Devonian Limestones Research Articles

Inflorescences and Flowers ofMauldinia angustilobasp. nov. (Lauraceae) from Middle Cretaceous Karst Infillings in the Rhenish Massif, Germany

A new species of the extinct genus Mauldinia (Lauraceae) is described from middle Cretaceous karst infillings in Upper Devonian limestone of the Rhenish Massif, Germany. The new species, Mauldinia angustiloba, is based on exquisitely preserved flowers and fragments of inflorescences, in some cases with flowers attached. The compound inflorescences consist of an elongate axis with bilobed lateral inflorescence units in a spiral arrangement. Each lateral inflorescence unit typically bears five sessile and bisexual flowers on the adaxial surface but in rare cases has up to nine flowers. Flowers are trimerous with two perianth whorls, nine fertile stamens in three whorls, three staminodia in an inner fourth whorl, and a superior unilocular gynoecium. Stamens of the third whorl have paired, almost sessile, lateral staminal appendages. Mauldinia angustiloba is distinguished from previously described species of the genus by the longer and narrower lobes of the lateral units and by the almost sessile staminal appendages. The excellent preservation of the fossils has yielded new information on floral structure and anatomy in Mauldinia.

DEVONIAN CARBONATES OF THE NIGEL PEAK AREA, ROCKY MOUNTAINS, CANADA: A FOSSIL PETROLEUM SYSTEM?

In this study we report on Devonian (Frasnian – Famennian) limestones and dolostones exposed near Nigel Peak in the Main Ranges of the Canadian Rocky Mountains. These carbonates are a proximal facies of the Southesk‐Cairn Carbonate Complex. The investigated strata are stratigraphically equivalent to the oil‐ and gas bearing Nisku Formation in the subsurface of the Western Canada Sedimentary Basin, about 300 km to the east. The rocks were investigated by polarisation and cathodoluminescence microscopy, total organic carbon analysis, Rock‐Eval pyrolysis, solid bitumen reflectance measurements, gas chromatography and fluid inclusion analysis.Thin section analyses showed that silt‐grade quartz and saddle dolomite increase upward from the base of the stratigraphic section, and that porosities are generally low. This is due to reduction of pore space due to early cementation and extensive dolomitization. Cathodoluminescence identified up to four generations of calcite cements. TOC values ranged from 0.2 to 2.4 %. Rock‐Eval pyrolysis of carbonate samples resulted in measurable S1 peaks but not S2 peaks, indicating that there was no residual petroleum generation potential. Organic petrographic analyses identified dispersed kerogen and migrabitumen, and calculated vitrinite reflectance values were around 4 % on average which implies peak temperatures of 234–262 °C (due to deep burial) or 309–352 °C (due to short term hydrothermal heating). Fluid inclusion data indicates at least one pulse of hot fluids with elevated homogenization temperatures of > 300 °C, and this may explain the high thermal maturity of the studied rocks.

The iron isotope composition of microbial carbonate

In a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve Fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation. It was found that δ 56Fe values of Archean stromatolites vary between − 2.1‰ and − 0.5‰, Devonian and Carboniferous microbiallites vary between 0‰ and − 1‰, and modern microbialites from the Great Barrier Reef show a range from − 0.12‰ to + 0.15‰. Five lines of evidence support the possibility that these compositions are potentially pristine. 1) Fe concentrations and Fe isotope ratios are not correlated. 2) The concentrations of other elements that are potentially mobile during carbonate diagenesis (Mg, Sr) do not correlate with δ 56Fe. 3) Mn concentrations are inversely correlated with δ 56Fe, which might hint at meteoric alteration. However, if Mn concentrations and Fe isotope signatures were pristine features, they might serve as a record of the reduction of these two metals in seawater. 4) Dolomitisation of two Devonian limestones does not shift their Fe isotope compositions. 5) Rare earth element and yttrium (REY) patterns in all limestones are similar to seawater REY sources. This suggests that the bulk of the Fe in the samples occurs in hydrogenous carbonates, not in primary or secondary Fe and Mn hydroxides, because the latter have different REY patterns. The modern microbialites are unfractionated in Fe relative to crustal rocks. Thus, these hydrogenous precipitates either incorporated unfractionated Fe of the ambient seawater composition, or they modified the Fe composition upon incorporation from a fractionated ocean Fe reservoir. In either case, provided diagenetic alteration has not modified the Fe isotope composition in a major way, a potential open ocean Fe isotope proxy may exist that may allow reconstruction of secular variations in seawater Fe isotope compositions. Our preliminary results suggest that Neoarchean microbial carbonate Fe may have been lighter than today's microbialite Fe, and that the carbonates' Fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time.

