pha Samyod) from the NE Thailand sector of the Loei fold belt, Indochina terrane. They include Ceratoikiscum sp. cf. planistellare Foreman, Helenifore laticlavium Nazarov & Ormiston, Astroentactinia sp. cf. paronae (Hinde), Stigmosphaerostylus herculea (Foreman), S. pusilla (Hinde), S. cf. vulgaris (Won), Trilonche davidi (Hinde), T. echinata (Hinde), T. elegans Hinde, T. hindea (Hinde), T. palimbola (Foreman), T. vetusta Hinde, Polyentactinia polygonia Foreman, Spongentactinella sp. cf. windjanensis Nazarov, Archocyrtium sp.?, Palaeoscenidium sp., Triaenosphaera sp., and others. Radiolarian faunas from these sections indicate a range from lower Frasnian to Famennian. Analyses of rare earth, trace and major elements suggest that the Upper Devonian chert and siliceous successions from Loei were deposited in a continental margin environment near to a volcanic arc. These successions contain distinctive weak negative Ce anomalies with strong positive Eu anomalies which differ from Upper Devonian cherts reported from northern Thailand, the Truong Son fold belt (Laos) and from South China. Negative Ce anomalies and low to moderate Lan/Cen ratios imply proximity between the Chom Noi and Phu Kham Phe (C), which were deposited in more distal parts of the depositional basin than the Pha Samyod and Phu Kham Phe (P) samples. Most of the Pha Samyod and Phu Kham Phe (P) samples exhibit high continental-derived trace element levels including Cr, Zr, Hf, Rb, Th and in part V, which supports close proximity to terrigenous sources. Paleozoic stratigraphic sequences of the Loei fold belt and geochemical results suggest that the Upper Devonian deep marine sequences in the Loei fold belt were deposited within a rifted, continental-margin basin, possibly a back-arc basin and not in a large oceanic basin as has been previously suggested.

Chapter 31 The pre-Cenozoic tectonic evolution of Myanmar

Over the past few decades a synthesis has been developed in which Southeast Asia has been interpreted as composed of a number of continental tectonic units. There is a general consensus that these tectonic units originated during the Precambrian in the Southern Hemisphere, as part of the Gondwana Supercontinent. During the Palaeozoic these tectonic units were rifted off the northern margin of Gondwana by the formation and expansion of successive segments of the Tethys Ocean. They were then accreted to the southern margin of Eurasia following the subduction of Tethys during the latest Palaeozoic to early Mesozoic (e.g. Metcalfe 1988, 1996, 2011; Barber et al. 2011). Major pre-Tertiary tectonic units recognized in Southeast Asia are the Indochina Block and Sibumasu (Sino or Siam–Burma–Malaysia–Sumatra) Block, the latter extending from Yunnan in China, through the Shan States and southern Myanmar, Thailand, western Peninsular Malaysia to eastern Sumatra (Metcalfe 1996) (Fig. 31.1). Boundaries between the major blocks can be recognized, where they are marked by belts of ophiolites, including serpentinites, cumulate gabbros, pillow lavas and radiolarian cherts, representing the debris of the subducted Tethyan ocean floor. Fig. 31.1. Tectonic units of Myanmar and the immediately adjacent areas. Note that the ‘Indochina Block’ includes the Sukhothai Volcanic Arc (Sone & Metcalfe 2008). In Thailand to the south and east of Myanmar the broad outlines of this scenario are well established (Metcalfe 1996; Sone & Metcalfe 2008; Barber et al. 2011), but it is not clear how the Indochina and Sibumasu blocks can be traced into Myanmar. In addition, in Myanmar the Mt Victoria (Mitchell 1989) or West Burma Block (Metcalfe 1996) has been identified in the literature, but the eastern margin of this block against Sibumasu, conventionally identified as the Sagaing Fault, has not been clearly defined. The original position …

Olistostrome and the mesozoic tectonic of the bantimala complex, South Sulawesi

The study aims to determine the presences and spreads of the olistostrome as one components of the tectonic complex of Bantimala area. The basement rocks of Bantimala area is a metamorphic rocks, which are unconformably overlain by Balangbaru Formation and radiolarian chert. Beneath in between cherts and basement rocks are presence breccia schists which give rise to various presumptions and interpretations of the environment and conditions of formation as submarine deposit, ideally cherts in the area underlain by oceanic crust.Schist breccias presence underneath of cherts in the Bantimala Complexes were suggested an olistostrome deposit. It was characterized by poorly sorting, unfoliated, shows deformed textures and composed olisthtolits which are embedded in sandy matrices, and in the cherts are presence layer sandstones and schist fragments. Olistostrome is sedimentary deposit as preserved in the trench, they will give us an interpretation that prior to formed cherts in the Bantimala area, initially tectonic subduction activity which are deforming and brecciation of the basement rocks subsequently as the constituent material of olistostrome.

