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1.
Fluid inclusion studies together with volcanological and petrochemical data allow reconstruction of the magma feeding system of basaltic-andesitic to andesitic activity during the oldest and intermediate stages of development of Lipari Island (223–81 ka). A major magma storage zone is active during the overall investigated time span at depths of 22 km, close to the crust-mantle Moho transition, at which mantle-derived mafic magmas tend to accumulate due to neutral buoyancy conditions. Beneath central-type volcanoes (M. Mazzacaruso, M. S.Angelo, M. Chirica-Costa d’Agosto), a shallower magma reservoir is located within the upper crust at 5.5–3.5 km, associated with a major lithological discontinuity. For fissural-type volcanoes (Timpone Ospedale, Monterosa, M. Chirica), tectonic structures are suggested to influence further magma ascent and storage at mid-crustal depths (∼14 km), with no ponding at shallower levels. Partial crustal melting processes at the roofs of the deep magma reservoirs (∼17 km) are invoked to explain the origin of cordierite-bearing lavas beneath M. S.Angelo and M. Chirica-Costa d’Agosto volcanoes, which were active during the intermediate stages of development of Lipari (105–81 ka). The generation of felsic anatectic melts in the lower crust could have created density and rheologic barriers to impede the passage of mafic melts and promote their ponding, with influence on the subsequent evolution of Lipari volcano.  相似文献   

2.
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   

3.
Peralkaline welded tuffs from the islands of Gran Canaria, Canary Islands, and Pantelleria, Italy, show abundant evidence for post-depositional flow. It is demonstrated that rheomorphism, or secondary mass flowage, can occur in welded tuffs of ignimbrite and air-fall origin. The presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process. Deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding. Internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets. It is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes, provided that the apparent viscosity of the magma is sufficiently low. Compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type.  相似文献   

4.
The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.  相似文献   

5.
This paper documents a complex sequence of interbedded lapilli-fall, base-surge, and pyroclastic-flow deposits, here named the Monte Guardia sequence, that erupted from volcanic centers in the southern part of Lipari (Aeolian Island Arc). Radiocarbon data from ash-flow tuffs above and below this sequence bracket its eruption between 22,600 and 16,800 years ago. Geologic evidence, however, suggests that this single eruptive cycle had a more restricted duration of years to tens-of-years. The basis for our interpretations comes from data measured at 38 detailed sections located throughout the island. The Monte Guardia sequence rests on a series of lower rhyolitic endogenous domes in the southern part of Lipari and it covers the oldest lavas, lahars, and pyroclastic flows in the north. Only in the northeast part of the island is it covered by younger deposits which there consist of lapilli tuffs and lavas of the Monte Pilato rhyolitic cycle. The deposit ranges in thickness from more than 60 m surrounding the vents in the south to less than a few decimeters at 10 km distance in the north. Throughout most of the island the Monte Guardia sequence overlies a thin andesitic lapilli-fall layer which is a key bed for correlation. This lapilli tuff probably erupted from a volcanic center on another island of the Aeolian Arc (possibly Salina). The principal activity of the Monte Guardia sequence started with an explosion that formed a continuous breccia blanket covering most of the island. Some pumiceous blocks within this breccia are composed of alternating bands of acidic and andesitic composition suggesting that the initiation of pyroclastic activity could have been triggered by magma mixing. Typical Monte Guardia sequence consists of explosive products that grade from magmatic (pumice-fall) to phreatomagmatic (base-surge) character. The eruptive cycle is characterized by a number of energy decreasing megarhythms that start with a lapilli-fall bed and end with a base-surge set that progresses through sand-wave, massive, and planar beds. Isopach maps of the fall and surge deposits indicate that both types were directed to the northwest by prevailing winds. Existing topographic relief was an additional factor that affected the emplacement of surge products. At the end of the cycle andesitic pyroclastic flows and rhyolitic endogenous domes were emplaced above the Monte Guardia deposits near the vent.  相似文献   