Mississippian Platform and Basin Successions from the Todrha Valley (northeastern Anti‐Atlas), southern Morocco

AbstractCarboniferous rocks of Mississippian age are exposed in Jebel Asdaf and Jebel Tisdafine, south and north respectively of the fault‐controlled Todrha Valley, between the towns of Tinerhir and Tinejdad in the Anti‐Atlas of southern Morocco. The age, as determined by microfossils, and facies of the dominantly siliciclastic successions of the two areas are different. The Jebel Asdaf Formation (245 m exposed; base and top not seen) is a shallow shelf sequence of late Tournaisian age. The Jebel Tisdafine Formation is a succession of turbidites (at least 1500 m thick), which rests unconformably on Devonian limestones and is of late Viséan age. Limestone beds in each formation have different origins. Skeletal grainstones and packstones in the Jebel Asdaf Formation formed when sand supply to the shelf was episodically reduced. Ooid‐rich skeletal packstones and grainstones in the Jebel Tisdafine Formation, characterized also by the presence of well‐rounded, coarse‐sand grade quartz grains, were derived from a contemporary carbonate platform. Individual beds in the Jebel Tisdafine Formation are essentially tabular and can be followed for long distances. Channelling is very rare. The turbidites, which are quartz‐rich, were transported from west to east across a basin floor with low slopes. Jebel Azguine, northwest of Tinejdad, is separated from Jebel Tisdafine by a major shear zone. It is made up of quartz sandstones that differ from those of both the Jebel Asdaf and Jebel Tisdafine formations in not containing any carbonate or body fossils, in its trace fossil assemblage dominated by vertical burrows, and in the presence of very thick beds. It is tentatively suggested that the Jebel Azguine sandstones are older than Carboniferous, to which they are assigned on some geological maps. The conclusions of this research are markedly different from accounts of the geology of the area published previously. Copyright © 2007 John Wiley & Sons, Ltd.

Structural and magnetic fabric study of the Marimanha granite (Axial Zone of the Pyrenees)

The structural and magnetic fabric study of the Marimanha granite, Axial Zone of the Pyrenees, provides new data to characterize the zonation and the internal structure of the pluton. The Marimanha granite intrudes Cambro–Ordovician clastic rocks and Silurian–Devonian limestones and slates. The zonation of the low field magnetic susceptibility, consistent with the petrological zonation of the igneous body, indicates a concentric arrangement of rock types, with more basic compositions at the external areas. This pluton is characterized by a low susceptibility, and rock–magnetic studies indicate a majority of “paramagnetic” samples. Magnetic foliations strike parallel to petrographic contacts and to contours of zonation of magnetic susceptibility, and show a dominant NE–SW strike, steeply dipping towards the North. Locally, in the northern border of the pluton foliation directions become perpendicular to petrographic contacts and depict sigmoidal trajectories. Magnetic lineations are characterized by the predominance of NE–SW trends with shallow plunges to the NE and SW. These foliations and lineations are parallel to the slight elongation of internal petrographic zonation. Magnetic fabric within the granitoid body and internal elliptical shape of petrographic zonation, suggest an intrusion contemporary with a transpressional regime and NNW–SSE shortening, syntectonic with the late stages of the Variscan orogeny. These results are in accordance with anisotropy of magnetic susceptibility studies of others plutons in the Pyrenees. To explain the origin of the Marimanha granite we propose magma ascent through faults at depth and emplacement by ballooning in situ at the rheological boundary between Cambro–Ordovician and Siluro–Devonian metasediments.