New Middle Triassic radiolarian species from the Monte Facito Formation (Lagonegro succession, Southern Apennines, Italy)

Rich and well preserved radiolarian assemblages from the Middle Triassic radiolarian cherts of the Monte Facito Formation, Lagonegro succession (Southern Apennines, Italy), contain new Late Anisian taxa. These radiolarian cherts represent the oldest deep-water sediments of the Lagonegro Basin, representing pelagic conditions since the Late Anisian. Six new species are here described: Tiborella tribullata, Baratuna lucana, Hozmadia lagonegroensis, Hozmadia (?) simplex, Bulbocyrtium multiporatum, Planispinocyrtis curta.

Radiolarian biochronology of upper Anisian to upper Ladinian (Middle Triassic) blocks and tectonic slices of volcano-sedimentary successions in the Mersin Mélange, southern Turkey: New insights for the evolution of Neotethys

The Mersin Ophiolitic Complex located in southern Turkey comprises two main structural units; the Mersin Mélange, and a well-developed ophiolite succession with its metamorphic sole. The Mersin Mélange is a sedimentary complex including blocks and tectonic slices of oceanic litosphere and continental crust in different sizes. Based on different fossil groups (Radiolaria, Conodonta, Foraminifera and Ammonoidea), the age of these blocks ranges from Early Carboniferous to early Late Cretaceous. Detailed fieldwork in the central part of the Mersin Mélange resulted in identification of a number of peculiar blocks of thick basaltic pillow-and massive lava sequences alternating with pelagic-clastic sediments and radiolarian cherts. The oldest ages obtained from the radiolarian assemblages from the pelagic sediments transitional to the volcano-sedimentary succession in some blocks are middle to late Late Anisian. These pelagic sediments are overlain by thick sandstones of latest Anisian to middle Early Ladinian age. In some blocks, sandstones are overlain by clastic and pelagic sediments with lower Upper to middle Upper Ladinian radiolarian fauna. Considering the litho- and biostratigraphical data from Middle Triassic successions in several blocks in the Mersin Mélange, it is concluded that they correspond mainly to the blocks/slices of the Beysehir-Hoyran Nappes, which were originated from the southern margin of the Neotethyan Izmir-Ankara Ocean. As the pre-Upper Anisian basic volcanics are geochemically evaluated as back-arc basalts, this new age finding suggest that a segment of the Izmir-Ankara branch of the Neotethys was already open prior to Middle Triassic and was the site of intraoceanic subduction.

Zircon U-Pb geochronological constraints on rapid exhumation of the mantle peridotite of the Xigaze ophiolite, southern Tibet

The Xigaze ophiolite outcrops in the central segment of the Yarlung Zangbo suture zone, southern Tibet. It is characterized by large amounts of ultramafic units with minor mafic rocks. The mafic rocks consist of gabbros, diabases and basalts. The gabbroic rocks of the Xigaze ophiolite occur as layered bodies or isotropic dikes intruding into mantle sections. Hectometric-sized gabbroic bodies are well-preserved in Dazhuqu, Baigang and Jiding. However, their formation time and generation mechanism are not systematically proposed or well understood. In this study, nine samples of mafic rocks from the Xigaze ophiolite, including seven gabbros and two rodingites (altered from diabases), were selected for in situ zircon U-Pb and Hf isotopic analyses. The geochemical feature suggested that these rocks formed by the intrusion of melts of normal mid-ocean ridge (N-MORB) type. The U-Pb data yielded identical ages of 124–129Ma within uncertainties. Positive zircon εHf(t) values indicated that these samples had an origin of depleted mantle source. Combined with previous studies on mafic dikes, amphibolite blocks and radiolarian cherts, it can be concluded that the Yarlung Zangbo ophiolites formed over a short period of time from 119 to 132Ma. Hence, a rapid exhumation of the mantle peridotites and gabbroic rocks of the Xigaze ophiolite may have occurred to get intrusion of the diabase dikes and sills. It excludes the existence of a long-term ancient magma chamber or lens. It is more likely that the gabbroic rocks are a series of plutonic intrusions beneath a fossil slow-spreading ridge, rather than products of magma chambers. Therefore, the “Chapman detachment model” may be applied to the generation of the Yarlung Zangbo ophiolites.

