scholarly journals [MnO|SiO2,Al2O3,FeO,MgO] balanced log-ratio in chlorites: a tool for chemo-stratigraphic mapping and proxy for the depositional environment

Clay Minerals ◽  
2018 ◽  
Vol 53 (3) ◽  
pp. 351-375
Author(s):  
Annette Süssenberger ◽  
Solveig Pospiech ◽  
Susanne Theodora Schmidt

ABSTRACTThe [MnO|SiO2,Al2O3,FeO,MgO] balanced ratio (i.e. the isometric log-ratio of the MnO concentration relative to the concentration of SiO2, Al2O3, FeO and MgO) of chlorite and of whole-rock composition is an effective discriminant between Mesozoic stratigraphic formations in the Magallanes Basin (Chile). The MnO content in chlorite is only controlled by the host rock chemistry and is dependent on the geological environment. The MnO content in chlorite remains unchanged at low-grade metamorphic conditions. Single-grain chlorite analysis (n = 1042, electron microprobe) and whole-rock analysis (n = 40, X-ray fluorescence) were used to discriminate stratigraphic formations and to decipher differences in the depositional environment in the Magallanes Basin. The samples are from one Upper Jurassic and three Cretaceous sedimentary units that were affected either by low-grade regional metamorphism or by Miocene contact metamorphism. The highest [MnO|SiO2,Al2O3,FeO,MgO] values are recorded in the upper Zapata Formation. The Punta Barrosa, Cerro Toro and Tobífera Formations show slightly lower [MnO|SiO2,Al2O3,FeO,MgO] values. Elevated [MnO|SiO2,Al2O3,FeO,MgO] values at the transition between Zapata and Punta Barrosa Formations record an oxygenated shallow marine environment that can be linked to the closure of the Rocas Verdes Basin and the onset of fold-and-thrust belt formation. Decreasing [MnO|SiO2,Al2O3,FeO,MgO] values from the Punta Barrosa towards the Cerro Toro Formation indicate gradually increasing water depths during the Upper Cretaceous that correlate well with the global sea level.

1979 ◽  
Vol 43 (326) ◽  
pp. 201-209 ◽  
Author(s):  
P. K. Verma

SummaryThe Lower Cambrian Weymouth Formation at Nahant, Massachusetts, consisting of interbedded pelitic and calcareous rocks, was intruded by the Nahant Gabbro. The predominant metapelitic mineral assemblage of the contact aureole is quartz-muscovite-chlorite-magnetite-ilmenite. The calcareous hornfelses exhibit a varied mineral assemblage, and in some cases the variation can be spatially related to the intrusive. A number of cross-cutting thin mineral veins, many containing prehnite, are characteristically associated with these calcsilicate rocks.The minerals have been analysed by electron microprobe and this work indicates the presence of a possible solvus in the Fe3+-Al epidote solid solution series. At the physicochemical conditions of the formation of the Nahant hornfelses, the ferric mole fractions of coexisting epidotes are 0.49 and 0.98.Comparison with experimental work shows that the conditions of the contact metamorphism were T ≃ 500°C, Ptotal ≃ 2 kb, and XCO2 ≃ 0.2. However, the present assemblages are the result of a later low-grade regional metamorphism, the ultimate product of which was prehnite.


Clay Minerals ◽  
2004 ◽  
Vol 39 (2) ◽  
pp. 151-162 ◽  
Author(s):  
M. Belmar ◽  
D. Morata ◽  
F. Munizaga ◽  
C. Pérez de Arce ◽  
S. Morales ◽  
...  

AbstractK-Ar isotopic dating of very low-grade metamorphism affecting Triassic-Jurassic rocks in the Coastal Range of central Chile was carried out on whole rocks and their <2 mm size fractions. In the study area, a regional-burial low-grade metamorphism at anchizone conditions (T≤190°C) and low-pressure conditions (P≥1.3 kbar) has been described. The highest observed temperatures are related to a contact metamorphism produced by nearby Jurassic intrusions, with aP-Testimate at the immediate contact zone of ~650–690°C and 4 kbar.The whole-rock K-Ar age of 174±5 Ma is interpreted as belonging to the contact metamorphism due to the intrusion of Jurassic plutons (165±5 Ma to 175±5 Ma). A time-interval of ~20 Ma between the diagenesis (206 Ma) and the anchizonal very low-grade metamorphism (181–184 Ma) is obtained, and a rate of subsidence of ~120 m/Ma is proposed for these Triassic-Jurassic basins. A thermal influence on the burial, very low-grade, regional metamorphism is invoked.


