Radiolarites Research Articles

Petrography of Late Cambrian to Middle Ordovician radiolarian chert in Kazakhstan with special reference to the emergence of benthic animals in the pelagic realm

AbstractUpper Cambrian to Middle Ordovician radiolarian chert successions from Kazakhstan were studied to clarify the history of the emergence of benthic animals in ocean floor sediments. Radiolarian tests and clay were deposited for a period of 30 Myr without experiencing an influx of continent‐derived coarse clastic materials. Red, grey and black cherts of the Upper Cambrian to the upper mid‐Darriwilian are thinly laminated, and no trace of benthic animal activity is recognized in that time interval. Bioturbation structures and burrow traces in mid‐Darriwilian stage red chert in Kazakhstan suggest that benthic animals colonized the location where radiolarian chert formed, but that there was a significant delay in colonization when compared with similar reported occurrences in Australia and Canada.

Entrapment and transformation of post-bloom radiolarians in cyanobacterial mats as a factor enhancing the formation of black cherts in the Early Silurian sea

ABSTRACTBlack radiolarian cherts are widespread in the lithological records of the Silurian; however, the processes governing their formation remain unclear. Analyses of Early Silurian cherts of Poland have revealed that they are composed of degraded remains of cyanobacterial mats enclosing variable numbers of dissolved radiolarian tests. In optical and scanning electron microscope (SEM) images, this cyanobacterial necromass, containing entrapped radiolarians and forming massive or laminated cherty deposits, exhibits a complex taphonomic and diagenetic history. These cyanobacterial mats underwent syndepositional to early diagenetic silicification in a peculiar environment which was highly enriched with dissolved silica, due mainly to a massive deposition of opaline tests of post-bloom radiolarians, which settled on mats covering large areas of the sea bottom. It is assumed that precipitation of the chert precursor silica occurred in the highly porous mats and, to a great extent, was governed by the activity of sulfate-reducing bacteria (SRB), which resulted in changes in pH in the mat profile. This process was a key factor governing the dissolution of the radiolarian tests and subsequent reprecipitation of dissolved radiolarian silica in the mats' microenvironment. Photosynthesis carried out by cyanobacteria and the decompositional activity of sulfate-reducing bacteria resulted in: i) a shift of the oxygen gradient below the mat, and ii) strong pH fluctuation within the mat profile. During the first stage of bacterial degradation of the cyanobacterial necromass (SRB I stage), an increase in pH caused dissolution of the radiolarian tests. During the second stage of bacterial degradation (SRB II stage), as the decomposition of organic matter proceeded, a significant amount of organic acid was produced, causing a reduction in pH in the mat interior. Presumably this triggered rapid silica precipitation from the pore waters, which were highly saturated with silica. The availability of radiolarian tests and the rate of their dissolution was therefore a key factor governing the rate of silicification in Early Silurian cyanobacterial mats.

The Middle Jurassic and Early Cretaceous basalt-radiolarian chert association from the Tekelidağ Mélange, eastern İzmir-Ankara-Erzincan suture zone (northern Turkey)

Basaltic and deep-sea sedimentary rocks are widely exposed in the Tekelidağ Mélange, eastern part of the İzmir-Ankara-Erzincan (İAE) suture zone. In this study, we present petrogenetic features of the basaltic rocks and new micropalaeontological data from the radiolarian cherts. The basaltic rocks are divided into two different groups based on whole-rock geochemistry data. The first group of basaltic rocks displays MORB-like geochemical characteristics. The second group exhibits island-arc tholeiite (IAT) geochemical characteristics. The cherts, which coexist within the basaltic rocks, yielded Middle Jurassic to Early Cretaceous radiolarian assemblages. Late Bajocian (Middle Jurassic) to Tithonian (Late Jurassic) ages were obtained from the radiolarian cherts observed within the MORB-type basalts belonging to the first group. On the other hand, the supra-subduction zone (SSZ)-type basalts belonging to the second group are associated with upper Aalenian (Middle Jurassic) and lower Aptian (Lower Cretaceous) radiolarian cherts. The new Middle–Late Jurassic and Early Cretaceous radiolarian dates and MORB- and SSZ-type basaltic rocks are consistent with radio-isotopic ages previously obtained from ophiolites and ophiolitic mélanges along the İAE suture zone. The radiolarian ages associated with basaltic rock data indicate that the occurrence of the MORB- and SSZ-type magmatism were simultaneous. The coeval IAT and MORB-like basalt generation inferred to occur in the SSZ environment similar to the South Sandwich back-arc basin system from South Atlantic Ocean, the Mariana Trough (e.g., Fiji and Tonga basin) from the Western Pacific and Coast Range Ophiolite from California.

