Triassic Chert Research Articles

Collapse of Late Permian chert factories in the equatorial Tethys and the nature of the Early Triassic chert gap

Permian chert successions were geographically extensive, spanning from the palaeoequator to the northern high latitudes. Large-scale chert production was abruptly terminated in the latest Permian, resulting in a multi-million-year “chert gap” in the Early Triassic. In order to constrain the tempo of chert production changes and understand their nature, we combine proxy data with Si box model analyses and focus on the Talung Formation of South China—the most representative Upper Permian siliceous unit in the equatorial Tethys. Two deepwater sections from the northern margin of the Yangtze Platform were investigated, showing that bedded cherts had already become less common in the Clarkina changxingensis conodont zone. The waning of chert production coincided with water column deoxygenation and an increase in carbonate and siliciclastic components in the late Changhsingian. The final collapse of the chert factory in South China predated the negative δ13C excursion and climate warming but coincided with a sharp decrease in primary productivity. Together with Si box model output, we suggest that warming-induced expansion of the oceanic dissolved silica inventory (and decrease in burial efficiency) alone cannot maintain a multi-million-year chert gap. Instead, a loss of siliceous biomass during the end-Permian crisis is the primary cause of the Early Triassic chert demise.

Mineralogy of spessartine-quartz rocks of the Sikhote-Alin

Research subject. The article is devoted to the study of spessartine-quartz rocks of the Triassic chert formation of the Sikhote-Alin. The research objects involved the outcrops of spessartine-quartz rocks of the Gornaya and Shirokaya Pad areas from Malinovka and Olga ore districts (in the Samarka and Taukha terranes respectively). Materials and methods. The selection of stone materials was performed manually. The diagnostic of minerals was performed using the microscope in transmitted and reflected light and carried out by the determination of their composion. The polished sections of the minerals were investigated at the FEGI FEB RAS using JXA8100 microanalysers. Results. Spessartine-quartz and manganese silicate rocks occupy the same position in the section of the Triassic deposits of the Taukha and Samarka terrains. This indicates a synchronous accumulation of sediments (protoliths of these rocks) in a single sedimentary basin. The rocks formed by contact metamorphism in the Late and Early Cretaceous large granitoid massifs. This process is manifested in the presence of a variety of such minerals, as spessartine, members of the ilmenite-pyrophanite series, titanite, monazite, bastnesite, allanite, apatite, zircon, baddeleyite, torianite and others. The rocks under study also include such rare minerals, as cheralite, greyite and a rare variety of fluorine-aluminous titanite. Conclusion. The occurrence (during contact metamorphism of siliceous-rhodochrosite rocks) of spessartine-quartz or manganesesilicate rocks composed mainly of pyroxenoids, was determined by both the relative amounts of carbonate and clay matter, as well as the concentrations of Ba and alkalis in the initial sediments.

Composition and Genesis of Accessory Mineralization in Manganese Silicate Rocks of the Triassic Sikhote-Alin Chert Formation