Petrogenesis of Middle Triassic volcaniclastic rocks from Balochistan, Pakistan: Implications for the break-up of Gondwanaland

Basaltic volcanic conglomerates near the Wulgai village in Balochistan occur in the undivided sedimentary rock unit of the Bagh complex which is the melange zone beneath the Muslim Bagh ophiolite. The presence of Middle Triassic grey radiolarian chert within the upper and lower horizon of the conglomerates suggests that the lavas, from which these conglomerates were principally derived, were eroded and re-deposited in the Middle Triassic. The Wulgai conglomerate contains several textural and mineralogical varieties of volcanic rocks, such as porphyritic, glomerophyric, intersertal and vitrophyric basalts. The main minerals identified in these samples are augite, olivine, plagioclase (An35-78) leucite and nosean, with apatite ilmenite, magnetite and hematite occurring as accessory minerals. These rocks are mildly to strongly-alkaline with low Mg# and low Cr, Ni and Co contents suggesting that their parent magma had undergone considerable fractionation prior to eruption. Trace element-enriched mantle-normalized patterns with marked positive Nb anomalies are consistent with 10%-15% melting of an enriched mantle source in a within-plate tectonic setting. It is proposed that this Middle Triassic intra-plate volcanism may represent mantle plume-derived melts related to the Late Triassic rifting of micro-continental blocks (including Afghan, Iran, Karakorum and Lhasa) from the northern margin of Gondwana.

Cretaceous (Albian to Turonian) radiolarians from chert blocks of the Moni Mélange (Southern Cyprus)

Well-preserved Cretaceous (Albian–Turonian) radiolarians were extracted from radiolarian-bearing chert olistoliths of the Monagroulli Member within the Moni Mélange (Campanian-Maastrichtian, Southern Cyprus). Four assemblages were distinguished: Middle Albian–Lower Cenomanian (Thanarla spoletoensis Zone), Upper Albian–Lower Cenomanian (Thanarla spoletoensis Zone, Dorypyle? anisa Subzone), lowermost Turonian (base of Alievium superbum Zone) and Lower Turonian (Alievium superbum Zone). The radiolarian assemblages are diverse and have taxonomic composition similar to coeval assemblages of Italy and Spain. The sediments of the Monagroulli Member differ from coeval rocks of the Mamonia Complex (western Cyprus) by the more common presence of radiolarian cherts and may have been formed in the distal part of a continental margin with less input of clastic material. A new spicular radiolarian genus Cyprothamnus with 2 new species (C. multifurcatus and C. moniensis) is described from the Lower Turonian strata.

Carboniferous and Permian evolutionary records for the Paleo‐Tethys Ocean constrained by newly discovered Xiangtaohu ophiolites from central Qiangtang, central Tibet

AbstractReconstructing the evolutionary history of the Paleo‐Tethys Ocean remains at the center of debates over the linkage between Gondwana dispersion and Asian accretion. Identifying the remnants of oceanic lithosphere (ophiolites) has very important implications for identifying suture zones, unveiling the evolutionary history of fossil oceans, and reconstructing the amalgamation history between different blocks. Here we report newly documented ophiolite suites from the Longmu Co‐Shuanghu Suture zone (LSSZ) in the Xiangtaohu area, central Qiangtang block, Tibet. Detailed geological investigations and zircon U‐Pb dating reveal that the Xiangtaohu ophiolites are composed of a suite of Permian (281–275 Ma) ophiolites with a nearly complete Penrose sequence and a suite of Early Carboniferous (circa 350 Ma) ophiolite remnants containing only part of the lower oceanic crust. Geochemical and Sr‐Nd‐O isotopic data show that the Permian and Carboniferous ophiolites in this study were derived from an N‐mid‐ocean ridge basalts‐like mantle source with varied suprasubduction‐zone (SSZ) signatures and were characterized by crystallization sequences from wet magmas, suggesting typical SSZ‐affinity ophiolites. Permian and Carboniferous SSZ ophiolites in the central Qiangtang provide robust evidence for the existence and evolution of an ancient ocean basin. Combining with previous studies on high‐pressure metamorphic rocks and pelagic radiolarian cherts, and with tectonostratigraphic and paleontological data, we support the LSSZ as representing the main suture of the Paleo‐Tethys Ocean which probably existed and evolved from Devonian to Triassic. The opening and demise of the Paleo‐Tethys Ocean dominated the formation of the major framework for the East and/or Southeast Asia.

