Tuff Beds Research Articles

The East European Platform in the late Ediacaran: new paleomagnetic and geochronological data

The paleogeography of the Earth, including the East European Platform, is very inaccurately defined for the interval 500–700 Ma. The quantity and quality of Late Precambrian–Cambrian paleomagnetic data on this platform are absolutely insufficient for reliable paleogeographical or paleotectonic reconstructions. Since there are almost no unstudied objects in the platform that could be used for paleomagnetic studies, it seems reasonable to consider the deformed platform margins. Of particular interest is the Bashkir anticlinorium (South Urals) with numerous Ediacaran sedimentary sections, some of which contain tuff beds suitable for isotope dating. We present paleomagnetic and geochronological data on the Upper Ediacaran Zigan Formation, sampled in the western part of the western limb of the Bashkir anticlinorium. The East European Platform must have been at near-equatorial latitudes at ∼550 Ma.

Correlation of Paleoproterozoic glaciations based on U–Pb zircon ages for tuff beds in the Transvaal and Huronian Supergroups

The rise in atmospheric oxygen between 2.45 and 2.2 Ga has been linked to the demise of a methane-rich atmosphere and the onset of multiple glaciations, culminating in a possible Snowball Earth. The glacial deposits represent possible global marker horizons, however, their correlation is uncertain because key sedimentary successions are undated, hampering paleoenvironmental reconstructions. Three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern Africa with those in North America. However, it has also been suggested that there were four glaciations and that the youngest was a Snowball Earth event recorded only in South African successions. In situ U–Pb zircon ages for tuffs in southern African and North American successions establish the existence of four glaciations between 2.45 and 2.22 Ga. Geochronological and stratigraphic data demonstrate that the three oldest glaciations predate ∼2.31 Ga and that the final glaciation, which is only recognized in South Africa, occurred between 2.26 and 2.22 Ga. The new age-calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations. At 2.31 Ga, the first marine sulfate evaporites were deposited contemporaneously with 13C-enriched carbonates, indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen. The appearance of “red beds” and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels, which culminated with the fourth glaciation between 2.26 and 2.22 Ga, after which the atmosphere remained irreversibly oxygenated.

U-Pb zircon age from the radiolarian-bearing Hitoegane Formation in the Hida Gaien Belt, Japan

The dating of radiolarian biostratigraphic zones from the Silurian to Devonian is only partially understood. Dating the zircons in radiolarian-bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian-bearing Hitoegane Formation, Japan, was undertaken. The Hitoegane Formation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones and felsic tuff. The felsic tuff and tuffaceous mudstone yield well-preserved radiolarian fossils. Zircon grains showing a U–Pb laser ablation–inductively coupled plasma–mass spectrometry age of 426.6 ± 3.7 Ma were collected from four horizons of the Hitoegane Formation, which is the boundary between the Pseudospongoprunum tauversi to Futobari solidus–Zadrappolus tenuis radiolarian assemblage zones. This fact strongly suggests that the boundary of these assemblage zones is around the Ludlowian to Pridolian. The last occurrence of F. solidus is considered to be Pragian based on the reinterpretation of a U–Pb sensitive high mass-resolution ion microprobe (SHRIMP) zircon age of 408.9 ± 7.6 Ma for a felsic tuff of the Kurosegawa belt, Southwest Japan. Thus the F. solidus–Z. tenuis assemblage can be assigned to the Ludlowian or Pridolian to Pragian. The present data also contribute to establishing overall stratigraphy of the Paleozoic rocks of the Fukuji–Hitoegane area. According to the Ordovician to Carboniferous stratigraphy in this area, Ordovician to Silurian volcanism was gradually reduced to change the sedimentary environment into a tropical lagoon in the early Devonian. And the quiet Carboniferous environment was subsequently interrupted, throwing it once more into the volcanic conditions in the Middle Permian.

