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1.
Ceboruco is a major composite volcano at the western end of the Mexican Volcanic Belt, near the junction between the North American and Pacific plates. The volcano is built from successive eruptions of andesite lavas and pyroclastic rocks, and major eruptions during its history have resulted in the formation of two concentric calderas. The youngest volcanic activity has included the extrusion of dacites within the inner caldera and a voluminous flank eruption of andesite during 1870–72. Fumarolic activity persists to the present day. Chemical analyses show that the lavas are of cale-alkaline type and rangs from andesite (SiO2=58–61%) to acid dacite (SiO2=68%) in composition. The rate of increase of K2O relative to SiO is greater than that in volcanic rocks from the Mexican Volcanic Belt as a whole. This indicates that simple models based on the application of such relationships may not be adequate to explain the petrogenesis of calc-alkaline lavas.  相似文献   

2.
Studies of ERT Satellite photographic documents and of acrial photographs with complementary lield work reveal the presence of recent very large nuée ardente deposits north-west of Socompa Volcano (Andean Cordillera of Atacama, northern Chile). Three zones are distinguishable from the bottom of Socompa Volcano to the front of the nuée ardente deposits: 1) pumice blocks are covered with parallel ridges of debris (lava blocks) from the north-western flank of Socompa Volcano, 2) pumice blocks lie upon small cones and flows from El Negrillar volcanoes located inside the graben of Negros de Aras, 3) pumice flow threads its way between cones and flows from El Negrillar volcanoes and stops more than 40 km away from the base of Socompa Volcano. The calculated thermal energy of this cruption is 7.9 × 1025 ergs, being in the range of of the most important recorded eruptions on earth. The pumice is almost aphyric (rare plagioclase, hypersthene and hornblende phenocrysts) and is of dacite composition lying pertectly on the K2O-SiO2 trend of the Socompa Volcano. Trace and major element data of the pumice are similar to those of two dacites from a pre-nuée lava flow and a post-nuée lava dome of Socompa Volcano and support a common magmatic origin with the Socompa Volcano lavas. A relative chronology is proposed.  相似文献   

3.
Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO2 content averages 51.5 wt.%. The flows are high in Al2O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na2O (mean 2.7 wt.%), and K2O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO2 (about 54 wt.%) and Al2O3, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K2O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K2O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K2O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time.  相似文献   

4.
Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO2 contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An95 to An60 in the basalts and An60 to An50 in the andesites, but, within each group, they are not correlated with SiO2 or Na2O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO2 variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.  相似文献   

5.
The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km3. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy.  相似文献   

6.
The Tertiary and Recent volcanics of Mexico occur in two provinces. The Cordillera Province is made up of about 1700 m of ignimbrite sheets overlain and intercalated in the upper part by olivine basalt and basaltic andesite. The Rio Lerma Province extends transversely across Mexico and in the Valley of Mexico the lavas consist mainly of andesite and dacite, 68 % of those analysed having 62 | 4.7 % SiO2. A total of 108 chemical analyses were made for the major elements, 90 of these including determinations of Cr, Ni, Cu, Zn, Rb, Sr, Ba, Th and Pb for two areas, the Valley of Mexico in the Rio Lerma Province and the Guadalajara region which lies at the intersection of the two provinces. Computer constructions of normative components in the basalt tetrahedron and other projections support an origin of partial melting of tholeiitic to pyrolitic material for the production of andesite. The Guadalajara lavas have consistently higher K/SiO2 and K/Rb ratios and lower Mg/SiO2 ratio than the Valley of Mexico rocks suggesting generation at greater depth.  相似文献   

