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1.
V N Sergeev A H Knoll S P Kolosova P N Kolosov 《Stratigraphy and Geological Correlation》1994,2(1):19-33
Silicified shallow-water marine carbonate deposits of the Proterozoic Debengda Formation (the Olenek Uplift, northeastern Siberia) contain well preserved microfossils. One or two distinct assemblages consists only of filamentous Siphonophycus microfossils, which are presumably the extracellular sheaths of hormogonium cyanobacteria. The other is dominated by coccoidal microfossils, first by the entophysalidacean cyanobacterium Eoentophysalis. The coccoidal assemblage was recognized in the layered carbonate precipitate structures of a superficially stromatolite appearance. Despite its simple composition, the microfossil assemblage supports the generally accepted Mesoproterozoic (middle Riphean) age of the Debengda Formation. This conclusion corresponds to the available data on isotopic geochronology, and to the composition of columnar stromatolites from the Dehengda Formation. Both the structural features and carbon isotopic composition of its rocks are comparable to those of rocks of known Mesoproterozoic age, but differ from the characteristics of definitely Neoproterozoic deposits. 相似文献
2.
The oldest filament- and colonial coccoid-containing microbial fossil assemblage now known is described here from drill core samples of stromatolitic cherty limestones of the Neoarchean, approximately 2600-Ma-old Campbell Group (Ghaap Plateau Dolomite, Lime Acres Member) obtained at Lime Acres, northern Cape Province, South Africa. The assemblage is biologically diverse, including entophysalidacean (Eoentophysalis sp.), probable chroococcacean (unnamed colonial coccoids), and oscillatoriacean cyanobacteria (Eomycetopsis cf. filiformis, and Siphonophycus transvaalensis), as well as filamentous fossil bacteria (Archaeotrichion sp.); filamentous possible microfossils (unnamed hematitic filaments) also occur. The Campbell Group microorganisms contributed to the formation of stratiform and domical to columnar stromatolitic reefs in shallow subtidal to intertidal environments of the Transvaal intracratonic sea. Although only moderately to poorly preserved, they provide new evidence regarding the paleoenvironmental setting of the Campbell Group sediments, extend the known time-range of entophysalidacean cyanobacteria by more than 400 million years, substantiate the antiquity and role in stromatolite formation of Archean oscillatoriacean cyanobacteria, and document the exceedingly slow (hypobradytelic) evolutionary rate characteristic of this early evolving prokaryotic lineage. 相似文献
3.
V. N. Sergeev M. A. Semikhatov M. A. Fedonkin N. G. Vorob’eva 《Stratigraphy and Geological Correlation》2010,18(6):561-592
A new suggested model outlining the evolution of the organic world from the mid-Early Proterozoic (∼2.0 Ga) to the Early Cambrian
is based on data characterizing the relevant chert-embedded and compression-preserved organic-walled microbiotas, impressions
of soft-bodied multicellular organisms, and biomarkers. Critical analysis of overall paleontological data resulted in the
distinguishing of seven successive assemblages of Proterozoic micro- and macrofossils. Being of global geographic range, the
assemblages correspond to the major stages in evolution of the organic world and typify global units which are termed the
Labradorian (∼2.0–1.65 Ga), Anabarian (1.65–1.2 Ga), Turukhanian (1.2–1.03 Ga), Uchuromayan (1.03–0.85 Ga), Yuzhnouralian
(0.85–0.635 Ga), Amadeusian (0.635–0.56 Ga), and Belomorian (0.56–0.535 Ga). Characteristic of the Labradorian unit are microfossil
assemblages of the Gunflint type including remains of morphologically bizarre prokaryotic microorganisms: star-like Eoastrion, umbrella-shaped Kakabekia, dumbbell-shaped Xenothrix, and some others. Fine-grained siliciclastic deposits of the same age yield the oldest remains of millimeter-sized eukaryotes:
spherical to ribbon-like Chuaria and Tawuia. Microfossils prevailing in shallow-water carbonate facies of the Anabarian unit are akinetes of nostocalean cyanophyceae
Archaeoellipsoides and entophysalidacean cyanobacteria Eoentophysalis, whereas acanthomorphic acritarchs Tappania and Shuiyousphaeridium dominate the assemblages of open-shelf facies, where they are associated with the first-found rare macroscopic multicellular
fossils Horodyskia. The distinguishing feature of the next Turukhanian unit is the first occurrence of filamentous red alga Bangiomorpha and the stalked cyanobacterium Polybessurus. The Uchuromayan unit is characterized by the appearance and worldwide radiation of structurally complicated eukaryotic microorganisms,
primarily of acanthomorphic acritarchs Trachyhystrichosphaera and Prolatoforma, branching thalli of green algae Aimophyton, Palaeosiphonella, Palaeovaucheria and Proterocladus, and of spiral-cylindrical cyanobacteria Obruchevella. In the Yuzhnouralian unit is recorded the first occurrence of vase-shaped testate amoebas Melanocyrillium, Cycliocyrillium, and others, and of scale microfossils Characodictyon, Paleohexadictyon, etc. As distinct from the others, the Amadeusian unit characterizes the global expansion of acanthomorphic acritarchs of
complex structure (the Pertatataka-like assemblage of Tanarium, Cavaspina, Appendisphaera, and others) and associated remains of red algae and cyanobacteria Obruchevella. The terminal Belomorian unit marks the extinction of Pertatataka-type microfossils, the appearance of soft-bodied multicellular
organisms on different continents, and the origin of diverse skeletal fossils in the terminal phase. 相似文献
4.
