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1.
It is proposed that fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows. Detailed field and petrographic analysis of such textures may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes. There is evidence at each conduit investigated for multiple seismogenic cycles, each of which involved four distinct evolutionary phases. In phase 1, shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow, forming the angular fracture networks that initiated faulting cycles. Transient pressure gradients were generated as the fractures opened, which led to fluidisation and clastic deposition of fine-grained particles that were derived from the fracture walls by abrasion. Fracture networks then progressively coalesced and rotated during subsequent slip (phase 2), developing into cataclasite zones with evidence for multiple localised slip events, fluidisation and grain size reduction. Phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction-controlled stick-slip behaviour. Increasing cohesion of cataclasites then led to aseismic, distributed ductile deformation (phase 3) and generated deformed cataclasite zones, which are enriched in metallic oxide microlites and resemble glassy pseudotachylite. Continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite-rich flow bands in obsidian (phase 4). Overall, the mixed brittle–ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle–ductile transition, with the rheological response mediated by strain-rate variations and frictional heating. Fault processes in highly viscous magma are compared with those elsewhere in the crust, and this comparison is used to appraise existing models of volcano seismic activity. Based on the textures observed, it is suggested that patterns of long-period and hybrid earthquakes at silicic lava domes reflect friction-controlled stick-slip movement and eventual healing of fault zones in magma, which are an accelerated and smaller-scale analogue of tectonic faults.Editorial responsibility: J. Stix 相似文献
2.
Microscopic shear zones have been found in the groundmass of glassy rocks of a Miocene submarine dacite dome in southwest Japan. Similar textures have been reported previously but only in dykes. These textures give a valuable insight into the deformation of the dome during its emplacement by recording the orientations of the principal strains. Detailed textural analysis indicates that the microshear zones formed as a result of flattening and stretching of the magma simultaneously with quenching. Measured stretching directions are near-parallel suggesting the magma flowed in one dominant direction rather than in a radial pattern. The strain is believed to also be influenced by high magma pressure inside the dome being imposed on the high viscosity outer part of the dome.
Present address: Faculty of Resource Science and Management, University of New England-Northern Rivers. P. O.Box 157 Lismore NSW 2480 Australia 相似文献
3.
At Mt. Etna volcano, the emission of plagioclase megacryst-bearing lavas, known locally as “cicirara”, has occurred rarely
and generally in association with unusual volcanological phenomena. In this work, we interpret the magma chamber processes
and the structural features of the plumbing system that led to the production of these peculiar volcanic rocks, based on a
detailed study of plagioclase megacrysts, including their oscillatory zoning, sieve textures, and fluid inclusions. Patchy
zoning suggests limited ascent in the deep levels of the plumbing system, based on the plagioclase nucleation threshold and
the volatile saturation depth. At intermediate, water-undersaturated levels of the plumbing system ascent is faster, as indicated
by crystals with coarse sieve textures. Storage at shallow, water-saturated levels (less than 6 km deep) is associated with
oscillatory zoning with very small changes in An. Slightly larger An variations coupled with different wavelengths provide
evidence of convection of crystals across distinct zones of the chamber. Stripes of melt inclusions formed at steps of magma
ascent and volatile loss, whereas layers of fluid inclusions may be related to episodes of volatile flushing into the magma
chamber. In contrast, strongly sieve-textured envelopes with An increase and constant FeO may be related to mixing with more
volatile-rich magmas of similar composition. We interpret the repeated occurrence of “cicirara” lavas as evidence that the
shallow portion of the plumbing system underwent a progressive coalescence of a complex network of dykes and sills in response
to increasing rates of magma supply from depth. Major magma withdrawals from this larger reservoir may be linked to episodes
of summit instability associated with major caldera collapses. 相似文献
4.