7.
Four-hundred and twenty-one analyses of quartz-normative, peralkaline, extrusive rocks have been collected from the literature and from unpublished sources and are used to examine chemical variation in this group of rocks. Comparisons are particularly made between the full body of data and the variations recorded in the non-hydrated obsidians alone byMacdonald andBailey (1973). It is argued that the compositions of the magmas which formed these obsidians and those which subsequently crystallised were similar as regards the major oxides SiO2, Al2O3, FeO + Fe2O3, Na2O and K2O. Marked variations in the abundances of the minor oxides CaO and TiO2 are shown to be a result of geographical location. Small but significant differences in the distribution of Al and Fe as a function of normative quartz can be recognised between various pantelleritic suites. A new classificatory scheme is proposed, based on the iron (as FeO) and Al2O3 contents. This is simpler than previously employed normative classifications, is more applicable to crystalline rocks, and, happily, in 95 % of cases gives the same rock name as the normative system.  相似文献   

8.
Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO2 content of ? 55%. The relation of SiO2 to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high PH2O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.  相似文献   

9.
The conductivity of four igneous rocks with, 49, 65, 77, and 84% SiO2 was measured as a function of temperature in the interval from 20° to 1280°C; measurements were made in a vacuum of 10?3 torr. No simple relationships were found between conductivity and SiO2 content or versus major element groupings such as Na2O=K2O=CaO and TiO2=Cr2O3=Al2O3=Fe2O3=FeO. An analytical expression was obtained between conductivity and the albite-quartz ratio, valid for temperatures between 300° and 1200°C. It was necessary to compute the CIPW norm in order to obtain the albite and quartz percentages. The onset of melting apparently occurred between 600° and 700°C. Petrography performed on two samples after cooling showed 70 and 85% partial melting. Three conduction regions were identified: 1) below 300°C, 2) between 300°C and 600°C, and 3) above 600°C. Different activation energies obtained for the heating and cooling intervals confirm that the sample undergoes textural changes in the heating-cooling cycle. Activation energy increments of 0.1 and 0.2 eV per decade of albite-quartz ratio were obtained.  相似文献   

10.
Volcanic rocks on the island of Filicudi are typical of the cale-alkaline association of the Aeolian archipelago, and range in composition from basalt to high-K andesite. All specimens contain abundant phenocrystic plagioclase (cores of An82–97, rims becoming progressively more sodic with increasing Differentiation Index), clinopyroxene, and magnetite; the most basic samples contain olivine, which is replaced by orthopyroxene at around D.I.=44. SiO2 generally increases with time, and this in conjunction with the continuity in mineralogy suggests that the lavas are the differentiation products of a single batch of magma. Least-squares mixing calculations are consistent with a shallow fractional model, as is the variation in trace element abundances.  相似文献   

11.
Pumiceous pyroclastic products are present as flows and falls at several stages of the evolution of the southern Guadeloupe volcanic island. An understanding of this volcanism had to rely on detailed petrochemical data of these products to complement similar data for effusive rocks so as to yield complete stratigraphical coverage. On the other hand most pumiceous rocks are more or less conspicuousily banded suggesting that mixing phenomena occurred to different degrees in their genesis. Three major classes of pumiceous products are found: (1) the Axial Chain deposits (2.0–0.5 My) are characterized by An90, 75-55 + En55 + Wo42En37 + Usp35-37 ± Fo68 ± Hble, SiO2 60%, SiO2/Th 35.6, and La/Th 3.9. Banded samples have components that differ in evolution indices by about 50%; (2) the Bouillante Chaine pyroclastics (0.3–0.1 My) consist of scattered deposits with variable mineralogical and geochemical compositions that seem to have erupted from a number of small eruptive centers. Qz-dacitic pumice is common with An90, 70-45 + En66-56 + Usp32-38 + Ilm94 + Hble ± Wo40-42En40-42, SiO2 62%, SiO2/Th 22.9, and La/Th >4. Mixed pumice samples have highly contrasted evolution indices differing by up to 120%; (3) the Pintade pumice flows and falls correspond to the major pyroclastic event (approx. 10 km3) in the southern Basse Terre area. They are characterized by An85-70 + En66-56 + Usp32-37 ± Wo42En42, SiO2/Th 18.7-22?6, and La/Th 3.0-4.0. Banded pumice lumps are scarce and show slight compositional contrasts; differences in evolution indices do not exceed 38%. Axial and Bouillante chain pyroclastics and Grande Découverte volcano pumice respectively, form two different families in trace element plots. Minor elements in pyroxenes also are distinctive. These trends are similar to those obtained for effusive rocks and define comagmatic series. Major and trace element data for the separated components of inhomogeneous pumice in each formation always plot in the corresponding series. These chemical discriminants can be used to attribute samples of unknown provenance to a given volcanic ensemble. An inverse relationship between differences in evolution indices in inhomegeneous pumice and the volume of any single eruptive sequence is noted. This is an indication that pumiceous pyroclastic rocks were erupted from a zoned magma chamber. We favor an interpretation where zonation is produced by influx of less envolved magma in superficial differentiated chambers which is a direct cause for eruption.  相似文献   