New U-Pb ages of zircons in the Owk Shale (Kurnool Group) with reflections on proterozoic porcellanites in India

Felsic tuff beds with some presumed sedimentary components were reported from the Owk Shale (Kurnool Group; bearing Neoproterozoic fossils) in the upper part of the sedimentary succession in the Cuddapah basin in India by Saha and Tripathy (2012a). Our optical and SEM petrographic study of three thin sections, however, indicates that the parent samples are sandy mudstones with variable amounts of a felsic volcaniclastic component. New highquality U-Pb (SHRIMP and LA-MC-ICPMS) ages of 133 detrital zircon grains from a sample show that one grain is ca. 1880 Ma, one grain is ca. 3300 Ma, and the ages of the remaining 131 grains fall between 2690 Ma and 2429 Ma, the population averaging 2522 ± 36 Ma. The data indicate that the zircons are detrital grains derived from the ca. 2.5 Ga granitic/gneissic/greenstone basement of the Dharwar cratons that also host minor older Archean enclaves. The single 1880 Ma grain could have come from a ca. 1.9 Ga LIP. In the absence of any younger magmatic zircon, the absolute age of the Owk Shale remains elusive.

Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA

The lower Pliocene Bouse Formation in the lower Colorado River Valley (southwestern USA) consists of basal marl and dense tufa overlain by siltstone and fine sandstone. It is locally overlain by and interbedded with sands derived from the Colorado River. We briefly review 87 Sr/ 86 Sr analyses of Bouse carbonates and shells and carbonate and gypsum of similar age east of Las Vegas that indicate that all of these strata are isotopically similar to modern Colorado River water. We also review and add new data that are consistent with a step in Bouse Formation maximum elevations from 330 m south of Topock Gorge to 555 m to the north. New geochemical data from glass shards in a volcanic ash bed within the Bouse Formation, and from an ash bed within similar deposits in Bristol Basin west of the Colorado River Valley, indicate correlation of the two ash beds and coeval submergence of both areas. The tuff bed is identified as the 4.83 Ma Lawlor Tuff derived from the San Francisco Bay region. We conclude, as have some others, that the Bouse Formation was deposited in lakes produced by first-arriving Colorado River water that entered closed basins inherited from Basin and Range extension, and estimate that first arrival of river water occurred ca. 4.9 Ma. If this interpretation is correct, addition of Bristol Basin to the Blythe Basin inundation area means that river discharge was sufficient to fill and spill a lake with an area of ∼10,000 km 2 . For spillover to occur, evaporation rates must have been significantly less in early Pliocene time than modern rates of ∼2–4 m/yr, and/or Colorado River discharge was significantly greater than the current ∼15 km 3 /yr. In this lacustrine interpretation, evaporation rates were sufficient to concentrate salts to levels that were hospitable to some marine organisms presumably introduced by birds.

Dating deposition and low-grade metamorphism by in situ U[sbnd]Pb geochronology of titanite in the Paleoproterozoic Timeball Hill Formation, southern Africa

Titanite coexists with zircon in a felsic tuff in the Paleoproterozoic Timeball Hill Formation, southern Africa. Two generations of titanite are preserved: euhedral, brown crystals with apatite inclusions, and colorless, matrix-filling cement. The brown titanite has elevated U, Th and Fe, and low Al, consistent with a magmatic origin, whereas the colorless titanite has higher Al and F contents, suggestive of a metamorphic paragenesis. In situ SHRIMP geochronology of zircons extracted from a polished thin section of the tuff bed suffer from inheritance and Pb-loss and yield an imprecise date (2259±35Ma), whereas coexisting brown titanites give a weighted mean 207Pb/206Pb age of 2278±7Ma. This date is consistent with UPb zircon ages for tuffs in the same stratigraphic unit from a nearby drill-hole. The brown titanite is interpreted to be magmatic and to provide a reliable estimate for the age of deposition. In situ UPb dating of matrix-filling titanite cement from the same stratigraphic interval yields an age of 2145±12Ma, which corresponds with previous estimates for a low-grade tectonothermal event in southern Africa. Our results demonstrate that titanite is a versatile chronometer that can be used to constrain depositional ages, and those of low-grade metamorphism, thus increasing the number of sedimentary rocks that can be dated and elucidating the low-temperature geological history of depositional basins.

