Facies Assemblages Research Articles

Metamorphic evolution of ultrahigh-pressure rocks from Chinese southwestern Tianshan and a possible indicator of UHP metamorphism using garnet composition in low-T eclogites

How to identify ultrahigh-pressure (UHP) metamorphism in the absence of coesite is a key problem to gain the correct P–T history for an orogenic belt. In this study, garnet composition combined with the pseudosection approach was used to identify ultrahigh-pressure metamorphism and to determine P–T paths for eclogites and metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both eclogites and metapelites develop pronounced chemical core-rim textures: the relatively homogeneous core with low pyrope [Prp; Mg/(Ca+Mn+Mg+Fe2+)×100] and grossular [Grs; Ca/(Ca+Mn+Mg+Fe2+)×100] content is overgrown by a thin rim with sharply increased Prp and Grs. Phase equilibria modeling indicates that the ultrahigh-pressure rocks have undergone a clockwise P–T path characterized by heating during early exhumation with peak P–T at 31–33kbar and 490–520°C. The P–T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend on temperature and pressure, respectively, especially in the stability fields of glaucophane–lawsonite-bearing eclogite facies assemblages. This indicates that garnet composition provides robust thermobarometric constraints. Consequently, we propose a Prp–Grs diagram which is subdivided into a high-pressure region and an ultrahigh-pressure region by the quartz–coesite–transition curve. Those garnet compositions which fall into the ultrahigh-pressure region are regarded to have experienced ultrahigh-pressure metamorphism. This approach is expected to be a useful tool to qualitatively identify ultrahigh-pressure metamorphism for glaucophane–lawsonite-bearing eclogites and its particular strength is the quick examination of large datasets comprising samples with similar bulk composition. Using this method, garnet compositions of eclogites and mafic blueschists from Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the Prp–Grs diagram and several possible ultrahigh-pressure eclogite occurrences are newly identified.

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

AbstractIn the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO2‐poor fluid with X(CO2) [CO2/(CO2 + H2O) in the fluid] <<0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO2) (>0.01) in the coexisting fluid at the epidote eclogite facies stage.

Lacustrine Carbonates From Lake Turkana, Kenya: A Depositional Model of Carbonates In An Extensional Basin

Abstract Lake Turkana, northern Kenya, serves as a long-lived end-member extensional basin that preserves a volcanically mediated, mixed siliciclastic and carbonate system. Stratigraphic data from the southeastern margin of Lake Turkana reveal a suite of carbonate facies assemblages. Lithologies of this system include mudstone, siltstone, sandstone, conglomerate, diatomite, carbonate mudstone, wackestone, grainstone, packstone, and boundstone. The boundstones are subdivided into framestones (stromatolites or oncoids) and display upward growth from a nucleus where found in association with cobbles and shells and bindstones (laminar tufa) where not. Three discrete, large (up to 7.5 m high), stacked, mounded boundstones are interpreted as subaerial spring deposits based upon overall shape, surficial texture, and orientation with respect to mapped faults and the distribution of modern springs. Petrographic analysis reveals peloids, ostracods, microbial sheaths, gastropods, pelecypods, diatoms, and detrital grains that include quartz, volcanic glass, and volcanic lithic fragments. All grains are micritized, and fossils tend not to retain their original microstructure. Some lithologies have porosities as high as 39%, and permeability averages 4 mD as a result of the dominance of moldic porosity and microporosity. The combination of lithologic data and facies associations is combined to produce a depositional model that is useful for comparison with ancient rift basins that host economic quantities of hydrocarbons. The controlling factors for carbonate development in Lake Turkana include the structural framework, water depth, and lake chemistry.

Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite‐rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

AbstractWe have investigated the effects of different Fe2O3 bulk contents on the calculated phase equilibria of low‐T/intermediate‐P metasedimentary rocks. Thermodynamic modelling within the MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system of chloritoid‐bearing hematite‐rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe2O3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe2O3 diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post‐compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock‐forming silicates that often yield important thermobaric information. In Fe2O3‐rich compositions, garnet‐ and carpholite‐in curves shift towards higher temperatures with respect to the Fe2O3‐free systems. The presence of a ferric‐iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe2O3 phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.

