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51.
52.
H.-J. Massonne 《Mineralogy and Petrology》2006,86(3-4):177-202
Summary Several granulite terrains are exposed in the Bohemian Massif of Central Europe. These were metamorphosed at pressures close
to 12 kbar and temperatures of more than 800 °C c. 340 Ma ago. The corresponding penetrative deformation almost totally erased
the record of the preceding metamorphic evolution. Nevertheless, rare relics such as mineral inclusions in large garnet grains
are witness of this earlier evolution, which was previously related to significantly higher pressures and, thus, to a subduction-related
event. The exemplary investigation of such mineral relics in a felsic granulite from the Granulitgebirge rather points to
pressures of 13–14 kbar only at relatively low temperatures of 620 °C and, thus, to considerable, nearly isobaric heating
before the exhumation of the granulites started at 800 °C or somewhat higher temperature.
The inferred P–T evolution is compatible with a geodynamic model of lithospheric delamination, with crustal material having
been involved. The delamination at c. 340 Ma ago followed long-lasting, continuous collision of Gondwana and Laurussia forming
the Variscan orogen. Within the thickened continental crust, the delamination concerned mainly the dense basic material in
the lower crust. This event also caused upwelling of the mantle asthenosphere. Both processes resulted in heating of the more
felsic lower portion of the continental crust, thinner than before delamination. Heating by 200 °C or more caused prograde
mineral reactions and created buoyancy forces, as the overlying crust became denser than the underlying hot and felsic granulites.
As a consequence, considerable volumes of felsic granulite could have reached shallow crustal levels (corresponding to 3 to
4 kbar), conditions documented in granulite bodies in the north-western Bohemian Massif. 相似文献
53.
Julio Sepúlveda Jens Wendler Arne Leider Hans-Joachim Kuss Roger E. Summons Kai-Uwe Hinrichs 《Organic Geochemistry》2009,40(5):553-568
We evaluated the structure of planktonic communities and paleoenvironmental conditions throughout the Cenomanian–Turonian Oceanic Anoxic Event (OAE2) by studying bulk geochemical properties and the molecular isotopic composition of source-specific hydrocarbons from organic-rich sediments deposited in an intrashelf basin at the Levant Platform, central Jordan. High concentrations of desmethyl and 4-methylsteranes as well as dinosteranes indicate that marine algae including dinoflagellates were the main primary producing organisms. The presence of 2-methylhopanes and 13C-enriched hopanes, in addition to isotopically enriched aryl isoprenoids, evidenced the contribution of cyanobacteria and green sulfur bacteria, respectively. Additionally, variable but fairly low δ15N values during OAE2 suggest the occurrence of diazotrophy as a likely important process fueling primary production during OAE2 in this stratified/anoxic continental platform. Variations in the relative contribution of biomarkers revealed changes in planktonic communities associated with sea level change and water column stratification. OAE2 was characterized by strong stratification, anoxic bottom waters and a deep chemocline, as evidenced by high gammacerane and homohopane indices and the absence of photic zone euxinia (PZE) markers, respectively. However, the presence of isorenieratane and its derivatives in post-OAE black shales points to a shoaling of the chemocline and to PZE. This interval was also characterized by an exceptionally high abundance of chlorophyll-derived pristane and phytane (up to 2 mg g?1 TOC), likely as a result of highly enhanced primary production and organic matter preservation. Remarkably, this high productivity event co-occurs with an exceptionally high abundance of calcispheres reported elsewhere to be part of a global bio-event. 相似文献
54.
Ying Li Hans‐Joachim Massonne Arne Willner Hong‐Feng Tang Cong‐Qiang Liu 《Island Arc》2008,17(4):577-590
Pseudosections for two sediments and one basalt calculated in the system K2O–Na2O–CaO–MgO–FeO–Fe2O3–Al2O3–TiO2–SiO2–H2O for the P–T range 10 to 35 kbar, 300 to 900°C give useful insights into the amount of H2O released from oceanic crust in subduction zones. In cold subduction zones (20 kbar–300°C to 35 kbar–500°C) hydrous minerals storing 3 to 4 wt% H2O are still present in metasediments at depths of 120 km. In the same environment, metabasite releases 1 wt% H2O in the depth range 100 to 120 km, but 4.5 wt% H2O is transported to greater depths. In hot subduction zones (300°C hotter than the cold subduction zone at 100 km depth), dehydration events of metasediments in the depth range 50 to 80 km correspond to the breakdown of chlorite and paragonite. In the calculations no further water is released at greater depths because the modal content of phengite, the only hydrous mineral phase at these depths, remains almost constant. For the same P–T path, metabasite shows continuous dehydration between 40 and 80 km releasing almost 3 wt% H2O. At 120 km depth less than 0.4 wt% of H2O remains. In an average modern subduction zone (~6°C/km) most dehydration of sediments occurs at depths of 70 to 100 km and that of basalts at depths of 80 to 120 km. Only 1.3 wt% H2O in metasediments and 1.6 wt% H2O in metabasalt has the potential to be subducted to depths greater than 120 km. The dehydration behavior of sediments concurs with the generally held idea that subduction zone fluids are most effectively transported to great depths by cold subduction. In hot subduction zones, such as those characteristic of early Earth, most H2O carried by oceanic crust is liberated at depths less than 120 km and, thus, would not contribute to island‐arc magmatism. 相似文献
55.
