Blocks Of Chert Research Articles

Late Jurassic to early Cretaceous radiolarian faunas from Changku, Gyangze, southern Tibet: Implications for the evolution of the Neo-Tethys

Radiolarians found in siliceous sediments within the Yarlung-Tsangpo Suture Zone in southern Tibet contain valuable geochronological and tectonic information for the evolution of the Neo-Tethys. Radiolarians, as the main microfossils contained in radiolarian cherts, are commonly used to determine the age of the chert. The study of radiolarian biostratigraphy is crucial in interpreting the tectonic nature of associated oceanic terranes, which can aid in restoring the evolution of the Neo-Tethys. A large number of poorly to well-preserved radiolarian fossils were recovered from a chert block near Changku, Gyangze, in southern Tibet. A total of 57 species attributed to 36 genera have been identified from eight productive samples. Four radiolarian assemblages have been identified by comparison with established radiolarian zones of the Neo-Tethyan realm and have been precisely dated to the late Kimmeridgian to earliest Hauterivian. The origin of the exotic blocks in the Zongzhuo Formation has been subject to different interpretations, and the chert block near Changku should be of tectonic origin.

The youngest matrix of 234 Ma of the Kanguer accretionary mélange containing blocks of N-MORB basalts: constraints on the northward subduction of the Paleo-Asian Kanguer Ocean in the Eastern Tianshan of the Southern Altaids

The Altaids accreted around, and grew southward, from the Siberian craton, but the time of final amalgamation of this orogen is still controversial. The Eastern Tianshan in the southernmost Altaids is characterized by multiple, late, accreted arcs and thus is an ideal tectonic environment to answer the time of final amalgamation of the Altaids. In this study we report the results of new field-based lithological mapping and structural analysis on the Kanguer melange in the Eastern Tianshan, which is composed of blocks of basalt, chert, limestone, and other rocks within a strongly deformed and cleaved matrix of sandstone and schist. Our geochemical and isotopic data of basaltic blocks from several parts of the Kanguer melange show they are relics of Normal-Mid-Ocean-Ridge (N-MORB)-type oceanic lithosphere, and U–Pb ages and Hf isotopes of detrital zircons from the matrix sandstones indicate they were derived only from the Dananhu arc to the north. Accordingly, our interpretation is that the Kanguer melange was part of an accretionary complex that fringed the Dananhu arc, and therefore the subduction polarity of the Kanguer Ocean was to the north (present coordinates). The maximum depositional ages (MDAs) of our three sandstone samples (08K01, 08K02, and 08K03) from the melange matrix were 234 ± 14 Ma, 242.5 ± 1.3 Ma, and 236 ± 2.0 Ma respectively, indicating that the Kanguer Ocean was still being subducted at ca. 234 Ma, and the accretion of the Kanguer melange must have lasted until that time, when the accretionary complex was still located opposite to the Yamansu-CTS accretionary complex to the south. Thus, the final amalgamation of the Dananhu and Yanmansu-CTS arcs took place by the welding of two accretionary complexes in the late Middle Triassic (Ladinian) in this part of the southern Altaids. Integration with relevant amalgamation histories throughout the Tianshan indicates that the time of terminal amalgamation in the southern Altaids was probably in the Middle-Late Triassic, which is much younger than previously envisaged.

Rock Assemblages and Formation Ages of the Baishuijiang Group in the Southwest Qinling Orogenic Belt, Northwest China

AbstractThe Baishuijiang Group, located in the southwest Qinling orogenic belt, is divided into three belts according to the characteristic of the matrix and rock blocks based on the large scale geological mapping. The north belt and south belt are composed of abyssal mudstone and siltstone, and limestone, chert and basic and ultrabasic rock blocks. The middle belt consists of a few limestone blocks and turbidites, which were formed in the trench environment. At present, in the Baishuijiang Group, many fossils were found in matrix and rock blocks, the fossils contain the Cambrian small shell fossils (Xiao, 1992; Tao et al., 1992), Silurian chitinozoas, scolecodonts and spores, and Ordovician graptolites, and middle Devonian Coral and conodonts in limestone and chert blocks (Wang et al., 2011a), and Permian radiolarians in the matrix (Wang et al., 2007). The volcanic rock blocks have undergone different degree of metamorphism. Their geochemical characteristics indicate that the rocks are similar to oceanic island arc and seamount (Wang et al., 2009), and SHRIMP U‐Pb dating yielded ages from Neoproterozoic to early Paleozoic (Yan et al., 2007; Wang et al., 2009, 2011b). Therefore, comprehensive analysis of regional data, the Baishuijiang group is an accretionary complex which was consisted of matrix and blocks, and was finally formed during Permian‐Triassic.