Facies Architecture and Stratigraphic Evolution of A River-Dominated Delta Front, Turonian Ferron Sandstone, Utah, U.S.A

Abstract Facies architectural analysis of a river-dominated deltaic parasequence in the Turonian Ferron Sandstone Member, near Factory Butte, Utah, was conducted to determine the hierarchical spatial arrangement of mouth bars and their role in constructing the delta. Five architectural elements were identified using 21 measured sections, paleocurrent measurements, and bedding diagrams. They are: (1) prodelta (PF), (2) frontal splay (FS), (3) detached distal bars (DBs), (4) distributary mouth bars (DMBs), and (5) terminal distributary channels (TDCs). Lower in the parasequence, detached bars consist of proximally located cross-bedded and quasi-parallel laminated subaqueous bars that onlap the delta-front slope and grade distally into finer-grained distal delta front splays. These bars translate downstream in a progradational–aggradational–degradational trajectory and are overlapped by adjacent bars, forming a bar assemblage. These bars are interpreted to have been deposited by descending hyperpycnal flows as a decelerating axial jet. Deposition of these bars occurs at the delta-front to prodelta chokepoint, presumably due to flow deceleration. The distal bar assemblage is capped by an extensive but discontinuous shale drape, suggesting local abandonment. Upper facies assemblages show a distinct switch to predominantly cross-bedded distributary mouth bars, corresponding to a decrease in water depth, an increase in friction, and a likely transition into planar versus axial jets. This is indicated by aggrading cross-bedded distributary mouth bars that overlie the distal bars. Laterally, the mouth-bar deposits are interstratified with terminal distributary-channel deposits with prominent scour surfaces at the base. Separate distributary channels and their bars coalesce, ultimately to build the entire delta lobe. The detached distal bars prograde locally west to northwest and lack fair-weather wave-formed sedimentary structures, suggesting a local embayment along the regionally northeast prograding shoreline. Low levels of ichnofaunal diversity indicate a stressed, river-dominated setting. At the base of the section, frontal splays and detached bars, dominated by storm-driven river-flood deposits, are deposited over muddy prodelta facies. Detached bars are likely initiated about 500 meters distal of the distributary channel, in water depths of ∼ 10–30 meters, based on the thickness of the parasequence. Once this subaqueous platform is constructed by the distal bar assemblage, more proximal distributary mouth bars fill the remaining shallow-water accommodation. As the system continues to shallow, distributary channels override and scour the bars, reflecting reduced accommodation.

Volcanic rift margin model for the rift-to-drift setting of the late Neoproterozoic-early Cambrian eastern margin of Laurentia: Chilhowee Group of the Appalachian Blue Ridge

New data support a model of a volcanic rifted margin for eastern Laurentia and the breakup of the supercontinent Rodinia. Upper Neoproterozoic–lower Cambrian rocks of the Chilhowee Group in the Blue Ridge Province of eastern North America are subdivided into two facies assemblages separated by an unconformity. Historically, the rocks have been correlated as a tripartite division: 1) basal sandstone and conglomerate (Cochran, Unicoi, and Weverton Formations), 2) middle siltstone and shale (Nichols Shale, Hampton Shale, and the Hampton and Harpers Formations), and 3) an upper sandstone and shale (Nebo Quartzite, Murray Shale, Hesse Quartzite, and Helenmode, Erwin, and Antietam Formations). Sedimentary analyses show that boundaries of the newly defined facies assemblages transect the named stratigraphic units. Assemblage A consists of fluvial-lacustrine deposits with interbedded subaerial basalt flows overlain by marine deposits. Fluvial strata formed in anastomosing braided-river channels and are interbedded with mudstones deposited in shallow lacustrine plains. Overlying shallow marine deposits consist of upward-coarsening successions capped by over-thickened sandstones. Shale-rich turbidite deposits characterized by subdued bioturbation and common slump features occur at the top. Assemblage B consists of sandstone and shale deposited on a stable shelf. These upward-coarsening parasequences are stacked to form transgressive-regressive system tracts reflecting long-term sea-level fluctuations. A working hypothesis is that these rocks are comparable to those present in seismic reflection profiles and drill cores of the volcanic rift margins of the Atlantic and Indian Oceans. Assemblage A deposits formed during rapid subsidence associated with the formation of seaward-dipping reflectors. Flood basalts within fluvial sandstones are the landward facies of more voluminous basalt flows in the opening basin. Overlying marine deposits of Assemblage A were coeval with basaltic volcanism and deeper-water deposits to the east. Assemblage B unconformably overlies Assemblage A rocks and laps onto much older continental rocks. It was deposited on the craton into the late Cambrian. Much of the lateral lithologic variability and differences in character and thickness of the Chilhowee Group is attributed to the depositional setting that was influenced by the underlying rift architecture. It was further complicated by juxtaposition of rocks telescoped along numerous Paleozoic thrust faults. This model predicts that coeval rocks in outboard thrust sheets are finer-grained, mafic volcanic-rich marine sequences (Assemblage A), overlain by shelf-margin and basin deposits that lack volcanic rocks (Assemblage B). This model provides more refined sedimentary criteria for examination of other volcanic rift margins.

