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1.
Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.  相似文献   

2.
 A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation. Received: 1 June 1998 / Accepted: 5 February 1999  相似文献   

3.
Sugarloaf Mountain is a 200-m high volcanic landform in central Arizona, USA, within the transition from the southern Basin and Range to the Colorado Plateau. It is composed of Miocene alkalic basalt (47.2–49.1?wt.% SiO2; 6.7–7.7?wt.% MgO) and overlying andesite and dacite lavas (61.4–63.9?wt.% SiO2; 3.5–4.7?wt.% MgO). Sugarloaf Mountain therefore offers an opportunity to evaluate the origin of andesite magmas with respect to coexisting basalt. Important for evaluating Sugarloaf basalt and andesite (plus dacite) is that the andesites contain basaltic minerals olivine (cores Fo76-86) and clinopyroxene (～Fs9-18Wo35-44) coexisting with Na-plagioclase (An48-28Or1.4–7), quartz, amphibole, and minor orthopyroxene, biotite, and sanidine. Noteworthy is that andesite mineral textures include reaction and spongy zones and embayments in and on Na-plagioclase and quartz phenocrysts, where some reacted Na-plagioclases have higher-An mantles, plus some similarly reacted and embayed olivine, clinopyroxene, and amphibole phenocrysts.Fractional crystallization of Sugarloaf basaltic magmas cannot alone yield the andesites because their ～61 to 64?wt.% SiO2 is attended by incompatible REE and HFSE abundances lower than in the basalts (e.g., Ce 77–105 in andesites vs 114–166?ppm in basalts; Zr 149–173 vs 183–237; Nb 21–25 vs 34–42). On the other hand, andesite mineral assemblages, textures, and compositions are consistent with basaltic magmas having mixed with rhyolitic magmas, provided the rhyolite(s) had relatively low REE and HFSE abundances. Linear binary mixing calculations yield good first approximation results for producing andesitic compositions from Sugarloaf basalt compositions and a central Arizona low-REE, low-HFSE rhyolite. For example, mixing proportions 52:48 of Sugarloaf basalt and low incompatible-element rhyolite yields a hybrid composition that matches Sugarloaf andesite well ? although we do not claim to have exact endmembers, but rather, viable proxies. Additionally, the observed mineral textures are all consistent with hot basalt magma mixing into rhyolite magma. Compositional differences among the phenocrysts of Na-plagioclase, clinopyroxene, and amphibole in the andesites suggest several mixing events, and amphibole thermobarometry calculates depths corresponding to 8–16?km and 850° to 980?°C. The amphibole P-T observed for a rather tight compositional range of andesite compositions is consistent with the gathering of several different basalt-rhyolite hybrids into a homogenizing ‘collection' zone prior to eruptions. We interpret Sugarloaf Mountain to represent basalt-rhyolite mixings on a relatively small scale as part of the large scale Miocene (～20 to 15 Ma) magmatism of central Arizona. A particular qualification for this example of hybridization, however, is that the rhyolite endmember have relatively low REE and HFSE abundances.  相似文献   

4.
Primitive andesites from the Taupo Volcanic Zone formed by magma mixing   总被引：1，自引：0，他引：1  
Andesites with Mg# >45 erupted at subduction zones form either by partial melting of metasomatized mantle or by mixing and assimilation processes during melt ascent. Primitive whole rock basaltic andesites from the Pukeonake vent in the Tongariro Volcanic Centre in New Zealand’s Taupo Volcanic Zone contain olivine, clino- and orthopyroxene, and plagioclase xeno- and antecrysts in a partly glassy matrix. Glass pools interstitial between minerals and glass inclusions in clinopyroxene, orthopyroxene and plagioclase as well as matrix glasses are rhyolitic to dacitic indicating that the melts were more evolved than their andesitic bulk host rock analyses indicate. Olivine xenocrysts have high Fo contents up to 94%, δ18O(SMOW) of +5.1‰, and contain Cr-spinel inclusions, all of which imply an origin in equilibrium with primitive mantle-derived melts. Mineral zoning in olivine, clinopyroxene and plagioclase suggest that fractional crystallization occurred. Elevated O isotope ratios in clinopyroxene and glass indicate that the lavas assimilated sedimentary rocks during stagnation in the crust. Thus, the Pukeonake andesites formed by a combination of fractional crystallization, assimilation of crustal rocks, and mixing of dacite liquid with mantle-derived minerals in a complex crustal magma system. The disequilibrium textures and O isotope compositions of the minerals indicate mixing processes on timescales of less than a year prior to eruption. Similar processes may occur in other subduction zones and require careful study of the lavas to determine the origin of andesite magmas in arc volcanoes situated on continental crust.  相似文献   

