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1.
The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km3 of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO2. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and 87Sr/86Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO2)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO2) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO2 content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics  相似文献   

2.
A voluminous (>600 km3) and long-lived (~520–75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U–Th zircon ages and δ18O values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-δ18O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-δ18O zircon values of 2.7–2.8‰ that are in equilibrium with low-δ18O host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-δ18O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.  相似文献   

3.
The petrogenesis of topaz rhyolites from the western United States   总被引：2，自引：1，他引：2  
High-silica topaz-bearing rhyolites of Cenozoic age are widely distributed across the western USA and Mexico. They are characteristically enriched in fluorine (>0.2 wt.%) and incompatible lithophile elements (e.g. Li, Rb, Cs, U, Th, Be). In addition to topaz, the rhyolites contain garnet, bixbyite, pseudobrookite, hematite and fluorite in cavities or in their devitrified groundmasses. Magmatic phases include sanidine, quartz, oligoclase and Fe-rich biotite. Allanite, fluorite, zircon, apatite and magnetite occur in most; pyroxene, hornblende, ilmenite and titanite occur in some. The rhyolites crystallized over a wide temperature interval (850° to 600° C) at \(f_{0_2 } \) that ranges from QFM to NNO. The REE patterns of most topaz rhyolites are almost flat (La/YbN=1 to 3) and have deep Eu anomalies (Eu/Eu*=0.01 to 0.02). Both parameters decrease with differentiation. Titanite-bearing rhyolites have prominent middle REE depletions. Topaz rhyolites appear to have evolved from partial melts of a residual granulitic source in the Precambrian lower crust. According to the proposed model, the passage of hot mafic magmas through the crust produced partial melts as a result of the decomposition of F-rich biotite or amphibole. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas. Some of the compositional differences between topaz rhyolites and peralkaline rhyolites may be attributed to the accumulation of fluorine and fluorphile elements (Al, Be, Li, Rb, U, Th, HREE) in melts which give rise to topaz rhyolites and chlorine and chlorophile elements (Ti, Fe, Mn, Zn, Zr, Nb and LREE) in melts which yield peralkaline rhyolites. Hence the F/Cl ratio of the melt or its source may determine the alumina saturation of the magma series. Topaz rhyolites are distinguishable from calc-alkaline rhyolites by lower Sr, Ba, Eu and higher F, Rb, U and Th. The usually low La/Yb ratios of topaz rhyolites distinguish them from both peralkaline and calc-alkaline rhyolite suites.  相似文献   