12.
The 2.1 b.y. old Rooiberg Felsite roofs and is intruded by the mafic layered rocks and granites of the Bushveld Complex. The felsite unit, which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300,000 km3, is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types. Volcanic rocks of more mafic composition occur towards the base of the sequence. The Rooiberg episode essentially terminated the volcanic activity in the Transvaal basin and heralded the emplacement of the Bushveld Complex. Despite the close spatial and temporal relationships between the Rooiberg Felsite and the Bushveld Complex, the precise nature of the petrogenetic link is obscure. Chemical analyses of felsite have been variously interpreted to suggest cyclic differentiation along a comagmatic trend or to demonstrate anomalous enrichment in SiO2. Several characteristics delineate the Rooiberg Felsite as a possibly unique occurrence of rhyolitic magmatism, notably the immense volume of the unit, the marked preponderance of lavas over pyroclastic types, and the unusually great thickness and lateral extent of the flows. The thesis that the Rooiberg Felsite represents a shock-produced, meteorite-impact melt cannot be supported on the available evidence.  相似文献   

13.
Preliminary data on major elements, Cs, Ba, Rb, Pb, Sr, REE, Y, Th, U, Zr, Ht, Sn, Nb, W, Mo, Cr, V, Sc, Ni, Co and Cu contents for eight samples coming from the Upper Cretaceous volcanic belt of the Pontic Chain (Northern Turkey) are reported. SiO, versus K2O relationship shows that the analyzed samples belong to the calc-alkaline and shoshonite series. The calc-alkaline rocks appear to represent two distinct magma types one close in composition to typical island are calc-alkaline magmas and one with high incompatible elements concentration and tractionated heavy REE patterns which suggest a genesis by partial melting at high pressure with a garnet bearing residue. Shoshonitic rocks show Na2O/K2O close to one, high incompatible elements concentration, and TiO2%. Al2O3%, Ni and Co contents, Ni/Co and V/Ni ratios and REE patterns similar to typical island are andesites which suggest for these rocks similar genetical processes as the island are calc-alkaline magmas.  相似文献   

14.
The Hilina Formation comprises the oldest sequence of lava flows and tuffs exposed on Kilauea Volcano. These rocks are only exposed in kipukas in younger Puna Formation lavas along cliffs on the south flank of Kilauea Volcano. Locally, tuffs and flows of the Pahala Formation separate the underlying Hilina Formation rocks rom the overlying Puna Formation rocks. Charcoal collected from the base of the Pahala Formation yielded a C14 age of 22.800±340 years B.P. which defines a minimum age for the Hilina Formation. Hilina Formation lavas crop out over a wide region and probably originated from the summit area and from both rift zones. The Hilina Formation contains both olivine-controlled and differentiated lavas (using the terminology ofWright, 1971). The olivine-controlled lavas of the Hilina Formation are distinguishable mineralogically and geochemically from younger olivine-controlled Kilauea lavas. The younger lavas generally contain discrete low-calcium pyroxene grains. greater glass contents, higher K2O/P2O5 ratios and lower total iron contents. Similar geochemical trends prevail for Manuna Loa lavas, and may typify the early lavas of Hawaiian shield volcanoes. Despite these similarities, the Hilina Formation (and all Kilauea) lavas have higher TiO2 and CaO, and lower SiO2 and Al2O3 contents than Mauna Loa Lavas. These differences have existed for over 30,000 years. Therefore, it is unlikely that the older lavas of Kilauea are compositionally similar to recent Mauna Loa lavas as was previously suggested. K2O, TiO2, Na2 and Zr contents of lavas from a stratigraphic sequence of Hilina Formation lavas are variable. These variations may be utilized to subdivide the sequence into geochemical groups. These groups are not magma batches. Rather, they represent lavas from batches whose compositions may have been modified by crystal fractionation and magma mixing.  相似文献   