Late Paleozoic tectonics of the Solonker Zone in the Wuliji area, Inner Mongolia, China: Insights from stratigraphic sequence, chronology, and sandstone geochemistry

The geology in the Wuliji area (including the Enger Us and Quagan Qulu areas) is important for understanding the Late Paleozoic tectonics of the Solonker Zone. Ultramafic/mafic rocks in the Enger Us area, previously interpreted as an ophiolitic suture, are actually lava flows and sills in a Permian turbiditic sequence and a small body of fault breccia containing serpentinite. Subduction zone features, such as accretionary complexes, magmatic arc volcanics or LP/HP metamorphism are absent. Early Permian N-MORB mafic rocks and Late Permian radiolarian cherts accompanied by turbidites and tuffeous rocks indicate a deep water setting. In the Quagan Qulu area, outcrops of the Late Carboniferous to Permian Amushan Formation are composed of volcano-sedimenary rocks and guyot-like reef limestone along with a Late Permian volcano-sedimentary unit. A dacite lava in the Late Permian volcano-sedimentary unit yields a zircon U-Pb age of 254Ma. The gabbros in the Quagan Qulu area are intruded into the Amushan Formation and caused contact metamorphism of country rocks. Sandstones in the Upper Member of the Amushan Formation contain detrital clasts of volcanic fragments and mineral clasts of crystalline basement rocks (i.e. biotite, muscovite and garnet). Geochemical analysis of volcaniclastic sandstones shows a magmatic affinity to both continental island arc (CIA) and active continental margin (ACM) tectonic settings. A Late Permian incipient rift setting is suggested by analyzing the lithostratigraphic sequence and related magmatism in the Wuliji area. The volcano-sedimentary rocks in the Wuliji area experienced a nearly N-S shortening that was probably related to the Early Mesozoic nearly N-S compression well developed in other areas close to the Wuliji area.

Zircon U-Pb geochronological evidence for the evolution of the Nan-Uttaradit suture in northern Thailand

The Nan-Uttaradit suture is marked by a narrow N-S trending and discontinuous ophiolite belt in northern Thailand. This suture zone is a melange composed of gabbro, tholeiitic metabasalt, andesite and radiolarian chert. Samples of gabbro and meta-basalt in the Nan-Uttaradit suture yield zircon U-Pb ages of 311±10 and 316±3 Ma, respectively, interpreted as the crystallization ages of the rocks, suggesting the Nan-Uttaradit Ocean existed in the Late Carboniferous. Our results indicate that the Nan-Uttaradit Ocean co-existed with the Ailaoshan-Jinshajiang Ocean to the north and was probably an along-strike extension of the latter.

Facies analysis of the Semanggol Formation, South Kedah, Malaysia: A possible Permian–Triassic boundary section

