ISSN:0148-0227    

DOI:10.1029/JB086iB11p10131

年代:1981

数据来源: WILEY

摘要:

The Sierra La Primavera, near Guadalajara, Mexico, is a Late Pleistocene rhyolitic center consisting of lava flows and domes, ash flow tuff, air fall pumice, and caldera lake sediments. All eruptive units are high‐silica rhyolites, but systematic compositional differences correlate with age and eruptive mode. The earliest lavas erupted approximately 145,000 years ago and were followed approximately 95,000 years ago by the eruption of about 20 km3of magma as ash flows that form the Tala Tuff. The Tala Tuff is zoned from a mildly peralkaline first‐erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Nb, Sb, HREE, Hf, Ta, Pb, Th, and U to a metaluminous last‐erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. Collapse of the roof zone of the magma chamber led to the formation of a shallow 11‐km‐diameter caldera in which lake sediments began to collect. The earliest postcaldera lava, the south‐central dome, is nearly identical to the last‐erupted portion of the Tala Tuff, whereas the slightly younger north‐central dome is chemically transitional from the south‐central dome to later, more mafic, ring domes. This sequence of ash flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine‐grained ashy sediments. Approximately 75,000 years ago a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only 10–20 m of sediments as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 30,000 years ago. The lavas that erupted 75,000, 60,000, and 30,000 years ago became decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U with time. They represent successive eruption of the uppermost magma in the postcaldera magma chamber. Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine ≥ quartz ≫ ferrohedenbergite>fayalite>ilmenite ± titanomagnetite. Major element compositions of sanidine, clinopyroxene, and fayalite phenocrysts vary only slightly between eruptive groups, but the concentrations of many trace elements change by factors of 5–10. This is reflected in phenocryst/glass partition coefficients that differ by factors of up to 20 between successively erupted units. Because the major element compositions of the phenocrysts and the pressure, temperature, and ƒO2of the magmas were essentially constant, the large variations in partitioning behavior are thought to result from small changes in bulk composition of the melt. Crystal settling and incremental partial melting are by themselves incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post‐95,000‐year lavas. Rather, diffusional processes in the silicate liquid are thought to have been the dominant differentiation mechanisms. The zonation in the Tala Tuff is attributed to transport of trace metals as volatile complexes within a thermal and gravitational gradient in a volatile‐rich but water‐undersaturated magma. The evolution of the postcaldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced octahedral

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10137

年代:1981

数据来源: WILEY

摘要:

Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients inTand ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in bothTand phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle‐derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large‐scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic‐dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less‐silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal‐rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large‐scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle‐derived volatiles in

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10153

年代:1981

数据来源: WILEY

摘要:

Major and trace element contents have been measured for sanidine‐glass pairs separated from eight high‐SiO2rhyolite lavas from the Yellowstone volcanic field. Sanidine/glass partition coefficients for most elements based on these data are uniform to within the cumulative analytical errors. This uniformity reflects similarities in major element compositions and inferred physical conditions for the analyzed samples, which are considered representative of most Yellowstone rhyolites. Of particular note are very high partition coefficients for Ba (∼22), Sr (∼28), and Eu (∼9), relative to the sparse data for other rhyolites available in the literature. With the exception of Na, K, and Pb, which have sanidine/glass partition coefficients of approximately unity, all other elements determined are strongly fractionated into the melt phase relative to sanidine. These data may be used in modeling crystallization of sanidine from Yellowstone or other rhyolites or anatexis of crustal rocks containing sanidine (or K‐feldspar sensu lato), but only with due caution. In particular, the wide range in reported partition coefficients for those elements (Sr, Ba, Eu) strongly incorporated in sanidine suggests that compositional and physical characteristics exert significant influence on these values. Accordingly, applicable partition coefficients are poorly known for these elements in most systems. Care must be taken to select appropriate values or to measure these partition coefficients directly in specific suites of rocks. Difficulties with direct experimental studies in silicic magmas seemingly preclude systemization of element partitioning

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10193

年代:1981

数据来源: WILEY

摘要:

Stratigraphic relationships and tephrochronologic information for volcanic ash beds (4.0–0.1 m.y.) of the Western Interior allow the recognition of at least 68 different ash falls. The average reoccurrence rate, based on the succession of the isotopically dated ash beds, is near one eruption every 57,000 years. The ash beds were grouped into two chemical types, rhyolitic and dacitic; the main property that distinguishes dacitic from rhyolitic ashes is the larger Ca and Mg and smaller K content in dacitic ashes. Dacitic volcanic ashes were defined to have more than 0.55 wt % Ca. Rhyolitic ashes were subdivided further into two types, W‐ and G‐type, based on the presence of biotite in the W‐type and its absence in G‐type. W‐type rhyolitic ashes are chalky white, have less than 0.55 wt % Fe, have colorless glass shards of pumiceous habit. G‐type rhyolitic ashes are light to medium gray, have from 0.55 to 2.0 wt % Fe in the glass shards, lack biotite, have colorless, platy, bubble wall, and bubble junction glass shards. Dacitic ashes are light gray to grayish brown, have more than 0.55 wt % Fe and Ca, and have a complex mixture of colorless to light brown shards, including pumiceous, fibrous, chunky, and bubble wall and bubble junction types. W‐type rhyolitic ashes possibly formed from magmas that were cooler (695° to 830°C) and thus more viscous than the hotter (870° to 1000°C) magma progenitors of G‐type ashes based on Fe‐Ti oxide mineral compositions. Primary microphenocrysts in the ash beds comprise assemblages of the following: quartz, sanidine, plagioclase, biotite, amphibole, clinopyroxene, orthopyroxene, fayalite, zircon, apatite, allanite, chevkinite, sphene, magnetite, and ilmentite. Fayalite and chevkinite only have been found in G‐type; biotite and sphene only have been found in W‐type. Rhyolitic ashes are highly evolved and contain 74–79 wt % SiO2; dacitic ashes vary from 67–77 wt % SiO2. Al ranges from 10.5 to 14.5 wt % Al2O3, Mg ranges from 0.06 to 1.0 wt % MgO, and Fe (as Fe3O4) ranges from 0.50 to 4.1 wt %. Dacitic ash beds have the most (0.80–3.8 wt % as Fe3O4) and rhyolitic ashes have the least (0.50–1.9 wt % as Fe3O4). Ca varies from an unusually low amount (0.25 wt % CaO) in a few rhyolitic ashes to as much as 2.2% in some dacitic ashes. Rhyolitic glasses show larger Eu deficiencies compared to dacitic glasses on rare‐earth element diagrams. W‐type rhyolitic glasses have moderately large La to Sm ratios and have La contents from about 30 to 100 times chondrite values; G‐type rhyolitic glasses also have moderately large La to Sm ratios and contain somewhat more La than do the W‐type rhyolitic ashes, general

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10200

年代:1981

数据来源: WILEY

摘要:

The high‐silica rhyolite domes and lava flows of the bimodal Pleistocene part of the Coso volcanic field provide an example of the early stages of evolution of a silicic magmatic system of substantial size and longevity. The rhyolites are sparsely porphyritic to virtually aphyric, containing qz + pl + san + bi + hb + mt ± allanite ± opx ± cpx ± fa ± il ± ap ± zircon phenocrysts. Major and trace element compositions of all 38 rhyolite extrusions are consistent with derivation from somewhat less silicic parental material by liquid state differentiation processes in compositionally and thermally zoned magmatic systems. Seven chemically homogeneous eruptive groups emplaced approximately 1.0, 0.6, 0.24, 0.17, 0.16(?), 0.09, and 0.06 m.y. ago can be distinguished on the basis of trace element and K‐Ar data. The oldest two groups are volumetrically minor and geochemically distinct from the younger groups, all five of which appear to have evolved from the same magmatic system. Erupted volume‐time relations suggest that small amounts of magma were bled from the top of a silicic reservoir at a nearly constant long‐term rate over the last 0.24 m.y. The interval of repose between eruptions appears to be proportional to the volume of the preceding eruptive group. This relationship suggests that eruptions take place when some parameter which increases at a constant rate reaches a critical value; this parameter may be extensional strain accumulated in roof rocks. Extension of the lithosphere favors intrusion of basalt into the crust, attendant partial melting, and maintenance of a long‐lived silicic magmatic system. Consideration of the mass of magma that must have been intruded into the crust in order to explain the anomalously high heat flow near the center of the rhyolite field and comparison of age, volume, mineralogic, and compositional characteristics of the rhyolites with those of caldera‐forming systems suggest that the Coso silicic system may contain a few hundred cubic kilometers of magma. The Coso magmatic system may eventually have the potential for producing voluminous pyroclastic eruptions if the safety valve provided by rapid crustal extension becomes inadequate to (1) defuse the system through episodic removal of volatile‐rich magma from its top and (2) prohibit migration of the reservoir to a s

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10223

年代:1981

数据来源: WILEY

摘要:

The Late Oligocene‐Early Miocene volcanism of this region is chemically strongly bimodal; the mafic lavas (volumetrically dominant) comprise basalts, hawaiites, and tholeiitic andesites, while the silicic eruptives are mainly comendites, potassic trachytes, and potassic, high‐silica rhyolites. The comendites and rhyolites have distinctive trace element abundance patterns, notably the extreme depletions of Sr, Ba, Mg, Mn, P, Cr, V, and Eu, and the variable enrichment of such elements as Rb, Zr, Pb, Nb, Zn, U, and Th. The trachytes exhibit these characteristics to lesser degrees. The comendites are distinguished from the rhyolites by their overall relative enrichment of the more highly charged cations (e.g., LREE, Nb, Y, and especially Zr) and Zn. The phenocryst mineralogy of the trachytes and rhyolites comprises various combinations of the following phases: sodic plagioclase (albite‐andesine), calcic anorthoclase, sanidine, quartz, ferroaugite‐ferrohedenbergite, ferrohypersthene, fayalitic olivine, ilmenite, titanomagnetite, and rarely biotite (near annite) and Fe‐hastingsitic amphibole. Accessories include apatite, zircon, chevkinite (ferrohedenbergite‐bearing rhyolites only), and allanite (amphibole and botite rhyolites only). The comendites generally contain Ca‐poor anorthoclase‐sanidine, quartz, fluorarfvedsonite, aegirine and aegirine‐augite (Zr‐bearing), aenigmatite, and ± ilmenite. Coexisting Fe‐Ti oxides are absent in the comendites and relatively uncommon in the rhyolites and trachytes. Where present, they indicate equilibration temperatures of 885°–980°C and fo2between QFM and WM buffers. The magmas are thus interpreted to have been strongly water undersaturated during phenocryst equilibration, which is also consistent with the general paucity of pyroclastics, the rarity of hydrous mineral phases, and the extreme Fe‐enriched ferromagnesian phenocryst compositions. The chemical and mineralogical data are interpreted to indicate the operation of extreme fractionation processes controlling the development of the silicic magmas, and the comendites, trachytes, and certain trachyte‐rhyolite series are considered to have evolved from a mafic parentage. The available oxygen, Sr, and Pb isotopic data, however, point to some modification of the magnas through crustal equilibration processes. The remaining high‐silica rhyolites are considered to most likely represent crustal partial melts but are again mod

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10242

年代:1981

数据来源: WILEY

摘要:

Rhyolite is found in several late Cenozoic volcanic areas in Papua New Guinea including (1) New Britain and islands to the north, where rhyolite compositions correspond to different depths to a Benioff zone, (2) the St. Andrew Strait area in the northern Bismarck Sea, where alkali rhyolites are the only rock types found on Tuluman, Lou, and Pam (TLP) islands, (3) the Moresby Strait area in the D'Entrecasteaux Islands, where alkali rhyolites similar in composition to the TLP rhyolites are found together with basalts, andesites, and dacites, and (4) the Dawson Strait area, also in the D'Entrecasteaux Islands, where the rhyolites are comenditic (peralkaline) and markedly different from all other rhyolite types known from Papua New Guinea. The TLP and Moresby Strait rhyolites are thought to have been formed by partial melting of mafic crust, whereas the peralkaline Dawson Strait suite has apparently originated by crystal fractionation. Crystal fractionation has also been proposed as the cause of rhyolite genesis on Willaumez Peninsula and at Rabaul in New Britain. However, the supporting evidence is not conclusive, and partial melting of young arc crust is a more acceptable alternative, at least for the abundant rhyolites of central Willaumez Peninsula.

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10257

年代:1981

数据来源: WILEY

摘要:

The andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49–50% SiO2to about 76% SiO2. The most primitive compositions, having contents of about 50% SiO2, are the basaltic andesites, which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust. The oceanic andesites cannot have assimilated material from the continental crusts, but their petrogenesis bears on the origin of the continental crust. The fractionation of andesitic magmas occurs at relatively low pressures, within the stability range of plagioclase. Basaltic andesites fractionate mainly orthopyroxene + plagioclase, while andesites fractionate clinopyroxene + orthopyroxene + plagioclase. The petrogenesis of the continental andesites is more complex, as the primary andesites contain sedimentary material from the continental crust

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10273

年代:1981

数据来源: WILEY

摘要:

Gravity, heat flow, and surface geology observations have been used as constraints for a thermal model of a late Tertiary silicic volcanic center at Twin Peaks, Utah. Silicic volcanism began in the area with the extrusion of the Coyote Hills rhyolite 2.74 ± 0.1 m.y. ago, followed by the Cudahy Mine obsidian, felsite, and volcanoclastics, and finally by a complex sequence of domes and flows that lasted until 2.3 ± 0.1 m.y. ago. Basalt sequences span the time 2.5 to 0.9 m.y. Terrain‐corrected Bouguer gravity anomalies at Twin Peaks are shaped by three features of varying characteristic dimensions: (1) a major north‐northeast trending −30 mGal gravity trough roughly 40 km wide caused by a thick sequence of Cenozoic sediments in the Black Rock Desert Valley, (2) a local roughly circular −7 mGal gravity low, 26 km across, probably related to an intrusive body in the basement, and (3) a series of narrow positive anomalies up to +10 mGal produced by the major Twin Peaks volcanic domes. The intrusive bodies have been modeled as three‐dimensional vertical cylinders; the total volume of intrusive material is estimated to be about 500 km3. Simple thermal models, assuming conductive heat transfer and using geometrical constraints from the gravity results, predict that a negligible thermal anomaly should exist 1 m.y. after emplacement of the intrusion. This prediction is consistent with an average heat flow of 96 mW m−2for the area, not significantly different from eastern Basin and Range values elsewhere. Magmatic longevity of this system, 2.7 to 2.3 m.y. for silicic volcanism or 2.5 to 0.9 m.y. for basaltic volcanism, does not seem to prolong the cooling of this system substantially beyond that predicted by condu

ISSN:0148-0227    

DOI:10.1029/JB086iB11p10287

年代:1981

数据来源: WILEY