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1.
Summary Pre- to early Variscan metamorphosed volcano-sedimentary series of the western (Bergstr?sser) Odenwald have been intruded
by Variscan calc-alkaline magmatites with plate margin affinities. Within the NE-SW trending metabasic and metapelitic series,
intercalations of anomalously aluminous- and iron-rich compositions representing former bauxites are distinguishable. Geochemical
data indicate that the Al-, Fe-rich rocks are the metamorphic equivalents of a former bauxitic-ferralitic weathering profile,
now comprising spinel fels (top) with relics of pisolitic structures, corundum-chlorite fels, corundum-cordierite-plagioclase
gneiss, sillimanite-cordierite-plagioclase gneiss, cordierite-plagioclase gneiss and kinzigite (base) displaying a decrease
of weathering in the order as listed. The evolution of such terrestrial sediments is related to specific climatic, environmental,
and physico-chemical conditions similar to those of the present-day tropical to subtropical humid regions with high seasonal
rains and intensive drainage. Comparison with palaeomagnetic and palaeoclimatic data indicate that the Odenwald metabauxites
originally formed during the Lower to Mid-Devonian. Our results imply that, during this geological time span, parts of the
precursors of the Mid-German Crystalline Rise not only transversed low latitudes but also were exposed to terrestrial weathering.
Received June 10, 1999; revised version accepted October 30, 2000 相似文献
Zusammenfassung Die Geochemie der Metabauxite im Bergstr?sser Odenwald (Mitteldeutsche Kristallinschwelle) und ihre Bedeutung für das Pal?omilieu Im westlichen (Bergstr?sser) Odenwald intrudierten variszische Plattenrand-Kalkalkali-Magmatite in pr?- bis früh-variszische, ehemalige vulkano-sediment?re Serien. Innerhalb der heute NE-SW streichenden Metabasit- und Metapelitzüge treten lokal Al-Fe-reiche Einschaltungen auf, die strukturell und geochemisch als metamorphe ?quivalente eines ehemaligen bauxitisch-ferralitischen Verwitterungsprofils klassifiziert werden k?nnen. Sie umfassen Spinellfelse mit erhaltenen, ehemaligen zonierten Pisolithen, Korund-Chloritfelse, Korund-Cordierit-Plagioklasgneise, Sillimanit-Cordierit-Plagioklasgneise, Cordierit-Plagioklasgneise und Kinzigite. Bauxite k?nnen als terrestrische Bildungen nur unter speziellen klimatischen und physiko-chemischen Bedingungen entstehen. Pal?omagnetische und pal?oklimatische Daten legen die Bildung der ehemaligen Bauxite im unteren bis mittleren Devon nahe. Daraus folgt, da? w?hrend dieser geologischen Zeitspanne die Vorl?ufer der mitteldeutschen Kristallinschwelle bei der Norddrift nicht nur ?quatorn?he passierten, sondern auch partiell ungest?rte, terrestrische, tropisch-subtropische Verwitterungsprozesse abliefen.
Received June 10, 1999; revised version accepted October 30, 2000 相似文献
2.
Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclasephenocrysts were dredged in the rift valley and adjacent flanksof the ultraslow-spreading Mohns and Knipovich ridges. The PUBsshow large variations in crystal morphologies and zoning. Thelarge variations suggest that single basalt samples containa mixture of plagioclase crystals that aggregated at differentlevels in the magma conduits. Resorbed crystals and repeatedreverse zones suggest that the magma reservoirs were replenishedand heated several times. Thin concentric zones with melt inclusions,and sharp reductions in the anorthite content of 3–7%,are common between the reverse zones. These zones, and skeletalcrystals with distinctly lower anorthite contents than massivecrystals, are interpreted to be the result of rapid crystalliztionduring strong undercooling. The changes between short periodsof cooling and longer periods with reheating are explained bymultiple advances of crystal-rich magma into cool regions followedby longer periods of gradual magma inflow and temperature increase.The porphyritic basalts are characterizd by more depleted andmore fractionated compositions than the aphyric basalts, withlower (La/Sm)N, K2O and Mg-numbers. This relationship, and theobservation that PUBs are sampled only close to segment centresalong these ridges, suggests that the PUBs formed by higherdegrees of melting and evolved in more long-lived magma reservoirs.We propose that the zoning patterns of plagioclase crystalsand crystal morphologies of these PUBs reflect the developmentand flow of magma through a stacked sill complex-like conduitsystem, whereas the aphyric equivalents represent later flowof magma through the conduit. The formation of voluminous higher-degreemelts may trigger the development of the magma conduits andexplain the generally depleted compositions of PUB magmas. KEY WORDS: basalt; mineral chemistry; MORB; magma mixing; magma chamber; major element 相似文献
3.
