Ground tilt measurements demonstrate that Askja is in a state of unrest, and that in the period 1988–1991 a maximum 48±3 rad tilt occurred down towards the centre of the caldera. This is consistent with 126 mm of deflation at the centre of the caldera with a 2.5–3.0 km depth to the source of deformation. The volume of the subsidence bowl is 6.2x106 m3. When combined with high precision microgravity measurements, the overall change in sub-surface mass may be quantified. After correction for the observed elevation change using the free air gradient of gravity measured for each station, the total decrease in mass is estimated to be less than 109 kg. A small residual ground inflation and net gravity increase in the southeastern part of the caldera may be caused by dyke intrusion in this region. The minimum dimensions of such an intrusion or complex of intrusions are 1 m width, up to 100 m deep and up to several hundred metres thick. 相似文献
The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle–ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement. 相似文献
The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, California, comprises a thick volcaniclastic sequence
deposited in a submarine island-arc setting and penetrated by numerous related hypabyssal intrusions. A composite andesite-diorite
intrusive complex ≥4.5 km long and ≥1.5 km thick was emplaced while the host Tuttle Lake sediments were still wet and unconsolidated.
Large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff
and tuff-breccia. The southern half of the complex consists of augite-phyric andesite containing peperite in numerous small,
isolated pockets and in more extensive, laterally continuous zones. The peperites comprise three main types recognized previously
in other peperite studies. Fluidal peperite consists of small (≤30 cm), closely spaced, at least partly interconnected, globular to amoeboid andesite bodies enclosed
by tuff. This peperite type developed during intrusion of magma into fine-grained wet sediment along unstable interfaces,
and fluidization of the sediment facilitated development of complex intrusive geometries. Blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic
stressing of rigid margins, possibly aided by small steam explosions. The northern half of the intrusive complex consists
predominantly of a different type of peperite, in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoidal,
elongate, or angular, polyhedral shapes are closely packed to widely dispersed within disrupted host sediment. Textural features
suggest the andesite clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits
are represented by elongate, sinuous bodies up to 30 m or more in length. Disruption and dispersal of the andesite clasts
are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits
penetrating the host sediments. Closely packed peperite is present adjacent to mappable intrusions of coherent andesite, and
along the margin of a large mass of coarse-grained diorite. These coherent intrusions are considered to be major feeders for
this part of the complex. Examples of magma/wet sediment interaction similar in scale to the extensive peperites described
here occur elsewhere in ancient island-arc strata in the northern Sierra Nevada. Based on these and other published examples,
large-scale peperites probably are more common than generally realized and are likely to be important in settings where thick
sediment sequences accumulate during active volcanism. Careful mapping in well-exposed terrains may be required to recognize
large-scale peperite complexes of this type.
Received: 8 June 1998 / Accepted: 4 December 1998 相似文献
The role of carbon dioxide in the dynamics of magma ascent in explosive eruptions is investigated by means of numerical modeling.
The model is steady, one-dimensional, and isothermal; it calculates the separated flow of gas and a homogeneous mixture of
liquid magma and crystals. The magma properties are calculated on the basis of magma composition and crystal content and are
allowed to change along the conduit due to pressure decrease and gas exsolution. The effect of the presence of a two-component
(water + carbon dioxide) exsolving gas phase is investigated by performing a parametric study on the CO2/(H2O+CO2) ratio, which is allowed to vary from 0 to 0.5 at either constant total volatile or constant water content. The relatively
insoluble carbon dioxide component plays an important role in the location of the volatile-saturation and magma-fragmentation
levels and in the distribution of the flow variables in the volcanic conduit. In detail, the results show that an increase
of the proportion of carbon dioxide produces a decrease of the mass flow rate, pressure, and exit mixture density, and an
increase of the exit gas volume fraction and depth of the fragmentation level. A relevant result is the different role played
by water and carbon dioxide in the eruption dynamics; an increasing amount of water produces an increase of the mass flow
rate, and an increasing amount of carbon dioxide produces a decrease. Even small amounts of carbon dioxide have major consequences
on the eruption dynamics, implying that the multicomponent nature of the volcanic gas must be taken into account in the prediction
of the eruption scenario and the forecasting of volcanic hazard.