Timing of deposition and deformation of the Moodies Group (Barberton Greenstone Belt, South Africa): Very-high-resolution of Archaean surface processes

Thin and largely discontinuous volcanic tuff beds in the alluvial, fluvial, and marginal-marine siliciclastic strata of the Middle Archaean Moodies Group, Barberton Greenstone Belt (BGB), contain rare to abundant zircon. Dating of magmatic zircon grains from five tuff beds and one cross-cutting dike yield ages which span the entire spectrum of BGB history from ca. 3550Ma to 3215Ma and show variable degrees of post-depositional alteration until ca. 2500Ma. The youngest pristine zircon grains in all tuff samples analyzed by CA-ID-TIMS (Chemical-Abrasion Ion-Dissolution Thermal Ion Mass Spectrometry) yield upper intercept and Concordia ages between 3227.6±0.3 and 3216±0.3Ma, and those grains analyzed by LA-SF-ICP-MS (Laser Ablation-Sector Field-Inductively Coupled Plasma-Mass Spectrometry) and SHRIMP (Sensitive High-Resolution Ion Microprobe) yield ages between 3228±9 and 3214±9Ma. Most of these ages are either identical (within error) or significantly older than a CA-ID-TIMS age of 3223.4±0.3Ma, obtained from zircon grains of a tuff horizon of the Schoongezicht Formation at the top of the underlying Fig Tree Group. We interpret this age to mark the time of volcanic eruption, whereas the significantly older and overlapping ages of the stratigraphically higher Moodies tuff zircon grains point either to a high degree of magmatic inheritance and/or a detrital contribution from surrounding sedimentary rocks during or after ash-tuff deposition. The minimum age for deposition of Moodies Group sediments and deformation is provided by a LA-ICP-MS zircon U-Pb age of 3219±9Ma obtained from an undeformed felsic dike which crosscuts folded Moodies strata. This age overlaps with a precise CA-ID-TIMS U-Pb zircon age of 3216.0±0.3Ma obtained from a tuff horizon of the upper Moodies Group. Overall, the new age data collectively imply that the up to 3.7km-thick, sandstone-dominated Moodies Group was deposited and deformed within <1–14Ma, and that Moodies strata thus hold the potential to offer a temporal resolution comparable to active- and passive-margin Quaternary strata (∼0.1–1mm/a).

The first record of Eocene tuff in a Paleogene rift basin near Nantou, Western Foothills, central Taiwan

Microfossils and a U–Pb age dating on zircon grains in the tuff beds exposed in the axial part of the Tsukeng anticline along the Pinglin River in the Western Foothills near Nantou, central Taiwan, show an occurrence of the Eocene volcanics unconformably beneath the uppermost part of the Latest Oligocene Wuchihshan Formation. This is the first discovery of the Eocene tuff exposed in the Western Foothills.The proposed Miocene “Tsukeng Formation” and “Takeng Formation” of Ho et al. (1956) named for sequences exposed in the Nantou area, Western Foothills, have to be abandoned and the standard Oligocene–Miocene lithostratigraphy used commonly in the Western Foothills of northern Taiwan is properly applicable in central Taiwan. The thick pink–brown–green colored volcanics unconformably beneath the uppermost Wuchihshan Formation is named for the first time as the Pinglin Tuff which contains Late Middle Eocene calcareous nannofossils (Zone NP16) consistent with a U–Pb age dating (38.8±1Ma) on zircon grains in the tuff. The Pinglin Tuff is overlying the Middle Eocene Chungliao Formation which contains indigenous larger foraminifera Discocyclina dispansa ex. interc. sella-dispansa and calcareous nannofossils of Zones NP14–15. The Middle Eocene Pinglin Tuff and Chungliao Formation represent the Paleogene syn-rift sequence unconformably overlain by the Latest Oligocene–Miocene post-rift sequence. This is the first document with conclusive paleontological data and age dating showing an occurrence of Paleogene marine rift basin exposed in the Western Foothills. This study also confirms similar Tertiary basin architecture between the Taiwan Strait–Pearl River Mouth Basin in the NE South China Sea and the Western Foothills onland central Taiwan.

Late Cretaceous U–Pb tuff ages from the Skunk Ranch Formation and their implications for age of Laramide deformation, Little Hatchet Mountains, southwestern New Mexico, U.S.A.