Although the Permian–Triassic Semanggol Formation is widely distributed in northwestern Peninsula Malaysia and is made of various lithofacies, its sedimentology and possible relation with the Permian–Triassic boundary (PTB) were not considered before. In this study, detailed facies analysis was conducted for two sections of the Semanggol Formation at the Bukit Kukus and Baling areas, South Kedah to clarify its sedimentology and relation to the PTB. Four facies from the Permian part of the Semanggol Formation that were identified at the Bukit Kukus section include laminated black mudstone, interbedded mudstone and sandstone, volcanogenic sediments, and bedded chert. In Baling area, the Triassic part of the formation is classified into three members. The lower member comprises of claystone and bedded chert facies, while the middle member is composed of sandstone and claystone interbeds (rhythmite). On the other hand, the upper member is grouped into two main units. The lower unit is mainly claystone and includes two facies: the varve-like laminated silt and clay and massive black claystone. The upper unit is composed of various sandstone lithofacies ranging from hummocky cross stratified (HCS) sandstone to thinly laminated sandstone to burrowed sandstone facies. The HCS sandstones occur as two units of fine-grained poorly sorted sandstone with clay lenses as flaser structure and are separated by a hard iron crust. They also show coarse grains of lag deposits at their bases. The laminated black mudstone at the lowermost part of the Semanggol Formation represents a reducing and quite conditions, which is most probably below the fairweather wave base in offshore environment that changed upwards into a fining upward sequence of tide environment. Abundance of chert beds in the volcanogenic sediments suggests the deposition of tuffs and volcanic ashes in deep marine setting which continues to form the Permian pelagic bedded chert and claystone. The bedded chert in the lower member of the Triassic section suggests its formation in deep marine conditions. The rhythmic sandstone and claystone interbeds of the middle member are suggestive for its formation as a distal fan of a turbidite sequence. Lithology and primary sedimentary structure of the upper member suggest its deposition in environments range from deep marine represented by the varve-like laminated silt and clay to subtidal environment corresponds to the massive black claystone to coastal environment represented by the hummocky sandstone units and reaches the maximum regression at the hiatus surface. Another cycle of transgression can be indicated from the second hummocky unit with transgressive lag deposits that develops to relatively deeper conditions as indicated from the formation of relatively thick laminated sandstone and bioturbated massive sandstone facies that represent tidal and subtidal environment, respectively. Late Permian lithological variation from the radiolarian chert into early Triassic claystone probably resulted from a decrease in productivity of radiolarians and might represent a PTB in the Semanggol Formation. Volcanogenic sediments in the studied section can be used as an evidence for volcanic activities at the end of the Permian, which is probably connected to the nearby volcanic ash layers in the eastern China, the ultimate cause of the PTB in this area. Black mudstone in the Permian part of the studied section may be interrelated to the Latest Permian Anoxia that started to build in the deep ocean well before the event on shallow shelves.

Age of the radiolarian chert from the Zhilong section in Gyangze, southern Tibet and its implications

Bedded cherts are well exposed in the Zhilong section near Zhilong Village, Gyangze County, southern Tibet, China. Abundant and moderately- to well-preserved Late Jurassic to Early Cretaceous radiolarians extracted from 31 cherty samples from this section are attributed to 95 species in 40 genera. As compared with the Mesozoic radiolarian zonation in other areas, especially in the West Tethys, the radiolarian assemblages from Zhilong are considered late Kimmeridgian–earliest Hauterivian in age and therefore Units 5 and 6 at the Zhilong section were deposited during the same interval. The stratigraphical sequence of the Zhilong section is considered to be reversed and the Jurassic/Cretaceous boundary interval probably occurring in the Unit 5.

Trace fossils in Ordovician radiolarian chert successions in the Southern Uplands, Scotland

ABSTRACTRadiolarian chert and associated siliceous claystone in the Southern Uplands of Scotland are examined, in order to study the Great Ordovician Biodiversification Event of benthic animals on the pelagic ocean bottom. Trace fossils which are uncommon, but convincing, are found in the grey chert and siliceous claystone of Gripps Cleuch. These observations constitute firm evidence that large benthic animals which could leave visible trace fossils had colonised the Iapetan Ocean by the late Middle Ordovician, confirming previous studies from Australia for Panthalassa, the other huge ocean. Red chert is, however, a poor recorder of trace fossils, probably because the highly oxidising environment breaks down organic matter, both inhibiting high-density activity of large benthic animals and removing clear traces of benthic animal life.

Jurassic metabasic rocks in the Kızılırmak accretionary complex (Kargı region, Central Pontides, Northern Turkey)