Fabric development in brittle-to-ductile shear zones 总被引：3，自引：0，他引：3
Carol Simpson 《Pure and Applied Geophysics》1986,124(1-2):269-288
Brittle-to-ductile shear zones from two separate geological settings are shown to have nucleated on zones of predominantly brittle deformation. The shear zones are not simply foliated cataclasites, since they contain abundant evidence of dynamic recrystallization of constituent minerals. A small quartz diorite lens in the Borrego Springs shear zone, southern California, contains centimeter-scale cataclasite zones that exhibit a gradual transition into foliated rock. Alteration of magnesiohornblende to actinolite, feldspar to white mica plus quartz, and biotite to chlorite, produced elongate minerals that define the foliation. During the later stages of deformation, intracrystalline slip and dynamic recrystallization of quartz and feldspar were important deformation mechanisms.The widespread occurrence of mineralized dilatant cracks predated the development of meter-to-decimeter-scale ductile shear zones in the Striped Rock granite, southern Virginia. Again, important deformation mechanisms in the final stages of deformation were intracrystalline slip and dynamic recrystallization of quartz.In both field areas the role of fluids has been important from the onset of brittle deformation. Fluids may have enhanced early fracturing in addition to causing the alteration and hydrolytic weakening of host rock minerals and the introduction of new mineral species. Each of these processes is thought to have contributed to the later localization of crystal plastic deformation in the rocks. 相似文献
5.
Patrizia Landi Nicole Mtrich Antonella Bertagnini Mauro Rosi 《Journal of Volcanology and Geothermal Research》2008,174(4):325-336
Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry. 相似文献
6.
The massive unit of a lava flow from Porri volcano (Salina, Aeolian Islands) displays many unusual structures related to
the physical interaction between two different magmas. The magma A represents approximately 80% of the exposed lava surface;
it has a crystal content of 51 vol.% and a dacitic glass composition (SiO2=63–64 wt.%). The magma B has a basaltic-andesite glass composition (SiO2=54–55 wt.%) and a crystal content of approximately 18 vol.%. It occurs as pillow-like enclaves, banding, boudin-like and
rolling structures which are hosted in magma A. Structural analysis suggests that banding and boudin-like structures are the
result of the deformation of enclaves at different shear strain. The linear correlation between strain and stratigraphic height
of the measured elements indicates a single mode of deformation. We deduce that the component B deformed according to a simple
shear model. Glass analyses of the A–B boundary indicate that A and B liquids mix together at high shear strain, whereas only
mingling occurs at low shear strain. This suggests that the amount of deformation (i.e. forced convection) plays an important
role in the formation of hybrid magmas. High shear strain may induce stretching, shearing and rolling of fluids which promote
both forced convection and dynamical diffusion processes. These processes allow mixing of magmas with large differences in
their physical properties.
Received: 15 July 1995 / Accepted: 30 May 1996 相似文献
7.
8.
The magmatic phase of the AD 79 eruption of Vesuvius produced alternations of fall and pyroclastic density current (PDC) deposits. A previous investigation demonstrated that the formation of several PDCs was linked with abrupt increases in the proportion of denser juvenile clasts within the eruptive column. Under the premise that juvenile clast density is controlled by vesiculation processes within the conduit, we investigate the processes responsible for these variations at or close to fragmentation levels. Pumice textures (vesicle sizes, numbers, and connectivity combined with crystal textures) from the AD 79 PDC deposits are compared to those from interbedded fall samples. Both PDC and fall deposits preserve textures that represent a full spectrum of degassing and outgassing processes, from bubble nucleation to collapse. Combining the textural and volatile (groundmass H2O) data, we derive a conduit model that satisfies all the textural and physical observations made for this phase of the eruption: lateral vesicularity/density stratifications are produced by maturing of bubble textures with superimposed localized shearing of bubble-rich magmas, which enhance outgassing of H2O. The incorporation of denser slower-moving magma from the conduit margins (??lateral magma density gradient??) is likely to be responsible for the higher abundances of dense juvenile pumice that triggered partial column collapses. We also illustrate how variations in the fragmentation depth (tapping a ??vertical magma density gradient??) can be responsible for variations in erupted clast density distributions, and potentially in the extent of degassing/outgassing. 相似文献
9.