LA-ICPMS U–Pb ages of individual zircon crystals from reworked ash-fall tuff beds in lacustrine strata of the Skunk Ranch Formation, a synorogenic unit deposited in a Laramide intermontane basin in the Little Hatchet Mountains, southwestern New Mexico, indicate an early Maastrichtian depositional age of 71–70 Ma. The dated beds are equivalent to nearby growth strata that are partially cut by, and folded over, a Laramide reverse fault. The strata thus record final displacement on the reverse fault and indicate an early Maastrichtian age for fault offset. The Skunk Ranch Formation was previously interpreted as Eocene in age and inferred to record the second event in a two-stage Laramide deformational history in the mountain range. In addition to significantly condensing the age range of the Laramide section and clarifying stratigraphic correlation in the Little Hatchet Mountains, the new ages of the formation also indicate that Laramide deformation in the range was likely restricted to a single Late Cretaceous episode.

A Late Eocene date for Late Triassic bird tracks

Arising from R. N. Melchor, S. De Valais & J. F. Genise , 936–938 (2002)10.1038/nature00818 Bird-like tracks from northwest Argentina have been reported as being of Late Triassic age1. They were attributed to an unknown group of theropods showing some avian characters. However, we believe that these tracks are of Late Eocene age on the basis of a new weighted mean 206Pb/238U date (isotope dilution–thermal ionization mass spectrometry method) on zircons from a tuff bed in the sedimentary succession containing the fossil tracks. In consequence, the mentioned tracks are assigned to birds and its occurrence matches the known fossil record of Aves.

High-precision U–Pb geochronology and a new chronostratigraphy for the Cañadón Asfalto Basin, Chubut, central Patagonia: Implications for terrestrial faunal and floral evolution in Jurassic

The fluvial, lacustrine and tuffaceous sedimentary succession of the Cañadón Asfalto continental basin exposed in the Argentinean Chubut Province of central Patagonia preserves an extraordinary record of Jurassic fauna and flora that marks key events in the evolution of Dinosauria, early mammals and major South American plant groups. However, basin-wide correlation of the fossiliferous units is complicated by fault displacements, unconformities and repetitive lithofacies. New U–Pb analyses of zircon (CA-TIMS method) from five primary tuff beds interstratified with the lacustrine strata establish a new chronostratigraphic framework for the sedimentary and volcanic units of Cañadón Asfalto Basin, constraining ca. 33m.y. of depositional history and biotic evolution that spans nearly all three epochs of the Jurassic. Five major vertebrate- and plant-rich stratigraphic intervals have been identified, and are being actively investigated, that range in age from Pliensbachian to Kimmeridgian (or younger). Our combined biostratigraphic and high-precision geochronologic results indicate that the major faunal turnover of the sauropodomorph dinosaurs which led to the rise of the eusauropods took place in the Early Jurassic, earlier than previously recognized. Similarly, the first successful radiation of the ornithischian dinosaurs (heterodontosaurids), as well as the evolutionary diversifications of the araucarian conifers and the osmundaceous ferns, all occurred before the end of the Early Jurassic. The diverse palynofloral assemblage of the Cañadón Calcáreo Formation that was once considered to be early Cretaceous in age, is now entirely constrained to the Late Jurassic.

U–Pb SHRIMP age for the Tuchengzi Formation, northern China, and its implications for biotic evolution during the Jurassic–Cretaceous transition

The Late Jurassic–Early Cretaceous Tuchengzi Formation is widespread in North China. Its clastic deposits indicate a tropical, dry, and hot paleoclimate, different from the subtropical, humid, and seasonal climate in the early Middle Jurassic. The sudden environmental change from the Middle to Late Jurassic resulted in a rapid disappearance of the Yanliao/Daohugou Biota, with more than 90% of the species of the Yanliao/Daohugou Biota dying out and replaced by the Early Cretaceous Jehol Biota. Eolian sandstones with large-scale cross-bedding occur within the uppermost part of the Tuchengzi Formation in North China. The sandstones are stratigraphically higher than the tuff previously dated as 139Ma. We obtained two SHRIMP zircon U/Pb ages from the tuff beds. One sample was collected from the tuff in the basal Tuchengzi Formation in northern Hebei and the other from the tuff intercalated in the eolian cross-bedded sandstones in the uppermost Tuchengzi Formation in western Liaoning. Our results show that 154Ma is the oldest age constraint for the Tuchengzi Formation and 137Ma is the youngest age estimate of the formation, providing an age constraint for the transition from the Yanliao Biota to the Jehol Biota.