4.
The Paran continental flood basalt province is a voluminousbimodal volcanic sequence, with <5% silicic rocks (‘rhyolites’)lying on top of the basalts, concentrated towards the SouthAtlantic margin. Petrographically, the rhyolites have an anhydrousmineralogy (plagioclase, pyroxene, Fe–Ti oxides), and.two distinct groups are defined on the basis of phenocryst abundance.The Palmas group rhyolites are almost aphyric (<5% phenocrysts),in contrast to the plagioclase-rith Chapec group rhyolites(<25% phenocrysts). The plagioclase and clinopyroxene phenocrystsin the Palmas group rhyolites are rounded and poorly preserved,and are compositionally less evolved than those in the Chapecgroup. Calculated eruption temperatures are unusually high forsilicic magmas (950–1100C), and lie within the rangeof temperatures for the associated flood basalts. Chemically,the Palmas and Chapec group rhyolites are clearly distinguishable,with the most striking feature being the higher high field strengthelements, notably Ti, in the Chapec group. This mirrors thewell-documented low- and high-Ti division of the Paran basalts,and in addition there is a geographic correlation between thelow- and high- Ti basalt and rhyolite provinces, with high-Tivolcanics predominating in the north of the Paran Basin, andlow-Ti in the south. The Chapec group have Sr and Nd isotoperatios which overlap with those of the high-Ti basalts (87Sr/86Sr1300•705–0•708), whereas the Palmas group exhibita range towards high Sr isotope ratios (87Sr/86Sr130 0•714–0•727),continuing the trend of the low-Ti basalts to more radiogenicvalues. This suggests that assimilation of radiogenic materialhas occurred. Both rhyolite groups plot away from the isotopicfields for crustal basement types beneath the Paran, thus anorigin by simple crustal melting is discounted. Based on petrographic,chemical and isotopic data, petrogenetic models for the tworhyolite groups are developed, focusing on the clear geneticlink between the Palmas rhyolites and the low-Ti basalts, andthe Chapec rhyolites and the high-Ti basalts. The Chapec rhyolitesare modelled as partial melts ( 30%) of underplated high-Tibasalts, rather than fractionates, primarily because of thetime gap between eruption of the high-Ti basalts and Chapecrhyolites. However, the Palmas rhyolites are almost coeval withthe low-Ti basalts, and are modelled as the products of open-systemfractional crystallization from these low-Ti basaltic magmas.In addition, this low-Ti suite shows a continuous trend frombasalt to rhyolite in highly incompatible elements such as Zrand Hf consistent with a liquid line of descent, whereas thehigh-Ti magmas have a substantial gap in the concentration ofthese elements between the basalts and rhyolites. Experimentaldata support the derivation of both Paran rhyolite groups frombasaltic parents with moderately low water contents. Pressurecalculations suggest shallower ponding for the Palmas magmasthan for the Chapec magma (<5 kbar vs 5–15 kbar),and the style of eruption inferred for the two groups is explosive(rheoignimbritic) for the Palmas group, and effusive (lava flows)for the Chapec group. KEY WORDS: Paran; Brazil; rhyolits; petrogenesis; geochemistry *Corresponding author  相似文献   

5.
The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma.  相似文献   

6.
Batholith-sized bodies of crystal-rich magmatic ‘mush’are widely inferred to represent the hidden sources of manylarge-volume high-silica rhyolite eruptive units. Occasionallythese mush bodies are ejected along with their trapped interstitialliquid, forming the distinctive crystal-rich ignimbrites knownas ‘monotonous intermediates’. These ignimbritesare notable for their combination of high crystal contents (35–55%),dacitic bulk compositions with interstitial high-silica rhyoliticglass, and general lack of compositional zonation. The 5000km3 Fish Canyon Tuff is an archetypal eruption deposit of thistype, and is the largest known silicic eruption on Earth. Ejectafrom the Fish Canyon magmatic system are notable for the limitedcompositional variation that they define on the basis of whole-rockchemistry, whereas 45 vol. % crystals in a matrix of high-silicarhyolite glass together span a large range of mineral-scaleisotopic variability (microns to millimetres). Rb/Sr isotopicanalyses of single crystals (sanidine, plagioclase, biotite,hornblende, apatite, titanite) and sampling by micromillingof selected zones within glass plus sanidine and plagioclasecrystals document widespread isotopic disequilibrium at manyscales. High and variable 87Sr/86Sri values for euhedral biotitegrains cannot be explained by any model involving closed-systemradiogenic ingrowth, and they are difficult to rationalize unlessmuch of this radiogenic Sr has been introduced at a late stagevia assimilation of local Proterozoic crust. Hornblende is theonly phase that approaches isotopic equilibrium with the surroundingmelt, but the melt (glass) was isotopically heterogeneous atthe millimetre scale, and was therefore apparently contaminatedwith radiogenic Sr shortly prior to eruption. The other mineralphases (plagioclase, sanidine, titanite, and apatite) have significantlylower 87Sr/86Sri values than whole-rock values (as much as –0·0005).Such isotopic disequilibrium implies that feldspars, titaniteand apatite are antecrysts that crystallized from less radiogenicmelt compositions at earlier stages of magma evolution, whereashighly radiogenic biotite xenocrysts and the development ofisotopic heterogeneity in matrix melt glass appear to coincidewith the final stage of the evolution of the Fish Canyon magmabody in the upper crust. Integrated petrographic and geochemicalevidence is consistent with pre-eruptive thermal rejuvenationof a near-solidus mineral assemblage from 720 to 760°C (i.e.partial dissolution of feldspars + quartz while hornblende +titanite + biotite were crystallizing). Assimilation and blendingof phenocrysts, antecrysts and xenocrysts reflects chamber-wide,low Reynolds number convection that occurred within the last10 000 years before eruption. KEY WORDS: Fish Canyon Tuff; Rb–Sr isotopes; microsampling; magmatic processes; crystal mush  相似文献   