Latest field research and palæoenvironmental reconstructions have revealed that within less than 6000 years the eastern Sahara experienced a dramatic climatic change similar to that in the western Sahara, passing from hyperaridity to semi-aridity (dry savanna) to its present hyperarid state. Groundwater levels started to rise about 9300 years before present (14C years BP), leading to the formation of a mosaic of freshwater lakes and swamps. Within a few decades, the aquifers were loaded and the palæopiezometric surface was as much as 25 m higher than it is today. The uplands generated up to 800 km long fluvial systems, which put an end to the endorheic drainage of the region and functioned as migration paths for large savanna mammals. These wetter conditions persisted in Western Nubia during the Holocene until ca 5000 years BP The climatic deterioration began around 5700 years BP as shown by evaporitic sediments. Reversal events prior to aridification during the Late Holocene were not recorded systematically in the sediments of the eastern Sahara because of the stability of the ecosystems. Changes in land-surface conditions such as palæolakes, swamps and vegetation created water vapour sources that generated local rainfall and buffered short dry spells. Radiocarbon-dated charcoal indicates that Neolithic human occupation culminated during this Early Holocene wet phase and ended ca 2000 years after the fading of the wet phase at about 3000 years BP, when the shallow aquifers were exhausted. 相似文献
56.
Owing to the hypercontinental location of Western Nubia, secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern Sahara. The study area is crossed by the 20-mm isohyet; between 9300 and about 4000 yr B.P., however, there were widespread lake and swamp environments with freshwater molluscs, ostracods, and diatoms, and a species-rich savanna mammal fauna. The center of the West Nubian Basin (approx. 18°N), an area of about 20,000 km2, was occupied by a semiaquatic landscape which was situated at the same latitude as Paleolake Chad. From extensive lake carbonates up to about 4 m thick, a long-term rise of the groudwater table is inferred. Environments developed that now exist at about latitude 13°N. Radiocarbon dates from lake sediment sequences cluster between 30,000 and 21,000 yr B.P., indicating a Pleistocene wet phase. A gap in radiocarbon dates between 21,000 and 11,000 yr B.P. signals a phase of hyperaridity, similar to the present hyperarid phase, with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation. 相似文献
57.
Petrogenesis of early cretaceous carbonatite and ultramafic lamprophyres in a diatreme in the Batain Nappes,Eastern Oman continental margin 总被引:1,自引:0,他引:1
S. Nasir S. Al-Khirbash H. Rollinson A. Al-Harthy A. Al-Sayigh A. Al-Lazki T. Theye H.-J. Massonne E. Belousova 《Contributions to Mineralogy and Petrology》2011,161(1):47-74
Allochthonous carbonatite and ultramafic lamprophyre occur in a diatreme at the beach of the Asseelah village, northeastern
Oman. The diatreme consists of heterogeneous deposits dominated by ‘diatreme facies’ pyroclastic rocks. These include aillikite
and carbonatite, which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation within the Batain
nappes. Both rock types are dominated by carbonate, altered olivine, Ti–Al–phlogopite and Cr–Al–spinel and contain varying
amounts of apatite and rutile. The carbonatite occur as fine-grained heterolithic breccias with abundant rounded carbonatite
xenoliths, glimmerite and crustal xenoliths. The aillikite consists of pelletal lapilli tuff with abundant fine-grained carbonatite
autoliths and crustal xenoliths, which resemble those in the carbonatite breccia. The aillikite and carbonatite are characterized
by low SiO2 (11–24 wt%), MgO (9.5–12.4 wt%) and K2O (<0.3 wt%), but high CaO (18–22 wt%), Al2O3 (4.75–7.04 wt%), Fe2O3tot (8.7–13.8 wt%) and loss-on-ignition (24–30 wt%). Higher CaO, Fe2O3total, Al2O3, MnO, TiO2, P2O5 and lower SiO2 and MgO content distinguish carbonatite from the aillikite. The associated carbonatite xenoliths and autoliths have intermediate
composition between the aillikite and carbonatite. Mg number is variable and ranges between 58 and 66 in the carbonatite,
66 and 72 in the aillikite and between 48 to 64 in the carbonatite autoliths and xenoliths. The Asseelah aillikite, carbonatite,
carbonatite xenoliths and autoliths overlap in most of their mineral parageneses, mineral composition and major and trace
element chemistry and have variable but overlapping Sr, Nd and Pb isotopic composition, implying that these rocks are related
to a common type of parental magma with variable isotopic characteristics. The Asseelah aillikite, carbonatite and carbonatites
xenoliths are LREE-enriched and significantly depleted in HREE. They exhibit similar smooth, subparallel REE pattern and steep
slopes with (La/Sm)
n
of 6–10 and relative depletion in heavy rare earth elements (Lu = 3–10 chondrite). Initial 87Sr/86Sr ratios vary from 0.70409 to 0.70787, whereas initial 143Nd/144Nd ratios vary between 0.512603 and 0.512716 (εNd
i
between 2.8 and 3.6). 206Pb/204Pb
i
ratios vary between 18.4 and 18.76, 207Pb/204Pb
i
ratios vary between 15.34 and 15.63, whereas 208Pb/204Pb
i
varies between 38.42 and 39.05. Zircons grains extracted from the carbonatite have a mean age of 137 ± 1 Ma (95% confidence,
MSWD = 0.49). This age correlates with large-scale tectonic events recorded in the early Indian Ocean at 140–160 Ma. Geochemical
and isotopic signatures displayed by the Asseelah rocks can be accounted for by vein-plus-wall-rock model of Foley (1992) wherein veins are represented by phlogopite, carbonate and apatite and depleted peridotite constitutes the wall-rock. The
carbonatite and aillikite magmatism is probably a distal effect of the breaking up of Gondwana, during and/or after the rift-to-drift
transition that led to the opening of the Indian Ocean. 相似文献
58.