Jennifer A. Goleski Florian Koch Maria A. Marcoval Charles C. Wall Frank J. Jochem Bradley J. Peterson Christopher J. Gobler 《Estuaries and Coasts》2010,33(5):1202-1215
Florida Bay is Florida’s (USA) largest estuary and has experienced harmful picocyanobacteria blooms for nearly two decades.
While nutrient loading is the most commonly cited cause of algal blooms in Florida Bay, the role of zooplankton grazing pressure
in bloom occurrence has not been considered. For this study, the spatial and temporal dynamics of cyanobacteria blooms, the
microbial food web, microzooplankton and mesozooplankton grazing rates of picoplankton, and the effects of nutrients on plankton
groups in Florida Bay were quantified. During the study, cyanobacteria blooms (>3 × 105 cells mL−1) persisted in the eastern and central regions of Florida Bay for more than a year. Locations with elevated abundance of cyanobacteria
hosted microzooplankton grazing rates on cyanobacteria that were significantly lower (p < 0.001) and less frequently detectable compared to sites without blooms. Consistent with this observation, cyanobacteria
abundances were significantly correlated with ciliates and heterotrophic nanoflagellates at low cyanobacteria densities (p < 0.001) but were not correlated during bloom events. The experimental enrichment of mesozooplankton abundance during blooms
yielded a significant decrease in the net growth rate of picoplankton but had the opposite effect when blooms were absent,
suggesting that the cascading effect of mesozooplankton grazing on the microbial food web was also altered during blooms.
While inorganic nutrient enrichment significantly increased the net growth rates of eukaryotic phytoplankton and heterotrophic
bacteria, such nutrient loading had no effect on the net growth rates of cyanobacteria. Hence, this study demonstrates that
low rates of zooplankton grazing and low rates of inorganic nutrient loading contribute to the persistence of cyanobacteria
blooms in Florida Bay. 相似文献
5.
6.
A new fossil microbiota yielding eukaryotic Tappania is described from the Mesoproterozoic Kamo Group of the Central Angara Basin, Siberian Craton. The assemblage also includes Valeria, Satka, Lophosphaeridium, Pulvinosphaeridium, Miroedichia, Osculosphaera, Tasmanites, Obruchevella, Glomovertella, as well as branching and anastomosing filamentous microfossils of putative fungal affinities and peculiar short trichomes, that together substantially increase the biodiversity of the Tappania Association. Stratigraphic position and geochronological constraints indicate a relatively young Late Mesoproterozoic age of the Tappania-bearing sediments. There appears to be a continuity between the Tappania Association and the Neoproterozoic microbiota, the latter being firmly rooted in the Late Mesoproterozoic. 相似文献
7.
贺兰山地区中元古代微生物席成因构造 总被引：23，自引：2，他引：21
贺兰山中段中元古界黄旗口组石英砂岩中发现丰富的微生物席成因构造(MISS),包括由微生物席生长、破坏和腐烂过程形成的3种类型、9种不同形态的构造;与华北大红峪组发现的同类构造在成因类型与多样性方面具有很强的可对比性.砂岩中发育双向交错层理、冲洗层理、高角度单斜层理系和波痕,泥质粉砂岩夹层中发育波痕与泥裂,表明微生物席主要发育于潮间带上部至潮上带下部环境.MISS构造在华北地台长城系下部砂岩中的广泛存在表明在1.6 Ga前以蓝细菌为主的微生物群在环潮坪碎屑环境也很活跃.可能代表了微生物由海洋向陆地环境发展的过渡阶段.具光合作用功能的制氧蓝细菌的蓬勃发展可能是引发中元古代海洋化学条件发生转变、含氧量增高的重要原因,并为真核生物及宏观藻类的兴起创造了条件.研究表明,黄旗口组与华北大红峪组大致同时,反映了Columbia超大陆裂解期华北地台开始拉伸-张裂、缓慢沉降的构造古地理背景. 相似文献
8.