Early Paleozoic tectonic evolution of the northern West Junggar (NW China): Constraints from Early Cambrian–Middle Silurian felsic plutons of the Chagantaolegai ophiolitic mélange

The early Paleozoic tectonic evolution of the Junggar Ocean (a major branch of the southern Paleo-Asian Ocean (PAO)) remains a topic of debate. This study mapped the Chagantaolegai ophiolitic mélange (COM) in the northern West Junggar (NWJ) on a large scale and identified several tectonically juxtaposed lithotectonic units, including the ophiolite suite and felsic plutons. The ophiolite suite mainly comprises serpentinized ultramafic rock, pyroxenolite, gabbro, dolerite, plagiogranite, basalt, volcanic rock, and radiolarian chert. Zircon U–Pb ages for two plagiogranite samples yielded ages of 515 ± 4 Ma and 513 ± 6 Ma, constraining the existence of the Junggar Ocean to the Early Cambrian at least. The felsic plutons can be subdivided into two groups. Group I (503–481 Ma), which intruded into the COM, shows low-K, calc-alkaline features with remarkable depletion of Nb, Ta, and Ti, resembling rocks formed in an intra-oceanic arc. Group II (435–428 Ma) was found on both sides of the COM and displays high-K, calc-alkaline series characteristics, similar to the Silurian to Early Devonian A-type granites in the NWJ, implying a post-collisional environment. Based on a combination of existing observations and our new observations and data with regional geological evidence, our new data indicate that the NWJ underwent a transition from the tectonic processes of seafloor spreading and subduction to arc–arc collision and post-collisional extension during the Early Cambrian–Early Devonian.

Reassessing the age of Karpathos ophiolite (Dodecanese, Greece): consequences for Aegean correlations and Neotethys evolution

AbstractWhile the Neogene history of the Eastern Mediterranean region is now fairly well understood, our knowledge of older regional palaeogeographies is less accurate, especially the positions of blocks and nappes constituting the Aegean Islands prior to the Cenozoic. Our study focuses on the ophiolite exposed on the island of Karpathos (Dodecanese), which is located in the Aegean fore-arc at a pivotal position between the ‘western’ and ‘eastern’ ophiolites of the Mediterranean region and where conflicting Late Jurassic and Late Cretaceous ages have led to diverging tectonic and palaeogeographic interpretations. To test these ages, we targeted the radiolarian cherts that depositionally overlie the ophiolite and extracted diagnostic radiolarian assemblages of Aptian (∼125−113 Ma), early–middle Albian (∼113−105 Ma) and Turonian (∼93.9−89.8 Ma) ages. These results suggest that previous Late Cretaceous K–Ar isotopic ages (from 95.3 ± 4.2 Ma to 81.2 ± 1.6 Ma) may have been reset by Late Cretaceous metamorphism or affected by argon loss. Overall, the new Early Cretaceous ages show that the Karpathos ophiolite should not be correlated with the Pindos Nappes of Greece or the ophiolites of Cyprus or Syria but rather with the Lycian Nappes of Turkey and their root located in the Izmir–Ankara–Erzincan Suture Zone. Therefore, the Karpathos ophiolite represents a remnant of the Northern Neotethys, not the Pindos Ocean or the proto-Eastern Mediterranean Basin.