Structural and isotopic constraints on fluid flow regimes and fluid pathways during upper crustal deformation: An example from the Taemas area of the Lachlan Orogen, SE Australia

Structural and stable isotope studies of calcite veins and host rocks in a 1000 m thick, Devonian limestone sequence in the Taemas area of the Lachlan Orogen in SE Australia indicate that externally derived fluids migrated through the sequence over a protracted interval during folding and associated reverse faulting at temperatures of ∼200°C. The evolution of fluid pathways was governed by growth of fold‐related and fault‐related fracture networks at transiently supralithostatic fluid pressures. The internal structures of veins indicate growth during repeated cycles of permeability enhancement and sealing. Systematic increase inδ18O of vein calcite upward through the limestone sequence, and the presence of an O isotope alteration front in the vein system, is related to buffering of fluid compositions by fluid‐rock reaction during upward migration of overpressured, lowδ18O fluids. Repeated switching of fractures between high‐permeability and low‐permeability states promoted episodic flow and changes in flow paths during progressive deformation. A lack of significant migration of the isotopic front during the flow history is consistent with fluid‐rock interaction being associated with numerous, separate pulses of fluid ascending through the percolation network. Irregular variations in fluidδ18O with time during vein growth are interpreted to be driven by repeated changes in reactive path lengths, reaction rates, and flow rates in a discontinuous flow regime. Systematic upward changes in veinδ18O over an area of 20 km2requires that on a time‐averaged basis, most of the vein system was hydraulically well connected to the external fluid reservoir and that growth of the fracture network occurred by fluid‐driven, invasion percolation processes. Repeated failure and sealing events in the fracture system are interpreted to have been driven by episodic migration of fluid pressure waves up through the fracture network, possibly in response to deeper level fault rupture events repeatedly breaching an overpressured fluid reservoir.

Rare earth element and yttrium analyses of sulfides from the Dachang Sn-polymetallic ore field, Guangxi Province, China: Implication for ore genesis

The Dachang Sn-polymetallic ore field (Guangxi, China) is one of the largest tin deposits in the world with a tin reserve of above 106 tons. Several types of orebody occur in this giant ore field, including mainly the Lamo Cu-Zn skarns at the contact of Devonian limestone and the Cretaceous Longxianggai granite, the Changpo-Tongkeng stratiform Sn-Pb-Zn ores in Devonian siliceous rocks, and the Gaofeng massive sulfide ore in Devonian reef limestone. Rare-earth element (REE) and yttrium concentrations in sulfides from different orebody types have been determined by ICP-MS. Sulfides from the Lamo skarn-type Cu-Zn mines display significant LREE-enriched pattern with large negative Eu anomalies. This is in agreement with the mineralizing fluid being of magmatic hydrothermal origin. However, sulfides from massive and stratiform orebodies show significantly different REE distributions. The Gaofeng massive sulfides have very low total REE concentrations and REE patterns range from (1) being strongly enriched in light REE with positive Eu anomalies, to (2) relatively flat patterns with strongly negative Ce and positive Eu anomalies. Pattern 1 resembles REE distributions in modern submarine hydrothermal fluids. Pattern 2 exhibits both seawater and hydrothermal vent-fluid characteristics. Sulfides from stratiform orebodies at Changpo-Tongkeng are LREE-enriched with small or no negative Eu anomalies. REE patterns of sulfides from different stratiform orebodies are slightly different, but they are similar to those of each host rock. It indicates that the sulfides and host rocks are synsedimentary and that their REE patterns are controlled by the local physical-chemical environments. Y/Ho atomic weight ratios of granite, skarn and sulfides at Lamo vary from 26.3 to 33.7, close to the chondritic Y/Ho ratio. But sulfides from massive and stratiform ores have more variable Y/Ho ratios (28.4-134) that are similar to those of many modern hydrothermal fluids. Hence, the ore-forming fluids responsible for mineralization of the stratiform and massive ores at Dachang were likely to be of exhalative hydrothermal origin. It is suggested that the Dachang deposits are formed through a two-stage hydrothermal process, i.e., the Devonian submarine hydrothermal ore-forming stage and the Yanshanian magmatic-hydrothermal ore-forming stage.