Abstract —Manganese silicate rocks together with silicate–magnetite ores and jaspers (Late Anisian–Ladinian) form lenticular or tabular bodies in the Triassic Sikhote-Alin chert formation. The lower part of the formation (Olenekian–Early Anisian) is enriched in clayey and organic matter. Nickel and cobalt compounds and other ore minerals in the Sikhote-Alin manganese silicate rocks belong to two genetic groups including minerals of the valence and ultimately reduced Ni, Co, and other metals. Minerals of the valence species of Ni, Co (sulfoantimonides, sulfoarsenides, sulfides, antimonides, arsenides, tellurides, and silicates), and other metals (galena, sphalerite, chalcopyrite, arsenopyrite, wolframite, scheelite, molybdenite, cassiterite, stannite, cinnabar, stibnite, boulangerite, jamesonite, bournonite, löllengite, bismuthite, fahlore, altaite, native Sb and Bi, etc.) formed from the protolith material during metamorphism under the same conditions as the rock-forming minerals. The presence of minerals of ultimately reduced Ni, Co (maucherite, native Ni, Ni phosphide, Ni and Co chromides, disordered solid solutions, and intermetallic compounds of Ni with Cu, Zn, Sn, and Pb), and other metals (native Pb, Zn, Fe, Sn, Se, Au, Pt, “cupriferous gold”, and intermetallic compounds of Cu, Sn, Pb, Sb, Al, and Zn) in the manganese silicate rocks is due to the influence of the organic matter of the underlying carbonaceous silicites. During metamorphism, the most volatile components (first of all, poorly bound water and hydrocarbons) were released from the heated carbonaceous rocks; as a result, a metal-enriched fluid with highly or ultrahighly reducing properties appeared, which migrated along fractures into other rocks. The manganese silicate rocks are the products of contact metamorphism of siliceous rhodochrosite rocks formed through the diagenesis of biogenic siliceous muds enriched in Mn oxides and organic matter. Erosion of the weathering crust of islands composed of gabbroids of the Kalinovka, Vladimiro-Aleksandrovskoe, and Sergeevka complexes (in the late Middle Triassic–Late Triassic) played the leading role in the formation of metalliferous sediments. Manganese silicate rocks localized in the stratigraphic column above the carbonaceous silicites of the Triassic chert formation are enriched in Au (up to 35.38 ppm), Pt (11.27 ppm), and Pd (5.33 ppm). They contain noble-metal minerals and a wide spectrum of native elements and intermetallic compounds. The presence of Au–Pd–Pt–Ni–Co association (typomorphic for basic and ultrabasic rocks) in the Triassic protoliths of the manganese silicate rocks and carbonaceous silicites is probably due to the sorption of these elements by Mn and Fe hydroxides and organic matter during the exogenous weathering of the ancient Sikhote-Alin gabbroids.

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.

Permian–Triassic back-arc basin development in response to Paleo-Tethys subduction, Sa Kaeo–Chanthaburi area in Southeastern Thailand

The Sa Kaeo Back-arc Basin in Southeastern Thailand developed behind the Sukhothai Arc along the western margin of the Indochina Block during the Permian to Triassic in response to the subduction of the Paleo-Tethys oceanic plate. To clarify its tectonic evolution of spreading, basin-infilling, and closure, we have investigated the provenance of its clastic sediments, the U-Pb ages of detrital zircons, the geochemistry of its basaltic rocks, and the structures in mélange. The rocks in the Sa Kaeo Back-arc Basin include two contrasting lithological units: the Thung Kabin Mélange, composed mainly of basaltic rocks, chert, limestone, and sandstone, along with mélange; and the Pong Nam Ron Formation, a thick succession of clastic sediments. Our observations suggest that the clastic sediments of the Pong Nam Ron Formation were deposited widely over the oceanic rocks (basaltic rocks, chert, and limestone) of the Thung Kabin Mélange. Geochemical analyses indicate that sandstones in both units (the Thung Kabin Mélange and the Pong Nam Ron Formation) are characterized by a provenance of mostly basaltic rocks. The U-Pb ages of detrital zircons from both units present similar age distributions: the youngest single-grain zircon (YSG) is early Late Triassic in age, the youngest cluster of zircons (YC1σ) is latest Permian to earliest Triassic in age, and there is a range of other Paleozoic and Proterozoic zircons. Based on these geochemical analyses and the U-Pb dating, the sandstones of both units had the same provenance. In addition, the basaltic rocks of the Thung Kabin Mélange are originated from back-arc basin basalts. These new observations have allowed us to reconstruct the tectonic evolution of the basin. In the Early Permian, spreading took place in the basin accompanied by basaltic magmatism. Subsequently, corresponding to an inactive period of the Sukhothai Arc into the Middle Triassic, the Sa Kaeo Back-arc Basin subsided, and a deep-sea environment prevailed with the formation of Permian cherts, Permian limestones, and Middle Triassic cherts. During the Triassic, the basin was filled by clastic sediments and eventually closed. The clastic sediments were sourced directly from the back-arc basin basalts, as well as from the felsic volcanic rocks of the active Sukhothai Arc (which yield Permian to Triassic zircons), and from the continental materials of the Indochina Block (Paleozoic and Proterozoic zircons). After the closure of the Sa Kaeo Back-arc Basin, the discussed area was affected by the termination fault-fan related to the Mae Ping Fault Zone.