Study and Use of Upper Cambrian to Lower Ordovician conodonts in central, southern, and western Laurentia, 1933–2018

The first descriptions of Laurentian Lower Ordovician conodonts were in the 1930s in the central USA. Authors boiled shales to free the conodont elements. The use of acetic acid began in 1940 at the University of Missouri. The first study of Cambrian conodonts was in 1959; part of the material was from the western USA and part from northern Europe. In the mid 1960s, research began to be based on measured sections and systematically collected samples; many new taxa still were being described. Histological studies resulted in recognising three major groups of Cambrian conodonts, protoconodonts, paraconodonts, and euconodonts. The latter group was the most successful. Preliminary biozonal frameworks were established in the early 1970s and were tied into biozonations based on other fossil groups. Conodont studies became more widespread geographically during the 1980s, biozonations were refined, and conodont zones began to be correlated globally based on many cosmopolitan taxa. In the 1980s, geochemists found that conodonts were useful source materials for chemostratigraphy. The Pander Society was founded in 1967. Regular meetings of the society have been held since 1968 in the USA and internationally, resulting in many important proceedings volumes. The International Working Group on the Cambrian–Ordovician Boundary decided to use conodonts as the primary fossil group to define that boundary, thus stimulating intensive study of conodonts of that age. Definition of that boundary was achieved in 2001, although problems remain because of redeposited conodonts at the stratotype section. A Working Group will recommend a GSSP for the base of the highest stage of the Cambrian, Stage 10. The lowest occurrence of the euconodont Eoconodontus notchpeakensis (Miller Journal of Paleontology, 43(2), 413–439 1969) is being considered to define the base of that stage. The species occurs in depositional environments from nearshore sandstones and dolostones to deep-ocean radiolarian cherts, indicating that conodonts had become adapted to many depositional environments during the late Cambrian. The alternative fossil for marking the base of Stage 10 is the lowest occurrence of an agnostoid arthropod, Lotagnostus americanus (Salter 1860), which has a controversial taxonomy.

The origin and emplacement of the Lajishankou ophiolite complex in the South Qilian belt: evidences from geological mapping

AbstractMany ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.

Tectonic, diapiric and sedimentary chaotic rocks of the Rakhine coast, western Myanmar

The western margin of Myanmar is the northern extension the active Sunda (India-Eurasia) subduction zone. Coastal regions and offshore islands have remarkable exposures of chaotic rock terranes along wave-cut terraces that allow characteristics of tectonic, sedimentary and diapiric mélanges to be recognized. Tectonic shear zones (tectonic mélanges) contain fragments of Cretaceous ophiolites (chrome-spinel-bearing peridotites and radiolarian cherts) that are in contact with thrust packets of Eocene turbidite units (broken formations). The turbidites contain shale-rich beds that have been sheared during soft-sediment deformation (sedimentary broken formations) and are sandwiched between undeformed thick sandy beds. These are mass transport deposits (MTDs) that most likely formed during deposition of the initial detritus of the Himalayan orogenic zone, probably trench slope basins on the accretionary prism. The ophiolitic and turbiditic thrust slices have been exhumed and are currently being intruded by active mud volcanoes that bring fragments of units up from depth to the surface, forming diapiric mélanges. These diapiric mélange bodies contain only small fragments (<50 cm) that are randomly oriented and do not exhibit shear fabrics. Because the region lacks superimposed deformation characteristic of most orogenic belts, the origins of all three rock bodies can easily be distinguished.

Cretaceous tectonic evolution of the Neo-Tethys in Central Iran: Evidence from petrology and age of the Nain-Ashin ophiolitic basalts

The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.