5.
Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.  相似文献   

6.
238U–230Th disequilibria and Sr and O isotope ratios have been measured in a suite of samples from most of the known prehistoric and historic eruptions of Hekla volcano, Iceland. They cover the compositional range from basaltic andesite to rhyolite. Recent basalts erupted in the vicinity of the volcano and a few Pleistocene basalts have also been studied. Geochemical data indicate that the best tracers of magmatic processes in Hekla are the (230Th/232Th) and Th/U ratios. Whereas most geochemical parameters, including Sr, Nd and O isotopes, could be compatible with crystal fractionation, (230Th/232Th) and Th/U ratios differ in the basalts and basaltic andesites (1.05 and 3.2, respectively) and in the silicic rocks, dacites and rhyolites (0.98 and 3.4–3.7, respectively). This observation precludes fractional crystallization as the main differentiation process in Hekla. On the basis of these results, the following model is proposed: basaltic magmas rise in the Icelandic crust and cause partial melting of metabasic rocks, leading to the formation of a dacitic melt. The basaltic magma itself evolves by crystal fractionation and produces a basaltic andesite magma. The latter can mix with the dacitic liquid to form andesites. At higher levels in the magma chamber, the dacitic melt sometimes undergoes further differentiation by crystal fractionation and produces subordinate volumes of rhyolites. Together all these processes lead to a zoned magma chamber. However, complete zoning is achieved only when the repose time between eruptions is long enough to allow the production of significant volumes of dacitic magma by crustal melting. This situation corresponds to the large plinian eruptions. Between these eruptions, the so-called intra-cyclic activity is characterized by the eruption of andesites and basaltic andesites, with little crustal melting. The magmatic system beneath Hekla most probably was established during the Holocene. The shape and the size of the magma chamber may be inferred from the relationships between the composition of the lavas and the location of the eruption sites. In a cross-section perpendicular to Hekla's ridge, a bell-shaped reservoir 5 km wide and 7 km deep appears the most likely; its top could be at depth of 8 km according to geophysical data.  相似文献   

7.
Andesites from northeastern Kanaga Island,Aleutians   总被引：1，自引：0，他引：1  
Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO2 contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO2 content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO2 contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO2 contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO2 content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO2) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with 63 wt % SiO2 that increase the thickness and lowers the overall SiO2 content of the upper silicic zone. It is suggested that the maximum SiO2 content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.  相似文献   

8.
Heterogeneous andesitic and dacitic lavas on Cordn El Guadalbear on the general problem of how magmas of differing compositionsand physical properties interact in shallow reservoirs beneathcontinental arc volcanoes. Some of the lavas contain an exceptionallylarge proportion (<40%) of undercooled basaltic andesiticmagma in various states of disaggregation. Under-cooled maficmagma occurs in the silicic lavas as large (<40 cm) basalticandesitic magmatic inclusions, as millimeter-sized crystal-clotsof Mg-rich olivine phenocrysts plus adhering Carich plagioclasemicrophenocrysts (An50–70), and as uniformly distributed,isolated phenocrysts and microphenocrysts. Compositions andtextures of plagioclase phenocrysts indicate that inclusion-formingmagmas are hybrids formed by mixing basaltic and dacitic melts,whereas textural features and compositions of groundmass phasesindicate that the andesitic and dacitic lavas are largely mechanicalmixtures of dacitic magma and crystallized basaltic andesiticmagma. This latter observation is significant because it indicatesthat mechanical blending of undercooled mafic magma and partiallycrystallized silicic magma is a possible mechanism for producingthe common porphyritic texture of many calc-alkaline volcanicrocks. The style of mafic-silicic magma interaction at CordonEl Guadal was strongly dependent upon the relative proportionsof the endmembers. Equally important in the Guadal system, however,was the manner in which the contrasting magmas were juxtaposed.Textural evidence preserved in the plagioclase phenocrysts indicatesthat the transition from liquid-liquid to solid-liquid mixingwas not continuous, but was partitioned into periods of magmachamber recharge and eruption, respectively. Evidently, duringperiods of recharge, basaltic magmas rapidly entrained smallamounts of dacitic magma along the margins of a turbulent injectionfountain. Conversely, during periods of eruption, dacitic magmagradually incorporated small parcels of basaltic andesitic magma.Thus, the coupled physical-chemical transition from mixed inclusionsto commingled lavas is presumably not coincidental. More likely,it probably provides a partial record of the dynamic processesoccurring in shallow magma chambers beneath continental arevolcanoes. KEY WORDS: Chile; commingling; magma mixing; magmatic inclusions *Present address: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA  相似文献   