Localized granulite and eclogite facies metamorphism at Flatraket and Kråkeneset, Western Gneiss Region: U–Pb data and tectonic implications

Abstract The Flatraket Complex, in the ultra-high-pressure (UHP) domains of the Western Gneiss Region (WGR) of Norway, preserves granulite facies assemblages, which were locally overprinted by eclogite and amphibolite facies metamorphism. Zircon and monazite indicate magmatic crystallization of the rocks at 1680–1640 Ma and constrain the timing of the granulite facies overprint at 1100 Ma. This age is older than previously reported ages of 1000–950 Ma for regional metamorphism reaching anatexis and locally granulite facies in the WGR. The granulites at Flatraket may have developed as a consequence of local metasomatism, perhaps linked to metasomatism occurring at the same time in the nearby Sandvik peridotite. Granitic rocks from neighbouring Kråkeneset indicate magmatic emplacement at ≥1650 Ma, during the event that formed the Flatraket Complex and the bulk of the WGR. A gabbro body at Kråkeneset is dated at 1255±8 Ma by baddeleyite, which was not affected by the granulite event, implying that the rock remained impermeable to fluids, reacting instead to some degree during the Caledonian UHP event.

Depositional environments and sequence stratigraphy of the Late Carboniferous−Early Permian coal-bearing successions (Shandong Province, China): Sequence development in an epicontinental basin

This study focuses on the Late Carboniferous−Early Permian coal-bearing successions in Shandong Province, North China in order to understand the depositional processes and sequence-stratigraphic framework in an epicontinental basin. Based on detailed analysis of eleven facies, five facies assemblages (FAs) were recognized in the studied succession. FA1–3 are present mainly in the Benxi and Taiyuan formations, and consist of mixed siliciclastic and carbonate lithofacies, representing eluvial-lagoon, barrier-lagoon, and tidal-flat environments. FA4 occurs in the Shanxi formation and consists mainly of interbedded medium to fine sandstone, siltstone, mudstone, and coal lithofacies, representing river-dominated deltaic environments. FA5 is characterized by interbeds of trough cross-stratified coarse sandstone, and silty mudstone, mainly in the Lower Shihezi Formation, which was deposited in meandering river channel and floodplain. Three third-order sequences were established based on the vertical arrangement of facies assemblages and identification of physical surfaces (i.e., subaerial unconformity, transgressive surface, and regressive surface). Each sequence comprises a transgressive systems tract (TST) and a highstand systems tract (HST). TST of sequence 1 is composed of eluvial lagoonal deposits (FA1), whereas HST formed in lagoon-barrier and tidal-flat settings (FA2 and FA3). TST of sequence 2 formed in a barrier-lagoon system (FA2), whereas HST is characterized by repetitive accumulation of interbedded limestone, sandstone, mudstone, and coal, deposited under lagoonal and tidal-flat settings (FA2 and FA3). TST of sequence 3 comprises FA2, and HST mainly FA4, deposited in a river-dominated shallow-water delta system. Sequence 3 is overlain by a fluvial sequence (FA5). The three third-order sequences in the Shandong region are generally correlated with those in the Taebaeksan Basin (South Korea), the eastern part of the North China Block. The relative sea-level curves established in the two regions show a generally similar long-term rising trend.