Structural architecture of the Western Alpine Ophiolites, and the Jurassic seafloor spreading tectonics of the Alpine Tethys

We present a regional synthesis of the structural architecture and tectonic evolution of the Western Alpine Ophiolites (WAO), exposed in NW Italy. The WAO represent the remnants of Alpine Tethys (Ligurian–Piedmont Ocean) that opened between Europe and Adria, and developed in four stages from the Middle Jurassic to Early Cretaceous. Emplacement of gabbroic intrusions into the extending lithospheric mantle of Europe–Adria marked the main magmatic event (Stage 1). Coalescent shear zones in the fossil upper mantle form lithospheric-scale detachment faults, which led to the exhumation of upper mantle peridotites and gabbros on the seafloor, and extensive serpentinization (Stage 2). Detachment faults, and serpentinized peridotites–gabbros in their footwalls, represent preserved fossil oceanic core complexes within the WAO. Emplacement of ophiolitic breccias and basaltic lava flows marked the syn-extensional phase (Stage 3). Radiolarian chert and limestone were deposited unconformably on this syn-extensional volcanic–sedimentary sequence, marking the post-extensional phase (Stage 4). Magmatic ages of gabbroic intrusions and mafic–felsic dykes, and depositional ages of post-extensional sequences in the WAO constrain the timing of the opening of the Ligurian–Piedmont Ocean to the Middle Jurassic ( c. 170–168 Ma), followed by a tectonic quiescence stage in the Late Jurassic–Early Cretaceous. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection' available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Geochemical Evidence for the Depositional Environment of the Esfandaghe Manganese Deposit, Kerman Province, Iran

The Esfandaghe manganese deposit is located in the east of Esfandaghe village, Kerman province in Iran. Late Jurassic basalt, lithic tuff, radiolarian chert, shale, flysch turbidite, sandstone, mudstone and dark shale as well as mid-early Jurassic volcanic rocks of rhyolitic crystal tuff and tuffaceous sandstone are the typical stratigraphic units in the study area. Mn ore occurs as interlayers with radiolarian chert and shale overlaying the late Jurassic volcanic rocks. Mn/Fe ratio in manganese ore bodies varies from 0.10 to 43.82 and the position of samples on Fe–Si × 2-Mn, Mn–Fe–(Co + Ni + Cu) × 10, Ni–Zn–Co and (Co + Ni)–(Zn + Cu + Pb + V) diagrams reveals that hydrothermal activity was the main process responsible for mineralization in the Esfandaghe manganese deposit. Cu, Zn, Ni, Co and HREEs are enriched in Esfandaghe manganese relative to Post-Archean Australian Shale. REE contents in the Esfandaghe Mn deposit vary from 22 to 313 ppm, conformable with REE concentration ranges of hydrothermal ferromanganese deposits.

THE LOWER BAJOCIAN GAETANI LEVEL: LITHOSTRATIGRAPHIC MARKER OF A POTENTIAL OCEANIC ANOXIC EVENT

In this paper we document in detail the transition from the Rosso Ammonitico Lombardo to the Radiolarites outcropping at Alpe Turati (Albavilla, Como) in the Lombardy Basin of the Southern Alps. At this location, the Jurassic succession was deposited on a flank of the Corni di Canzo paleohigh: as found on other structural shallow areas, the upper Pliensbachian–Toarcian–Aalenian interval is represented by pseudonodular to nodular marly limestones of the Morbio Limestone and Rosso Ammonitico Lombardo units. In the early Bajocian deposition of biosiliceous sediments is ubiquitously testified by the Radiolarites (Bajocian–Callovian). At Alpe Turati, the interval immediately below the basal green member of the latter unit, consists of a 10 cm thick black shale, pointing to oxygen-depleted bottom waters. Dysoxia persisted during the Bajocian–Callovian as evidenced by dark green stratified cherts. Calcareous nannofossil biostratigraphy indicate an early Bajocian age for the black shale that correlates with the core of a distinct C isotopic negative anomaly also known in various basins outside the Southern Alps. Very dark grey to black lithologies of early Bajocian age have been documented in a few sections from Poland, Corsica and Morocco. We name Gaetani Level the black shale interval in recognition of Maurizio Gaetani’s pioneering and extensive work on Jurassic sedimentary successions of the Lombardy Basin. The association of the Gaetani black shale with a C isotopic anomaly suggests that it could be the sedimentary record of an Oceanic Anoxic Event whose regional to global extension must be ascertained.