Microtextural characteristics of fresh ejecta from Stromboli volcano were examined from three periods of differing eruption
style and intensity in 2002. Activity shifted from relatively weak and infrequent ash-charged explosions during January through
May into two broad cycles of waxing activity in June through late September, and late September through December, followed
by the onset on 28 December of the 2002/2003 effusive eruption. Analyzed sets of lapilli from May, September/October, and
28 December show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity.
Three distinct textures are observed among the analyzed pyroclasts: low density (LD) with an abundance of subspherical bubbles,
the presence of large, irregularly shaped bubbles, and a light-to-transparent glass matrix; transitional texture (TT) with
an intermediate number of subspherical bubbles, a high frequency of large, irregularly-shaped bubbles, and a honey colored
glass matrix; and high density (HD) with sparse relatively small bubbles, conspicuous large irregular bubbles, and a dark
glass matrix. Observational and quantitative data (density, vesicle size) indicate that these textures are linked through
variable residence time in Stromboli’s shallow conduit, with an ongoing evolution from LD to HD magma. Calculations suggest
that residual LD magma will evolve to HD texture in a period of hours to days. Contrasting amounts of the LD, TT, and HD magmas
are present in each sample, with the most TT in May, the most LD in September/October, and the most HD in December. This implies
that the shallow magma had a different rheology at each collection period. The viscosity of LD and HD magmas are calculated
to be in the range of 2,000 to 2,600 and 3,000 to 5,000 Pa s, respectively, which, with their changing proportions, must have
implications for rates of bubble slug ascent and processes of fragmentation. This study suggests that an increasing maturity
of magma in Stromboli’s shallow conduit (with resultant increase in viscosity) feeds back to reduce the intensity of explosions,
whereas a steady flux of LD magma favors more powerful explosions. 相似文献
10.
We present a visco-elastic bubble growth model, accounting for viscous and elastic deformations and for volatile mass transfer
between bubbles and melt. We define the borders between previous bubble growth models accounting for incompressible viscous
melt, and our new model accounting also for elastic deformation; this is done by a set of end-member analytical solutions
and numerical simulations. Elastic deformation is most prominent for magma of small vesicularity, where the growth regime
depends on the shear modulus. For high shear modulus, bubble growth is slow and follows an exponential law in a viscous growth
regime, while for low shear modulus bubbles quickly follow a square-root diffusive solution. Our model provides all the elastic
components (stresses, strains and strain rates) required for defining criteria for failure and magma fragmentation. We suggest
two failure criteria, a stress related one based on the internal friction and the Mohr-Coulomb failure theory, and a strain
related one based on fibre elongation experiments. We argue that both criteria are equivalent if we consider their shear modulus
dependency and its effect on magma rheology. Last, we apply our model to the process of bubble nucleation. In the incompressible
case, following nucleation, growth is slow and leads to long incubation times during which bubbles may be dissolved back into
the melt. The elastic response in magmas with low shear modulus results in a short incubation time, increasing the probability
of survival. The above effects emphasize the significance of visco-elasticity for the dynamic processes occurring in magmas
during volcanic activity. 相似文献
11.
Ivan Alejandro Petrinovic Ulrich Riller Jos Affonso Brod 《Journal of Volcanology and Geothermal Research》2005,140(4):295-320
This petrologic analysis of the Negra Muerta Volcanic Complex (NMVC) contributes to understanding the magmatic evolution of eruptive centres associated with prominent NW-striking fault zones in the southern Central Andes. Specifically, the geochemical characteristics and magmatic evolution of the two eruptive episodes of this Complex are analysed. The first one occurred as an explosive eruption at 9 Ma and is represented by a strongly welded, fiamme-rich, andesitic to dacitic ignimbrite deposit. The second commenced with an eruption of a rhyolitic ignimbrite at 7.6 Ma followed by effusive discharge of hybrid lavas at 7.3 Ma and by emplacement of andesitic to rhyodacitic dykes and domes. Both explosive and effusive eruptions of the second episode occurred within a short time span, but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber. Our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber, pressurising the magmatic system and triggering explosive eruption of rhyolitic magma. Chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition, which could have been strong enough to inhibit the interaction between the two magmas. After the initial explosive stages of the eruption at 7.6 Ma, the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation. Restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber. At 7.3 Ma, increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera. Assimilation during turbulent ascent (ATA) is invoked to explain a conspicuous reversed isotopic signature (87Sr/86Sr and 143Nd/144Nd) in the entire volcanic series. Therefore, the 7.6 to 7.3 Ma volcanic rocks of the NMVC resulted from synchronous and mutually interacting petrological processes such as recharge, fractional crystallization, hybridisation, and Assimilation during Turbulent Ascent (ATA).Geochemical characteristics of both volcanic episodes show diverse type and/or depth in the sources and variable influence of upper crustal processes, and indicate a recurrence in the magma-forming conditions. Similarly, other minor volcanic centres in the transversal volcanic belts of the Central Andes repeated their geochemical signatures throughout the Miocene. 相似文献
12.