9.
The Medicine Lake shield volcano is part of the Oregon high alumina plateau basalt petrologic province, as defined by Waters (1962) and Higgins (1973). The early eruptions are basaltic andesites and they constitute a significant portion of the shield-forming lavas. These lavas are characterized by a mild iron enrichment trend produced by fractionation of plagioclase and olivine, together with lesser amounts of clinopyroxene. Siliceous andesites of less areal extent form the shield-capping lavas. Their formation is initiated by the appearance of titanomagnetite as a liquidus phase which prevents further iron enrichment. Additional fractionation of plagioclase, clinopyroxene, orthopyroxene, and minor olivine continued during this interval.An origin for the basaltic andesites which involves the derivation of a liquid by partial melting of lithosphere composed of low Sr87/Sr86 material previously subducted along the continental margin is favored. This magma subsequently fractionated under low pressure conditions, a conclusion supported by least squares mixing calculations.  相似文献   

10.
Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low 87Sr/86Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes  相似文献   

11.
我们对采自于加勒比海地区小安德列斯岛弧(Lesser Antilles Arc)Kick’em Jenny(KEJ)海底火山玄武岩中的斜长石斑晶进行了矿物形态和成分分析。利用电子探针(EMPA)和LA-ICP-MS测定了具有环带结构的斜长石斑晶中主量元素的空间分布,同时也利用LA-ICP-MS分析了斜长石中Sr的分布。结果表明,在不同的矿物斑晶中,元素含量均表现出和环带结构相联系的空间分布变化。斜长石斑晶中最主要的结构为韵律环带以及熔蚀结构,所测定的矿物边缘都存在An值从由内向外迅速降低的致密韵律环带,可能反映了快速结晶时的不平衡;而晶体内部的稀疏韵律环带结构是由岩浆填充或对流活动导致的。部分斜长石的熔蚀层An值由内向外升高,反映了高Ca岩浆填充的过程。这说明斜长石斑晶的矿物形态和元素环带可以用来制约俯冲带海底火山岩浆从源区上升到岩浆房再到喷发的复杂过程,包括岩浆演化、熔体多次填充、熔体与结晶矿物之间的反应、以及矿物再熔融等。这对于理解海底火山的喷发以及岛弧岩浆岩的演化有重要意义。  相似文献   

12.
St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An97 and An95, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO2 = NNO ? 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt KdCa–Na and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H2O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.  相似文献   

13.
This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H2O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 14C years Before Present (14C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H2O contents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ∼3000 14C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H2O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine Lake system have not always been low H2O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence of H2O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H2O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H2O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes. Received: 4 January 1999 / Accepted: 30 August 1999  相似文献   

14.
One-atmosphere, anhydrous phase equilibria determined for alkali basalt/high-silica rhyolite mixtures provide a model for crystallization of natural calc-alkaline mixed magmas. The compositional trend defined by these mixtures mimics the trends of many continental calc-alkaline volcanic suites. As with many naturally occurring suites, the mixtures studied straddle the low-pressure olivine-plagioclase-augite thermal divide. Magma mixing provides a convenient method for magmas to cross this thermal divide in the absence of magnetite crystallization. For the mixtures, Mg-rich olivine (Fo82–87) coexists alone with liquid over an exceptionally large range of temperature and silica content (up to 63 wt% SiO2). This indicates that the Mg-rich olivines found in many andesites and dacites are not necessarily out of equilibrium with the host magma, as is commonly assumed. Such crystals may be either primary phenocrysts, or inherited phenocrysts derived from a mafic magma that mixed with a silicic magma. For the bulk compositions studied, the distribution of Fe and Mg between olivine and liquid (K D ) is equal to 0.3 and is independent of temperature and composition. This result extends to silicic andesites the applicability of K D arguments for tests of equilibrium between olivine and groundmass and for modeling of fractional crystallization. In contrast, the distribution of calcium and sodium between plagioclase and liquid varies significantly with temperature and composition. Therefore, plagioclase-liquid K D s cannot be used for fractional crystallization modeling or as a test of equilibrium. Calcic plagioclase from a basalt will be close to equilibrium with andesitic mixtures, but sodic plagioclase from a rhyolite will be greatly out of equilibrium. This explains the common observation that calcic plagioclase crystals in hybrid andesites are generally close to textural equilibrium with the surrounding groundmass, but sodic plagioclase crystals generally show remelting and armoring with calcic plagioclase.  相似文献   