15.
Agrigan is the tallest (965 m a.s.l.) and largest (44 km2) of the volcanoes of the northern Mariana Islands. Its slopes are asymmetric to the east; a small caldera (4 km2) dominates the interior. The volcanic edifice has been disrupted along three sets of faults: 1) exterior slump faults, 2) radial faults, and 3) interior faults related to caldera-collapse. The rocks of the volcano are characterized by porphyritic clinopyroxene-olivine-plagioclase basalts and subordinate andesites. Cumulate xenoliths composed of Fo81, An95 and diopside are common in the basalts. Development of the volcano began with 3–4 km of submarine growth. The earliest recognizable flows are the result of fissural Hawaiian- and Strombolian-type eruptions. These were followed by the eruption of more viscous lavas from above the present summit. Flank eruptions of basalt and andesite preceded voluminous outpourings of andesitic pyroclastics contemporaneous with caldera-collapse. Subsequent magmatic resurgence is localized along a N10E rift zone. Violent ejection of lapilli and ash occurred in 1917.  相似文献   

16.
A brief report is made of current laboratory investigations on phase relations among olivine, pyroxene, anorthite, magnetite, tridymite, liquid and gas in the system Mg2SiO4-CaAl2Si2O8-FeO-Fe2O2-SiO2 over a wide range of oxygen partial pressures. Courses of fractional crystallization under various conditions of oxygen partial pressure are depicted using an anorthite saturation diagram. Starting with a basalt-like composition in the system, fractional crystallization at a moderate oxygen partial pressure (10 atm.) results in an andesite-like residual liquid of composition 55 SiO2, 14 iron oxide, 6 MgO, 9 CaO, 16 Al2O3 at a temperature of 1155°C. With fractional crystallization in a closed system, the end liquid approaches the composition of 45 SiO2, 38 iron oxide, 6 CaO and 11 Al2O3, at a temperature of 1050°C and oxygen partial pressure of about 10?12 atm. The andesitic final liquid in this system would be expected to further differentiate toward dacitic and rhyolitic compositions if alkalies and water were present in the system. On the basis of these studies, the derivation of liquids of andesitic, dacitic or rhyolitic composition from primary basalts by fractional crystallization seems entirely possible if the oxygen partial pressure is maintained at a moderate or high level.  相似文献   