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.

Provenance of eclogitic metasediments in the north Qilian HP/LT metamorphic terrane, western China: Geodynamic implications for early Paleozoic subduction-erosion

In this contribution, we present new petrological, geochemical, zircon U–Pb and Hf isotopic data for eclogitic metasediments from the north Qilian orogenic belt, in which early Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasediments contain eclogite facies assemblages reflecting P–T conditions of 450–560°C and 1.9–2.4GPa, consistent with those of adjacent eclogites. REE compositions of the eclogitic metasediments overlap those of average upper continental crust. The SHRIMP and LA-MC-ICPMS U–Pb data of zircons from metasediments demonstrate input from sources with major age components about 500Ma and 1600–1900Ma (peak at ~1800Ma), with a smaller population at 660–800Ma (peak at 750Ma), and minor >1.9Ga zircon ages. The youngest detrital zircons suggest a maximum depositional age of ca. 475Ma, and combined with P–T conditions and previously determined metamorphic age suggest a burial rate of >0.6–0.7cm/yr. Zircon Hf isotopic analyses show that 500Ma zircons have positive εHf(t) (mainly between +8 and +16). By contrast, Meso- to Paleoproterozoic detrital zircons show a broad spectrum of εHf(t) (mainly between −5 and +10) with TDMHf of 1800–2500Ma. These data suggest that eclogitic metasediments are derived from a mixture of Proterozoic continent crust and juvenile early Paleozoic oceanic and/or island arc crust, and their protoliths likely were deposited in a terrigenous-dominated continental margin basin rather than a pelagic oceanic crust environment. The new results are consistent with subduction erosion of the active continental margin during south-dipping subduction, as these sediments, formed in a fore-arc environment close to the Qilian block to the south, were transported in the subduction zone to 60–70km depth prior to their exhumation.

Underplated seamount in the Narooma accretionary complex, NSW, Australia

The Wagonga Group at Batemans Bay and Narooma contains basalt clasts in mélange and basaltic breccia units. At Batemans Bay the basalt breccia unit is associated with Middle to Late Cambrian parautochthonous limestone beds and clasts. They have been altered by hydrothermal fluids and during tectonic burial producing prehnite pumpellyite facies assemblages. Subsequently, they were taken to greater depths, deformed and partially recrystallised resulting in the development of lower greenschist facies assemblages. Despite alteration, immobile elements reveal distinctive ocean island basalt (OIB), seamount‐like signatures in most basalt samples. This is indicated by their alkaline basaltic affinity, LREE-enriched, chondrite and LILE-enriched, primitive mantle normalised patterns, and immobile element ratios. The OIB‐like basalts represent plume-derived melts from a deep, HIMU-type source that has undergone low to moderate degrees of melting. The studies confirm that the basalts in the Wagonga Group have been derived from a seamount and suggest that they were subducted and sequentially altered under changing P–T conditions.

Geochemistry of komatiites in the Southern Cross Belt, Youamni Terrane, Western Australia