Geochemistry of the Samarka terrane cherts (Sikhote-Alin) and the size of the accreted paleo-oceanic plate

We have studied the geochemistry of Late Triassic cherts from tectonic sedimentary complexes of different structural levels of the Samarka terrane. The results show that the contents and distribution patterns of major rock-forming oxides, trace elements, and REE in the cherts of the upper and lower structural levels are somewhat different, which is due to different facies environments of chert accumulation. In particular, the contents of Al2O3, TiO2, and K2O gradually decrease and the contents of Fe2O3 and MnO increase downsection. The contents of Zr, Rb, Hf, Th, and Cr, which were supplied into the bottom sediments with a terrigenous suspension of heavy-mineral fragments, are twice to ten-fold lower than their average contents in the upper crust. In contrast, the contents of Pb, Cu, and Ni, which got into the sediments mostly with metalliferous hydrothermal solutions, are rather high and even exceed their average contents in the middle and upper crust. That is, the contents of the first-group trace elements gradually decrease and the contents of the second-group trace elements increase from upper to lower structural level. The calculated negative Ce anomaly gradually decreases from lower (0.75) to upper (0.88) structural level. The geochemical parameters altogether indicate that the cherts accumulated in the same pelagic depositional environment but in its different parts. The Katen complex composing the lower structural level is the most remote from the continental margin, and the Amba-Matay complex forming the upper structural level is the most proximal. Based on the geochemical and biostratigraphic data and on the time of accretion of paleo-oceanic fragments, we have established the location of each complex within the not yet subducted oceanic plate and estimated the extensionof this plate. Throughout the Jurassic, about 6000 km of the oceanic lithosphere was subducted beneath the eastern margin of the Paleo-Asian continent and partly accreted to it.

Geochemical evidence for consecutive accretion of oceanic fragments: The example of the Samarka Terrane, Sikhote-Alin

The data of geochemical study of Late Triassic cherts from tectonic–sedimentary complexes from different structural levels of the Samarka Terrane are reported. It is shown that the concentration and character of distribution of the major petrogenic oxides and minor and rare-earth elements in cherts of the upper and lower structural levels differ significantly, which results from differences in the facies environments of chert deposition. All the geochemical characteristics of cherts show that their deposition proceeded in the pelagic area of sedimentation, but in different parts. The Katen Complex composing the lower structural level is the most distant from the continental margin. The closest is the Amba-Matay Complex composing the upper structural level. Based on the geochemical and biostratigraphic data and the age of accretion of paleooceanic fragments, the length of the subducted oceanic plate (>6000 km) is calculated.

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.

Volcanic rocks of the Khabarovsk accretionary complex, southern Far East Russia

The results of study of the volcanic rocks of the Khabarovsk accretionary complex, a fragment of the Jurassic accretionary prism of the Sikhote Alin orogenic belt (the southern part of the Russian Far East), are presented. The volcanic rocks are associated with the Lower Permian limestones in the melange blocks and Triassic layered cherts. The petrography, petrochemistry, and geochemistry of the rocks are characterized and their geodynamic formation conditions are deduced. The volcanic rocks include oceanic plume basalts of two types: (i) OIB-like intraplate basalts formed on the oceanic islands and guyots in the Permian and Triassic and (ii) T(transitional)-MORBs (the least enriched basalts of the E-MORB type) formed on the midoceanic ridge in the Permian. In addition to basalts, the melange hosts suprasubduction dacitic tuff lavas.

Metamorphic evolution of the Karakaya Complex in northern Turkey based on phyllosilicate mineralogy

The Triassic Karakaya Complex (KC) of the Sakarya Composite Terrane in northern Turkey is traditionally subdivided into two units. The Lower Karakaya Complex (LKC) consists of a tectonic melange with blocks of metabasic rocks, metacarbonates, meta-arenites and metapelites that have been affected by high pressure/low temperature metamorphism. It is followed by a low pressure/low temperature metamorphic overprint; the latter is the only metamorphic event in some tectonic slivers of the LKC. The Upper Karakaya Complex (UKC) units are primarily composed of diagenetic to low-grade metamorphic rocks, comprising Late Permian and Triassic cherts and blocks of OIB-type volcanic rocks interfingering with Anisian limestones. LKC slide-blocks of variable sizes are frequently observed within the UKC. Phyllosilicates of LKC and UKC were examined for their abundance, crystallinity, polytype and b cell dimension. Trioctahedral chamositic chlorites have IIb polytype and phengites 2 M1 polytypes in the LKC units and 2 M1 + 1 M + 1Md polytypes of phengitic dioctahedral illites in the UKC units. Kubler index data correspond to the low anchizone and epizone for the LKC units, and to the high diagenesis-low anchizone, and in part to the epizone for the UKC units. The b values of illites are consistent with a high-pressure facies series for the LKC, but only intermediate-pressure facies for the UKC. According to textural features, mineral paragenesis, clay transformations, index minerals, and b values, the lower-middle parts of the LKC represent an accretional tectonic setting, whereas the UKC units reflect pressure temperature conditions of an extensional basin affected by high heat flow.