A set of Palaeozoic diopside–titanite veins are present in Mesoproterozoic metagranites and metasediments that constitute the basement (Mt Painter Inlier) of the Adelaide Fold Belt (South Australia). These massive veins (up to 1 m) of pegmatitic nature contain large crystals of diopside, LREE–Y-enriched titanite (up to 40 cm in length) and minor amounts of quartz. They can be used to trace the system’s development from a high-temperature magmatic stage through to a massive hydrothermal event. The pegmatitic origin of these veins is evident from a complex fluid-melt inclusion assemblage, consisting of a highly saline inhomogeneous fluid and relicts of melt. Immiscibility of melt and heterogeneous highly saline fluids (exceeding 61 eq. mass% NaCl) is preserved in primary inclusions in diopside and secondary inclusions in titanite, indicating relatively shallow conditions of formation (510 ± 20°C and 130 ± 10 MPa). Graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites. The system evolved into hydrothermal conditions, which can be deduced from a later population of only fluid inclusions (homogeneous and less saline, ≈ 40 eq. mass% NaCl), trapped around 350 ± 20°C and 80 ± 10 MPa. During quartz crystallization, the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals, which were trapped at 200 ± 20°C and 50 ± 10 MPa. Brecciation and a palaeo-geothermal system overprinted the pegmatitic veins with an epithermal hematite–quartz assemblage and lesser amounts of bladed calcite and fluorite, in an intermittently boiling hydrothermal system of fairly pure H2O at 100–140°C and 1–5 MPa. Remobilization of LREEs and Y from titanite and/or the granitic host rock is evidenced by precipitation of apatite, allanite and wakefieldite in an intermediate stage. Occasional incorporation of radioactive elements or minerals, presumably U-rich, in the fluorite is responsible for radiolysis of H2O to H2. 相似文献
9.
Extensive high-grade polydeformed metamorphic provinces surroundingArchaean cratonic nuclei in the East Antarctic Shield recordtwo tectono-thermal episodes in late Mesoproterozoic and lateNeoproterozoic–Cambrian times. In Western Dronning MaudLand, the high-grade Mesoproterozoic Maud Belt is juxtaposedagainst the Archaean Grunehogna Province and has traditionallybeen interpreted as a Grenvillian mobile belt that was thermallyoverprinted during the Early Palaeozoic. Integration of newU–Pb sensitive high-resolution ion microprobe and conventionalsingle zircon and monazite age data, and Ar–Ar data onhornblende and biotite, with thermobarometric calculations onrocks from the H.U. Sverdrupfjella, northern Maud Belt, resultedin a more complex P–T–t evolution than previouslyassumed. A c. 540 Ma monazite, hosted by an upper ampibolite-faciesmineral assemblage defining a regionally dominant top-to-NWshear fabric, provides strong evidence for the penetrative deformationin the area being of Pan-African age and not of Grenvillianage as previously reported. Relics of an eclogite-facies garnet–omphaciteassemblage within strain-protected mafic boudins indicate thatthe peak metamorphic conditions recorded by most rocks in thearea (T = 687–758°C, P = 9·4–11·3kbar) were attained subsequent to decompression from P >12·9 kbar. By analogy with limited U–Pb singlezircon age data and on circumstantial textural grounds, thisearlier eclogite-facies metamorphism is ascribed to subductionand accretion around 565 Ma. Post-peak metamorphic K-metasomatismunder amphibolite-facies conditions is ascribed to the intrusionof post-orogenic granite at c. 480 Ma. The recognition of extensivePan-African tectonism in the Maud Belt casts doubts on previousRodinia reconstructions, in which this belt takes a pivotalposition between East Antarctica, the Kalahari Craton and Laurentia.Evidence of late Mesoproterozoic high-grade metamorphism duringthe formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics ofgranulite-facies metamorphism recorded from other parts of theMaud Belt. The polymetamorphic rocks are largely derived froma c. 1140 Ma volcanic arc and 1072 ± 10 Ma granite. KEY WORDS: Maud Belt; Pan-African orogeny; geochronology; P–T–t path, East Antarctica 相似文献
10.