7.
The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial 87Sr/86Sr ratios (0·7034–0·7080).Quartz phenocryst 18O values indicate that the rhyolite magmashad 18O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with 18O values between4·8 and 6·2. The low-18O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ 18O magmavalues. These low-18O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low 18O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the 18O magma value. It is proposed that the low-18Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-18O magmas; partial melting  相似文献   

8.
Voluminous (3·9 x 105 km3), prolonged (18 Myr) explosivesilicic volcanism makes the mid-Tertiary Sierra Madre Occidentalprovince of Mexico one of the largest intact silicic volcanicprovinces known. Previous models have proposed an assimilation–fractionalcrystallization origin for the rhyolites involving closed-systemfractional crystallization from crustally contaminated andesiticparental magmas, with <20% crustal contributions. The lackof isotopic variation among the lower crustal xenoliths inferredto represent the crustal contaminants and coeval Sierra MadreOccidental rhyolite and basaltic andesite to andesite volcanicrocks has constrained interpretations for larger crustal contributions.Here, we use zircon age populations as probes to assess crustalinvolvement in Sierra Madre Occidental silicic magmatism. Laserablation-inductively coupled plasma-mass spectrometry analysesof zircons from rhyolitic ignimbrites from the northeasternand southwestern sectors of the province yield U–Pb agesthat show significant age discrepancies of 1–4 Myr comparedwith previously determined K/Ar and 40Ar/39Ar ages from thesame ignimbrites; the age differences are greater than the errorsattributable to analytical uncertainty. Zircon xenocrysts withnew overgrowths in the Late Eocene to earliest Oligocene rhyoliteignimbrites from the northeastern sector provide direct evidencefor some involvement of Proterozoic crustal materials, and,potentially more importantly, the derivation of zircon fromMesozoic and Eocene age, isotopically primitive, subduction-relatedigneous basement. The youngest rhyolitic ignimbrites from thesouthwestern sector show even stronger evidence for inheritancein the age spectra, but lack old inherited zircon (i.e. Eoceneor older). Instead, these Early Miocene ignimbrites are dominatedby antecrystic zircons, representing >33 to 100% of the datedpopulation; most antecrysts range in age between 20 and 32 Ma.A sub-population of the antecrystic zircons is chemically distinctin terms of their high U (>1000 ppm to 1·3 wt %) andheavy REE contents; these are not present in the Oligocene ignimbritesin the northeastern sector of the Sierra Madre Occidental. Thecombination of antecryst zircon U–Pb ages and chemistrysuggests that much of the zircon in the youngest rhyolites wasderived by remelting of partially molten to solidified igneousrocks formed during preceding phases of Sierra Madre Occidentalvolcanism. Strong Zr undersaturation, and estimations for veryrapid dissolution rates of entrained zircons, preclude coevalmafic magmas being parental to the rhyolite magmas by a processof lower crustal assimilation followed by closed-system crystalfractionation as interpreted in previous studies of the SierraMadre Occidental rhyolites. Mafic magmas were more probablyimportant in providing a long-lived heat and material flux intothe crust, resulting in the remelting and recycling of oldercrust and newly formed igneous materials related to Sierra MadreOccidental magmatism. KEY WORDS: ignimbrite; rhyolite; Sierra Madre Occidental; Tertiary; U–Pb geochronology; zircon; antecryst; crustal melting  相似文献   