This paper examines the role of the position and orientation of a regional fault in the roof of a magma chamber on stress
distribution, mechanical failure, and dyking using 2D finite element numerical simulations. The study pertains to the magma
chamber behavior in the relatively short time intervals of several hundreds to thousand of years. The magma chamber is represented
as an elliptical inclusion (eccentricity, a/b = 0.12) at a relative depth, H/a, of 0.9. The fault has a 45° dip and is represented by a frictionless fracture. The temperature field in the host rock is
calculated assuming a quasisteady-state thermal regime that develops through periodic episodes of magma supply. The rheology
of the surrounding rocks is treated using viscoelasticity with temperature activated strain-rate dependent viscosity. Strain
weakening of the rocks in the ductile zone is described within the frame of the Dynamic Power Law model . The magma pressure
is coupled with the deformation of the rock mass hosting the chamber, including the fault. The variation of magma pressure
in response to magma supply and chamber deformation is calculated in the elastic and viscoelastic regimes. The latter corresponds
to slow filling, while the former represents a filling time much less than the viscous relaxation time scale. The resulting
“equation of state” for the magma chamber couples the magma pressure with the chamber volume in the elastic regime, and with
the filling rate for the viscoelastic regime. Analysis of stresses is used to predict dyke propagation conditions, and the
mechanical failure of the chamber roof for different fault positions and magma overpressures. Results show that an outward
dipping fault located on the periphery of the chamber roof hinders the propagation of dykes to the surface, causing magma
to accumulate under the footwall of the fault. At high to moderate overpressures (30–40 MPa), the fault causes localized shear
failure and chamber roof collapse that might lead to the first stage of a caldera-forming eruption. 相似文献
13.
Eruptions from the top of a dyke containing two layers of magma can selectively withdraw the upper layer, leaving the dense lower layer undisturbed. Alternatively, if the upper layer is thinner than some critical depth, d, then both layers will be tapped simultaneously. Laboratory experiments yield an equation giving the draw-up depth, d, as a function of dyke geometry, eruption rate, and magma properties. This equation is valid for low to moderate Reynolds numbers and applies to dykes which are much longer than the draw-up depth. Short dykes will yield larger draw-up depths than are predicted by the equation. A large draw-up depth is favoured when the eruption rate, upper layer magma viscosity, or dyke length/breadth ratio is large or the density difference is small. Calculations show that rhyolite-capped dykes can contain several hundred metres thickness of rhyolite when a lower layer is first tapped. Draw-up depths in a dyke are as much as an order of magnitude greater than those for an identical eruption from a large cylindrical chamber tapped by a central vent. Nonetheless, for low effusion rate eruptions from small dykes, as at Inyo Domes, California, relatively small draw-up heights are calculated (e.g. 70 m). This is compatible with the small amounts of mixed magmas found at the transition between the two rhyolite magmas erupted there [11]. 相似文献
14.