EARLY TITHONIAN DEEP-WATER COLONIZATION BY BENTHIC FORAMINIFERA IN THE MAGURA BASIN (PIENINY KLIPPEN BELT, WESTERN CARPATHIANS): A CLUE TO THE ORIGINS OF DEEP-WATER FORAMINIFERA.

Deep-water benthic foraminifera were reported from lower Tithonian abyssal deposits lying above a barren interval of radiolarites in the Magura Basin (Western Carpathians). The analysis of morphogroups shows that the assemblage is represented by increased globular chambered morphogroups of both agglutinated and calcareous benthic foraminifera and planoconvex calcareous benthic foraminifera. The composition of the benthic foraminiferal assemblage points to a stressed setting particularly occurring in environments with increased organic flux and depleted oxygen in bottom and pore waters, as indicated by the scarcity of strictly epifaunal agglutinated foraminifera and the absence of strictly epifaunal calcareous benthic foraminifera. The assemblage is compared with Oxfordian – Tithonian assemblages reported from deep-water facies and shelf assemblages from the Boreal, Tethys and Atlantic realms.

Phenomenon of schistic breccia, autoclastic breccia and radiolarian chert, a Mesozoic tectonic trace in Bantimala Tectonic Complex Area, Pangkep Regency South Sulawesi Province

The presence of the schistic breccia and the autoclastic breccias and the radiolarian chert on the Bantimala tectonic complex is one of the interesting geological phenomena to study. In addition to knowing the position and stratigraphic relationships between radiolarian chert with autoclastic breccia as basement rock, the secular breccia may reveal Bantimala’s tectonic environment in the Mesozoic time. This research uses secondary data assessment method from previous researcher and field investigation in the form of observation and rock sampling for petrographic and chemical analysis (X-Ray Fluorescence) to the chemical elements of rock. The appearance of a field indicating the presence of an alternating of the radiolarian chert with a schistic breccias and a sandstone insertion at the bottom of which is in the same composition as the schistic breccia and the autoclastic breccia as the source of some of its components, indicating the stratigraphy relation between radiolarian chert with schistic breccias are conformably, while the radiolarian chert with an autoclastic breccia is unconformably. The metamorphic rock components present in the schistic breccia and the autoclastic breccia are varied in the schistic breccias of about 42.94 - 91.44% SiO2, while the autoclastic breccia 33 - 92.02% SiO2 showing of protolith a mixing of sedimentary rocks, ultramafic, basalt and intermediates rocks. The result of plotting on the spider diagram of trace elements and rare earth elements indicates that the autoclastic breccias originate from the Mid-oceanic ridge basalt (MORB), while the schistic breccias are derived from MORB, OIB and active continental margin.

Blueschist facies overprint of late Triassic Tethyan oceanic crust in a subduction–accretion complex in north-central Anatolia, Turkey