9.
Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle.  相似文献   

10.
We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ≤230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ~85 ka rhyolites yielded ages between ~100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies.  相似文献   

11.
Mafic inclusions present in the rhyolitic lavas of Narugo volcano,Japan, are vesiculated andesites with diktytaxitic texturesmainly composed of quenched acicular plagioclase, pyroxenes,and interstitial glass. When the mafic magma was incorporatedinto the silica-rich host magma, the cores of pyroxenes andplagioclase began to crystallize (>1000°C) in a boundarylayer between the mafic and felsic magmas. Phenocryst rim compositionsand interstitial glass compositions (average 78 wt % SiO2) inthe mafic inclusions are the same as those of the phenocrystsand groundmass glass in the host rhyolite. This suggests thatthe host felsic melt infiltrated into the incompletely solidifiedmafic inclusion, and that the interstitial melt compositionin the inclusions became close to that of the host melt (c.850°C). Infiltration was enhanced by the vesiculation ofthe mafic magma. Finally, hybridized and density-reduced portionsof the mafic magma floated up from the boundary layer into thehost rhyolite. We conclude that the ascent of mafic magma triggeredthe eruption of the host rhyolitic magma. KEY WORDS: mafic inclusion; stratified magma chamber; magma mixing; mingling; Narugo volcano; Japan  相似文献   

12.
Relicts of silicate-iron fluid media were found in the Early Cretaceous rhyolites of the Nilginskaya depression, Central Mongolia. They are localized in matrix cavities and in the inclusions in quartz and sanidine phenocrysts. The mineral composition of rhyolites and aggregates of silicate-iron phases has been studied. Calculations showed that crystallization of ilmenite and magnetite in a matrix occurred within a temperature range of 593–700°C and oxygen fugacity $\Delta \log f_{O_2 }$ NNO from ?2.29 to 1.68. The average compositions of the rhyolites and residual glasses in melt inclusions (MI) have A/CNK index of 1.03–1.05. The compositions of MI glasses define a trend from agpaitic to plumasitic types (A/NK and A/CNK change from 0.8–0.9 to 1.1–1.2). According to calculations, the rhyolitic melt was solidified at 640–750°C. Based on cathodoluminescent study, inclusions with silicate-iron phases are observed separately or together with MI in the early and intermediate growth zones of quartz and sanidine crystals. Aggregates found in the inclusions are represented by loose matter consisting of silica with small admixture of Al, Na, K, and Cl; silicate-iron aggregates with wide variations of Fe and Si; essentially Fe-rich micaceous and mica-silicate-iron aggregates. They usually have variable composition (wt %): 30–60 SiO2, 10–25 Al2O3, 10–30 FeO, up to 3 TiO2, 1.5–4 MgO, up to 3 CaO, up to 3 Na2O, up to 3 K2O, and up to 4 P2O5. They presumably contain up to 10–15 wt % H2O. Some inclusions comprise large segregations of siderophyllite enriched in F (3–10 wt %) and Cl (0.1–3.3 wt %). Evolution of the rhyolitic melt from magmatic chamber to its vitrification after ejection led to the decrease of F content. The highest F content (1–1.8 wt %) is typical of MI glasses, while the lowest content (0.05–0.1 wt %) was found in the glassy matrix and rhyolitic samples. The melt degassing was accompanied by the release of F-rich fluid containing up to 1.3 wt % F (based on partition coefficient fluid/meltDF) or 0.2–0.8 mol/dm3 HF (based on composition of micas from matrix and inclusions). Segregations of silicate-iron media existed in the rhyolitic magma. During formation of rhyolitic pile, these media were in a liquid state. The silicate-iron fluid media captured in MI could not be true fluids or silicate melts. They were likely formed during fluid-magmatic interaction and transformation of fluid phases of different density (vapor and liquid true solutions) that existed in a F-rich melt. The high concentrations of F and Cl and elevated alkalinity of fluids contribute their enrichment in silica and other elements, which could lead to the formation of hydrosilicate liquids. It is suggested that such liquids (gels) in dispersed (colloidal) state extracted F and many trace elements (P, Ti, Mg, Ca, REE, As, Nb, Th, and V) from surrounding rhyolitic magma.  相似文献   