Sulfur Isotope Geochemistry of the Supergiant Xikuangshan Sb Deposit, Central Hunan, China: Constraints on Sources of Ore Constituents

. The supergiant Xikuangshan Sb deposit is located in the Middle to Upper Devonian limestone of central Hunan, China. Primary ores are composed of early-stage stibnite and calcite with rare pyrite, early main-stage stibnite and quartz, and late main-stage stibnite and calcite. New sulfur isotope data reveal the clustering of δ34S values (+5 ∼ +8 %) for both early and late main-stage stibnite; a single early-stage stibnite exhibits δ34S value (+7.5 %) identical to its main ore-stage counterparts and the coexisting calcite has almost unmodified carbon isotope composition (-4.4 %). The data suggest a probable common source of sulfur for stibnite that was deposited at different paragenetic stages. A much wider variation in δ34S values for early main-stage stibnite (+3.5 to +16.3 %, av. +7.5 %) compared to that for late main-stage stibnite (+5.3 to +8.1 %, av. +6.2 %) can be interpreted to be due to local interaction of earlier ore fluid with Devonian host rocks. The previous studies show that the Precambrian basement contains elevated Sb concentrations, and two distinctive sulfur reservoirs with δ34Spyrite values at ca. +11 ∼ +24 % and -7.0 ∼-11 %. The homogenizing effect for sulfur hydrothermally leached from the two reservoirs might have provided ore constituents for the Xikuangshan fluids.

The pink topaz-bearing calcite, quartz, white mica veins from Ghundao Hill (North West Frontier Province, Pakistan): K/Ar age, stable isotope and REE data

In the area of the Ghundao Hill (Northern Frontier Province, Pakistan) an orange-yellow to cherry-red topaz is found in calcite, quartz, white mica veins crosscutting the schistosity of probably Silurian to Devonian gray limestones. Topaz with such a range of colours is traded as Imperial Topaz. Low fluorine contents of about 15 wt.%, oxygen isotope thermometry, K/Ar age determination on white mica, fluid inclusion data and mineral textures indicate that the topaz from Ghundao Hill crystallized at temperatures of about 230 °C during the Eocene Himalayan tectonothermal event and not from a late to postmagmatic granite-related fluid. The pink Topaz from Ghundao Hill shares the coexistence with carbonates, low fluorine content and a crystallization at low temperature and pressure during a regional tectonothermal event with the Imperial Topaz from Ouro Preto (Brazil) and from the Sanarka/Kamenka rivers (South Urals, Russia). The efficiency of topaz to remove fluorine from fluids at low temperature explains how topaz can be formed from metamorphic fluids that are typically poor in fluorine. High CO2 activity produced in the fluids by metamorphic decarbonatisation reactions and Al buffering by white mica prevented fluorination of carbonates stabilising topaz relative to fluorite.

Placoderm fauna from the Connemarra Formation (?late Lochkovian, Early Devonian), central New South Wales

Two new placoderm taxa, based on isolated dermal plates from the head and trunk shields and body scales, are described from ?late Lochkovian (Early Devonian) limestones of the Connemarra Formation, central New South Wales. The taxa are Connemarraspis youngi gen. et sp. nov. and Narrominaspis longi gen. et sp. nov., for which the holotype specimens are ossified optic capsules. The fauna also includes several indeterminate acanthothoracids. All species were small fish, estimated to have been less than 10 cm long. This placoderm fauna shows a wide range of affinities, encompassing taxa with similarities to species from the Lochkovian of Arctic Canada, Europe and Vietnam.