The late Cretaceous accretionary complex along the İzmir–Ankara–Erzincan suture zone, northern Turkey, includes various types of metamorphic rock together with radiolarite, sandstone, mudstone, serpentinite, basalt and limestone. Meta-plagiogranite blocks (up to 5 m in diameter) and a meta-gabbro slice (800 × 500 m) cross-cut by meta-plagiogranite are observed in a matrix of serpentinite, mudstone and radiolarian chert. These meta-gabbros and meta-plagiogranites show subduction-related geochemical characteristics. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons from two meta-plagiogranites yielded 222.3 ± 1.5 and 227.2 ± 1.6 Ma crystallization ages. This study shows that the meta-gabbro and meta-plagiogranite from the accretionary complex are remnants of the metamorphosed equivalents of the late Triassic lower crustal rocks of the subducted Tethyan oceanic crust. They are the oldest lower crustal rocks observed in the İzmir–Ankara–Erzincan suture zone. Both the meta-gabbros and meta-plagiogranites have Na–Ca amphiboles, indicating blueschist facies metamorphism. However, phengites from a blueschist facies rock yielded an 40 Ar/ 39 Ar age of 104.3 ± 0.7 Ma, indicating that blueschist metamorphism occurred during the Albian. All these data suggest that the oceanic crust formed during the Norian, metamorphosed during the Jurassic to early Cretaceous and then dismembered during accretionary complex formation in the late Cretaceous. Supplementary material: A complete description of the analytical methods ( electron microprobe, LA-ICP-MS zircon U–Pb and 40 Ar/ 39 Ar age analyses, pseudo-section modelling and THERMOCALC) are available at https://doi.org/10.6084/m9.figshare.6509984 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Early Cretaceous origin of the Woyla Arc (Sumatra, Indonesia) on the Australian plate

Key to understanding the plate kinematic evolution of the Neotethys oceanic domain that existed between the Gondwana-derived Indian and Australian continents in the south, and Eurasia in the north, is the reconstruction of oceanic plates that are now entirely lost to subduction. Relics of these oceanic plates exist in the form of ophiolites and island arcs accreted to the orogen that stretches from Tibet and the Himalayas to SE Asia that formed the southern margin of Sundaland. The intra-oceanic Woyla Arc thrusted over western Sundaland – the Eurasian core of SE Asia – in the mid-Cretaceous. The Woyla Arc was previously interpreted to have formed above a west-dipping subduction zone in the Early Cretaceous, synchronous with east-dipping subduction below Sundaland. The oceanic ‘Ngalau Plate’ between the Woyla Arc and Sundaland was lost to subduction. We present paleomagnetic results from Lower Cretaceous limestones and volcaniclastic rocks of the Woyla Arc, Middle Jurassic radiolarian cherts of the intervening Ngalau Plate, and Upper Jurassic–Lower Cretaceous detrital sediments of the Sundaland margin. Our results suggest that the Woyla Arc was formed around equatorial latitudes and only underwent an eastward longitudinal motion relative to Sundaland. This is consistent with a scenario where the Woyla Arc was formed on the edge of the Australian plate. We propose a reconstruction where the Ngalau Plate formed a triangular oceanic basin between the N–S trending Woyla Arc and the NW-SE trending Sundaland margin to account for the absence of accreted arc rocks in the Himalayas. As consequence of this triangular geometry, accretion of the Woyla Arc to the western Sundaland margin was diachronous, accommodated by a southward migrating triple junction. Continuing convergence of the Australia relative to Eurasia was accommodated by subduction polarity reversal behind the Woyla Arc, possibly recorded by Cretaceous ophiolites in the Indo-Burman Ranges and the Andaman-Nicobar Islands.

MESOZOIC TETHYAN RADIOLARITE AGE DETERMINATION, ZAGROS SUTURE ZONE, KURDISTAN, NE IRAQ

A regional belt of Mesozoic Tethyan radiolarites extends along the northeastern Arabian plate's margin. Outcrops of this radiolarite were documented from Hawasina series in Oman, and northeastwards to Pichakun Series of southern Iran, to Kermanshah of northwest Iran passing onto the Qulqula Group of northeast Iraq up until the Kocali Series of Turkey. The age of most of these radiolarite suites was already constrained. It is the purpose of this work to investigate the age of the radiolarite Suites in northeast Iraq using Radiolarian microfossils chronostratigraphy. Patches of the Mesozoic radiolarite suites are exposed at the Zagros suture zone in Kurdistan Region of northeast Iraq and known as the Qulqula Group. It consists of complexly deformed and thick slices of pelagic radiolarian chert and siliceous limestone, radiolarian red claystone and siliceous mudstone, and reddish-green mudstone and limestone. These units were sampled from three areas (Naurabab, Penjween, and Hawraman) located to the south and east of Sulaimani city in Kurdistan, for radiolarian micropaleontological analysis. Radiolaria microfossils identification and age determination is done at specialized laboratories. Results of the radiolarian age constraint show that these sediments range from Bajocian to the end of the Cenomanian.