13.
陈井胜  邢德和  刘淼  李斌  杨浩  田德欣  杨帆  汪岩 《岩石学报》2017,33(9):2792-2810
辽宁省辽阳地区出露有大面积的古元古代辽河群地层,在辽河群里尔峪组及高家峪组中识别出一套酸性火山岩。本文通过对酸性火山岩同位素年代学及地球化学研究,用以限定辽河群形成时代,并进一步探讨其形成的动力学背景。这套酸性火山岩岩性包括流纹岩、变流纹质含角砾凝灰岩、角岩化流纹岩和弱硅化流纹岩,对其中6件样品利用LA-ICP-MS锆石U-Pb方法测年。CL图像显示辽河群酸性火山岩中的锆石均具典型的岩浆振荡环带结构和较高的Th/U比值,显示为岩浆锆石。锆石U-Pb年龄可分为3期:2190~2180Ma、2110~2100Ma、1970~1960Ma,结合其中存在继承锆石的结果认为辽河群至少存在3期岩浆活动,将辽河群形成时代限制于2.19~1.96Ga,其形成时代大约持续了230Myr左右。野外接触关系的观察及酸性火山岩的年代学特征从另一个方面揭露辽河群总体为一套无序、局部有序的构造地层,各岩组间不存在上下关系。前人研究表明"辽吉花岗岩"形成年龄为2.16Ga,通过野外观察其与辽河群之间均为构造接触。D1442-1和D1444-2两个样品锆石U-Pb测年结果为2180±12Ma、2190±13Ma,均略早于"辽吉花岗岩",结合其与辽河群野外关系,认为"辽吉花岗岩"应该不是作为沉积基底,而是与辽河群为同一构造运动的产物。辽河群流纹岩Al_2O_3含量16.33%~16.90%,K_2O含量4.62%~4.77%,K_2O+Na_2O含量5.97%~7.54%,含铝指数(A/CNK)比较高为1.27~1.50,为过铝质岩石,显示富铝、高钾特征。此外流纹岩轻稀土富集,重稀土亏损,轻重稀土分馏明显,具有明显的负铕异常,亏损Ba、Sr、Ti,较高的Zr及(Zr+Nb+Ce+Y)含量、10000Ga/Al比值等特点都显示为A型花岗岩,说明其形成于地壳减薄环境。本文在辽河群中识别出一套酸性火山岩与前人在南辽河群识别出的变质流纹岩组成了酸性端元(2201~1960Ma),同时期存在的基性火成岩(2198~1869Ma)为基性端元,证明了辽河群"双峰式火山岩"的存在,暗示辽河群形成于伸展拉张环境。  相似文献   