6.
Thirteen pumice samples from the D and E ignimbrite units of Kalymnos Tuff have been analyzed for their biotite and feldspar phenocryst mineral chemistry and for bulk major and 20 trace, including 14 Rare Earth elements, to define and compare their petrochemistry with the Kos Plateau Tuff (KPT). For the same purpose major element analyses were obtained from Kalymnos Tuff and KPT glasses. Both KPT and Kalymnos pumice lapilli are rhyolites characterized by a well-developed ‘silky’ texture and roundish quartz. Phenocrysts of biotite and feldspars (sanidine, oligoclase) from both tuffs display compositional overlap. Crystals are charac-terized by undulatory extinction (quartz), fractures (sanidine, oligoclase) and bent cleavages (biotite) due to the explosive origin of their host. Both tuffs show well-defined petrogenetic trends and extensive compositional overlaps on major and trace element variation diagrams suggesting that they are consanguineous. However, D ignimbrite samples are more evolved than those obtained from E ignimbrite as indicated from major elements, alkali earths (Ba, Rb, Sr), immobile (Zr, Y), compatible (V) and hygromagmatophile trace element (Th) distributions. This evidence indicates a stratified magma chamber under a ～16 Km caldera superstructure which is mostly submarine.  相似文献   

7.
The recent finding of mafic enclaves in the Rocche Rosse (RR) lava flow, the last magmatic product on Lipari (Aeolian Islands, Italy) (AD 1230 ± 40), opens the possibility to investigate in detail the most recent magmatic system of the island, an important issue for the volcanic hazard assessment of the area. The RR lava flow is an aphyric rhyolitic coulée consisting of grey and black pumice and black and grey obsidian. Enclaves have ellipsoidal to spheroidal shape and vary from mm-sized in the central portion of the flow, to cm-sized, at the top and in the flow front, where they are also more abundant. Enclaves are shoshonitic-latitic (group A) and trachytic (group B) in composition. The mineralogy of group A consists of dominant clinopyroxene crystals with minor abundance of feldspar (plagioclase > K-feldspar), olivine and biotite, while group B is composed of feldspar (K-feldspar > plagioclase) with minor clinopyroxene, olivine and biotite. Geochemical modeling suggests that the host rhyolitic rocks could be the product of AFC (Assimilation plus Fractional Crystallization) of a magma compositionally similar to the associated shoshonitic-latitic enclaves, which, in turn, could be obtained, through an AFC process, from the primitive melts erupted as olivine hosted melt inclusions during the last 15 ka at Vulcano. The already-known last 42 ka relationship between Lipari and Vulcano Islands is here reinforced until historical time, especially for the last 1 ka. The geochemical and petrological overlap between Lipari and Vulcano is interpreted to reflect the existence of a similar magmatic system underneath the two islands. The nearly aphyric RR rhyolites are interpreted to be the products of a superheated (temperature far above the liquidus) and initially water-undersaturated magma that underwent degassing close to the surface inhibiting microlite crystallization.  相似文献   

8.
Pillow talk     
Three distinct types of pillows and pillow lava sequences with different modes of origin have been recognized in the extrusive sequences comprising the upper parts of ophiolite complexes that represent the mafic portion of the floor of an Early Cretaceous back-arc basin in southern Chile. One type of pillow formed by non-explosive submarine effusion. A second type formed by magmatic intrusion into pre-existing aquagene tuff formed by explosive eruption. The third type of pillow occurs within dikes, forming pillowed dikes, possibly as a result of vapor streaming within a cooling dike. Where studied in southern Chile, aquagene tuffs and intrusive pillows decrease and water-lain pillows increase in relative abundance from north to south. This variation corresponds with a north-to-south decrease in both the relative volume of extrusives to extensional dikes and the range and volume of differentiated rocks, suggesting a southward increase in rate of extension relative to rate of magma supply within the spreading ridges at which the ophiolites formed. In the northern part of the original basin where the rate of extension was small relative to the rate of magma supply, magma remained in magma chambers longer, resulting in a greater range and volume of differentiated rocks. The larger volume of more differentiated, cooler and more viscous magmas, in conjunction with the likelihood of more violent eruption of volatile-rich differentiates, may have been responsible for the large volume of aquagene tuff in the northern part of the original basin. These observations in southern Chile suggest that ophiolites which contain a great abundance of aquagene tuffs and intrusive pillow lavas formed in tectonic environments in which the rate of extension was small relative to the rate of magma supply (island arcs, embryonic marginal basins). Ophiolites with predominantly water-lain pillowed and massive lavas formed in tectonic environments in which the rate of extension was large relative to the rate of magma supply (mid-ocean ridges, mature back-arc basins). Thus geologic field data may supplement geochemical data as a tool in distinguishing the original igneous-tectonic environments in which ophiolites originate.  相似文献   