Magnetostratigraphic and archaeological records at the Early Pleistocene site complex of Madigou (Nihewan Basin): Implications for human adaptations in North China

The Nihewan Basin in North China contains the densest concentration of early Pleistocene Paleolithic sites outside Africa. This paper introduces a new archaeological site complex at Madigou (MDG) that was systematically excavated from 2011 to 2014 in the northeastern part of the Nihewan Basin. The site contains fossils and well-preserved stone artefacts in fluvio-lacustrine sediments. Our magnetostratigraphic results situate the MDG sedimentary sequence in the early Brunhes normal chron and the late Matuyama reverse chron, including the Jaramillo normal subchron. The MDG artifact layers are positioned within the pre-Jaramillo Matuyama chron, with an estimated age of ca. 1.2 Ma, close to the onset of Mid-Pleistocene climate transition. The MDG core and flake technology includes bipolar flaking of siliceous dolomite cobbles, and freehand flaking of chert and brecciated chert block fragments. Mammalian fauna and pollen compositions indicate that the MDG hominins lived in an open habitat varying from lightly-wooded grassland to an ecosystem dominated by sparse steppe near the shore of the Nihewan paleolake. Our combined results in the fields of archaeology, paleontology, palynology and magnetochronology suggest that innovations in technological behavior may correlate with adaptations to high environmental variability during the start of Mid-Pleistocene climate transition.

Lajishan Accretionary Complex: Implications for Paleozoic Accretionary and Collisional Events in the South Qilian Belt

AbstractThe Qilian orogen along the NE edge of the Tibet‐Qinghai Plateau records the evolution of Proto‐Tethyan Ocean that closed through subduction along the southern margin of the North China block during the Early Paleozoic. The South Qilian belt is the southern unit of this orogen and dominated by Cambrian‐Ordovician volcano‐sedimentary rocks and Neoproteozoic Hualong complex that contains similar rock assemblages of the Central Qilian block. Our recent geological mapping and petrologic results demonstrate that volcano‐sedimentary rocks show typical rock assembles of a Cambrian‐early Ordovician arc‐trench system in Lajishan Mts. along the northern margin of the Hualong Complex. Island arc rocks including basalt, andesite, dacite, rhyolite, and breccia is in fault contact with ophiolite complex consisting of mantle peridotite, serpentinite, gabbro, dolerite, plagiogranite, and basalt. Accretionary complexes are tectonically separated from the ophiolite‐arc rocks, with various rock assemblages spatially. They consist of pillow basalt, basalt breccia, tuff, chert, and limestone blocks with a seamount origin within the scaly shale in Dingmaoshan and Donggoumeikuang areas, and basalt, chert, and sandstone blocks within muddy shale matrix and mélange at Lajishankou area. Abundant radiolarians occur in red chert, and trilobite, brachiopod, and coral fossils occur within Dingmaoshan limestone blocks. Although partial basalt or chert blocks are highly disrupted, duplex, thrust fault, rootless intrafolial fold, tight fold, and penetrative foliation are well‐developed at Donggoumeikuang area. Spatially, accretionary complexes lie structurally beneath ophiolite complex and above the turbidites of the Central Qilian block. Ophiolite and accretionary complexes are also overlapped by late Ordovician molasse deposits sourced from Cambrian arc‐trench system and the Central Qilian block. These observations demonstrate that a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt formed during the early Paleozoic southward subduction of the South Qilian Ocean collided with the Central Qilian block prior to the late Ordovician.

Manganese Mineralization in the Mesozoic Siliceous Deposits of the Central Sikhote-Alin and Nadanhada-Alin