Summary High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism
and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote
± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile
+ quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite
(∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry
and THERMOCALC average-P–T calculations yield peak eclogite-facies P–T conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P–T of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin P–T trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies
recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks,
P–T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism.
Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of
∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes. 相似文献
11.
Kathleen A. McFadden Shuhai Xiao Chuanming Zhou Micha&#x; Kowalewski 《Precambrian Research》2009,173(1-4):170
Doushantuo acanthomorphic acritarchs are large morphologically complex organic-walled microfossils broadly constrained between the 635 Ma Nantuo glaciation and the 551 Ma Miaohe Biota. They are potential biostratigraphic tools for subdivision and correlation of the Ediacaran System in South China. However, major variations in sedimentary facies and stratigraphic thickness present challenges in understanding the spatial and temporal distribution of these acritarchs. Further, the distribution of acritarchs in the Doushantuo Formation is associated with the presence of early diagenetic chert and phosphate, implying a certain degree of preservational bias and/or environmental control. The purpose of this paper is to document the stratigraphic distribution of Doushantuo acritarchs and to quantitatively evaluate their biostratigraphic significance and possible taphonomic–environmental biases, based on high-resolution paleontological data from six sections over 100 km in the Yangtze Gorges area, South China.A total of 1082 acritarch fossils were recorded from 84 chert horizons in the six sections of the study area. These chert horizons are not uniformly distributed throughout the Doushantuo Formation, thus the presence/absence of early diagenetic chert does play a role in controlling the distribution of acritarchs. The sampled chert horizons can be grouped into two stratigraphic intervals in the lower and upper Doushantuo Formation, respectively, based on regional stratigraphic correlation. Quantitative analysis shows that the two intervals are distinct taxonomically and largely independent of taphonomic or facies controls. Thus, the two intervals can be regarded as assemblage biozones. The lower biozone (biozone 1) is numerically dominated by Tianzhushania spinosa (n = 587; 68.3%) and Meghystrichosphaeridium magnificum (n = 74; 8.6%), whereas the upper biozone (biozone 2) is dominated by Ericiasphaera rigida (n = 104; 47.1%) and Tianzhushania spinosa (n = 34; 14.1%). The three most common genera, Meghystrichosphaeridium, Tianzhushania, and Ericiasphaera, have been identified in all sections. Correspondence analysis (CA), detrended correspondence analysis (DCA), discriminant analysis (DA), and pairwise comparisons of samples (Spearman rank coefficient and Jaccard–Chao index), all consistently support biostratigraphic zonations. Thus, the distribution of the Doushantuo acanthomorphs is primarily controlled by biostratigraphic position of samples, with facies and taphonomic differences playing a secondary role. Our case study suggests that acanthomorphic acritarchs can offer a viable tool for regional correlation of the Ediacaran System. 相似文献
12.
Cryptic Blooms: Are Thin Layers the Missing Connection? 总被引：1，自引：0，他引：1
Margaret A. McManus Raphael M. Kudela Mary W. Silver Grieg F. Steward Percy L. Donaghay James M. Sullivan 《Estuaries and Coasts》2008,31(2):396-401
Harmful algal blooms (HABs) are common in Monterey Bay, CA, and have resulted in repeated closures of shellfish fisheries
and the poisoning and death of marine mammals. In the majority of instances, HAB events in this region are first detected
by the presence of sick or dying animals. The phrase “cryptic blooms” was adopted to denote the appearance of poisoning at
higher trophic levels with no prior evidence of a large phytoplankton bloom. We hypothesize that the onset of many HAB events
goes undetected because the bloom is initially concentrated in discrete thin subsurface layers in the water column that are
easily missed by conventional sampling and monitoring methods. In this paper, we report on the detection and monitoring of
a subsurface layer of phytoplankton in northern Monterey Bay, CA, using a high-resolution, autonomous profiler. This ‘thin
layer,’ which measured from 10 cm to 3 m in thickness (85% < 2 m; 54% < 1 m), persisted over a 7-day period near the base
of the pycnocline. The phytoplankton assemblage in the layer was primarily composed of a multi-species assemblage of Pseudo-nitzschia including the toxin-producing species Pseudo-nitzschia australis. Concentrations of toxic phytoplankton (P. australis), cyanobacteria, and bacteria in the layer were significantly higher than outside the layer (P < 0.05). Counts of total Pseudo-nitzschia spp. showed similar levels of enrichment in the layer compared to outside the layer. Our findings indicate that, when monitoring
for HABs, it is critical to sample at scales appropriate to resolve thin layers. Thin layers have been identified as a common
recurrent feature in a variety of coastal systems, suggesting that the use of autonomous high-resolution vertical profilers
coupled with targeted sampling, could allow more timely detection of HABs in many coastal environments. 相似文献
13.