14.
EWART  A. 《Journal of Petrology》1982,23(3):344-382
The magmas of the Tertiary volcanic province of S. Queenslandare chemically bimodal, and occur in numerous volcanic centres,at least three representing original shield volcanoes. The maficlavas are dominantly hawaiites and tholeiitic andesites, whereasthe silicic magmas comprise mainly trachytes, rhyolites, andcomendites. The silicic rocks exhibit variable trace element abundance patterns.There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn,and P, through the trachytes to the rhyolites and comenditeswhile the behaviour of Zr, Nb, LREE, Y and Zn is very variable.Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour,becoming most generally concentrated in the comendites and rhyolites.These trace element patterns are modelled by application ofthe Rayleigh distillation model, using partition coefficientsbased on analysed phenocrysts from the S. Queensland siliciclavas. Trace mineral phases, namely zircon, chevkinite, andallanite, are shown to be important in the probable controlof LREE, Zr, and Th abundances, while Nb and Zn are probablycontrolled during fractionation by magnetite. Trace elementdata for the hawaiites and tholeiitic andesites also indicateextensive although variable levels of fractional crystallizationof these magmas. The Sr and O isotopic compositions of the mafic lavas, trachytes,comendites and rhyolites are as follows: initial 87Sr/86Sr ratios;0.70357–0.70456, 0.70432–0.70589, 0.70495–0.70917,and 0.70708–0.70863 respectively. 18O range between 5.6–7.0(mafic lavas), 4.9–8.7 (trachytes), 5.0–7.6 (comendites)and 8.1–10.4 per mil (rhyolites). Pb isotopic compositionsare variable, showing a variation of 6.7 per cent for 206Pb/204Pbratios through the range of volcanic compositions. The rhyolitesexhibit a much greater divergence in their O, Sr, and Pb isotopiccompositions compared with those of associated mafic lavas,than is found in the trachytes and comendites. Within the silicicvolcanics, positive correlations exist between 18O and initialSr ratios, and between Pb isotopic compositions and initialSr ratios (with one group of trachytes providing a noteworthyexception). These correlations are not so clearly defined forthe mafic lavas, although these do exhibit positive correlationsbetween differentiation index, 18O, and initial Sr isotope ratios. The development of the silicic magmas, excepting two groups,is interpreted in terms of a model in which assimilation andfractional crystallization occur concurrently, involving a basaltor hawaiite magma component and a crustal component (modelledon the analysed Carboniferous basement greywackes outeroppingin the region); the data indicate, however, that differentiationcontinued in isotopically closed systems (i. e. isolated fromthe wallrocks). The highly depleted Sr and Ba abundances ofthe rhyolites and comendites suggest that contamination didnot occur after differentiation had ceased. The rhyolites havethe highest isotopic input of the crustal components and areinterpreted as crustal anatectic melts, produced locally withinthe crust in response to basalt/hawaiite magma intrusion, whereasmost of the trachytes and comendites are interpreted as primarilythe differentiated products from original mafic parental magmas,with variable assimilation of crustal wallrock components. Theisotopic data suggest that only the Minerva Hills trachyticlavas, and a Glass House comendite, have not been significantlymodified by wallrock assimilation processes. The erpted maficmagmas were also evidently modified by isotopic crustal wallrockinteractions, which independent petrological data suggest hasoccurred at intermediate to lower crustal depths.  相似文献   

15.
Understanding the processes of differentiation of the Yellowstone–Snake River Plain (YSRP) rhyolites is typically impeded by the apparent lack of erupted intermediate compositions as well as the complex nature of their shallow interaction with the surrounding crust responsible for their typically low O isotopic ratios. A pair of normal-δ18O rhyolitic eruptions from the Heise eruptive centre in eastern Idaho, the Wolverine Creek Tuff and the Conant Creek Tuff, represent unique magmatic products of the Yellowstone hotspot preserving abundant vestiges of the intermediate differentiation steps leading to rhyolite generation. We address both shallow and deep processes of magma generation and storage in the two units by combining high-precision ID–TIMS U–Pb zircon geochronology, trace element, O and Hf isotopic studies of zircon, and Sr isotopic analyses of individual high-Mg# pyroxenes inherited from lower- to mid-crustal differentiation stages. The zircon geochronology confirms the derivation of both tuffs from the same rhyolitic magma reservoir erupted at 5.5941 ± 0.0097 Ma, preceded by at least 92 ± 14 ky of continuous or intermittent zircon saturation approximating the length of pre-eruptive magma accumulation in the upper crust. Some low-Mg# pyroxenes enclosing zircons predate the eruption by at least 45 ± 27 ky, illustrating the co-crystallisation of major and accessory phases in the near-liquidus rhyolitic melts of the YSRP over a significant period of time. Coeval zircon crystals are isotopically heterogeneous (two populations at εHf ~?5 and ?13), requiring the assembly of isotopically distinct melt pockets directly prior to, or during, the eruption. The primitive Mg# 60–90 pyroxenes are out of isotopic equilibrium with the host rhyolitic melt (87Sr/86Sri = 0.70889), covering a range of 87Sr/86Sri = 0.70705–0.70883 corresponding to ratios typical of the most radiogenic YSRP basalts to the least radiogenic YSRP rhyolites. Together with the low εHf in zircon, the Sr isotopic ratios illustrate limited assimilation dominated by radiogenic Archean crustal source materials incorporated into variably evolved YSRP melts as they progress towards rhyolitic compositions by assimilation–fractional crystallisation.  相似文献   