The stratiform ferromanganese bodies in the Mesozoic chert formations of central Sikhote-Alin (Katen R., J3) and Nadanhada-Alin (Khuntseling settlement, J2–3), and the vein manganese deposit (Ktab) at the Glubinny settlement area (central Sikhote-Alin) are characterized for the first time. Jurassic primary ores occur as interlayers in clayey–siliceous units. They consist of grains and globules of Mn and Fe oxides of the psilomelane–pyrolusite group and goethite. The interlayers also contain the allogenic products of eroded sialic rocks (quartz, potassium feldspar, albite, kaolinite, muscovite, paragonite, illite, and zircon) and accessory ore minerals (monazite, sphalerite, galena, apatite, cassiterite, barite, pyrite, titanite, rutile, native silver, auricupride, cerianite, tetrahedrite, and intermetallic Cu5Zn3 and Ni7Cu4). In the vein ores, spessartine and rhodonite fill veinlets and form a matrix of tectonic breccia. Manganese oxides form crusts (up to 4 cm) on the Triassic chert lumps and blocks. They likely represent the products of oxidation and breakdown of former hydrothermal–metasomatic manganese carbonates and silicates in a supergene environment. In contrast to the manganese minerals from the stratiform ores, those of the vein ores do not contain Co, Ni, Zn, and Pb, which are involved in the sulfides and sulfoarsenides (Co-gersdorffite and Ni-cobaltite, smaltite, pentlandite, and other minerals). Magnetite; titanomagnetite; barite; chalcopyrite; pyrrhotite; galena; argentite; zircon; titanite; wollastonite; anhydrite; REE phosphates, silicates, and oxides; bismuthine; native Ag, W, Cu, Zn, and Ni; and intermetallic compounds (Cu5Zn3, (Pd, Pt)5Cu and Pd4Cu) are also recognized in the vein manganese ores.

A giant late Precambrian chert-bearing olistostrome discovered in the Bohemian Massif: A record of Ocean Plate Stratigraphy (OPS) disrupted by mass-wasting along an outer trench slope

An intriguing example of chert–graywacke olistostrome is exceptionally well preserved within the late Neoproterozoic to early Cambrian Blovice accretionary wedge, Bohemian Massif. The olistostrome exhibits a block-in-matrix fabric defined by chert blocks isolated within the graywacke matrix. The major and trace element composition indicates two distinct types of cherts that formed either in a hydrothermal pelagic or hemipelagic environment supplied with a distal terrigenous material. The former is documented by elevated contents of Fe, Co, Zn, Ni, and Ti whereas the latter by high Al2O3 contents, relatively lower LaN/CeN ratios, and higher Eu/Eu* and Ce/Ce* values. Based on these geochemical data integrated with field observations and detrital zircon U–Pb ages of the host graywackes (determined using laser ablation ICP-MS), a new model for the origin of chert–graywacke association is proposed. The cherts are interpreted as representing pelagic and hemipelagic members of the Ocean Plate Stratigraphy (OPS) that formed in a sedimentary basin, carried on top of a subducting plate towards the trench. While moving over the outer swell (rise), the chert basin was intensely fractured and disrupted into large blocks or slabs. Subsequent motion of the plate brought the blocks onto an outer trench slope where they became gravitationally unstable to slide down and mix in the trench with distal, ca. 580–570 Ma turbidites derived from the overriding plate. Finally, this chert–graywacke olistostrome was covered by younger, ca. 560–547 Ma trench-fill turbidites (devoid of chert blocks) and accreted to the accretionary wedge toe, deformed, buried, and exhumed back to the wedge surface. We propose that such an olistostrome composed of pelagic/hemipelagic chert blocks and terrigenous, arc-derived graywacke matrix represents a rarely documented case of submarine, outer trench slope mass-wasting deposits and may be considered a new type of subduction-related mélanges. We coin the term outer-trench-slope mélange.

Emplacement of the ophiolitic mélanges in the west Karamay area: Implications for the Late Paleozoic tectonic evolution of West Junggar, northwestern China

The Darbut and Karamay ophiolitic mélanges in the west Karamay area at the southeastern margin of West Junggar, southern Altaids, represent the relict fragments of the Paleozoic Junggar ocean basin. The mélanges contain structurally complex tectonic blocks of ultramafic rocks, gabbro, basalt, and chert that are dispersed within an extensively sheared serpentinite matrix. This study presents new field observations, structural analyses, geochronological and geophysical data for the mélanges and the overlying Carboniferous strata. Three stages of deformation (D1–D3) have been identified. D1 is only locally preserved in the mélanges and is characterized by thrust-imbricated faults, recording the subduction and accretion of the paleo-oceanic crust in the Early Carboniferous. D2 mainly consists of map-scale upright folds (F1) in the Carboniferous strata and NE-SW-striking sub-vertical right-lateral shear zones in the mélanges. D3 is characterized mainly by NE-SW-striking transcurrent left-lateral strike-slip faults and “S-shaped” drag folds (F2). The extensive tectonic reworking during D2 and D3 deformations eradicated most of the D1 structures. Our structural observations, combined with geophysical data, suggest that the basement of the west Karamay area consists of trapped oceanic crust or dismembered ophiolitic fragments and that the Darbut and Karamay ophiolitic mélanges were extruded through right-lateral shear zones. Our results and the published data further suggest that the emplacement of the mélanges and the formation of F1 folds represent regional-scale amalgamation processes between different tectonic blocks and/or plates in the Late Carboniferous. These processes resulted in the closure of the West Junggar remnant ocean basin and other sub-oceanic basins in the Paleo-Asian Ocean, leading to the formation of the Kazakhstan orocline. Furthermore, the D3 deformation was associated with anticlockwise rotation of the West Junggar region in a post-collisional setting in the Permian.