2017 ◽  
Vol 54 (11) ◽  
pp. 1165-1178 ◽  
Author(s):  
Nabil A. Shawwa ◽  
Robert P. Raeside ◽  
David W.A. McMullin ◽  
Christopher R.M. McFarlane

At Kellys Mountain, Cape Breton Island, Nova Scotia, the late Neoproterozoic Glen Tosh formation (a low-grade metapsammite–metapelite unit of the George River Metamorphic Suite) has been intruded by diorite, granodiorite, and granite plutons, and the diorite hosts a narrow contact metamorphic aureole. New mapping and sampling in the contact aureole reveals that the metasedimentary rocks have reached amphibolite-facies metamorphism resulting in the development of neoformed biotite, muscovite, cordierite, ilmenite, garnet, andalusite, sillimanite, monazite, and spinel within the meta-pelite, a mineral assemblage also found in the Kellys Mountain Gneiss as a result of low-pressure regional metamorphism. Neoformed minerals and the disappearance of foliation defines a contact metamorphic aureole within 300 m of the pluton contacts. Petrographic and microprobe analyses of equilibrium assemblages in metapelitic units of the contact aureole yielded metamorphic pressures of 250 MPa, implying an intrusion depth of ∼9 km, with temperatures ranging from 365 to 590 °C. The presence of earlier-formed andalusite and garnet indicates the rocks may have initially undergone a low-pressure regional metamorphic event prior to contact metamorphism. Monazite in the contact aureole was dated using in-situ U–Pb methods and yielded an age of 480.9 ± 3.7 Ma, interpreted as the time of formation of the contact metamorphic aureole.


1972 ◽  
Vol 9 (2) ◽  
pp. 158-171 ◽  
Author(s):  
B. W. Robinson ◽  
R. D. Morton

In the Echo Bay region of Great Bear Lake the Echo Bay Group has been intruded by Precambrian granites, granodiorites, aplites, and diabases. This group, which has undergone zeolite-facies regional metamorphism, has a lower dominantly sedimentary-pyroclastic sequence (about 1320 m thick) and an upper volcanic sequence (> 1500 m thick). The Lower Echo Bay sub-Group consists of tuffs and cherts with a 190 m thick intermediate lava flow and occasional thin limestones. A sequence of andesites and pyroclastics comprises the Upper Echo Bay sub-Group. Contact metamorphism of the hornblende hornfels and albite–epidote hornfels facies has been induced by the granitic intrusions. A Rb–Sr isochron for the Upper Echo Bay sub-group volcanics gives an age of 1770 ± 30 m.y. K–Ar apparent ages for the granite, granodiorite, and volcanics average around 1650 m.y., possibly reflecting local argon loss up to this date. Mineralized veins with a pitchblende-Co,Ni arsenide-silver-Fe, Cu, Ag, Bi, Zn. Pb sulfide para genesis are mainly confined to the sedimentary-pyroclastic horizons of the Lower Echo Bay sub-Group. It is possible that the mineralization was effected by fluids generated during the low-grade regional metamorphism and that some contribution was made to ore genesis by the thermal regime developed during the diabase intrusion.


1938 ◽  
Vol 75 (4) ◽  
pp. 160-174 ◽  
Author(s):  
F. J. Turner

During the past ten years a number of papers dealing with progressive regional metamorphism in the southern portion of New Zealand have been published. In the following pages a brief summary of the assemblages of minerals typical of the various metamorphic zones is given, but the writer's main object is to draw attention to certain mineralogical and structural peculiarities that appear to differ in some degree from what are usually regarded as the normal features of regional metamorphism in such classic areas as the Scottish Highlands and the Caledonian chain of Norway. The possibility that such departures from the normal may in some instances be connected with chemical peculiarities in the parent rock is suggested by such phenomena as the well-known general limitation of chloritoid, staurolite, and low-grade garnets to pelitic rocks of special chemical compositions. Other unusual features, especially when found to recur in widely separated regions, may well be governed by some particular combination of physical rather than chemical conditions.


GeoArabia ◽  
2009 ◽  
Vol 14 (4) ◽  
pp. 91-108 ◽  
Author(s):  
Thamer K. Al-Ameri ◽  
Amer Jassim Al-Khafaji ◽  
John Zumberge