Conodont biostratigraphy across the Permian-Triassic boundary at the Xinmin Section, Guizhou, South China

A continuous sedimentation at the Xinmin Section, Anshun, Guihzou Province, with the Upper Permian Talung Formation dominated by bedded siliceous rocks, and the Lower Triassic Luolou Formation consisting mudstones and marls as well as siliceous mudstones at its basal part indicates that it represents a deeper-water basinal facies across the Permian-Triassic boundary. Based on a systematic conodont biostratigraphic work, nine conodont species belong to two genera have been identified in this study. It enable us to establish five conodont zones at this section, in ascending order, they are: Clarkina changxingensis Zone (beds 1-3-4-2), Clarkina yini Zone (beds 4-3-5-1-1), Clarkina meishanensis Zone (beds 5-1-2-5-2), Hindeodus changxingensis Zone (beds 5-3-1-5-3-2) and Hindeodusparvus Zone (beds 5-3-3-5-3-4), respectively. According to the first occurrence of Hindeodus parvus in bed 5-3-3, the Permian-Triassic boundary is placed at the base of bed 5-3-3. This conodont zonation of the Xinmin Section provides precise biostratigraphic framework for further investigations on the geological events across the Permian-Triassic boundary at this section. In addition, the new conodont data also reveals that several siliceous beds occurred at the basal Triassic. It provides an exception of Early Triassic Chert Gap.

Change in sedimentation rates of the Upper Triassic chert based on conodont datums

Change in sedimentation rates of the Upper Triassic chert based on conodont datums

Late Permian and Early to Middle Triassic radiolarians from the Hat Yai area, southern peninsular Thailand: Implications for the tectonic setting of the eastern margin of the Sibumasu Continental Block and closure timing of the Paleo-Tethys

Abstract The fine clastic and siliceous sedimentary successions distributed in the Hat Yai area, southeastern peninsular Thailand were examined using radiolarian biostratigraphy, lithology, and stratigraphy. The studied succession was essentially divided into two kinds of fine-grained sedimentary rock units: the lower shale unit and the upper chert unit. The lower shale unit and the upper chert unit yielded late Middle to early Late Permian and Early to Middle Triassic radiolarians, respectively. We clarified that the sedimentary rocks distributed in the study area are not entirely Carboniferous, but contain some sediments from the Permian and Triassic. This age determination suggests that the clastic–chert succession distributed in the study area should be distinguished from the Carboniferous Yaha Formation and correlated with the Permian to Triassic Semanggol Formation. Lithological change with increasing siliceous composition was observed in the uppermost part of the lower shale unit, and the geological age based on the radiolarians indicates a nearly conformable stratigraphic relationship for the lower shale unit and the upper chert unit. The stratigraphy and lithology of the Permian to Triassic succession in the study area, together with geological correlation around southernmost Thailand and northern Malaysia, suggest that the Triassic chert should be interpreted as continental slope sediments overlying Permian clastic and/or calcareous facies, rather than typical pelagic deep-water sediments formed on an abyssal plain. The depositional environment of the chert in this area was likely restricted to the vicinity of a continental slope. Considering with the wide distribution of Triassic platform carbonates over southeastern peninsular Thailand, the continental margin of the Sibumasu along the Paleo-Tethys was represented by a stable passive margin during the Middle–early Late Triassic time. On the basis of the sedimentary setting and stratigraphy in the Permian and Triassic, it is suggested that the closure of the Paleo-Tethys between the Sibumasu and Indochina continental blocks took place at least after the Middle Triassic in southeastern peninsular Thailand.