14.
V. López Sánchez-Vizcaíno J. A. D. Connolly M. T. Gómez-Pugnaire 《Contributions to Mineralogy and Petrology》1997,126(3):292-302
Phase relations and metamorphic conditions have been studied in metacarbonate rocks from the Nevado-Filábride Complex (Cordilleras
Béticas) through forward modeling. In many rock samples, the assemblage titanite + rutile + calcite + quartz + graphite buffered
the composition of the C-O-H fluid present during metamorphism. Over a wide range of P-T conditions, fluid compositions computed for this buffer are essentially binary H2O-CO2 mixtures. This buffer also constrains the chemical potentials of TiO2, CaO and SiO2. Consequently it is possible to make a thermodynamic projection through these components to predict the stable phase relations
consistent with the buffer. Using this method, phase relations have been analyzed in a rock containing the buffer assemblage
and paragonite, albite, phengite, epidote, and chlorite. The equilibrium P-T conditions for this assemblage are constrained, by minimization of the differences between predicted and observed mineral
compositions, to be 560 ± 15 °C and 9.5 ± 1 kbar. Conditions obtained compare well with those estimated from other studies
in different lithologic units. The inferred metamorphic fluid composition is H2O-rich ().
Received: 11 October 1995 / Accepted 5 August 1996 相似文献
15.
1 Introduction The stromatolites of the Jiawengmen area in the southern belt of the Eastern Kunlun orogen were initially interpreted as vortex structures by the Regional Geological Survey Team, Qinghai Bureau of Geology and Mineral Resources in 1973; these samples were then identified as algal fossils of Sinian age by the Nanjing Institute of Geology and Paleontology (Qinghai Bureau of Geology and Mineral Resources, 1973). In 1994, Chen and Luo (1998) discovered some stromatolites, i… 相似文献
16.
Silicified carbonates of the latest Mesoproterozoic Sukhaya Tunguska Formation, northwestern Siberia, contain abundant and diverse permineralized microfossils. Peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of Lyngbya and Phormidium. In subtidal to lower intertidal settings, mat-dwelling microbenthos and possible coastal microplankton are abundant. In contrast, densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats; the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth. Eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface. Most microbenthic populations are cyanobacterial, although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton. Protists are more securely represented by large (up to 320 micrometers in diameter) but poorly preserved acritarchs in basinal facies. The Sukhaya Tunguska assemblage contains 27 species in 18 genera. By virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions, including substrates, Proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and, thereafter, resist competitive replacement. The resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of Phanerozoic plants and animals. 相似文献
17.
The upper Qigeblaq Formation （Fm） dolostones and the Yurtus Fm phosphatic cherts, black shales, limestones, and dolostones are widely distributed in the Precambrian/Cambrian transitional succession of the Aksu-Wushi area. Negative δ13C excursion above the Yurtus Fm/ Qigeblaq Fm boundary was determined in this study. The pronounced negative carbon isotope excursion occurs in the phosphatic chert layers at the bottom of the Cambrian Yurtus Fm, below which the first appearance of the Asteridium- Heh＇osphaeridium-Comasphaeridium （AHC） acritarch assemblage zone. The δ13C curve of the lower part of the Yurtus Fm in the Aksu-Wushi area was found to be correlated with the early Cambrian δ13C curves of the Zhujiaqing Fm （Daibu Member）, the lower part of the Yanjiahe Fm on the Yangtze Platform in China, the lower Tal Fm in India, the Sukharikha Fm in Siberia, and the upper part of the Tsagaan Oloom Fm in Mongolia through biostigraphy. The lower part of the Yurtus Fm in the Tarim Basin is at the Nemakit-Daldynian stage, and the Precambrian/Cambrian boundary of the Aksu-Wushi area may be located in the phosphatic chert unit which just below the first appearance AHC acritarch assemblage zone. The negative δ13C excursion （N1） across the Precambrian/Cambrian boundary in the studied section may have resulted from oceanic overturning and sea level rise. 相似文献
18.
19.
Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X
A as well as P and T. The predicted mineral associations for the low- and high-X
A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X
A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X
A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X
A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X
A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X
A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased 相似文献
20.
A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M1) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M1; 1090–1030 Ma) and again during the Pan-African orogeny (M2a, M2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma. 相似文献