Fluid control of localized mineral domains in limestone pressure solution structures

Grain-to-grain and stylolitic solution structures in two central Appalachian Siluro–Devonian limestone macroscale folds contain one of four distinct mineral assemblages that are characterized by the dominant iron-phase mineral present: (1) chlorite±illite± pyrite±calcite±quartz±TiO 2±goethite, (2) chlorite±illite± pyrite altered to iron oxide/hydroxide±calcite ±quartz±TiO 2, (3) chlorite±illite± magnetite±calcite±quartz, and (4) chlorite±illite± goethite±calcite±quartz±TiO 2. Optical reflectance microscopy and SEM–EDS was used to characterize the mineralogy and mineral morphology of these structures. Geochemical modeling was used to constrain the conditions of formation and preservation. The primary control on solution structure mineral assemblage was the redox conditions present in the solution structures during burial and deformation. The redox conditions on the microscale may have been controlled by the local fluid chemistry and the presence–absence of hydrocarbons and organic acids within the formation fluids, and the influx of externally derived fluids by fracture formation during the folding process. The wide variation in mineralogy of the solution structures shows that they were ‘chemical factories’ where a variety of chemical reactions took place during rock dissolution. In particular, the formation of authigenic magnetite in solution structures has significant implications for paleomagnetic applications, and use of anisotropy of anhysteretic remanent magnetization and anisotropy of magnetic susceptibility fabrics.

X-ray microtomography of 410 million-year-old optic capsules from placoderm fishes

The structure of two small ossified optic capsules from mid-Palaeozoic placoderm fishes has been revealed in fine detail, by the use of X-ray microtomography analysis and 3D visualisation software. These two specimens are 410 million-year-old; they were collected from an Early Devonian (Lochkovian) limestone in central New South Wales, and are the oldest known optic capsules from jawed fishes. The capsules show attachment areas for seven extrinsic eye muscles, rather than the six until recently deemed universal for gnathostomes. The analysis also revealed structures within the ossified cartilage which covered the medial surface of the eyeball, including nerve tracts, vascular canals, and possibly a choroid rete mirabile.

Proterozoic magmatic and tectonometamorphic evolution of the Taratash complex, Central Urals, Russia

The Taratash Complex (TC) in the northernmost Bashkirian Anticlinorium (Middle Urals) is unique among the pre-Uralian polymetamorphic complexes along the eastern margin of the East European Craton because it experienced granulite facies peak metamorphic conditions (850–900°C/10 kbar). Herein, we constrain the post-granulite facies polystage evolution of the complex, which records various increments of the geodynamic history of the East European continental margin. Formation of granite and migmatite associated with amphibolite facies events are dated at 2,344±29 and 2,044±8 Ma (U–Pb, zircon) in different structural units. At 1,810±41 Ma, the TC was affected by a greenschist facies retrogressive metamorphism which was probably related to a stage of granite formation in the eastern part of the East European Craton. This is confirmed by a U–Pb–zircon age of 1,848±8 Ma obtained from a sheared granite in the adjacent Alexandrovskiy Complex (AC). Greenschist facies shear zones which separate different structural units of the TC formed before 1,350 Ma. Partial re-equilibration of Rb–Sr- and K–Ar-isotope systems between 1,350 Ma and 1,200 Ma is attributed to fluid flow probably induced by anorogenic magmatism in the Bashkirian Anticlinorium. Meso- to Neoproterozoic basaltic dykes indicate that the TC had been exhumed to upper crustal levels at that time. Evidence for a Grenvillian event or for the Timanian orogeny which affected other pre-Uralian complexes in the Urals is lacking. Uralian orogenic shortening and thrusting on Devonian limestones is recorded by shear zones in the AC to the east of the TC and has been dated at 300 Ma (Rb–Sr, 40Ar/39Ar).