Osmium isotope evidence for a large Late Triassic impact event

Anomalously high platinum group element concentrations have previously been reported for Upper Triassic deep-sea sediments, which are interpreted to be derived from an extraterrestrial impact event. Here we report the osmium (Os) isotope fingerprint of an extraterrestrial impact from Upper Triassic chert successions in Japan. Os isotope data exhibit a marked negative excursion from an initial Os isotope ratio (187Os/188Osi) of ∼0.477 to unradiogenic values of ∼0.126 in a platinum group element-enriched claystone layer, indicating the input of meteorite-derived Os into the sediments. The timing of the Os isotope excursion coincides with both elevated Os concentrations and low Re/Os ratios. The magnitude of this negative Os isotope excursion is comparable to those found at Cretaceous–Paleogene boundary sites. These geochemical lines of evidence demonstrate that a large impactor (3.3–7.8 km in diameter) produced a global decrease in seawater 187Os/188Os ratios in the Late Triassic.

The radiolarian evidence for the accretion of the Fu-saki Formation with the inferred oceanic plate stratigraphy: A case of weakly-metamorphosed accretionary complex in Ishigaki Jima, southern Ryukyu Arc, Japan

The island of Ishigaki Jima, located in the western part of the southern Ryukyu Arc, Japan, is underlain by a basement comprising the Tumuru and Fu-saki formations. The former is a pelitic glaucophane schist with a metamorphic age of 220–190Ma, and the latter is a weakly metamorphosed accretionary complex, composed mainly of chert, mudstone and sandstone with minor amounts of limestone and mafic rocks. The Fu-saki Formation was weakly metamorphosed at ∼140Ma. Latest Carboniferous–Early Jurassic microfossils have been obtained from the limestones, cherts and siliceous mudstones of this formation, but no fossils have been collected from the phyllitic mudstones. The radiolarian fauna of the phyllitic mudstones described herein indicates a late Pliensbachian–early Toarcian (Early Jurassic) age. This result, when combined with existing data, enables the reconstruction of an oceanic plate stratigraphy, showing a succession of (in ascending order) Upper Carboniferous–Triassic cherts, Sinemurian–lower Pliensbachian siliceous mudstones and upper Pliensbachian–lower Toarcian phyllitic mudstones and sandstones. The radiolarians from the phyllitic mudstones are important in constraining the timing of the accretion of the Fu-saki Formation to the base of the Tumuru Formation.

A newly discovered earliest Triassic chert at Gaimao section, Guizhou, southwestern China

The Permian–Triassic (PTB) boundary section at Gaimao (Huaxi District, Guiyang, Guizhou Province) records a unique lithological transition between the Permian and Triassic in South China. Thus, bioclastic limestones of the Changxing Formation are overlain by radiolarian-bearing siliceous rocks of the Dalong Formation and ultimately limestone/mudstone alternations of the Shabaowan Formation. Conodont sampling of the section revealed well-preserved elements that enabled a conodont biostratigraphy to be established. The occurrences of Hindeodus julfensis and Hindeodus typicalis in Changxing Formation bioclastic limestone indicate a Late Permian age. Four conodont zones have been established in Dalong and Shabaowan Formations. In ascending order those are the Isarcicella isarcica Zone, Neogondolella (or Clarkina) planata Zone, Neogondolella krystyni Zone and Neospathodus dieneri Zone. The presence of typical Griesbachian conodonts I. isarcica and Isarcicella staeschei in upper Dalong Formation indicates that this formation's radiolarian-bearing siliceous mudrocks are of earliest Triassic age and not restricted to the Permian as previously thought. Thin section analysis reveals that the upper Dalong Formation contains spheroidal radiolarians but not a diverse assemblage of Permian holdover taxa. The new data proves that, following the end-Permian mass extinction of radiolarian, the abundance of these siliceous planktons was maintained at least locally enabling chert deposition to persist. Thus, the Gaimao section provides an exception to the ‘Early Triassic Chert Gap’, the first of its kind in Palaeo-Tethys, and indicates that the end of chert deposition does not always mark the end-Permian mass extinction.