15.
Amphibole-bearing mafic inclusions (low to medium-K high-alumina basalt to basaltic andesite) comprise 4.1 vol% of calc-alkaline rhyolite and rhyodacite lavas on Akrotiri Peninsula, Santorini, Greece. Physical features indicate a magmatic origin for the inclusions, involving mingling with the host silicic magma and quenching. Water contents of the mafic magmas are estimated to have been above 4% at water pressures of 1.8 kbars or more at temperatures of approximately 950–1,000 °C. Three evolutionary stages are inferred in their petrogenesis. In the first stage infiltration of slab fluids promotes partial melting in the mantle to generate primitive wet basaltic magmas enriched in LREE, LILE, Th and U in comparison to N-type MORB. In the second stage storage and crystal differentiation of primitive magmas occurred in the lithospheric mantle or deep crust, involving olivine, spinel and clinopyroxene followed by amphibole and plagioclase. In the third stage differentiated mafic magma intrudes into porphyritic silicic magma at shallower crustal levels (estimated at 7–10 km). Mingling and quenching of the mafic magmas within the silicic host causes chemical or physical interactions between the inclusions and the host prior to and during eruption. The silicic lavas have geochemical affinities with the mafic inclusions, but are relatively depleted in MREE, HREE and Y and enriched in Rb relative to Ba and K. These observations are consistent with involvement of amphibole in magma genesis due either to crystal differentiation from wet basalt or to partial melting of mafic rocks with residual amphibole. Crystallization of wet basalt in the deep crust is preferred on the basis of physical considerations.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: I. Parsons  相似文献   

16.
The Miocene Karamağara volcanics (KMV) crop out in the Saraykent region (Yozgat) of Central Anatolia. The KMV include four principal magmatic components based on their petrography and compositional features: basaltic andesites (KMB); enclaves (KME); andesites (KMA); and dacites (KMD). Rounded and ellipsoidal enclaves occur in the andesites, ranging in diameter from a few millimetres to ten centimetres. A non‐cognate origin for the enclaves is suggested due to their mineralogical dissimilarity to the enclosing andesites. The enclaves range in composition from basaltic andesite to andesite. Major and trace element data and primitive mantle‐normalized rare‐earth element (REE) patterns of the KMV exhibit the effects of fractional crystallization on the evolution of the KME which are the product of mantle‐derived magma. The KMA contain a wide variety of phenocrysts, including plagioclase, clinopyroxene, orthopyroxene, hornblende and opaque minerals. Comparison of textures indicates that many of the hornblende phenocrysts within the KMA were derived from basaltic andesites (KMB) and are not primary crystallization products of the KMA. Evidence of disequilibrium in the hybrid andesite includes the presence of reacted hornblendes, clinopyroxene mantled by orthopyroxene and vice versa, and sieve‐texture and inclusion zones within plagioclase. The KMV exhibit a complex history, including fractional crystallization, magma mixing and mingling processes between mantle and crust‐derived melts. Textural and geochemical characteristics of the enclaves and their hosts require that mantle‐derived basic magma intruded the deep continental crust followed by fractional crystallization and generation of silicic melts from the continental material. Hybridization between basic and silicic melts subsequently occurred in a shallow magma chamber. Modelling of major element geochemistry suggests that the hybrid andesite represents a 62:38 mix of dacite and basaltic andesite. The implication of this process is that calc‐alkaline intermediate volcanic rocks in the Saraykent region represent hybrids resulting from mixing between basic magma derived from the mantle and silicic magma derived from the continental crust. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

17.
The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K2O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas.  相似文献   

18.
Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO2), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60–62% SiO2, <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase phenocrysts are absent, and evenly distributed but sparse (0.5–3.5%) quartz xenocrysts are present in all the lavas. In order to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages (either olivine + augite or opx + augite) are temperatures in excess of 1000 °C, with water saturated liquids (>3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have ∼2 wt% lower Al2O3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al2O3 concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators of mantle melting processes. Received: 29 July 1997 / Accepted: 29 January 1998  相似文献   

19.
Reconciling the diverse records of magmatic events preserved by multiple crystals and minerals in the same sample is often challenging. In the case of basaltic–andesites from Volcán Llaima (Chile), Mg zoning in olivine is always simpler than Ca zoning in plagioclase. A model that explains a number of chemical patterns is that Llaima magmas stall in the upper crust, where they undergo decompression crystallization and form crystal-mush bodies. Frequent magma inputs from deeper reservoirs provide the potential for remobilization and eruption. The records of multiple recharge events in Llaima plagioclase versus an apparent maximum of one such event in coexisting olivine are addressed by using trace element zoning in olivine phenocrysts. We have integrated elements that (1) respond to changes in magma composition due to recharge or mixing (Mg, Fe, Ni, Mn, ±Ca), with (2) elements that are incorporated during rapid, disequilibrium crystal growth (P, Ti, Sc, V, Al). A more complex history is obtained when these elements are evaluated considering their partition coefficients, diffusivities, and crystal growth rates. The olivine archive can then be reconciled with the plagioclase archive of magma reservoir processes. Olivine (and plagioclase) phenocrysts may experience up to three or more recharge events between nucleation and eruption. Diffusion modeling of major and trace element zoning in two dimensions using a new lattice Boltzmann model suggests that recharge events occur on the order of months to a couple of years prior to eruption, whereas crystal residence times are more likely to be on the order of a few years to decades.  相似文献   

20.
The volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

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