05/00569 Geochemistry of oil in fluid inclusions in a middle Proterozoic igneous intrusion: implications for the source of hydrocarbons in crystalline rocks

Reefal carbonate deposits in South China preserve a record of community structure and environmental change across the Frasnian-Famennian boundary and through the Late Devonian, detailing a regional response to one of the largest mass extinction events in Earth history. The Upper Devonian (Frasnian to Famennian) limestone outcrops in the Maoying area of northern Ziyun County, South China, record a carbonate platform depositional setting with some patch reef development. The Upper Devonian carbonate deposits in Maoying area are distinctive from those in correlative sections in Dushan County with rare argillaceous limestone and dolomitic limestone as well as less dolostone. Exposures to the south in the Guilin area of northeastern Guangxi Province are similar to those seen in the study area. Upper Devonian carbonates in the Maoying area are divided into two lithologic groups, reef limestones or dolostone beds, composed of five lithologically distinctive intervals. The Upper Devonian strata have a cumulative thickness of 530 m. The Upper Devonian reefs in the Maoying area were developed on a shell pavement of brachiopod- and bivalve-shell shoal packstones and evolved into patch reefs in the interior of the carbonate platform. Patch-reef development was limited to early Frasnian time. Reef-building organisms are dominated by massive and tabular stromatoporoids, Amphipora sp., Stachyodes sp., and coral Thamnopora sp., while brachiopods and algae accumulated between stromatoporoid skeletal frameworks. Three evolutionary stages of the Frasnian patch reefs, including an initial development stage, maturity stage and demise stage, can be recognized according to sedimentary structures and fossil assemblage characteristics. The Frasnian strata overlying the patch reefs represent a well-bedded back-reef depositional setting and are mainly composed of Amphipora floatstone and packstone containing a few bulbous stromatoporoids and small fasciculate corals. Famennian strata are characterized by laminated limestone and fenestrate limestone that are interbedded with infrequent mudstone and argillaceous limestone horizons. A prominent microbialite bed developed in the laminated limestone of the Famennian carbonate platform consists of stromatolite and thrombolite framestone. This bed can be correlated, in horizon and age, with microbial reefs and mounds in the Famennian offshore carbonate platform in the Huanan Epeiric Sea (such as Famennian microbial reefs and mounds in Guilin). This represents a biogenic carbonate formation characterized by microbial fabrics that accumulated in a nearshore carbonate platform setting after the global Frasnian/Famennian biotic extinction event, which promoted the growth of microbialites during the latest Devonian in South China.

Mineral chemistry of chromite from the Permian Jinbaoshan Pt–Pd–sulphide-bearing ultramafic intrusion in SW China with petrogenetic implications

The Jinbaoshan ultramafic intrusion is a member of the late–middle Permian (260 Ma) Emeishan Large Igneous Province in Yunnan Province, SW China and hosts an economical Pt–Pd–sulphide deposit. The intrusion consists mainly of wehrlite (92 vol.%), with minor olivine clinopyroxenite and clinopyroxenite (8 vol.%). The ores, defined as rocks having >0.5 ppm Pt and Pd, contain abundant chromite (up to 20 vol.%) and minor sulphide (<3 vol.%). Sulphide aggregates occur along the silicate grain boundaries and thus clearly crystallized after olivine and chromite. Olivine occurs as euhedral, subhedral or rounded crystals, and varies from 0.3 to 4 mm in size. The olivine has almost constant Fo contents (83 wt.%), and variable Ni contents (0.13 to 0.34 wt.%). There are two types of chromite: (1) relatively homogeneous chromite, as euhedral inclusion in olivine; and (2) zoned chromite as interstitial phase between silicate minerals, and occasionally, as inclusion in olivine pseudomorphs. The zoned chromite can be further divided into type 2a with magnetite rim and type 2b without magnetite rim. Primary chromites, including type 1 chromites and the core of type 2a chromites, have relatively high Cr#'s (51 to 70), a relatively wide range of Mg#'s (13 to 57), variable TiO 2 (1.55 to 9.39 wt.%) and low Ni (NiO≤0.35 wt.%) and Zn (ZnO≤0.72 wt.%). Type 2 chromites formed during serpentinization and were chemically modified toward magnetite rim. The modified chromites have generally lower Cr 2O 3 and Al 2O 3 and higher FeO t (total Fe) than primary chromites. The high Cr contents of the primary chromites and the high MgO/FeO molar ratio (1.47) of the calculated melt composition in equilibrium with olivine in the Jinbaoshan intrusion demonstrated that the Jinbaoshan chromites most likely crystallized from a low-Ti and high-Mg tholeiitic magma derived from a mantle plume in an intra-continental setting. The relatively large amount of chromite in the ores may indicate a direct connection between the chromite crystallization and the ore formation. We infer that the parental magma was S-undersaturated, and remained rich in highly siderophile elements such as Pt and Pd. The parental magma may have experienced early olivine crystallization such that Ir and Os were depleted. This magma intruded the Devonian limestone, sandstone, and slate. Assimilation of the country rocks induced temporary supersaturation of chromites and also triggered S-saturation in the magma. The Jinbaoshan intrusion represents a magma conduit in which magmas deposited excess ultramafic components with Pt–Pd–sulphide enriched horizons due to delayed S-saturation during crystallization.