16.
Pleistocene lavas from Monte S. Angelo and Chiesa Vecchia volcanoes on Lipari contain two suites of inclusions. A metapelitic suite consists of gneisses and granulites with combinations of cordierite, garnet, corundum, hercynite, andalusite, sillimanite, orthopyroxene, ilmenite, magnetite, biotite, plagioclase, and quartz. A gabbroic suite has cumulus texture and contains plagioclase, orthopyroxene, clinopyroxene, and magnetite. All megacryst phases in the lavas appear to be derived from rock fragments, with the exception of euhedral strongly zoned calcic plagioclase, and none has grown by homogeneous nucleation from liquid represented by the groundmass, which is peraluminous rhyolite (>70 wt% SiO2, >6 wt% K2O). Ground-mass microcrysts were nearly all derived from disaggregated metapelites; overgrowths of alkali feldspar on plagioclase and of orthopyroxene on clinopyroxene, and quartz intergrown with alkali feldspar, are the only phases that grew from the rhyolitic liquid. Euhedral cordierite, hercynite, and plagioclase at the margins of some rock fragments grew by reaction of metapelite with liquid.For grains in contact within metapelite inclusions, geothermometers and geobarometers yield estimates of equilibration conditions in the range of 800±100° C and 5±1 kbar. Compositions of phases in the same thin section, but not in the same inclusion, yield broadly erratic P and T estimates indicating disequilibrium among metapelite inclusions. Pyroxene thermometry in the gabbro suite indicates a crystallization temperature of 1020±50° C and a lack of subsequent thermal equilibration with the rhyolitic liquid.The metapelite suite may partly be restite, but much is xenolithic, derived from a vertical interval of perhaps several kilometers, and may have undergone a much earlier episode of melting. The gabbro fragments are accidental xenoliths incorporated as the magma rose. Contaminants (metapelite and gabbro) account for 50 vol.% of the lavas, and cause them to be classified as high-K andesite according to whole-rock major element analysis.The rhyolitic liquid may have originated by partial fusion of metapelites in the lower crust, or by fractional crystallization of mafic mantle-derived magma combined with assimilation of metapelite; the bulk of the evidence favors assimilation-fractional crystallization. Miocene and younger metapelite-contaminated rhyolites also occur in Tuscany, SE Spain, E Morocco, and NW Tunisia, and are associated in each region with mafic silica-undersaturated lavas, implying crustal underplating around the western Mediterranean before, during, and after formation of the Tyrrhenian basin.  相似文献   

17.
Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km3 magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of  0.3 km3. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.  相似文献   

18.
The Suguti volcanic rocks of the southern Musoma-Mara greenstone belt in northern Tanzania comprise mainly of a bimodal suite of tholeiitic basalts-basaltic andesites and calc-alkaline rhyolites with a subordinate amount of intermediate rocks. Zircon U–Pb and whole rock Sm–Nd geochronology suggests that the two suites are cogenetic and were emplaced at 2755 ± 1 Ma with a common initial Nd value of 2.1.The tholeiitic basalts are characterised by relatively flat chondrite-normalised REE patterns with La/YbCN ratios of 0.8–1.6 (mean = 1.0). The basalts also exhibit negative Ti and Nb anomalies in primitive mantle-normalised multi-element diagrams. The flat REE patterns, the presence of prominent negative Nb anomalies and the positive initial Nd value of 2.1 suggest that the basalts were formed by low pressure melting of a mantle wedge in an active continental margin setting.Compared to the tholeiitic basalts, the calc-alkaline rhyolites are characterised by low abundances of the transition elements (Cr < 20 ppm, Ni < 20 ppm) and moderately high HFSE (e.g. Zr = 111–250 ppm) abundances. The rhyolites display strongly fractionated, slightly concave upward chondrite normalised REE patterns that are characterised by a slight depletion of the MREE relative to the HREE and minor to large negative Eu anomalies (Eu/Eu* = 0.3–0.9) and their epsilon Nd values range from +2.05 to +2.33. The depletion of the MREE relative to the HREE is an indication of fractionation of clinopyroxene and hornblende during petrogenesis whereas the negative Eu anomalies indicate plagioclase fractionation. The rhyolites are interpreted to have formed from the parental magma of the basalts by fractional crystallization and/or partial melting of a relatively young basaltic crust.  相似文献   