New radiolarian data from the Jurassic ophiolitic mélange of Avala Mountain (Serbia, Belgrade Region)

An ophiolitic melange of Avala Mountain located south of Belgrade was studied to obtain age data from blocks of radiolarian cherts and to enable better understanding of age and nature of this melange. The studied area is located south of Avala Mt. and its geotectonic position is rather obscure. The obtained data allow us to determine radiolarians from five samples; one is of Late Anisian to Early Ladinian, and four of them are of Middle Jurassic (Late Bathonian–Early Callovian) age. The finding of Triassic as well as Jurassic age radiolarians in one and the same melange suggests that this melange formed during obduction of the Western Vardar ophiolites.

Accreted seamounts in the South Tianshan Orogenic Belt, NW China

The South Tianshan Orogenic Belt (STOB) is situated in the southwest margin of the Central Asian Orogenic Belt and is characterized by thedistributionof ophiolitic mélanges and metamorphic rocks. In this study, we focus on 5 ophiolitic mélanges with ocean island basalt (OIB) and aim to discuss the tectonic evolution of the South Tianshan Ocean. The Kynda, Kainar, Uchkel, Heiyingshan, and Guluogou ophiolitic mélanges in STOB mainly consist of metamorphic peridotite, gabbros, basaltic lavas (pillows), abyssal radiolarian cherts, and limestones. The basaltic rocks mainly occur as tectonic blocks within the mélanges. These basaltic rocks are characterized by LREE enrichment and HREE depletion, very weak or no Eu anomalies, and no obvious Nb, Ta, and Ti negative anomalies, positive εNd(t) values, relatively high initial87Sr/86Sr ratios, and wide206Pb/204Pb ratios, showing the signature of OIB. The OIB‐type rocks from Kynda, Kainar, and Guluogou are likely to be derived from garnet lherzolite by relatively high degrees of melting (10% to 20%). But the basaltic rocks of Uchkel and Heiyingshan formed at significantly lower degrees of partial melting (1% to 5%) of spinel and/or garnet lherzolite. The OIB‐type rocks in these ophiolitic mélanges are considered as accreted seamount fragments and may represent mantle plume‐related magmatism within the South Tianshan Ocean. In combination with previous work, we suggest an updated model, which is bidirectional oceanic subduction with seamounts accretion and subduction system, is suitable for Palaeozoic tectonic evolution of the South Tianshan Ocean.

Radiolarian and Detrital Zircon in the Upper Carboniferous to Permian Bancheng Formation, Qinfang Basin, and the Geological Significance

The Bancheng Formation exposed along the Shiti Reservoir nearby Bancheng Town, Qinfang Basin, southern Guangxi, is mainly composed of thin-bedded chert. The radiolarian assemblages in the studied section suggest it was a pelagic setting and the age of the Bancheng Formation is Late Carboniferous to Early Permian. The detrital zircon U-Pb ages from the section are characterized by a Permian peak at ~282 Ma. Detrital zircon provenance analysis suggests that the Permian detrital zircons in the Bancheng Formation were likely from volcanic-magmatic arc rocks related to the subduction of the Paleo-Tethyan Ocean. The long-lived deposition (from Upper Devonian to Middle Permian, about 125 Ma) of the radiolarian cherts in the Qinfang Basin was comparable with that deposited in Ailaoshan Ocean. The radiolarian assemblages in Qinfang Basin show a Tethyan affinity. Together with the Permian subduction-related arc volcanic rocks and the E-MORB type basalts to the northwest of the study area, our data support the existence of a Permian arc-related basin in the Qinfang area.