Groundwater resources assessment using integrated geophysical techniques in the southwestern region of Peninsular Malaysia

Combined geophysical techniques such as multi-electrode resistivity, induced polarization, and borehole geophysical techniques were carried out on volcano-sedimentary rocks in the north of Gemas as part of the groundwater resource’s investigations. The result identifies four resistivity units: the tuffaceous mudstone, tuffaceous sandstone, the tuff bed, and the shale layer. Two types of aquifer systems in terms of storage were identified within the area: one within a fracture system (tuff), which is the leaky area through which vertical flow of groundwater occurs, and an intergranular property of the sandy material of the aquifer which includes sandstone and tuffaceous sandstone. The result also reveals that the aquifer occupies a surface area of about 3,250,555 m2 with a mean depth of 43.71 m and a net volume of 9.798 × 107 m3. From the approximate volume of the porous zone (28 %) and the total aquifer volume, a usable capacity of (274.339 ± 30.177) × 107 m3 of water in the study area can be deduced. This study provides useful information that can be used to develop a much broader understanding of the nature of groundwater potential in the area and their relationship with the local geology.

Re–Os geochronology of black shales from the Neoproterozoic Doushantuo Formation, Yangtze platform, South China

In South China, the Doushantuo Formation represents 90% of the time interval of the Neoproterozoic Ediacaran Period (635–542Ma). However, subdivision and correlation of strata in this formation is limited by sporadic occurrences of fossils of uncertain age and a scarcity of radiometric time constraints. In order to obtain Re–Os ages and initial 187Os/188Os (Osi) data, black shale samples from the Doushantuo Formation (Yangtze Platform, South China) were analyzed using the CrO3-H2SO4 digestion technique. A Re–Os date of 595±22Ma (Osi=0.85±0.18, MSWD=29, n=7) was obtained for black shale samples from the basal part of the fossil-bearing Doushantuo Member 4 (“Miaohe Member”) at Jiulongwan section, Three Gorges area, South China. A subset of these samples define a more precise isochron age of 591.1±5.3Ma (Osi=0.874±0.038, MSWD=1.3, n=4). We interpret the high MSWD of 29 and larger uncertainties for the 595±22Ma date to reflect heterogeneity of the Osi of samples related to temporal change of 187Os/188Os ratio of seawater when the sedimentary rocks were deposited. Taking 591.1±5.3Ma as the age for the base of the Doushantuo Member 4, together with the age of a tuff bed on the top of Member 4 (551.1±0.7Ma, zircon U–Pb dating, Condon et al., 2005), a minimum time difference of 35m.y. between the base and top of Member 4 is obtained. This estimate suggests very low sedimentation rates during black shale deposition (∼0.4mm/ka). The time difference of 35m.y. also represents a minimum duration of the Shuram–Wonoka δ13C excursion in South China. Black shale samples from Doushantuo Member 2 at Baiguoyuan section yield an imprecise date of 592±68Ma (Osi=0.534±0.057, MSWD=77) due to limited spread in 187Re/188Os ratio of the samples and a relatively large sampled stratigraphic interval (∼6m). If deposition occurred in water masses with access to the open ocean, Osi of the hydrogenous fraction from these organic matter rich sedimentary rocks may record the Os isotopic composition of coeval seawater. Thus, combined with previously published Re–Os data, the highly radiogenic initial 187Os/188Os ratio of 0.874±0.038 derived from Re–Os isochron for the base of Doushantuo Member 4, and even more radiogenic values near the top of Member 4 (1.08–1.17, calculated at 551Ma) indicate a dramatic increase of the flux of 187Os into the ocean during the late Ediacaran. The enhanced 187Os flux can only result from enhanced continental weathering and oxidation rates during that period.

Tuff beds in Kurnool subbasin, southern India and implications for felsic volcanism in Proterozoic intracratonic basins

A first report on tuff beds from the Owk Shale in the Proterozoic Kurnool sub-basin in southern India is presented. The rhyolitic to rhodacitic tuffs, overlying shelfal limestones formed at depths below storm wave base, have rheomorphic features indicative of viscoplastic flow, and geochemical signatures of rhyolitic to rhyodacitic unwelded to welded tuffs, similar to those described from other Proterozoic intracratonic basins like Vindhyan and Chhattisgarh basins in India. Fragmentary nature of altered glass with perlitic cracks and local admixture with intrabasinal sediments suggest phreatomagmatic reactions. The widespread and repeated occurrences of felsic tuffs in these basins, possibly derived from low degree melting of continental crust, suggest intermittent tectonothermal instability which likely influenced basinal topography and cyclic development of the carbonate platforms.