An aberrant bi-apicalFollicucullus(Albaillellaria) from the late Guadalupian (Middle Permian), with the possible oldest evidence of double malformation in radiolarians

Abstract. An aberrant bi-apical Follicucullus specimen (Albaillellaria, Radiolaria) has been discovered from an upper Guadalupian (Middle Permian) chert block of the Kamiaso Unit of the Mino terrane, central Japan. If this specimen was formed with double malformation, it would be the oldest record of this phenomenon in radiolarians and the first record of its kind in Albaillellaria.

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.

Detrital zircon geochronology of Devonian quartzite from tectonic mélange in the Mianlue Suture Zone, Central China: provenance and tectonic implications

ABSTRACTDevonian quartzite occurs as blocks within a phyllite matrix in Puziba area of the Mianlue Suture Zone (MLSZ) in central China. The depositional time of the quartzite is younger than 425 Ma (mainly Early Devonian), constrained by the zircon U–Pb geochronology data from the quartzite, cross-cutting relationships with granite, and palaeontology evidence. The detrital zircons in the quartzite show typical magmatic features with four main age peaks at: 2676–2420 Ma (11.6% of the population), 1791–1606 Ma (4.8%), 997–817 Ma (26.5%), and 597–425 Ma (17.5%). In combination with the zircon εHf(t) values, we propose that the quartzite in the MLSZ was sourced from Neoproterozoic and Palaeozoic magmatic and sedimentary rocks in the South Qinling Block and the South China Block (particularly from the Bikou Terrane), with minor contributions from Archaean and Palaeoproterozoic magmatic units from both of the South and North China blocks. The blocks of quartzite, slate, marble, metasandstone, and chert blocks in the phyllite matrix in the Puziba area show a typical block-in-matrix texture in a tectonic mélange, and provide significant evidence from sedimentary rock blocks rather than ophiolite or volcanic rock for the existence of the MLSZ.

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.

Macro to micro aspects of the plant preservation in the Early Devonian Rhynie cherts, Aberdeenshire, Scotland

ABSTRACTMaterial excavated from a trench dug to expose the Rhynie Cherts Unit of the Dryden Flags Formation included blocks of Rhynie chert up to 50 cm thick and comprising the full thickness of plant-bearing chert beds. These blocks, and others collected as float, display a variety of macro-textures typical of silicification at the terrestrial surface and in shallow water. On sandy terrestrial surfaces, autochthonous and allochthonous plant debris and plant rhizomes are well preserved, but aerial axes generally decayed prior to silicification. In shallow water, clonal plants, particularlyRhynia, are preserved with aerial axes in growth position, supported by microbial mats and meshworks. Preservation of such delicate fabrics required the creation of a load-bearing framework early in the silicification process, to prevent crushing during early burial.On the microscale, plant taphonomy reveals characteristic features due to plant decay prior to silicification in both wet and dry conditions, and also during the silicification process. Silicification of plants was frequently very rapid, preserving delicate transient features such as spore germination and ejection of sperm cells from antheridia. In situations where plant tissue continued to decay during silicification, the process was slower.