9.
Welded tuffs and related pyroclastic deposits are distributed at many localities in northeastern Japan, especially around the volcanoes of the Nasu volcanic zone running from north to south, but they are absent from the region along the Japan Sea. Their geological age varies from the Miocene to the Holocene, those of the Pleistocene being predominant in amount. Petrographically they cover rather a wide range from andesite to rhyolite, among which dacite is most common. The welded tuffs are always compact and hard, with well-developed columnar jointing, carrying parallel-layered obsidian lenticules; and various stages are observed from loose pyroclastic deposits to lava-like welded tuffs. Petrological, petrochemical, and physical properties of these deposits are studied in some detail. From these data some genetic consideration is given for the mechanism of welding, and also for the relation between the nature of parental magma and the formation of such pyroclastic deposits.  相似文献   

10.
Bedded tuffs in diatremes and related volcanic vents have been reported from a wide range of localities. Most of the stratified pyroclastic material is interpreted as subaerial in origin, although local, thin deposits are clearly subaqueous and apparently were laid down in crater lakes. Bedded tuff fillings of pipes commonly attain thicknesses of 300–500 m, and some estimates in excess of 1200 m appear to be reliable. Most explanations for the presence of bedded tuffs at considerable depths in pipes involve a cauldron or caldera-like subsidence mechanism that relies heavily upon withdrawal of magma from lower pipe regions. Compaction and marginal slumping of ejecta may be important factors in subsidence, but are probably significant as a « mechanism » only near the surface. Based on experimental studies, a fluidization mechanism of subsidence is proposed as a viable explanation for the development of at least some bedded pipe fillings, particularly those that occur at greatest depths. Bedded materials may subside in a quiescent bed fluidized state when upward gas flow is less than free fall velocity of pyroclastic particles. A similar volume of material is simultaneously carried upward by higher velocity gases through the central portion of the vent and erupted at the surface to contribute more air-fall ash to developing surface beds.  相似文献   

11.
The Latera caldera is a well-exposed volcano where more than 8 km3 of mafic silica-undersaturated potassic lavas, scoria and felsic ignimbrites were emplaced between 380 and 150 ka. Isotopic ages obtained by 40Ar/39Ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera. The initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite. They were closely followed by eruption of evolved phonolitic magma. After roughly 25 ky, several phonolitic ignimbrites were deposited, and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash-flow units at 191 ka. Compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs, tephriphonolitic spatter, and scoria-bearing ignimbrites. After 40 ky of mafic surge deposit and scoria cone development around the caldera rim, a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the Latera caldera. This activity marks the end of large-scale explosive eruptions from the Latera volcano at 156 ka.  相似文献   

12.
Brown Tuffs (BT) are volcaniclastic ash deposits prominently represented in the stratigraphic profiles of all the Aeolian Islands (and Capo Milazzo on the northern coast of Sicily). Detailed stratigraphy and tephrochronology together with available radiometric ages suggest that they were emplaced over a long time interval spanning from the end of the last interglacial period (ca. 80 ka BP) up to 4–5 ka BP (age of the overlying Punte Nere pyroclastic products on Vulcano). The most complete BT succession is documented on Lipari where 14 distinct and successive units are subdivided by the interbedding of widespread tephra layers, local volcanic products, paleosols and epiclastic deposits and the occurrence of local erosive surfaces. Inter-island occurrence of Ischia-Tephra (a widely known tephra layer in the Aeolian archipelago dated at 56 ka BP) and Monte Guardia pyroclastics from Lipari (dated at 22–20 ka BP) subdivides the BT succession in Upper (UBT), Intermediate (IBT) and Lower BT units (LBT), which can be correlated at regional level: the LBT was emplaced between 80 and 56 ka BP, the IBT between 56 and 22 ka BP and the UBT between 20 and 4–5 ka BP. On the basis of stratigraphy, similarity in lithology and textural features, morphology of glass fragments, composition and consistency of thickness and grain-size variations, UBT units correlate with Piano Grotte dei Rossi tuffs on Vulcano island. They were generated by pulsating hydromagmatic explosive activity giving rise to pyroclastic density currents spreading laterally from a source located inside the La Fossa caldera on Vulcano island. Composition is in agreement with this hypothesis since UBT compositional features match those of Vulcano magmas erupted in that period. The effect of co-ignimbrite ash clouds (or associated fallout processes from sustained eruptive columns) is seen to explain the presence of UBT in areas further away from the suggested source (e.g. Salina and Lipari islands and Capo Milazzo). The origin of UBT exposed on Panarea island is still a matter of debate, due to contrasting compositional data. Due to large uniformity of lithological, textural and componentry characters with respect to the UBT, the lower portions of the BT succession (LBT-IBT) are considered to be the result of recurrent, large scale hydromagmatic eruptions of similar type. Moreover, for the IBT units, the correlation with Monte Molineddo 3 pyroclastics of Vulcano island (on the basis of lithological, compositional and stratigraphic matching) again suggests source(s) related to the Vulcano plumbing system and located within the La Fossa Caldera.  相似文献   