The Southern Cross belt is located in the Youanmi Terrane of the Yilgarn Craton, directly north of the Forrestania belt, which contains economic komatiite-hosted Ni–Cu mineralisation. The mafic and ultramafic greenstone assemblage in the Southern Cross belt, encompasses a continuum from olivine-rich cumulates through to evolved high MgO komatiitic basalts and gabbros. Inspection of geochemical data shows that komatiites can be divided into a high-Al group with Al2O3/TiO2 17–19, slightly lower than typical Munro-type komatiites, and a low-Al group with Al2O3/TiO2 17–19, intermediate between typical Munro-type and Barberton-type komatiites. Parent magmas for the high-Al and low-Al groups had MgO contents ranging between 10 and 15 wt% and between 15 and 25wt%, respectively. Volcanic facies show a transition from distal thin flow facies in the north, changing to lava lakes and sills further south. Olivine-rich channel or conduit facies assemblages are absent from both suites, suggesting low prospectivity for nickel sulfide mineralisation. The high-Al group displays flat to very weakly LREE enriched patterns with flat HREE that are typical of Munro-type komatiites. The low-Al group is weakly LREE enriched, has distinctly negative sloping REE patterns from La to Lu, and minor HREE depletion relative to MREE. Direct comparison with the Forrestania belt shows that the Southern Cross komatiites overlap with the transitional Munro-type komatiites of upper Central and Takashi belts that represent the unmineralised portion of the Forrestania belt. The predominance of Munro-type komatiites in the Southern Cross belt implies that the Southern Cross komatiites were derived from shallower melting than the Barberton-type komatiites that host mineralisation in Forrestania greenstone belt. Furthermore, the variation between Al/Ti and V/Ti implies that the Southern Cross ultramafic suite may represent the transition from Barberton-type to Munro-type komatiites, and formed close to thelower pressure limit of stability of majorite garnet in the mantle residue. It is suggested that the transitional character of the Southern Cross komatiites may be associated with a fundamental difference in the crustal architecture and craton keel thickness underlying the southeastern Youanmi Craton.

High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at >10–11kbar and 450–650°C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900°C and at 14 to 30kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22kbar and 1060–1040°C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a >90km×40km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.

Mechanisms of volcaniclastic aggradation in fluvial systems influenced by explosive volcanism: An example from Neoproterozoic Hammamat Group, Wadi Queih area, Central Eastern Desert, Egypt

Wadi Queih basin records different continental settings that interacted with explosive volcanism. This paper discusses the contrasting aggradational mechanisms in fluvial systems strongly influenced by explosive volcanism which took place during sedimentation of the Queih basin. Six main facies associations composed of 12 lithofacies have been recognized in the Neoproterozoic succession filling the Queih basin: (1) lava flows and pedogenically modified pyroclastics facies association, (2) debris-flow-dominated alluvial fan facies association, (3) sheetflood-dominated alluvial fan facies association, (4) crevasse-splay facies association, (5) lacustrine facies association, and (6) loess facies association. These facies assemblages are typical of low-sinuosity rivers flowing through unsymmetrical half graben basin. Petrographical and geochemical data of the Queih sediments indicate a predominantly continental volcanic block provenance and stable craton to fault-bounded basement uplift. Low values of Chemical and Plagioclase Index of Alteration (CIA and PIA) are consistent with low intensity source rock weatherning under prevailing semi-arid to arid climate.During deposition of the lower member of the Queih basin, common fall and flow tuff events occurred, indicating syn-eruptive conditions. In contrast, deposition of the upper member occurred in a fluvial–aeolian setting without input of primary volcaniclastic detritus, indicating inter-eruptive conditions. The change in depositional mode from the lower to upper is considered to be due to a change in the balance between the sedimentation rate and the rate of lake-level rise. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected the Queih basin by volcanic input. Thus, the Queih basin records the response of fluvial system to large, volcanism-induced sediment loads.

Ichnology, sedimentology, and sequence stratigraphy of outer-estuarine and coastal-plain deposits: Implications for the distinction between allogenic and autogenic expressions of the Glossifungites Ichnofacies

Integration of sedimentologic, ichnologic and sequence-stratigraphic data supports that the upper member of the Río Turbio Formation records sediment accumulation in tide-dominated, outer-estuarine and coastal-plain environments. Outer-estuarine assemblages comprise facies that were deposited in channel-thalweg, subtidal-sandbar complex, oyster-reef and reef-margin environments. Coastal-plain facies assemblages include distributary-channel, floodplain, salt-marsh, swamp, and swamp margin deposits.Twelve ichnogenera assigned to soft-, stiff- and firmground suites were identified. Surfaces demarcated by autogenic and allogenic expressions of the Glossifungites Ichnofacies were recognized. The trace fossils Thalassinoides, Psilonichnus, and Bergaueria were documented within the autogenic stiffground suites, whereas Gastrochaenolites was found in both autogenic and allogenic firmground suites. Allogenically produced surfaces occur on a regional scale, creating major discontinuities, while minor autogenically produced surfaces extended laterally only over a few meters. Integrating the data derived from the substrate-controlled ichnofacies with the vertical and lateral distribution of the paleoecological data provided a more thorough understanding of the importance and scale of the erosional discontinuities.Estuarine and coastal-plain deposits contain low-diversity trace fossil assemblages inferred to have been produced by opportunistic, impoverished marine infauna, indicative of brackish-water settings. The overall distribution of ichnocoenoses within the lower portion of the upper member of the Río Turbio Formation appears to have been controlled largely by the salinity gradient, with other factors such as energy and sedimentation rate playing a more limited role.

Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation

AbstractThe Archean (2.8 Ga) Banded Iron Formation (BIF) of the Bell Lake region of Yellowknife greenstone belt, Canada is recrystallized to metamorphic assemblages of the amphibolite facies. This BIF is characterized by centimetre‐scale Fe‐rich and Si‐rich mesobands. In the Si‐rich mesobands, thin layers of magnetite microbands are developed in a quartz matrix. The Fe‐rich mesobands are composed mainly of Ca‐amphibole (hornblende), Fe–Mg amphibole (grunerite), and magnetite. The metamorphic foliation locally cuts across the mesoband boundaries, indicating the mesobanding was formed prior to peak metamorphism. Variations in mineral modal proportions between Fe‐rich mesobands and microbands are diagnostic of depositional compositional differences between beds. Micro‐X‐ray fluorescence imaging reveals metamorphic differentiation within Fe‐rich mesobands, with segregation of Fe–Mg amphibole, and the incompatible element Mn is concentrated at the margins of the Fe‐rich mesobands during the amphibole‐forming reactions. Ti was relatively immobile during metamorphic segregation and its distribution provides a record of the original structures in the Fe‐rich mesobands.

Evidence for a Caledonian amphibolite to eclogite facies pressure gradient in the Middle Allochthon Lindås Nappe, SW-Norway

The Proterozoic anorthosite–mangerite–charnockite complex dominating the Lindås Nappe in the Scandinavian Caledonides was locally eclogitized in the southwestern part of the nappe during the Caledonian orogeny, whereas only amphibolite facies assemblages are recorded in the rest of the nappe. Sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite-free exposures of the nappe exhibit large garnet phenoblasts (ca. 900°C) that are fractured and partly replaced by a Caledonian symplectitic amphibolite facies assemblage (ca. 515°C). Metamorphic zircon attributed to this garnet breakdown is dated by ID-TIMS U–Pb at 430 ± 3 Ma, suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south, probably implying that the section across the nappe represents a Caledonian pressure gradient. The rocks also preserve a complex Sveconorwegian history including an age of 969 ± 6 Ma, which we interpret as dating magmatic emplacement of jotunitic–anorthositic portions of the complex, 936 ± 12 Ma reflecting the granulite facies metamorphism, and 908 ± 16 Ma, representing a late generation of zircon best explained as having formed by metasomatic processes. Caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone, resetting their U–Pb systems, and forming new ones, hereby also demonstrating a case of resetting and recrystallization of low-U zircon. Our data, gained from diverse lithologies, illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the Lindås Nappe and the Jotun Nappe Complex.

Cambrian sponge spicules and Chancelloriid sclerites from the Argentine Precordillera: A Review