19.
The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P total and . The values of P total and depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases P total.The rhyolite magma contained over 9 per cent water (by weight) when the cummingtonite phenocrysts precipitated, and possibly as much as 12 per cent, so that it is surprising that one of these rhyolites is a coherent lava. The calculated values of P total and are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that >0.7–0.8 P total for cummingtonite to precipitate in rhyolites, and that iron-rich olivine and cummingtonite could only exist in rhyolites over a small temperature range at a pressure near 5 kilobars. Hornblende phenocrysts co-existing with fayalitic olivine in rhyolites accordingly have a very low activity of Mg7Si8O22(OH)2.  相似文献   

20.
The Loch Ba ring-dyke in the Tertiary igneous central complex of Mull, N.W. Scotland is composed predominantly of a banded rhyolitic welded tuff. The rhyolite contains numerous inclusions of dark aphanitic rock. The textural relationships between the different rocks indicate rapid, violent and intimate mixing during emplacement of the dyke. The dark glassy component varies continuously from basaltic andesite to andesite, dacite and rhyolite. These glasses are enriched in FeO and depleted in MgO at a given SiO2 content in comparison to other tholeiitic highly differentiated volcanic rocks. The rhyolite contains an average of 4% phenocrysts and is associated with the mineral assemblage plagioclase (An32 to An21)-sanidine(Or50–60)-hedenbergite-fayalite-magnetite-ilmenite-apatite-zircon. Mineral aggregates involving either plagioclase-hedenbergite-ilmenite or plagioclase-fayalite-magnetite are common, but aggregates containing fayalite and hedenbergite together are scarce. The dark glassy components are either phenocryst free or contain less than 0.2% phenocrysts. The main phenocrysts associated with the dark glasses are plagioclase (An65-An30), high calcium clinopyroxene ranging continuously from augite to pure hedenbergite, pigeonite, magnetite, ilmenite and rare apatite. Zoning in minerals is generally weak or absent. The plagioclase feldspar, high calcium clinopyroxenes and pigeonites have similar compositional ranges to the minerals observed in the Middle and Upper Zones of the Skaergaard Intrusion. The mineral compositions are systematically related to SiO2 content and Mg number of the glasses. The data demonstrate that mineral compositions and assemblages similar to the Skaergaard form from silica-rich andesitic to rhyolitic liquids. The various mafic glasses are interpreted to have been derived from a zoned magma chamber underlying an upper layer of rhyolitic magma. Differentiation is attributed to fractional crystallization of the observed mineral assemblages causing SiO2 enrichment and FeO depletion. However, glasses with less than 57% SiO2 have unusual compositions with very low MgO and P2O5 as well as variable Al2O3 and TiO2. Their peculiarities could be explained by andesitic magmas assimilating cumulate mineral aggregates precipitated from more differentiated dacite and rhyolite magmas. The bulk compositions of these cumulates have high FeO, low SiO2 and negligible MgO and P2O5. It is suggested that the high density of the mineral aggregates containing fayalite-hedenbergite-magnetite and ilmenite caused them to settle through the zoned chamber to be assimilated by high temperature, less differentiated magmas.  相似文献   

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