13.
Distinguishing strongly rheomorphic tuffs from extensive silicic lavas   总被引：2，自引：6，他引：2  
High-temperature silicic volcanic rocks, including strongly rheomorphic tuffs and extensive silicic lavas, have recently been recognized to be abundant in the geologic record. However, their mechanisms of eruption and emplacement are still controversial, and traditional criteria used to distinguish conventional ash-flow tuffs from silicic lavas largely fail to distinguish the high-temperature versions. We suggest the following criteria, ordered in decreasing ease of identification, to distinguish strongly rheomorphic tuffs from extensive silicic lavas: (1) the character of basal deposits; (2) the nature of distal parts of flows; (3) the relationship of units to pre-existing topography; and (4) the type of source. As a result of quenching against the ground, basal deposits best preserve primary features, can be observed in single outcrops, and do not require knowing the full extent of a unit. Lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix; such deposits are unique to lavas. Because the chilled base of an ashflow tuff generally does not participate in secondary flow, primary pyroclastic features are best preserved there. Massive, flow-banded bases are more consistent with a lava than a pyroclastic origin. Lavas are thick to their margins and have steep, abrupt flow fronts. Ashflow tuffs thin to no more than a few meters at their distal ends, where they generally do not show any secondary flow features. Lavas are stopped by topographic barriers unless the flow is much thicker than the barrier. Ash-flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit. Lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents. Tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits. These criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava-like ignimbrite. The latter have characteristics of lavas except for great areal extents, up to many tens of kilometers. These rocks have been interpreted as ash-flow tuffs that formed from low, boiling-over eruption columns, based almost entirely on their great extents and the belief that silicic lavas could not flow such distances. However, we interpret the best known examples of lava-like ignimbrites to be lavas. This interpretation should be tested through additional documentation of their characteristics and research on the boiling-over eruption mechanism and the kinds of deposits it can produce. Flow bands, flow folds, ramps, elongate vesicles, and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic. Pumice and shards also occur in both tuffs and lavas, although they occur throughout ash-flow tuffs and generally only in marginal breccias of lavas. Dense welding, secondary flow, and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick, rheomorphic tuffs and thick lavas. High-temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts. Extensive silicic lavas could be appropriately termed flood rhyolites.  相似文献   

14.
Historical volcanic rocks of the Aeolian islands range in composition from shoshonitic basalts to rhyolites, which might reflect fractional crystallization of a shoshonitic parent magma. However Sr and Pb isotopic data indicate a more complex history. The shoshonitic basalts at present erupted at Stromboli, although chemically similar to the postulated parent magma, are genetically unrelated to the other studied rocks. Sr isotopes indicate that Vulcano, Vulcanello and Lipari had independent magma sources. It is proposed that crustal contamination raised the Sr isotopic composition of the Lipari rhyolites. The rocks of these island are related by a common very steep trend of207Pb/204Pbvs. 206Pb/204Pb. Such a trend is a common feature of orogenic magmas and shows that Pb was derived by mixing of at least two components. Presently it is impossible to constrain precisely either the timing or the physical meaning of the Pb end members. The Pb isotopic trend in the Eolian island is very distinct from those recorded in volcanic rocks both from behind the arc (Etna, Iblean Mts.) and from Central and Southern Italy.  相似文献   

15.
Detailed stratigraphic analysis of the Green Tuff of Pantelleria shows that this formation can be divided into several members designateda throughh from base to top. These members have a coherent pattern when traced from outcrop to outcrop throughout the island shedding light on their origin. Only memberg completely mantles the entire island. The distribution of the other members is controlled by prevailing wind direction or by topography. Membera is entirely of fall origin. Membersc ande are of fall and/or surge type. Membersb,d, andh have the characteristics of thin welded ash-flow tuffs. Membersf andg are ash-flow tuffs with textural characteristics of compound cooling units. Most of the ash-flow tuffs exhibit characteristics of ignimbrites: vertical fluidization pipes, local concentrations of lithic lapilli, imbrication of clasts, and valley ponding. Memberg is unusual in that it is highly-welded, exhibits large-scale rheomorphic structures, contains huge lithic clasts, and has near-vertical foliation where it adheres to cliffs and caldera walls.Granulometric data from the members identified in the field as ignimbrites confirms this conclusion, as do density profiles through the various members.  相似文献   