Evolution of geochemistry and depositional settings of Early Palaeozoic siliciclastics of the Barrandian (Teplá-Barrandian Unit, Bohemian Massif, Czech Republic)

Cambrian and Ordovician-Middle Devonian sequences of two successive Early Palaeozoic basins of the Barrandian unconformably overlie Cadomian basement in the Bohemian Massif NW interior (Tepla-Barrandian unit) which is the easternmost peri-Gondwanan remnant within the Variscides. Correlation of stratigraphy and geochemistry of the Early Palaeozoic siliciclastic rocks elucidated sediment provenances. Sandstones of the Middle Cambrian Přibram-Jince Basin were derived from a Cadomian Neoproterozoic island arc. The source area of the Ordovician shallow-marine siliciclastics of the successor Prague Basin is a dissected Cadomian orogen. Late Cambrian acid volcanics of the Barrandian and Cambrian (meta)granitoids emplaced in the W part of the Tepla-Barrandian Cadomian basement are also discernible in these sediments. Old sedimentary component increased during the Ordovician. Early Llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of Silurian transgression. Emsian siliciclastics (intercalated in the Late Silurian to Early Devonian limestone suite) presumably comprise an addition of coeval basic/ultrabasic volcaniclastics. Middle Devonian flysch-like siliciclastics indicate reappearance of Cadomian source near the Barrandian during early Variscan convergences of Armorican microplates that preceeded accretion of the Tepla-Barrandian unit within the Bohemian Massif terrane mosaic.

Petroleum systems in the eastern and central Taurus region, Turkey

The Autochthonous Taurus ‘known’ petroleum system is geographically located in the eastern Taurus region and includes the Allochthonous Units and Miocene–Pliocene sequences in the Adana Basin. The total thickness of the Early Palaeozoic to Cenozoic sediments reaches 9500–10400 m. The eastern Taurus region is a major petroleum source area with three source rocks: Upper Devonian limestones, Lower Carboniferous shales and dark carbonates, and Upper Permian algal limestones and shales. The Burdigalian–Serravillian sandstones and limestones and the Late Tortonian–Messinian evaporites and shales of the Adana Basin constitute the reservoir–seal rock combinations in this petroleum system. The main periods for oil generation from these source rocks occurred during the Aptian to Bartonian, Maastrichtian and Lutetian to the present. However, the main periods for oil generation for the effective source rocks (Lower Carboniferous and Upper Permian formations) began 40 mybp and continued to 20 mybp. Oil and gas traps in this petroleum system are formed by thrusting due to Late Cretaceous and Late Eocene tectonism, strike–slip faults and unconformity surfaces.

Precise Sm–Nd and U–Pb isotopic dating of the supergiant Shizhuyuan polymetallic deposit and its host granite, SE China

The supergiant Shizhuyuan W–Sn–Bi–Mo deposit is hosted by the Qianlishan granite, a small, highly fractionated granitic pluton (∼10 km2) with multiple phases of intrusions within the Early Yanshanian granitoid province of SE China. Strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and Devonian limestone is responsible for the polymetallic mineralizations. SHRIMP U–Pb zircon analysis indicates that the two early phases of the Qianlishan granite formed contemporaneously at 152±2 Ma. Metasomatic minerals (garnet, fluorite and wolframite) separated from the skarn and greisen yield a Sm–Nd isochron age of 149±2 Ma that is interpreted as the formation age of the Shizhuyuan deposit. Therefore, the mineralization of the supergiant Shizhuyuan polymetallic deposit formed contemporaneously with, or very shortly after, the intrusion of the small, highly fractionated Qianlishan granite.