Spicules of sponges and sclerites of chancelloriids have been collected from in situ Cambrian sections of the La Laja Formation (San Juan Precordillera). They have also been collected from carbonate olistoliths, which were emplaced from the Cambrian units in the Early Ordovician Los Sombreros Formation (San Juan Precordillera) and Empozada Formation (Mendoza Precordillera). The Cambrian La Laja Formation sequences were deposited in a shallow carbonate platform in the eastern part of the Precordillera. The Cambrian carbonate rocks, which make up the olistoliths in the Los Sombreros and Empozada Formations, record deposition in the slope environments which developed in the western Precordillera. Cambrian Porifera and sponge-like chancelloriid assemblages are known mainly from fragments of skeletal nets, dissociated spicules and sclerites, which enable us to define two assemblages. The first assemblage of spicules was defined from the upper Lower to Middle Cambrian sequences of the La Laja Formation. This assemblage consists of a variety of stauractines and sclerites of Chancelloria WALCOTT. The family Protospongiidae is represented by triradiate prodianes, pentactines and hexactines, all belonging to Kiwetinokia WALCOTT. The second assemblage, which consists of isolated hexactines, pentactines, monaxons and skeletal nets, was defined in the Cambrian derived olistholiths of the Los Sombreros and Empozada Formations. Early hexactinellid Protospongiidae with body preservation (Diagoniella ? and Kiwetinokia) also occur. Demosponges have a very limited record in the Cambrian of the Precordillera. Anthaspidellid sponges had been reported from the La Laja Formation and now from the San Martin olistolith (Empozada Formation). The occurrences of these two assemblages of spicules and sclerites in both the Cambrian platform and slope facies of the Precordillera contribute additional data for a better understanding of the relationships between the eastern and the western facies assemblages, which developed in the Precordillera during the Cambrian. Therefore, they are useful for paleoenvironmental and paleogeographical interpretations of the Cambrian depositional framework.

High frequency subtidal-peritidal cycles of the Callovian Calabozo Formation (Neuquén Basin, Western Argentina): Preliminary approach

The Calabozo Formation (Cuyo Mesosequence, Neuquen Basin) is made up by a thin, up to 30-m thick Middle Jurassic carbonate-dominated sequence. The minor, thin siliciclastic facies in this formation (i.e. lowermost granule-pebble shoreface conglomerate and fine-grained sandstones interbedded among the lower carbonates) record early transgressive shoreline and shallow lagoonal-subtidal sedimentation, respectively. The major carbonate deposits have been split into six facies and seven microfacies on the base of macroscopic field observation and petrological analysis. The carbonate facies have been grouped into three major assemblages: 1) Tidal flat facies assemblage (including the planar laminated algal boundstone facies; 2) Inner platform facies assemblage (including peloidal and bioclastic wackestone, coral floatstone and bioclastic and intraclastic packstone facies; and 3) Platform margin assemblage (including oolitic-peloidal packstone-grainstone facies). Afossil algal record made up by dasycladacean species (Salpingoporella annulata, Cylindroporella sp. and Heteroporella sp.) and blue-green algae (Cayeuxia (Rivularia) piae, and Cayeuxia (Rivularia) kurdistanensis) has been reported for the first time in this unit. The analysis of the intracycle facies and microfacies arrangements, as well as the nature of the cycle bounding surfaces, enabled one to identify two kinds of cycles. 1) Widespread subtidal cycles capped by shallow to intermediate subtidal facies, with no evidence of subaerial exposure at cycle tops; and 2) a minor subtidal cycle with similar shallowing-upward trend but capped by peritidal facies. The stacking pattern and thickness trend of these cycles was studied to determine possible allocyclic and autocyclic controls on their evolution. The Fischer plots reveal that the deposition of the Calabozo Formation subtidal-peritidal cycles took place under upward decreasing accommodation space conditions which may be related to eustatic lowering.

Calcareous nannofossils and sedimentary facies in the Upper Cretaceous Bozeş Formation (Southern Apuseni Mountains, Romania)

The lithology, sedimentology and biostratigraphy of the Bozes Formation, which crop out in the SE Metaliferi Mountains (Apuseni Mts.) have been investigated in order to establish the age of the deposits and the depositional environment. The sedimentary structures and facies are interpreted as indicating a deep-water depositional environment, representing part of a submarine fan lobe. Three facies assemblages have been identified and described. Calcareous nannofossils were used to determine the age of the investigated deposits. The presence of Lucianorhabdus cayeuxii and Calculites obscurus indicates the CC17 biozone, while UC13 Zone is pointed out by the continuous occurrence of Arkhangelskiella cymbiformis and the absence of Broinsonia parca parca. Thus, the age of the studied deposits is Late Santonian -?Early Campanian.