The occurrence of micron to millimeter size globular heterogeneities in igneous rocks is frequently explained by processes of liquid immiscibility. However, such textures have also been documented in miscible magmatic pairs. In this study, the ability of miscible magmas to develop transient surface tensions and mimic the behavior of immiscible liquids is tested for the whole spectrum of magmatic compositions. We implemented a numerical model that includes the effect of gradient stresses (namely Korteweg stress) in order to investigate the role of such stresses in the evolution of diffusive interfaces. The results show that an initially elongated heterogeneity surrounded by a miscible and compositionally diverse magma will tend to minimize its contact surface by relaxing to a spherical shape, advected by a Korteweg stress driven flow. If the initial aspect ratio of the heterogeneity exceeds a critical value, surface minimization may be achieved by drop breakup. In addition, it is shown that two neighboring heterogeneities may coalesce to a single spherical drop. These results imply that even for fully miscible magmas, rheological barriers may prevent efficient mechanical intermingling and induce the formation of small-scale globular textures, analogous to those commonly observed in immiscible liquids. A better understanding of the role of Korteweg stress may be of the utmost importance for deciphering the textures generated by the interaction of compositionally diverse magmas. 相似文献
15.
Calc-alkaline intermediate rocks are spatially and temporally associated with high-Mg andesites (HMAs, Mg#>60) in Middle Miocene Setouchi volcanic belt. The calc-alkaline rocks are characterized by higher Mg# (strongly calc-alkaline trend) than ordinary calc-alkaline rocks at equivalent silica contents. Phenocrysts in the intermediate rocks have petrographical features such as: (1) coexisting reversely and normally zoned orthopyroxene phenocrysts in single rock; (2) sieve type plagioclase in which cores are mantled by higher An%, melt inclusion-rich zone; and (3) reversely zoned amphibole phenocrysts with opacite cores. In addition, mingling textures and magmatic inclusions were observed in some rocks. These petrographic features and the mineral chemistry indicate that magma mixing was the most important process in producing the strongly calc-alkaline rocks. The core composition of normally zoned orthopyroxene phenocrysts and the mantle composition of reversely zoned orthopyroxene phenocrysts have relatively high Mg# (85–90) in maximum. Although basaltic and high-Mg andesitic magmas are candidate as possible mafic end-member magmas, basaltic magma is excluded in terms of phenocryst assemblage and bulk composition. HMA magmas are suitable mafic end-member magmas that precipitated high Mg# (90) orthopyroxene, whereas andesitic to dacitic magma are suitable felsic end-members. In contrast, it is difficult to produce the strongly calc-alkaline trend through fractional crystallization from a HMA magma, because it would require removal of plagioclase together with mafic minerals from the early stage of crystallization, whereas the precipitation of plagiolase is suppressed due to the high water content of HMA magmas. These results imply that Archean Mg#-rich TTGs (>45–55), which are an analog of the strongly calc-alkaline rocks in terms of chemistry and magma genesis, can be derived from magma mixing in which a HMA magma is the mafic end-member magma, rather than by fractional crystallization from a HMA magma. 相似文献
16.
We propose that the fluid mechanics of magma chamber replenishment leads to a novel process whereby silicic magmas can acquire an important part of their chemical signatures. When flows of basaltic magma enter silicic magma chambers, they assume a ‘fingered' morphology that creates a large surface area of contact between the two magmas. This large surface area provides an opportunity for significant chemical exchange between the magmas by diffusion that is enhanced by continuous flow of silicic liquid traversing the basalt through thin veins. A quantitative analysis shows that a basaltic magma may thereby impart its trace-element and isotopic characteristics to a silicic magma. Depending on concentration differences and diffusion coefficients for the given components, this new mechanism may be as important as crystal fractionation and assimilation in producing the compositional diversity of silicic magmas. It may explain concentration gradients in silicic ash-flow tuffs and should be considered when interpreting the isotopic signatures of silicic rocks, even in the overt absence of mixing. For example, we show that, for several well studied, compositionally graded ash-flow tuffs, the concentrations and isotopic ratios of important geochemical tracers such as strontium could be largely due to this flow-enhanced diffusion process. 相似文献
17.
Ken Thomson 《Bulletin of Volcanology》2007,70(2):183-201
Three-dimensional seismic data from the Faeroe-Shetland Basin provides detailed information on the relationships between sills,
dykes, laccoliths and contemporaneous volcanic activity. The data shows that sills are predominantly concave upwards, being
complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat
outer rim by a steeply inclined sheet. Such morphologies are only partially modified by pre-existing faults. Sills can be
sourced from dykes or the steep climbing portions of deeper sills. Both sills and dykes can provide magma to overlying volcanic
fissures and sills can be shown to feed shallow laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest
that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves
emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to volcanic fissures, other sills and
laccoliths, the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and
level of sill emplacement. Instead, the data suggests that the presence of lithological contrasts, particularly ductile horizons
such as overpressured shales may permit sill formation at any level below the neutrally buoyant level.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Ken Thomson–deceased, April 2007 相似文献
18.
Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana 总被引：1，自引：0，他引：1
C. Aubourg G. Tshoso B. Le Gall H. Bertrand J.-J. Tiercelin A.B. Kampunzu J. Dyment M. Modisi 《Journal of Volcanology and Geothermal Research》2008,170(3-4):247-261
To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the west. 相似文献
19.
《Journal of Volcanology and Geothermal Research》1999,91(1):65-78
The tectonic stresses can significantly affect the propagation of a magma-filled crack. It has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress. However, it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states. We have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow. The stress profiles were estimated by a simple model of lithospheric deformation. We employed a three-layer model of the lithosphere; the upper crust, the lower crust and the upper mantle have different rheological properties. A constant horizontal force is applied to the lithosphere, and the horizontal strain is assumed to be independent of depth. When realistic tectonic forces (>1011 N/m) are applied, the rheological boundaries mainly control the emplacement of magma. The emplacement is expected at the MOHO, the upper–lower crust boundary, and the brittle–ductile boundary. For lower tectonic forces (<1011 N/m), the tectonic stress no longer plays an important role in the emplacement of magmas. When the tectonic stress controls the emplacement, the roles of rheological boundaries strongly depend on the surface heat flow. When the surface heat flow is relatively high (>80 mW/m2), the stress in the mantle is quite low and the MOHO cannot trap ascending magmas. For relatively low heat flow (<80 mW/m2), on the other hand, the MOHO acts as a magma trap, and the upper–lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high. Our results suggest that the emplacement depth can change responding to the change in the tectonic force and/or that in the surface heat flow. This may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and/or thermal history. 相似文献
20.
Crystal fractionation of granitic magma during its non-transport processes: A physics-based perspective 总被引：2，自引：0，他引：2
Chen Chen Xing Ding Rui Li WeiQi Zhang DongJian Ouyang Lei Yang WeiDong Sun 《中国科学:地球科学(英文版)》2018,61(2):190-204
Granitic continental crust distinguishes the Earth from other planets in the Solar System. Consequently, for understanding terrestrial continent development, it is of great significance to investigate the formation and evolution of granite.Crystal fractionation is one of principal magma evolution mechanisms. Nevertheless, it is controversial whether crystal fractionation can effectively proceed in felsic magma systems because of the high viscosity and non-Newtonian behavior associated with granitic magmas. In this paper, we focus on the physical processes and evaluate the role of crystal fractionation in the evolution of granitic magmas during non-transport processes, i.e., in magma chambers and after emplacement. Based on physical calculations and analyses, we suggest that general mineral particles can settle only at tiny speed(~10~(-9)–10~(-7) m s~(-1))in a granitic magma body due to high viscosity of the magma; however, the cumulating can be interrupted with convection in magma chambers, and the components of magma chambers will tend to be homogeneous. Magma convection ceases once the magma chamber develops into a mush(crystallinity, F~40–50%). The interstitial melts can be extracted by hindered settling and compaction, accumulating gradually and forming a highly silicic melt layer. The high silica melts can further evolve into high-silica granite or high-silica rhyolite. At various crystallinities, multiple rejuvenation of the mush and the following magma intrusion may generate a granite complex with various components. While one special type of granites, represented by the South China lithium-and fluoride-rich granite, has lower viscosity and solidus relative to general granitic magmas, and may form vertical zonation in mineral-assemblage and composition through crystal fractionation. Similar fabrics in general intrusions that show various components on small lengthscales are not the result of gravitational settling. Rather, the flowage differentiation may play a key role. In general, granitic magma can undergo effective crystal fractionation; high-silica granite and volcanics with highly fractionated characteristics may be the products of crystal fractionation of felsic magmas, and many granitoids may be cumulates. 相似文献