전남일원 점토광상의 광물 및 지화학적 특성과 효과적 탐사를 위한 건층의 선정

전남지역의 점토광상은 그 구성광물이 단사정계형 엽납석(pyrophyllite), 1T형 고령석(kaolinite)을 주로 하고 부구성 광물로서 견운모, 석영 및 장석류를 포함한다. 이들 점토광상은 유사한 시기에 유사한 성분을 가진 화산암류로부터 생성되었으며, 각 광상은 광물학적으로 엽납석과 고령석 중의 하나를 우세하게 포함하는 경향이다. 고령토 광상은 주로 다이아스포나 강옥을 포함하지 않지만, 광체 상부에 명반석을 흔히 포함한다. 반면에 강옥이나 다이아스포는 납석 광상에서 흔히 수반된다. 점토광체 상부의 규화응회암류와 응회암류 등 지질시대가 젊어짐에 따라 엽납석 및 고령석의 양은 감소하는 경향이며, 마침내는 나타나지 않게 된다. 반면에 견운모, 석영 및 사장석류는 미약한 상부층에서 흔히 나타난다. 대부분의 점토광체는 상단부에 저색 응회암층이 오고, 그 상부로는 규화대층, 응회암층, 그리고 최상부에 각력 응회암층이 온다. 화학성분 분석에 의하면, <TEX>$Al_2O_3$</TEX>는 중-저급의 점토광체뿐 아니라 응회암류에서도 다소 높게 나타내며, 그 이유는 응회암류의 주요 구성광물인 사장석 때문이다. 광체와 모암에서 <TEX>$Na_2O$</TEX>와 CaO는 비교적 낮은 함량을 나타내나, <TEX>$K_2O$</TEX>는 광체에서 좀더 낮게 나타난다. 점토의 작열 감량은 명반석이나 다이아스포를 포함하는 부분에서는 특히 높게 나타나는 것은 이들이 포함하는 <TEX>$Al_2O_3$</TEX> 및 <TEX>$SO_4$</TEX> 성분에 의한다. 희토류함량분석에 의하면, 경희토류가 중희토류보다 많이 포함되는 경향이며, 다소 미약하지만 부의 유러피움(Eu)이상대를 나타낸다. 대부분의 점토광체는 주로 유백-담회색의 미립이며 층리가 양호하여 야외에서 구분이 어렵지 않은 경우가 많지만, 괴상광체 등이 나타나는 곳에서의 효과적인 탐사를 위해서는 광체 상부 지층 중에서 확인되는 특징적 지층을 선정하는 것이 바람직하다. 전남지역에서 광체를 배태하는 층의 상단부에 연속성이 양호한 저색점토/응회암과 함께 그 상부로 층리와 연장성이 지극히 좋은 규화대가 확인되며, 광물조성 상으로도 광체 및 인근 지층과의 구분이 확실하므로 효과적인 점토광체 탐사를 위한 건층(key bed나 mark horizon)으로서의 역할이 가능하다. Clayey ores of the Jeonnam province mainly consist of pyrophyllite (monoclinic), kaolinite (1T), and minor amounts of quartz, muscovite, and feldspars. Mineralogical studies revealed that two kinds of clay minerals were mainly produced from the volcanic sediments with similar ages and compositions. Kaolinite deposits sometimes contain neither diaspore nor corundum, but alunites are often found in the upper portions of the kaolin ore bodies. On the other hand, corundum and diaspore are commoner in the pyrophyllite deposits than the kaolin deposits. As ages of rock formations are becoming younger, amounts of pyrophyllite and kaolinite are rather radically decreased, and finally disappeared. But muscovite, quartz, and plagioclase feldspars are inclined to be preserved because of weak alteration. Most of clay ore bodies contain purple tuff beds on the uppermost portion, and silicified beds, tuff, and lapillistone are found in an ascending order in the most of clay quarries. Chemical analyses show that higher contents of <TEX>$Al_2O_3$</TEX> might not necessarily be due to the argillization, since some tuffs contain higher <TEX>$Al_2O_3$</TEX> contents originated from feldspars. <TEX>$SiO_2$</TEX> contents are fairly higher in the silicified beds than in those of adjacent formations, which might have been introduced from the ore bodies. And <TEX>$K_2O$</TEX> contents are obviously lower than those of <TEX>$Na_2O$</TEX> and CaO in the ores and their vicinities. Ignition losses of some of clays represent much higher contents than those of the ordinary ones because of the sporadic presence of alunite, diaspore and corundum which are accompanied with lots of <TEX>$SO_4$</TEX> and <TEX>$Al_2O_3$</TEX> contents. REE (rare earth element) abundances of most of volcanics and clay ores show rather higher LREE (light rare earth elements) contents, and represent small to moderately negative Eu anomalies. Though most of ores ususally show milky white color, fine-grained and well bedded formations which could be easily discernible in the most of outcrop. But more distinct characteristics are desirable where rather massive ore bodies exist. Purple tuffs and silicified beds above the ore bodies would be useful as marker horizons/key beds since they have rather obvious lithology, extension and mineralogy than those of other adjacent formations.