The Kızılırmak accretionary complex near Kargı is tectonically bounded by the Jurassic and Early Cretaceous metamorphic massives of the Central Pontides. It consists mainly of serpentinite, serpentinized peridotite, gabbro, basalt, metabasite and deep-marine sedimentary rocks. The metabasites in the Kızılırmak accretionary complex are tectonically located within a serpentinite, radiolarian chert, spilitized basalt, gabbro association and commonly display a steep contact with serpentinites. Amphiboles from metabasites yielded robust 40Ar/39Ar plateau ages ranging between 159.4±0.4Ma and 163.5±0.8Ma. These are interpreted as cooling ages of the metabasites. The metabasites have 87Sr/86Sr(i) between 0.7035 and 0.7044 and 206Pb/204Pb(i) ranging between 18.18 and 18.92. The gabbros have higher 87Sr/86Sr(i) between 0.7044 and 0.7060 and 206Pb/204Pb(i) ranging between 17.98 and 18.43. Three basalt samples display 87Sr/86Sr(i) between 0.7040 and 0.7059. Their 206Pb/204Pb(i) are unrealistically low (15.42 and 15.62), suggesting, most likely, Pb loss which results in over-corrected values for decay through time. Pb–Sr–Nd isotopic compositions for all samples consistently plot between the fields of MORB or the Depleted MORB Mantle reservoirs and enriched mantle reservoirs (EMII rather than EMI). All the samples (except one dolerite dike) have negative ɛNdDM(t=160Ma) values, suggesting derivation from a reservoir more enriched than the depleted mantle. The protoliths of metabasites correspond to diverse sources (N-MORB, E-MORB, OIB and IAT) based on whole rock major and trace element composition. An IAT-like protolith for the metabasites indicates that the İzmir–Ankara–Erzincan ocean domain was subducting and the tectonic regime was compressional during Late Jurassic and before. The protoliths of these rocks were metamorphosed during the subduction/accretion processes, as observed in the metamorphic rocks located along the Balkan, Northern Turkey and Armenia/Iran ophiolites and/or accretionary complexes.IAT-like geochemistry for the gabbro/dolerites indicates that the non-metamorphosed basaltic rocks occurred in a supra-subduction tectonomagmatic environment and is in agreement with their radiogenic isotope compositions.

Tracing of hydrothermal ore forming process in the Sorkhvand manganese deposit, Kermanshah Province, Iran

The Sorkhvand manganese deposit is located in the Zagros thrust belt, close to the Kermanshah ophiolite melange. This ophiolite records the early stage of Neo-Tethys evolution in the western part of Tethyan realm. In the study area, Mn-mineralization occurs as irregular lenses or interlayers with radiolarian cherts within the Cretaceous radiolarite-bearing mudstones. The present work is an attempt in order to characterize the geochemical process involved in manganese mineralization. Mineralogically and texturally, the Sorkhvand manganese deposit is analogous to that of Tethyan manganese occurrences, comprising three generation of Mn-ores including syngenetic (pyrolusite), diagenetic (braunite-pyrolusite), and late-stage epigenetic mineralization (pyrolusite). The significant geochemical characteristics such as high Mn content (6.35–67.64 wt%; average 33.4 wt%), low concentration of Fe (0.18 to 8.4 wt%; average 2.26 wt%), high Ba (14–3445 ppm, average 1038.3 ppm), LREE > HREE, and negative Ce anomalies (Ce/Ce* = 0.58–1.02; average 0.74) with a negative Eu anomalies (Eu/Eu* = 0.48–0.91, average 0.67) reveal a primary distal hydrothermal source for mineralization. Also, the prevailing of oxic conditions (V/V + Ni = 0.11–0.52), absence of sulfide minerals coupled with relative enrichment of some bioessential elements (e.g., Co) suggest that manganese segregation from hydrothermal ore-bearing solutions may be related to microbial activity influenced by bacterially mediated reactions.

Rare-earth element distribution and genesis of manganese ores associated with Tethyan ophiolites, Iran: A review

AbstractThe Zagros orogenic and metallogenic belt is characterized by the widespread occurrence of manganese and ferromanganese deposits. These deposits are spatially associated with radiolarian cherts and basaltic rocks, which cap the ophiolite sequences. The present work provides a review on the rare-earth element (REE) geochemistry coupled with major- and trace-element geochemical characteristics of the Nasirabad and Abadeh Tashk manganese deposits (associated with the Neyriz ophiolite), and Sorkhvand manganese deposit (associated with the Kermanshah ophiolite). These data are used to gain an insight into the primary ore-forming processes that control the deposition of manganese ores. All of the selected manganese deposits have consistently high Ba contents and low concentrations of trace elements (Co, Cu and Ni) with high Mn/Fe ratios typical of hydrothermal activity. A relatively low REE abundance, Lan/Ndn ratios (>3), and position on a Lan/Cen vs. Al2O3/(Al2O3 + Fe2O3) discrimination plot indicate a distal hydrothermal source for almost all of the selected manganese deposits. Most of the deposits are characterized by Ceanom < –0.1 which reflects the prevailing oxidative conditions during the deposition of manganese ores. Importantly, this is consistent with the occurrence of non-sulfide oxic Mn mineralization in all the manganese deposits of the Zagros orogeny. The comparison of the Sorkhvand, Abadeh Tashk and Nasirabad manganese deposits with other manganese deposits elsewhere in the world indicates that major- and trace-element characteristics, as well as the REE composition of the Zagros manganese deposits are analogous to those typical of hydrothermal deposits.