57Fe Mössbauer spectroscopic analysis of deep-sea pelagic chert: Effect of secondary alteration with respect to paleo-redox evaluation

Chemical states of iron in the pre-Jurassic pelagic deep-sea cherts are used as one of redox indicators of lost deep-oceans, e.g. the Permian–Triassic boundary case. Primarily red hematite-bearing cherts were often altered secondarily into greenish gray cherts. We examined the pattern of secondary change in chemical state of iron, associated with color change, in the Middle Triassic chert beds at Hisuikyo in central Japan by 57Fe Mössbauer spectroscopy. Three sets of chert bed with the lateral color change from red to greenish gray were analyzed. The analyses confirmed that the red cherts contain hematite, paramagnetic Fe 3+, and paramagnetic Fe 2+, whereas the greenish gray cherts contain paramagnetic Fe 3+ and paramagnetic Fe 2+ without hematite. The greenish gray parts contain larger amounts of paramagnetic Fe 2+ component of amorphous siderite-like mineral, in contrast to the red parts have lesser amounts. These results confirmed that the primary hematite has changed into paramagnetic Fe 2+-bearing minerals, in accordance with the color change, by the secondary alteration. Further comparison with the pyrite-bearing primary black to dark-gray cherts was discussed with respect to the evaluation for primary redox change in the past oceans.

Preservation of larval bivalve shells in a radiolarian chert in the Late Triassic (Early Norian) interval of the Malampaya Sound Group, Calamian Island, western Philippines

Abstract Silicified thin larval bivalve shells occur in Upper Triassic radiolarian chert (Early Norian) in the Liminangcong Formation of the Malampaya Sound Group, part of a Late Jurassic to Early Cretaceous subduction-related accretionary complex in the North Palawan Block, western Philippines. The bivalve-bearing radiolarian chert, which we term “bivalve chert,” is considered to be a deep-sea sediment that accumulated in an open-ocean realm of the Panthalassa Ocean. Radiolarian and conodont biostratigraphies indicate an early Norian deposition of the thin-shelled bivalves. The bivalve morphology, thin shells (less than 30 μm thick), smooth ornamentation, and small size (less than 1.3 mm long), suggests they represent larval shells. Although their taxonomic identity cannot be established with certainty, halobiid bivalves that lived in Norian times apparently had such a planktonic larval mode of life.

The intermetallic compound Ni3Au and gold-nickel solid solutions in metalliferous sediments of the triassic chert formation in Sikhote-Alin in the Far East

The intermetallic compound Ni3Au and Au-Ni solid solutions (native nickel Au and Au-bearing native Ni) were found in the contact metamorphosed metalliferous sediments of the Triassic chert formation in Sikhote-Alin. The metalliferous rocks are characterized by high contents of Au, Ag, and PGE, as well as the presence of diverse minerals of precious metals. Nickel gold (Au0.91-0.88 Ni0.09-0.12) is found as grains and crystals (3 to 4 im in size) in Au-bearing cherts with hematite, which are conventionally defined as “itabir-ites” and in the altered siliceous rocks of the Dal’nerechensky district (the upper reaches of the Gornya River). The nickel gold is associated with copper Au, pure native Au (Au1.00), and Au-Ag and Au-Ag-Pb solid solutions (Au0.86/0.84Ag0.14_0.16 and Au0.78Ag0.19Pb0.03, respectively). The Au-bearing Ni is found in the metamorphosed carbonaceous mudstones and Au-bearing “itabirites” of the Shirokopadninsky area (Olgin-sky district). The Au content varies from 6.09 wt % (Ni0.98Au0.02) to 11.30 wt % (Ni0.96Au0.04) in some Au-bearing Ni grains (about 10—15 im in diameter) taken from the metamorphosed mudstones. The grains of Au-bearing Ni (Co0.001_0.00Au00.2-0.17 Ni 0.98_0.83) in the “itabirites” are also characterized by their heterogeneous composition and the fine impregnation of the Ni3 Au intermetallic compound (Ni2.99Au1.01 based on the microprobe analysis).

Discovery of Ni-rich spinels in Upper Triassic chert of the Mino Terrane, central Japan

The magnetic fraction of a lower Norian claystone layer (ca. 5 cm thick) in a radiolarian chert succession within the Mino Terrane, Inuyama area, Central Japan, contains a large number of small euhedral to subhedral crystals of oxidized Ni-rich spinels. The stratigraphic position of this concentration of Ni-rich spinels is clearly indicated by a sharp increase in magnetic susceptibility at the claystone layer. These spinels are distinguished from typical igneous spinels by their high Ni and Fe3+ contents, and show large variations in composition. The textures and chemical compositions of the spinel crystals are similar to those of Ni-rich spinels at the Cretaceous-Paleogene boundary. The discovery of Ni-rich spinels in a lower Norian claystone suggests an important sedimentary record of an extraterrestrial impact in the Late Triassic.