ABSTRACT Five oil samples reservoired in the Cretaceous Mishrif Formation from the Ratawi, Zubair, Rumaila North and Rumaila South fields have been analysed using Gas Chromatography – Mass Spectroscopy (GC-MS). In addition, fifteen core samples from the Mishrif Formation and 81 core samples from the Lower Cretaceous and Upper Jurassic have been subjected to source rock analysis and palynological and petrographic description. These observations have been integrated with electric wireline log response. The reservoirs of the Mishrif Formation show measured porosities up to 28% and the oils are interpreted as being sourced from: (1) Type II carbonate rocks interbedded with shales and deposited in a reducing marine environment with low salinity based on biomarkers and isotopic analysis; (2) Upper Jurassic to Lower Cretaceous age based on sterane ratios, analysis of isoprenoids and isotopes, and biomarkers, and (3) Thermally mature source rocks, based on the biomarker analysis. The geochemical analysis suggests that the Mishrif oils may have been sourced from the Upper Jurassic Najma or Sargelu formations or the Lower Cretaceous Sulaiy Formation. Visual kerogen assessment and source rock analysis show the Sulaiy Formation to be a good quality source rock with high total organic carbon (up to 8 wt% TOC) and rich in amorphogen. The Lower Cretaceous source rocks were deposited in a suboxic-anoxic basin and show good hydrogen indices. They are buried at depths in excess of 5,000 m and are likely to have charged Mishrif reservoirs during the Miocene. The migration from the source rock is likely to be largely vertical and possibly along faults before reaching the vuggy, highly permeable reservoirs of the Mishrif Formation. Structural traps in the Mishrif Formation reservoir are likely to have formed in the Late Cretaceous.


Fossil Record ◽  
1999 ◽  
Vol 2 (1) ◽  
pp. 25-61 ◽  
Author(s):  
W.-D. Heinrich

Tendaguru is one of the most important dinosaur localities in Africa. The Tendaguru Beds have produced a diverse Late Jurassic (Kimmeridgian to Tithonian) dinosaur assemblage, including sauropods (<i>Brachiosaurus, Barosaurus, Dicraeosaurus, Janenschia</i>), theropods (e.g., <i>Elaphrosaurus, Ceratosaurus, Allosaurus</i>), and ornithischians (<i>Kentrosaurus, Dryosaurus</i>). Contrary to the well studied skeletal anatomy of the Tendaguru dinosaurs, the available taphonomic information is rather limited, and a generally accepted taphonomic model has not yet been established. Assessment of unpublished excavation sketches by the German Tendaguru expedition (1909–1913) document bone assemblages of sauropod and ornithischian dinosaurs from the Middle Saurian Bed, Upper Saurian Bed, and the Transitional Sands above the <i>Trigonia smeei</i> Bed, and shed some light on the taphonomy of the Tendaguru dinosaurs. Stages of disarticulation range from incomplete skeletons to solitary bones, and strongly argue for carcass decay and post-mortem transport prior to burial. The sauropod bone accumulations are dominated by adult individuals, and juveniles are rare or missing. The occurrence of bones in different superimposed dinosaur-bearing horizons indicates that skeletal remains were accumulated over a long time span during the Late Jurassic, and the majority of the bone accumulations are probably attritional. These accumulations are likely to have resulted from long-term bone imput due to normal mortality events caused by starvation, seasonal drought, disease, old age and weakness. The depositional environment of the Middle and Upper Saurian Bed was mainly limnic to brackish in origin, while the palaeoenvironment of the Transitional Sands was marginal marine. <br><br> Tendaguru zählt zu den bedeutendsten Dinosaurier-Lagerstätten Afrikas. Aus den Tendaguru-Schichten sind zahlreiche Skelettreste von Sauropoden (<i>Brachiosaurus, Barosaurus, Dicraeosaurus, Janenschia</i>), Theropoden (z.B. <i>Elaphrosaurus, Ceratosaurus, Allosaurus</i>) und Ornithischiern (<i>Kentrosaurus, Dryosaurus</i>) geborgen worden. Sie stammen aus der späten Jura-Zeit (Kimmeridge — Tithon). Während der Skelettbau der Tendagurusaurier gut untersucht ist, wirft die Taphonomie des Sauriervorkommens von Tendaguru noch immer Fragen auf. Unklar ist bislang, wie die enormen Anreicherungen von Dinosaurierknochen in den Tendaguru-Schichten zustandekamen. Unveröffentlichte Grabungsskizzen der Deutschen Tendaguru Expedition (1909–1913) erweitern unsere Kenntnisse über die Taphonomie der Tendagurusaurier. In den ausgewerteten Grabungsskizzen sind Knochenansammlungen von Sauropoden und Ornithischiern aus dem Mittleren und Oberen Sauriermergel sowie aus den Übergangsschichten über der <i>Trigonia smeei</i>-Schicht dokumentiert. Die Lage und der Erhaltungszustand der Funde lassen auf erheblichen Zerfall der Kadaver und post-mortalen Transport von Skelettelementen vor der Einbettung schließen. Das Vorkommen von Saurierknochen in mehreren übereinanderliegenden Profilabschnitten der Tendaguru-Schichten zeigt, daß Skelettreste während der späten Jura-Zeit über einen längeren Zeitraum hinweg akkumuliert wurden. Die Ansammlungen von Skelettresten gehen wahrscheinlich auf „normale” Sterbe-Ereignisse zurück, wie z. B. Verhungern, Verdursten, Kankheit, Altersschwäche und jahreszeitliche Dürre. Als Ablagerungsraum der Mittleren und Oberen Saurierschicht kommt ein küstennaher limnischer, zeitweise wohl auch brackischer Küstenstreifen in Betracht. Die knochenführenden Übergangsschichten unter- und oberhalb der Saurierschichten sind randlich marine Ablagerungen. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.1999.4860020102" target="_blank">10.1002/mmng.1999.4860020102</a>