Deng X Wagner HJ Popper AN 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(1):27-37
The inner ear structure of Antimora rostrata and its coupling to the swim bladder were analyzed and compared with the inner ears of several shallow-water species that also have similar coupling. The inner ear of Antimora has a long saccular otolith and sensory epithelium as compared to many other fishes. Some parts of the membranous labyrinth are thick and rigid, while other parts are thinner but attached tightly to the bony capsule. The partially rigid membranous labyrinth, along with its intimate connection to the swim bladder, may help the inner ear follow the sound oscillations from the swim bladder with better precision than would occur in a less rigid inner ear. In addition, the saccular sensory epithelium has an elaborate structure and an anterior enlargement that may be correlated with increased hearing sensitivity. Some of the features in the inner ear of Antimora may reflect the functional specialization of deep-water living and support the hypothesis that there is enhanced inner ear sensitivity in some deep-sea fishes. 相似文献
59.
Hans‐Joachim Massonne Gabriele Cruciani Marcello Franceschelli Giovanni Musumeci 《Journal of Metamorphic Geology》2018,36(1):55-77
To better understand the evolution of deep‐seated crust of the Variscan orogen in the Sardinia‐Corsica region, we studied garnet‐bearing micaschists which were sampled 3 km east and 15 km northeast of Porto Vecchio, south‐eastern Corsica. After a careful investigation of the textural relations and compositions of minerals, especially of zoned garnet, a P–T path was reconstructed using contoured P–T pseudosections. U–Th–Pb dating of monazite in the micaschists was undertaken with the electron microprobe. The micaschists from both localities were formed along similar anticlockwise P–T paths. The prograde branch of these paths starts at 3 kbar close to 600°C in the P–T field of sillimanite and reaches peak conditions at 7 kbar and 600 (15 km NE of Porto Vecchio) to 630°C (3 km E of Porto Vecchio). The metamorphism at peak P–T conditions happened c. 340 Ma based on low‐Y (<0.65 wt% Y2O3) monazite. Ages of monazite with high‐Y contents (>2 wt% Y2O3), which probably have formed before garnet, scatter around 362 Ma. The retrograde branch of the P–T paths passes through 4 kbar at ~550°C. We conclude that the micaschists belong to a common metasedimentary sequence, which extends over the Porto Vecchio region and is separated from other metamorphic rock sequences in the north and the south by major tectonic boundaries. This sequence had experienced peak pressures which are lower than those determined for metamorphic rocks, such as micaschist and gneiss, from north‐eastern Sardinia. At present, we favour a continent–continent collisional scenario with the studied metasedimentary sequence buried during the collisional event as part of the upper plate. The contemporaneous high‐P metamorphic rocks from NE Sardinia were part of the upper portion of the lower plate. The addressed rocks from both plates were exhumed in an exhumation channel. 相似文献
60.
Small oval‐shaped, unshielded monazite grains found in a Variscan garnet–muscovite‐bearing mylonitic paragneiss from the Liegendserie unit of the Münchberg Metamorphic Complex in the northwestern Bohemian Massif, central Europe, yield only pre‐Variscan ages. These ages, determined with the electron microprobe, have maxima at c. 545, 520 and 495 Ma and two side‐maxima at 455 and 575 Ma, and are comparable with previously determined ages of detrital zircon reported from paragneisses elsewhere in the NW Bohemian Massif. The pressure (P)–temperature (T) history of this mylonitic paragneiss, determined from contoured P–T pseudosections, involved an initial stage at 6 kbar/600 °C, reaching peak P–T conditions of 12.5 kbar/670 °C with partial melting, followed by mylonitization and retrogression to 9 kbar/610 °C. The monazite, representing detrital grains derived from igneous rocks of a Cadomian provenance between 575 and 455 Ma, has survived these Variscan metamorphic/deformational events unchanged because this mineral has probably never been outside its P–T stability field during metamorphism. 相似文献