Geology of the volcano-sedimentary complex of the Kaminnyi Potik Unit on Chyvchyn Mountain (Ukrainian Carpathians): preliminary results

The Kamyanyi (Kaminnyi) Potik Unit (Nappe) is the most internal and structurally-highest unit of the Fore-Marmarosh units, and in many places is directly covered by the Marmarosh Nappes of the Marmarosh Crystalline Massif. Chyvchyn Mount is built of this unit and forms a separate tectonic cap (Chyvchyn Mt. Klippe). It consists of the Upper Jurassic/lowermost Cretaceous Chyvchyn Formation, composed mainly of basic volcanic rocks, and the Tithonian-Early Cretaceous Kamyanyi Potik Formation, represented by calcareous and/or turbiditic deposits containing volcanic material. Geological mapping showed that this complex forms a tectonic klippe, which consists of four small tectonic thrust slices. Structurally, the lowermost one is represented by thin-bedded micritic limestones with cherts, and is interbedded with coarse/fine-grained calcareous pyroclastic turbidites (flysch). The second thrust slice is composed of calcareous-pyroclastic breccia with blocks of limestone, basalt, and chert (radiolarite?), which occur within a pyroclastic matrix and of coral limestones with basalt fragments and pyroclastic intercalations. The third thrust slice is constructed of breccia with a pyroclastic and volcanic matrix and clasts of effusive rocks and limestone. The fourth thrust slice – the highest – is represented by massive basaltic pillow lavas. Sedimentologically, the volcano-sedimentary complex represents a whole spectrum of marine mass-movement deposits, from debris flows through proximal turbidites to distal ones, which were formed during latest Jurassic/earliest Cretaceous time in the Outer Dacide-Severinide part of the Carpathian basins.

Central Tibetan Meso-Tethyan oceanic plateau

We report the occurrences of the remnants of a Meso-Tethyan oceanic plateau, encompassing an area of ~ 2 × 10 5 km 2 in central Tibet. The plateau remnants include large volumes of pillow basalt formed largely by emergent to subaerial eruption, minor ultramafic intrusives and cumulates, exotic blocks of limestone, radiolarian chert, graywacke, and shale. Isotopic and paleontological dating suggest two major plateau eruptive events at 193–173 Ma and at 128–104 Ma, respectively. The basalts are characterized by enrichment of incompatible elements and a wide range of Sr–Nd isotope composition (initial ε Nd from –3.71 to + 7.9, initial 87 Sr/ 86 Sr from 0.703927 to 0.707618). The trace element and Sr–Nd isotopic data suggest that these basalts are of affinity with those from the Kerguelen and Tethyan plumes, indicative of a plume mantle upwelling origin with involvement of continental material. The wholesale obduction of the Meso-Tethyan oceanic plateau, along with the dismembered normal oceanic crustal fragments, over the Tibetan continental crust could have given rise to perhaps 2 km elevation of central Tibet during the Late Cretaceous. • Remnants of Meso-Tethyan oceanic plateau in central Tibet covers an area of ~ 2 × 10 5 km 2 . • Plateau remnants are dominated by basalt sections with a thickness of at least 13 km. • Twohree major plateau eruptive events occurred at 193–173 Ma, at 167–162 Ma, and at 128–104 Ma. • Basalts are enriched in incompatible elements, with a wide range of and heterogeneous in Sr–Nd isotopes. • Obduction of the oceanic plateau likely triggered a 2-km elevation of central Tibet.

Examples of Franciscan Complex mélanges in the northernmost California Coast Ranges, a retrospective

A variety of mélanges exist within the Franciscan Complex of the northernmost California Coast Ranges. The Cowan mélange classification is employed in their description. Classic turbidite broken formation is Type I mélange that has suffered coaxial layer-parallel extension prior to folding. Type II mélange (stratally disrupted interbedded sequences of radiolarian chert, black argillite, hyaloclastite, and pillow basalt) exists as mélange blocks and/or thrust fault-bounded slabs. Type III mélange (block-in-matrix mixes of greenstone, chert, limestone, keratophyre, tonalite, turbidites, and blueschist) may have emplaced diapirically and/or by sediment gravity-flow. Type IV mélange (block-in-matrix mixes developed along shear zones) with blocks of chert, greenstone, gabbro, sandstone, and conglomerate in metapelitic or serpentinite gouge is associated with major faults. An example of the latter consists of omphacite-bearing tectonic blocks in serpentinite in an area of imbricate thrust faulting of South Fork Mountain Schist over unfoliated turbidites adjacent to the South Fork fault, which separates the Coast Ranges and Klamath Mountains provinces. This serpentinite-schist Type IV mélange has an affinity to the Coast Range ophiolite, which crops out to the south opposite the Great Valley, and perhaps formed during western translation of the Klamath Mountains.