17.
The Queershan composite granitic pluton is located in the north of the late Paleozoic Yidun arc collision-orogenic belt, eastern Tibetan Plateau. The main rock types are coarse-grained porphyritic alkalic-monzonite granite with minor fine-grained porphyritic monzogranite and granodiorite distributed in the eastern and southwestern regions. Here we report their zircon U-Pb ages and geo- chemical data. The intrusive contact relations indicate that granodiorite was formed earlier than the alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma). These suggest that the Queershan composite granitic pluton was formed through three-stage magmatic events. The alkalic-monzonite granite(105.9±1.3 Ma) and monzogranite(102.6±1.1 Ma) are characterized by high SiO2(73.5%–77.7%), K2O+Na2O(6.9%–8.5%), Ga/Al ratios(2.6–3.4) and low Al2O3(11.8%–14.5%), CaO(0.25%–1.5%), MgO(0.18%–0.69%), negative Ba, Sr and Eu anomalies, showing A-type granite affinities. The granodiorite exhibits lower SiO2, P2O5 and K2O+Na2O contents, but higher Al2O3, CaO and MgO contents than alkalic-monzonite granite and monzogranite, showing I-type granite affinity. 176Hf/177 Hf ratios of the alkalic-monzonite granite and the monzogranite are 0.282692–0.282749 and 0.282685–0.282765, respectively, and with similar ?Hf(t) values(?0.56 to 1.43 and ?0.87 to 1.90 respectively). They also present similar TDM2 model ages(1.04–1.22 and 1.07–1.2 Ga respectively), indicating they may be sourced from a similar rock source, mostly like Kangding Complex. The homogeneity of the Hf isotopic compositions and the absence of the MMEs demonstrate that little depleted mantle materials have contributed to the source. We propose that the Mesoproterozoic crust materials of the Yangtze Craton exist beneath the Yidun arc terrane and support it was a dismembered part of the Yangtze Craton. The A-type granites of Queershan composite granitic pluton are most probably related to the closure of the Bangong-Nujiang Tethys ocean.  相似文献   

18.
Application of the silica geothermometer to over 70,000 non-thermal groundwaters from the United States has shown that there is a correlation between the average silica geotemperatures for a region (T SiO2 in °C) and the known regional heat flow (q in mW m?2) of the form: 1 $$TSiO_2 = mq + b$$ wherem andb are constants determined to be 0.67°C m2 mW?1 and 13.2°C respectively. The physical significance of ‘b’ is the mean annual air temperature. The slope ‘m’ is related to the minimum average depth to which the groundwaters may circulate. This minimum depth is estimated to be between 1.4 and 2.0 km depending on the rock type. A preliminary heat flow map based on equation (1) is presented using theT SiO2 for new estimates of regional heat flow where conventional data are lacking. Anomalously high localT SiO2 values indicate potential geothermal areas.  相似文献   

19.
Emuruangogolak is a Quaternary basalitrachyte volcano situated in the Suguta graben of the northern Kenva rift, and probably erupted last early in this century. Following the construction of an early trachytic shield volcano, two episodes of caldera collapse occurred. each preceded by explosive pvroclastic activity. Post-calelera volcanism consisted of alternating phases of basalt and trachyte eruption. The basic lavas are high-Ti ferrobasalts of a mildly alkaline ‘transitional’ composition and the trachytes are peralkaline and oversaturated. A distinct compositional bimodality exists and no rocks in the range 49–59°. SiO, have been found. Major and trace element analyses suggest that the trachytes are genetically related to the basalts. Associations of almost identical lavas occur in Ethiopia. Pantelleria and the Azores but with the presence of intermediate terms Fractional crystallization is the mechanism currently preferred to account for the origin of the trachytes. The ‘Daly gap’ may be a consequence of a crystallization process which limits the volume of intermediate magma available at any time. In addition, the physical properties and spatial distribution of the different magmas probably discriminate against the cruption of lavas of intermediate composition.  相似文献   

20.
The volcanic centre of Monte Seda Oro, N. W. Sardinia, representative of a Cenozoic calc-alkaline andesitic suite of rocks is composed of a variety of rocks ranging from high alumina basalts to dacites. The minerals of basaltic, andesitic and dacitic rocks show only limited variation in chemical composition. The geochemical data suggest that the various rock-types are related by a crystal-liquid fractionation. Least-squate numerical calculations, using major element data, support the derivation of andesites with SiO3 content ranging from 53.8 to 59.0% from basalts having about 48.7% of SiO2 by low pressure crystal fractionation of the phenocryst phases present in these rocks. However, the origin of dacites cannot be readily explained by this mechanism.  相似文献   

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