16.
Petrological studies of 12 volcanic rock units in the northeast segment of the Taum Sauk Caldera, the major structural feature in the western part of the St. Francois Mountains, indicate that they were probably derived from the same magma chamber. These calc-alkalic rocks become progressively silica and alkali rich and calcium poor from the base to the top of the stratigraphic column. In the part of the northeast segment of the caldera studied in detail, the extrusives are over 5 thick and have a volume of over 500 km3. Rock units consisting of ash-flow tuffs, bedded airfall tuffs and lava flows were apparently deposited within a single episode of volcanic activity, since no signs of extensive erosion were observed among them. Although the rocks are completely devitrified, the preservation of pyroclastic and flow features is excellent. These volcanics are exposed representatives of a 1.3–1.4 b.y. old belt of volcanics and associated plutons which extends from southern Ohio to the Texas Panhandle any may represent a belt of continental accretion.  相似文献   

17.
Extreme fractionation of minor and trace elements commonly accompanies very modest changes in major element concentrations in highly felsic igneous sequences. In such sequences, Si increases by only a few percent while, for example, Sr, Ba, Mg, and light rare earth elements decrease drastically, commonly by a factor of 10 or more. It has been argued, most notably by Hildreth (e.g. [1]), that such trends observed in tuffs were not induced by fractional crystallization (FC), but rather are a manifestation of compositional gradients in parental magma chambers which form via liquid-state thermogravitational diffusion (LSTD). The strongest arguments against FC are that (1) crystal settling is not a viable mechanism for crystal-liquid separation, and (2) extensive recrystallization is required to produce the observed trends, yet the tuffs are relatively crystal-poor. Many workers have noted trends in plutonic as well as volcanic rocks which are strikingly similar to those for which LSTD has been proposed, and some have concluded that LSTD was the fractionating mechanism.Several lines of evidence lead us to the conclusion that FC is the dominant differentiating process in high-silica magmas: (1) elemental trends are strikingly consistent with those predicted for FC; it would be a remarkable coincidence if diffusion-induced trends mimicked FC so closely; (2) large phenocryst assemblages in high-silica tuffs indicate low-variance liquid compositions that would be improbable if crystal-liquid equilibria were not controlling differentiation; (3) highly evolved plutonic rocks in many cases do not form the caps expected for LSTD, but rather occur in dikes and pods where they apparently segregated as late liquids; (4) recent experimental studies suggest that trends induced by diffusion differ drastically from observed felsic igneous trends.We do not believe that the principal arguments against FC in high-silica systems (unlikelihood of crystal settling; crystal-poor nature of tuffs) refute the reality of the chemical process, but rather emphasize the need for a better understanding of the physical mechanisms of crystal-liquid fractionation and eruption.  相似文献   

18.
The Angareb Ring Dike Complex is located in Begemder Province in northwestern Ethiopia, 55 kilometers NNW of the provincial capital Gondar. The structure was first identified in 1973 from Landsat 1 satellite imagery analysis of the East African Rift System. The complex is exposed on the floor of the Tana Graben, a feature bearing structural affinity to the East African Rift. A post-early Miocene age is ascribed to the complex. Association with other as yet univestigated circular features may define a volcanic province of early continental rifting in Ethiopia. Contained within the ring structure is a heterogenous tuff breccia of over 100 cubic kilometers in volume, composed of massive agglomorates, ash flows, welded tuffs, and subaqueous tuffs. The tuff is interpreted as filling a subsiding caldron above an eviscerating magma chamber. Mineralogy and chemical analyses depict a fractionating basaltic magma of transitional-mildly alkaline composition. Partial melting of the Precambrian basement is envisioned to explain the subalkaline granites. Comparison of chemical data with regional petrochemical interpretations suggests that the complex represents the beginning of central vent activity in the waning stages of Tertiary volcanism in Ethiopia.  相似文献   

19.
Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km3) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates.The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km3. It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO2) is poorly correlated with phenocryst abundance.These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit.The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.  相似文献   

20.
An attempt is made to reproduce by numerical simulation the last 50,000 years of the Phlegraean Fields volcanic history in terms of a magma chamber with volume progressively reduced by magma extraction to the surface and without refillings from depth. Since the aim is just to verify the plausibility of such an assumption, attention is focused on the major volcanic events, and a rather crude model is adopted. It turns out that the main features of the Phlegraean Fields thermal history, namely the Campanian Ignimbrite, the yellow tuffs emission and the high temperatures measured in the geothermal wells drilled inside the caldera, can reasonably be reproduced under the not-refilled-system assumption. The magmatic body is predicted to have an average temperature of 1000°C at present.  相似文献   

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