1976 ◽  
Vol 13 (3) ◽  
pp. 405-421 ◽  
Author(s):  
Lee C. Pigage

Pelitic metasediments immediately southwest of Yale, British Columbia contain mineral assemblages characteristic of staurolite through sillimanite zones of the Barrovian facies series. Microprobe analyses of major constituent phases in the pelites are combined with linear regression techniques to formulate probable kyanite- and sillimanite-forming reactions.A zone some 3 km wide contains the assemblage staurolite–kyanite–garnet–biotite–muscovite–quartz–plagioclase-ilmenite-rutile, which is univariant in AFM projection. Within precision limits of microprobe analysis, this assemblage is also univariant using linear regression techniques. Univariant relations are possible if [Formula: see text] with the composition of the fluid phase being buffered by the progressing reaction. This assemblage emphasizes the need for precise analyses when using the regression method, since minor components are often within permissible error limits rather than being balanced.Pelitic and calc-silicate assemblages from the metasediments restrict estimates of pressure–temperature conditions during regional metamorphism to 6–8 kbar and 550–770 °C. Pseudomorphs after andalusite indicate that contact metamorphism preceded regional upgrading of the pelites.


2020 ◽  
Vol 57 (4) ◽  
pp. 542-552 ◽  
Author(s):  
Ramon S. Nagesan ◽  
James A. Campbell ◽  
Jason D. Pardo ◽  
Kendra I. Lennie ◽  
Matthew J. Vavrek ◽  
...  

Western North America preserves iconic dinosaur faunas from the Upper Jurassic and Upper Cretaceous, but this record is interrupted by an approximately 20 Myr gap with essentially no terrestrial vertebrate fossil localities. This poorly sampled interval is nonetheless important because it is thought to include a possible mass extinction, the origin of orogenic controls on dinosaur spatial distribution, and the origin of important Upper Cretaceous dinosaur taxa. Therefore, dinosaur-bearing rocks from this interval are of particular interest to vertebrate palaeontologists. In this study, we report on one such locality from Highwood Pass, Alberta. This locality has yielded a multitaxic assemblage, with the most diagnostic material identified so far including ankylosaurian osteoderms and a turtle plastron element. The fossil horizon lies within the upper part of the Pocaterra Creek Member of the Cadomin Formation (Blairmore Group). The fossils are assigned as Berriasian (earliest Cretaceous) in age, based on previous palynomorph analyses of the Pocaterra Creek Member and underlying and overlying strata. The fossils lie within numerous cross-bedded sandstone beds separated by pebble lenses. These sediments are indicative of a relatively high-energy depositional environment, and the distribution of these fossils over multiple beds indicates that they accumulated over multiple events, possibly flash floods. The fossils exhibit a range of surface weathering, having intact to heavily weathered cortices. The presence of definitive dinosaur material from near the Jurassic–Cretaceous boundary of Alberta establishes the oldest record of dinosaur body fossils in western Canada and provides a unique opportunity to study the Early Cretaceous dinosaur faunas of western North America.


1982 ◽  
Vol 19 (1) ◽  
pp. 129-143 ◽  
Author(s):  
Peter H. Thompson ◽  
Jean-Pierre Bard

Detailed petrography across a metapelitic sequence in the eastern axial zone of the Montagne Noire, France, is the basis for a sequence of isograds marking the first appearance of biotite–cordierite, staurolite, andalusite, and sillimanite. The juxtaposition of low-grade biotite-free rocks against medium-grade rocks at the gently dipping biotite–cordierite isograd is attributed to tectonic telescoping of the metamorphic sequence. Study of mineral assemblages with respect to an AFM reaction sequence indicates the staurolite isograd is related to changes in rock composition, and complex assemblages in the sillimanite zone may be the result of unstable persistence of minerals formed when metamorphic grade was lower. These assemblages are interpreted to contain a record of part of the P–T history during which pressure decreased as temperature increased. P–T profiles show that temperature gradients of 200–300 °C/km suggested by previous workers are not required to explain the isograd pattern; gradients of 37 °C/km or less are sufficient.


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