Cryogenian volcanic arc in the NW Indian Shield: Zircon SHRIMP U–Pb geochronology of felsic tuffs and implications for Gondwana assembly

The NW domain of the Indian lithosphere witnessed multiple cycles of the birth and demise of ocean basins associated with the history of Proterozoic supercontinents. Here we present field, petrological and geochemical evidence for imbricated ocean plate stratigraphy from the Sindreth Group in Rajasthan characterized by pillow basalt, bedded chert, rhyolite, sandstone and conglomerate suggesting the accretion of oceanic sediments together with continental detritus. The petrographic and geochemical characteristics of basalts and associated rhyolites indicate a volcanic arc setting for the Sindreth Group. The SHRIMP U–Pb analyses of zircons from a siliceous tuff and a rhyolitic tuff intercalated with chert yield well-defined 206Pb/238U concordia ages of 765.1±7.2 (MSWD=0.76) and 768.2±6.9 (MSWD=2.3), constraining the timing of arc volcanism associated with mid Neoproterozoic subduction. A younger population of zircons in the rhyolitic tuff yield ages in the range of 666 to 644Ma, suggesting that the arc magmatism continued to late Cryogenian. Our results correlate the Sindreth Group with the Malani volcanics and we interpret the tuff beds to have been derived from ash flows from a volcanic arc associated with mid Neoproterozoic subduction realm. The data from this study together with those from the recent works in other regions including the Syechelles, Madagascar, southern India and Sri Lanka suggest that Cryogenian magmatic arcs were widely distributed along the margins of the East African Orogen associated with the subduction history of the Mozambique Ocean lithosphere, prior to the final amalgamation of the Gondwana supercontinent in the latest Neoproterozoic–Cambrian.

The age and tectonic setting of the Puncoviscana Formation in northwestern Argentina: An accretionary complex related to Early Cambrian closure of the Puncoviscana Ocean and accretion of the Arequipa-Antofalla block

Abstract TIMS and SHRIMP U–Pb zircon geochronology of selected parts of the Puncoviscana Formation suggest its deposition took place mainly during the Early Cambrian, coeval with 540–535 Ma calc-alkaline Pampean arc volcanism mainly preserved as tuff beds in the oldest identified parts of this unit. Syn- to post-tectonic plutons constrains the Tilcarian–Pampean orogeny to have occurred between ca. 530 Ma and deposition of the unconformably overlying Middle-Upper Cambrian Meson Group. Deposition of the Puncoviscana Formation continued after the onset of the Tilcarian–Pampean orogeny. We propose that the Puncoviscana Formation rocks older than 530 Ma were deposited in the arc-trench gap of the west-facing Pampean arc and/or the associated trench, whereas the rocks younger than 530 Ma were deposited in a syn-collision foreland basin. The Puncoviscana Formation rocks were progressively assembled into a west-younging accretionary complex, consistent with the style of deformation and low-grade metamorphism. The age of the syn-collision plutons (≤530 Ma) suggest the foredeep deposits record the transition from trench to foreland basin, due to arrival of the Arequipa-Antofalla block at the west-facing trench at ca. 530 Ma. Our geochronological and Pb-isotope investigations suggest that the Arequipa-Antofalla terrane was a coherent, ribbon-shaped crustal block that also included the western part of the Pampia terrane. A compilation of existing U–Pb zircon studies suggests that the Pampean arc extended along the length of the proto-Andean margin of West Gondwana, represented by the previously amalgamated Amazonia and Rio de La Plata cratons, and probably was initiated during the late Ediacaran after 600 Ma. Following earlier workers, we reaffirm that the Arequipa-Antofalla block was originally separating Laurentia and Amazonia in Rodinia. It probably rifted from Laurentia during the Ediacaran between 600 and 570 Ma, following an earlier departure of Amazonia (∼650 Ma?). The separation of Arequipa-Antofalla from Amazonia and Laurentia opened the Puncoviscana and Iapetus oceans respectively.