When Did the Emeishan Mantle Plume Activity Start? Geochronological and Geochemical Evidence from Ultramafic-Mafic Dikes in Southwestern China

SHRIMP zircon U-Pb dating of an ultramafic-mafic dike intruded into Lower Devonian limestones from southwestern China gives a mean 238U/206Pb age of 262 ±3 Ma (n = 12, MEWD = 5.7). This is the oldest age hitherto reported for the Emeishan large igneous province (ELIP). These and related dikes have OIB-like trace-element signatures (e.g., low LILE/HFSE, LREE/HFSE, and Zr/Nb ratios) and strong geochemical and Sr-Nd isotopic similarities to the least contaminated ELIP high-Ti basalts. A compilation of published Ar/Ar and SHRIMP zircon U-Pb ages for the ELIP shows that plume activity probably started at ~262 Ma, and attained its maximum at 253-251 Ma as a result of continuous plume-lithosphere interaction. The ~10 m.y. interval of the ELIP magmatic episode favors a plume-impaction model, wherein the plume head impinged on the base of the lithosphere over a time scale predicted by laboratory experiments.

Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: confirmation of a seawater REE proxy in ancient limestones

Rare earth element and yttrium (REE+Y) concentrations were determined in 49 Late Devonian reefal carbonates from the Lennard Shelf, Canning Basin, Western Australia. Shale-normalized (SN) REE+Y patterns of the Late Devonian samples display features consistent with the geochemistry of well-oxygenated, shallow seawater. A variety of different ancient limestone components, including microbialites, some skeletal carbonates (stromatoporoids), and cements, record seawater-like REE+Y signatures. Contamination associated with phosphate, Fe-oxides and shale was tested quantitatively, and can be discounted as the source of the REE+Y patterns. Co-occurring carbonate components that presumably precipitated from the same seawater have different relative REE concentrations, but consistent REE+Y patterns. Clean Devonian early marine cements ( n = 3) display REE+Y signatures most like that of modern open ocean seawater and the highest Y/Ho ratios (e.g., 59) and greatest light REE (LREE) depletion (average Nd SN/Yb SN = 0.413, SD = 0.076). However, synsedimentary cements have the lowest REE concentrations (e.g., 405 ppb). Non-contaminated Devonian microbialite samples containing a mixture of the calcimicrobe Renalcis and micritic thrombolite aggregates in early marine cement ( n = 11) have the highest relative REE concentrations of tested carbonates (average total REE = 11.3 ppm). Stromatoporoid skeletons, unlike modern corals, algae and molluscs, also contain well-developed, seawater-like REE patterns. Samples from an estuarine fringing reef have very different REE+Y patterns with LREE enrichment (Nd SN/Yb SN > 1), possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged LREE from a nearby riverine input source. Hence, Devonian limestones provide a proxy for marine REE geochemistry and allow the differentiation of co-occurring water masses on the ancient Lennard Shelf. Although appropriate partition coefficients for quantification of Devonian seawater REE concentrations from out data are unknown, hypothetical Devonian Canning Basin seawater REE patterns were obtained with coefficients derived from modern natural proxies and experimental values. Resulting Devonian seawater patterns are slightly enriched in LREE compared to most modern seawaters and suggest higher overall REE concentrations, but are very similar to seawaters from regions with high terrigenous inputs. Our results suggest that most limestones should record important aspects of the REE geochemistry of the waters in which they precipitated, provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high, non-seawater-like REE contents. Hence, we expect that many other ancient limestones will serve as seawater REE proxies, and thereby provide information on paleoceanography, paleogeography and geochemical evolution of the oceans.