The polyphasal magmatic evolution of the Caledonian Karmøy Ophiolite Complex includes: (1) formation of an axis sequence from island-arc tholeiitic (IAT) and more MORB-like magmas (493+7/-4 Ma); (2) intrusion of magmas of boninitic affinity (485±2 Ma); (3) intrusion of MORB- and IAT-like magmas; (4) intrusion and extrusion of calc-alkaline magmas (470+9/-5 Ma); (5) intrusion and extrusion of basalts with alkaline trace-element affinity. Repeated intrusion of MORB and IAT-like magmas may be explained by intermittent magmatism involving magma-chamber solidification and remelting of a source characterized by initial Nd of approximately +6.5. The boninitic rocks may have formed from two LREE-depleted sources: the primary source of the axis-sequence magmas and the residual source left after extraction of these magmas. These sources have been enriched in LREE, Th and Zr from subducted material exhibiting a continental Nd-isotope signature with initial Nd less than-8. Covariation between Nd and Th, Zr, Nd, Y and Yb may be explained by metasomatic enrichment of a LREE-depleted mantle source by a LREE-enriched subduction component, followed by partial melting during which the degree of melting of the metasomatized mantle source increased linearly with the amount of subduction component added to the mantle source. The calc-alkaline magmas may have formed by remelting of a highly depleted source, which became enriched in some trace elements derived from the source of the subsequent alkaline magmatism. The geology and geochemistry of the Karmøy Ophiolite Complex suggest growth of an island-arc upon newly-formed oceanic crust, followed by arc-splitting and the development of a new basin. 相似文献
4.
Role of the subducted slab,mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone 总被引:78,自引:2,他引:78
All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49–54°S) have erupted exclusively adakitic andesites
and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole
and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some
arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively
thin crust (<35 km) within which plagioclase rather than garnet is stable. The source for AVZ adakites is more likely to be
subducted oceanic basalt, recrystallized to garnet-amphibolite or eclogite. Geothermal models indicate that partial melting
of the subducted oceanic crust is probable below the Austral Andes due to the slow subduction rate (2 cm/year) and the young
age (<24 Ma) of the subducted oceanic lithosphere. Geochemical models for AVZ adakites are also consistent with a large material
contribution from subducted oceanic crust (35–90% slab-derived mass), including sediment (up to 4% sediment-derived mass,
representing approximately 15% of all sediment subducted). Variable isotopic and trace-element ratios observed for AVZ adakites,
which span the range reported for adakites world-wide, require multistage models involving melting of different proportions
of subducted basalt and sediment, as well as an important material contribution from both the overlying mantle wedge (10–50%
mass contribution) and continental crust (0–30% mass contribution). Andesites from Cook Island volcano, located in the southernmost
AVZ (54°S) where subduction is more oblique, have MORB-like Sr, Nd, Pb and O isotopic composition and trace-element ratios.
These can be modeled by small degrees (2–4%) of partial melting of eclogitic MORB, yielding a tonalitic parent (intermediate
SiO2, CaO/Na2O>1), followed by limited interaction of this melt with the overlying mantle (≥90% MORB melt, ≤10% mantle), but only very
little (≤1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of
Burney (52°S), Reclus (51°S) and northernmost AVZ (49–50°S) andesites and dacites require melting of a mixture of MORB and
subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal
assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant
northwards in the AVZ as the angle of convergence becomes more orthogonal.
Received: 1 March 1995 / Accepted: 13 September 1995 相似文献
5.
6.
Friedrich Lucassen Peter Dulski Rainer Abart Gerhard Franz Dieter Rhede Rolf L. Romer 《Contributions to Mineralogy and Petrology》2010,160(2):279-295
Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry. 相似文献
7.
Friedrich Lucassen Gerhard Franz Rolf L. Romer Frank Schultz Peter Dulski Klaus Wemmer 《Lithos》2007,99(3-4):312-338
Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial 143Nd/144Nd ( 0.5127–0.5128) and 87Sr/86Sr ( 0.7032–0.7040). The initial 206Pb/204Pb ratios are variable (18.5–19.7) at uniform 207Pb/204Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (143Nd/144Nd 0.5123, 87Sr/86Sr 0.704, 206Pb/204Pb 17.5–18.5, and 207Pb/204Pb 15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.
Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism. 相似文献
8.
9.
Gerald Schubert 《天文和天体物理学研究(英文版)》2019,(4):7-22
With a great deal of humility I attempt in the following to recall important events in my life and in my scientific career of more than five decades. I am not through yet. I continue to do research though, I admit, not with the energy and fervor I once had. Still, I hope to contribute to science in meaningful ways. 相似文献
10.
Rolf Thienhaus 《International Journal of Earth Sciences》1953,42(1):63-79
Ohne Zusammenfassung 相似文献