Chert blocks in the ophiolitic mélange of Zlatibor Mt. (SW Serbia): Age and geodynamic implications

Cherts are quite frequently occurring rocks in the Internal Dinarides, an extremely complex area composed of several tectonostratigraphic units in which oceanic sediments, ophiolites and partly metamorphosed parts of the distal continental margin of Adria are preserved. Therefore, these cherts differ in age and the original depositional environment in which they were formed. Results of investigations carried out in the chert blocks found in the melange in the vicinity of Jasenovo village on SE slopes of Zlatibor Mt. are presented here. Radiolarian cherts from the studied localities represent blocks in melange of the East-Bosnian-Durmitor Unit, exposed in a large tectonic window below the Triassic carbonates of Drina-Ivanjica Unit. Biostratigraphic data revealed Callovian-early Kimmeridgian ages of the studied chert blocks, thus implying a Kimmeridgian or younger age of obduction of the West Vardar ophiolites.

Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr–Nd–Hf isotopic characterization

The Dongqiao ophiolite occurs in the central segment of the Bangong–Nujiang suture zone, in north-central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30years’ studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive εNd(t) (+1.6 to +6.7) and εHf(t) (+8.1 to +13.9) values. Zircon U–Pb dating yielded ages of 188±1Ma for the layered gabbro and 181±1Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong–Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the Bangong–Nujiang suture zone. If so, the Tethys Ocean of the Bangong–Nujiang area probably existed as a fore-arc oceanic basin during the Late Triassic to Early Jurassic.

Palaeoenvironmental implications of geochemistry and radiolarians from Upper Devonian chert/shale sequences of the Truong Son fold belt, Laos

A new locality of Frasnian radiolarian chert and siliceous shale is reported in the Vilabouly area, within the Truong Son fold belt of central Laos, in the eastern part of the Indochina Terrane. Samples from this locality, together with previously described radiolarian–bearing siliceous shale from Ban Phonxai, 100 km further north, have been geochemically analysed. Most of the Ce anomalies from Vilabouly and Ban Phonxai exhibit negative normalized values suggesting deposition in an open–marine basin and in a transition zone between open–marine and continental margin environments, respectively. NASC and chondrite-normalized REE and La/Ce plots and trace element contents support this interpretation. Normalized La/Yb values indicate removal of LREEs relative to HREEs from the water column, suggesting deposition far from terrigenous sources for the Vilabouly samples. The Eu/Eu* values are not pronounced which, with the high values of Al2O3/(Al2O3 + Fe2O3), indicate minimal hydrothermal influence. Both sections are characterized by a distinctive Frasnian radiolarian and tentaculitid assemblage. The new radiolarian fauna from Vilabouly includes 23 species including Ceratoikiscum sp. cf. C. bujugum, Archocyrtium sp., Astroentactinia cf. stellata, Helioentactinia aster, H. perjucunda, Spongentactinella corynacantha, S. veles, Spongentactinia concinna, S. exquisita, Stigmosphaerostylus profundisulcus, Trilonche davidi, T. echinata, T. elegans, T. hindea, T. palimbola, T. vetusta, Palaeoscenidium cladophorum and others. The tentaculitids, Homoctenus ultimus and Costulatostyliolina vesca, have been previously described from South China and from Ban Phonxai. The similarity of the radiolarian fauna to those from South China, northern Thailand and Western Australia suggests free interchange of species. Based on geochemistry, stratigraphic sequences and structural geology, the radiolarian chert/siliceous shales are interpreted as highstand deposits in pull-apart basins. This was possibly caused by transtension along the NW–SE–trending strike–slip fault of the Thakhek–Danang Shear Zone during the Late Devonian. Copyright © 2015 John Wiley & Sons, Ltd.