Received: 6 March 1998 / Accepted: 28 October 1998 相似文献
The Western Qinling Orogen(WQO) is characterized by voluminous distribution of Indosinian granitoids,the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny.Here we investigate a group of intrusions termed "Five Golden Flowers" based on petrological,geochemical,zircon U-Pb geochronological and Lu-Hf isotopic studies on the granitoids and their mafic microgranular enclaves(MMEs).Our results show that these intrusions are genetically divided into two types,namely,magma-mixing and highly fractionated.The Jiaochangba,Lujing,Zhongchuan,and Luchuba granitoids are biotite monzogranites(220±0.8 Ma to 217±2.6 Ma) with abundant coeval MMEs(220±.1 Ma to 217±2.7 Ma).The rocks contain moderate to high SiO_2,high MgO,Rb,Sr,Ba,and Th contents,but low TiO_2,P_2 O_5,and Sc values,A/CNK of 1.1,and a range of ε_(Hf)(t) values of-11.7 to +2.23 with corresponding T_(DM2)values of 1967-1228 Ma.The MMEs possess K-feldspar megacrysts,abundant acicular apatites,and show lopsided textures.They have lower SiO_2,Al_2 O_3,and Th contents,but higher MgO,TiO_2,and Sc,with ε_(Hf)(t) values of-18.0 to +3.18 and T_(DM1) of 849-720 Ma.The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle.We suggest that these host granitoids were produced by partial melting of latePaleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas.The Baijiazhuang pluton is dominantly composed of leucogranite(muscovite granite and twomica monzogranite,216±1.5 Ma) without MMEs.The rocks are peraluminous with high A/CNK(1.06-1.27).Compared with the other four granitoids,the Baijiazhuang leucogranite shows higher SiO_2 content,markedly lower concentrations of TiO_2,MgO,Al_2 O_3,CaO,and Fe_2 O_3~T,and lower LREE/HREE and(La/Yb)N values.These leucogranites are also rich in Rb,Th,and U,and display marked depletions in Ba,Sr,Ti,and Eu,indicating that they experienced significant fractionation.Zircon ε_(Hf)(t) values(-10.2 to-3.27) and T_(DM2)(1868-1424 Ma),as well as the Nb/Ta and K_2 O/Na_2 O values are similar to the other four granitoids,indicating that they are likely to have been derived from a similar source;with sediments playing only a minor role in the magma generation.The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling.Based on the above features,we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures,within magma chambers at mid-crust depths.We propose that the granitoid suite was formed in the transitional setting from synto post-collision during the collisional orogeny between the SCB and NCB,following break-off of the subducted South China Block lithosphere during 220-216 Ma. 相似文献
Orogenic granitoids often display mineralogical and geochemical features suggesting that open-system magmatic processes played a key role in their evolution. This is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass, the occurrence of strong disequilibrium textures in mineralogical phases, and/or extreme geochemical and isotopic variability.
In this contribution, intrusive rocks constituting the Sithonia Plutonic Complex (Northern Greece) are studied on the basis of mineral chemistry, whole-rock major, trace element geochemistry, and Sr and Nd isotopic composition. Sithonia rocks can be divided into a basic group bearing macroscopic (mafic enclaves), microscopic (disequilibrium textures), geochemical, and isotopic evidence of magma interaction, and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process, but macroscopic and microscopic features are lacking.
A two-step Mixing plus Fractional Crystallization (MFC) process is considered responsible for the evolution of the basic group. The first step explains the chemical variation in the mafic enclave group: a basic magma, represented by the least evolved enclaves, interacted with an acid magma, represented by the most evolved granitoid rocks, to give the most evolved enclaves. The second step explains the geochemical variations of the remaining rocks of the basic group: most evolved enclaves interacted with the same acid magma to give the spectrum of rock compositions with intermediate geochemical signatures. A convection–diffusion process is envisaged to explain the geochemical and isotopic variability and the lack of macroscopic and petrographic evidence of magma interaction in the acid group.
The mafic magma is presumably the result of melting of a mantle, repeatedly metasomatized and enriched in LILE due to subduction events, whereas the acid magma is considered the product of partial melting of lower crustal rocks of intermediate to basaltic composition.
It is shown that Sithonia Plutonic Complex offers the opportunity to investigate in detail the complex interplay between geochemistry and magma dynamics during magma interaction processes between mantle and crustal derived magmas. 相似文献
Igneous garnets have the potential to strongly fractionate rare earth elements (REE). Yet informations on partition coefficients
are very scant, and criteria for distinguishing between hydrothermal and magmatic garnets are ambiguous. To fill this gap,
we present trace element and isotopic data for two types of Ca-rich garnets from phonolites (Mt. Somma-Vesuvius). Both Ca-garnet
populations are different in their style and dynamics of fractionation: one population is progressively strongly depleted
in HREE from core to rim, reflecting REE fractionation in the host phonolite via earlier-crystallized garnets. Such examples
for extreme changes in HREE in garnets are only known for hydrothermal grandites by REE-bearing fluids. The second garnet
population is homogeneous and formed in a closed system. Near-flat patterns between Sm and Lu confirm experimental data indicating
lower D(Sm)/D(Lu) for Ca-rich garnets than for e.g. pyrope-rich garnets. It follows: DGrt/PhMelt for La = 0.5, Sm = 48 and Yb = 110. 相似文献