Chert blocks in Bentong-Raub Suture Zone: A heritage of Palaeo-Tethys

Bentong-Raub Suture Zone is considered as a collision zone between the Sibumasu and East Malaya (Indochina) terranes. Sibumasu was a part of the Cimmerian plate, which was attached to Gondwana during the Carboniferous. East Malaya was attached to the Indochina plate. From the Devonian to the Permian the Sibumasu and East Malaya blocks were separated by an ocean called Palaeo-Tethys. The closure of Palaeo-Tethys was completed during the Triassic. The major part of the history of the Palaeo-Tethys was destroyed and only a small portion is still preserved in blocks of oceanic sedimentary rocks such as cherts. The chert blocks are exposed in several localities along the Bentong-Raub road (3°35'N, 101°54'E), Gua Musang-Cameron Highland road (4°45'N, 101°45'E) and at a road-cut in Langkap (2°38'N, 102°21'E). The chert blocks consist of thinly bedded chert interbeded with mudstone. Some chert layers contain radiolarians. The oldest chert block found at Bentong-Raub road-cut yielded an assemblage of radiolarians belonging to the Trilonche minax Zone of early Frasnian, (early Late Devonian) age. A chert block from Langkap, Negeri Sembilan yielded radiolarians belonging to the Albaillella deflandrei Assemblage Zone, Tournaisian, (Early Carboniferous) age. Permian radiolarians were retrieved from several chert blocks in the vicinity of Pos Blau, Ulu Kelantan. The youngest radiolarian assemblage in the area is from the Follicucullus monacanthus Assemblage Zone indicating Wordian, (Middle Permian) age. The occurrence of radiolarian chert suggests high plankton productivity during the Late Devonian, Early Carboniferous and Permian. These chert blocks are the natural heritage of the Paleo-Tethys which needed to be conserved as National Heritage sites.

A late-Carboniferous to early early-Permian subduction–accretion complex in Daqing pasture, southeastern Inner Mongolia: Evidence of northward subduction beneath the Siberian paleoplate southern margin

A subduction–accretion complex is identified from previously defined late-Carboniferous and early-Permian strata in Daqing pasture, southern Xiwuqi, Inner Mongolia. The subduction–accretion complex is composed of a matrix of siltstone and exotic blocks of bioclastic limestone, pillow basalt, foliated basalt, chert and asbestos. The pillow basalt possesses the geochemical characteristics of mid-ocean ridge basalt (N-MORB), whereas the foliated basalt displays the geochemical characteristics of island arc basalt (IAB), indicating that these basalts are of different origins. U–Pb (zircon) dating indicates that the foliated basalt formed in the late-Carboniferous (314.5 to 318.4Ma) and the bioclastic limestone formed in the early early-Permian. Combined with regional geological data, the subduction–accretion complex and coeval calc-alkaline granitic belt to the north constitute the essential elements of the late-Carboniferous to early early-Permian subduction zone on the southern margin of the Siberian paleoplate. The zircon εHf(t) values of the foliated basalt are positive (+14.4 to +23.9), suggesting that this basalt originated directly from depleted mantle. The temporal–spatial distribution of the subduction–accretion complex and ophiolite belts in southeastern Inner Mongolia indicates that there was significant lateral crustal growth on the southern margin of the Siberian paleoplate in the late Paleozoic.

Jurassic and Cretaceous radiolarian assemblages from the Bornova mélange in northern Karaburun Peninsula (western Turkey) and its connection to the İzmir–Ankara mélanges

Two large blocks of red bedded chert identified within the Late Cretaceous–Paleocene Bornova mélange in northern Karaburun Peninsula yielded Jurassic (late Bathonian–early Oxfordian) and Cretaceous (middle–late Albian) radiolarian assemblages. These new data confirm the correlation of the Bornova mélange with the Bornova Flysch Zone (BFZ) and the İzmir–Ankara mélanges. A review of all previously obtained ages in chert blocks of the BFZ and the İzmir–Ankara mélanges is provided in order to strengthen this correlation.

Subduction initiation along transform faults: The proto-Franciscan subduction zone

The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones present special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduction initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite melange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that reflect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that reflect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting. The Tehama-Colusa serpentinite melange consists of blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. The melange matrix represents hydrated refractory peridotites with forearc affinities, and blocks within the melange consist largely of upper-plate lithologies (harzburgite, arc volcanics, and arc-derived sediments). Lower-plate blocks within the melange include oceanic basalts and chert with rare blueschist and amphibolite. The abyssal peridotites have low pyroxene equilibration temperatures that are consistent with formation in a fracture-zone setting. However, the current melange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault or fracture zone that evolved into a subduction-zone melange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems and for the Franciscan–Coast Range ophiolite system. Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite melange.