Episodic Precambrian crust growth: Evidence from U–Pb ages and Hf–O isotopes of zircon in the Nanhua Basin, central South China

We report here the first integrated U–Pb dating and hafnium–oxygen isotope analyses of detrital zircon grains from sedimentary and volcaniclastic rocks in the Neoproterozoic Nanhua Basin, central South China. These data provide vital information on the depositional cycles of the Cryogenian strata and their bearings on the breakup of the Rodinia supercontinent, and the characteristics of Precambrian crustal growth in the South China Block and neighboring continents. Zircon SIMS U–Pb ages indicate that the Qingshuijiang Formation was deposited at 772±5Ma and the Fanzhao Formation was deposited between 800 and 770Ma. The onset time of the Sturtian (Jiangkou) glaciation is constrained to be no older than 730Ma. Together with compilations of all available high precision zircon U–Pb data on tuff beds in the Nanhua Basin, our work demonstrates that the Cryogenian deposition in South China began at about 850Ma and had three depositional cycles at ca. 850–820Ma, 820–720Ma and 720–635Ma. Hf isotope analyses and U–Pb dating, combined with existing O isotope analyses, of detrital zircon have revealed three striking features for grains with 1.0–0.7Ga ages: (1) about 22% of total analysed grains have Hf isotope compositions that plot between the new continental crust and depleted mantle growth curves; (2) about 80% of zircon grains have δ18O values below 6.5‰; and (3) 950–700Ma is the most dominant age group in the total of 1616 concordant detrital zircon ages from the Cryogenian sedimentary rocks. The Early Neoproterozoic (1.0–0.7Ga) thus appears to have witnessed significant juvenile crustal growth in South China. In addition, zircon analyses of the Cryogenian sedimentary rocks revealed 3.2–2.9Ga and possibly 2.7–2.4Ga as two additional episodes of juvenile crust growth in South China and neighboring continents.

Charcoalified plant remains from the Lashly Formation of Allan Hills, Antarctica: Evidence of forest fire during the Triassic Period

Microscopic charcoal preserved in sediments of the Lashly Formation (Middle to Late Triassic) of South Victoria Land (SVL), Antarctica provides a proxy record of palaeofire. These charred fragments derived from wood and leaf pieces along with spores and pollen grains are part of the buried plant biomass preserved in the shale and carbonaceous shale of fluvial/lacustrine deposits. The evidence of paleofire in Antarctica during the Triassic Period is supported by the presence of a thin bed of silicic tuff interlayered with Dicroidium- bearing shale. The detailed morphology and comparative anatomy of least to high thermally altered plant fragments observed under the light, fluorescence and scanning electron microscope demonstrates the evidence of effect of fire on the vegetation during Middle to Late Triassic (240-200 million years). Most of the charred particles retain identifiable structures with well-preserved to distorted cellular details. Such plant fragments are dark brown, blackish brown, quadrangular-multiangular, equi-dimensional, opaque up to the edges and non- fluorescent. The pollen grains recorded from several sedimentary sequences (but the new data come from only one level i.e., member C of Lashly Formation) show distorted exinal sculptures and are light to dark brown in colour indicating intensive thermal effect. The detailed microscopic images of such fire-affected plant remains also explain their preservation biases in the sediments. The magnitude of the charred pieces and their abundance peaks from various locations in Allan Hills is related solely to the production and amount of burnt material embodied in the sediments. The high peaks indicate severity of fire and production of maximum charcoal of the standing vegetation in Allan Hills.