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1.
Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO2) to felsic (65–67 wt% SiO2), temperature has varied by about 150°C (770°–920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period.  相似文献   

2.
The 1.3 Ma Purico complex is part of an extensive Neogene-Pleistocene ignimbrite province in the central Andes. Like most other silicic complexes in the province, Purico is dominated by monotonous intermediate ash-flow sheets and has volumetrically minor lava domes. The Purico ignimbrites (total volume 80-100 km3) are divided into a Lower Purico Ignimbrite (LPI) with two extensive flow units, LPI I and LPI II; and a smaller Upper Purico Ignimbrite (UPI) unit. Crystal-rich dacite is the dominant lithology in all the Purico ignimbrites and in the lava domes. It is essentially the only lithology present in the first LPI flow unit (LPI I) and in the Upper Purico Ignimbrite, but the LPI II flow unit is unusual for its compositional diversity. It constitutes a stratigraphic sequence with a basal fall-out deposit containing rhyolitic pumice (68-74 wt% SiO2) overlain by ignimbrite with dominant crystal-rich dacitic pumice (64-66 wt% SiO2). Rare andesitic and banded pumice (60-61 wt% SiO2) are also present in the uppermost part of the flow unit. The different compositional groups of pumice in LPI II flow unit (rhyolite, andesite, dacite) have initial Nd and Sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice ()Nd=-6.7 to -7.2 and 87Sr/86Sr=0.7085-0.7090). However, two lines of evidence show that the andesite, dacite and rhyolite pumices do not represent a simple fractionation series. First, melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples. Second, mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre-eruptive temperatures (780-800 °C), whereas the rhyolites and andesites yield consistently higher temperatures (850-950 °C). Hornblende geobarometry and pressure constraints from H2O and CO2 contents in melt inclusions indicate upper crustal (4-8 km) magma storage conditions. The petrologic evidence from the LPI II system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than, and chemically unrelated to, the underlying dacite. We suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic Purico magma chamber. Rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite. Isotopic and compositional variations in the Purico system are typical of those seen throughout the Neogene ignimbrite complexes of the Central Andes. These characteristics were generated at moderate crustal depths (<30 km) by crustal melting, mixing and homogenization involving mantle-derived basalts. For the Purico system, assimilation of at least 30% mantle-derived material is required.  相似文献   

3.
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   

4.
Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.  相似文献   

5.
The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.  相似文献   

6.
Volcán Ceboruco, Mexico, erupted ~1,000 years ago, producing the Jala pumice and forming a ~4-km-wide caldera. During that eruption, 2.8 to 3.5 km3 of rhyodacite (~70 wt% SiO2) magma and 0.2 to 0.5 km3 of mixed dacite (~67 wt% SiO2) magma were tapped and deposited as the Jala pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis dome, a low-silica (~64 wt% SiO2) dacite dome with a volume of ~1.3 km3. Petrographic evidence indicates that the Jala dacite and Dos Equis dacite originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Linear least-squares modeling and detailed modal analysis indicate that the Jala dacite is predominantly a bimodal mixture of rhyodacite and dacite with a small additional mafic component, whereas the Dos Equis dacite is composed of mostly dacite mixed with subordinate amounts of rhyodacite and mafic magma. According to Fe–Ti oxide geothermometry, before the caldera-forming eruption the rhyodacite last equilibrated at ~865 °C, whereas the dacite was originally at ~890 °C but was heated to ~960 °C by intrusion of mafic magma as hot as ~1,030 °C. Zoning profiles in plagioclase and/or magnetite phenocrysts indicate that mixing between mafic and dacite magma occurred ~34–47 days prior to eruption, whereas subsequent mixing between rhyodacite and dacite magmas occurred only 1–4 days prior to eruption. Following the caldera-forming eruption, continued inputs of mafic magma led to effusion of the Dos Equis dome dacite. In this case, timing between mixing and eruption is estimated at ~93–185 days based on the thickness of plagioclase overgrowth rims.Editorial responsibility: T.L. Grove  相似文献   

7.
Quizapu is part of a linear system of active volcanos in central Chile. The volcanic petrology and geology have been used to infer the plumbing system beneath the volcano. The 1846–1847 eruption (~5 km3) started with small flows of dacite, then changed to a range of andesite–dacite compositions and finally terminated with large flows of dacite. Andesitic enclaves (<10 %) occur in some of these flows. Activity restarted explosively in 1932 (~4 km3 DRE) with an initial andesite–dacite ash, followed by uniform dacite ash and then a terminal andesite ash. All samples, including the enclaves, have chemical compositions that lie on an almost perfect mixing line, with a few exceptions. The abundant plagioclase macrocrysts in the matrix were divided into five petrographic classes on the basis of colour in cold-cathode cathodoluminescence images and zonation in visible light. All populations of macrocrysts have CSDs characteristic of coarsening, although they differ in detail. Two classes can be ascribed to growth in andesite and dacite magmas, but the three other classes are associated with particular magma batches. A model is developed which started with ponding of andesite magma in the crust. This differentiated to produce a dacite magma, most of which probably solidified to make a granodiorite batholith. Activation of a N–S fault enabled volcanism: andesite magma traversed the dacite-filled chamber, heating and raising it up into storage areas hosted by the fault, where it mixed to form a homogeneous magma. A short time before the 1846–1847 eruption, more andesite magma was injected into the shallow part of the system where it mingled with existing mixed magmas. The first magma to be erupted from Quizapu was a dacite, but soon other storage areas along the fault started to feed the system—first mixed magmas, then back to dacites. The eruption then terminated until 1932 when renewed injection of andesite into the system created a conduit that tapped an undegassed dacite chamber and resulted in a strong explosive eruption. The whole story is one of continual andesite magmatism, modulated by storage, degassing and mixing.  相似文献   

8.
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   

9.
Between 1953 and 1974, approximately 0.5 km3 of andesite and dacite erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest (or New) Trident. Field, analytical, and experimental evidence shows that the eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. Four lava flows (58.3–65.5 wt% SiO2) are the dominant products of the eruption; these contain discrete andesitic enclaves (55.8–58.9 wt% SiO2) as well as micro- and macro-scale compositional banding. Tephra from the eruption spans the same compositional range as lava flows; however, andesite scoria (56–58.1 wt% SiO2) is more abundant relative to dacite tephra, and is the explosively erupted counterpart to andesite enclaves. Fe–Ti oxide pairs from andesite scoria show a limited temperature range, clustered around 1000 °C. Temperatures from grains found in dacite lavas possess a wider range; however, cores from large (>100 μm) magnetite and coexisting ilmenite give temperatures of ∼890 °C, taken to represent a pre-mixing temperature for the dacite. Water contents from dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ∼3.5 wt% H2O, equivalent to 3 km depth if saturated. Unzoned pyroxene and sodic plagioclase in the dacite suggest that it likely underwent significant crystallization at this depth; highly resorbed anorthitic plagioclase from the andesite suggests that it originated at greater depths and underwent relatively rapid ascent until it reached 3 km, mixed with dacite, and erupted. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of earliest lava by approximately one month. The lack of a compositional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly, via disaggregation of enclaves, phenocryst transfer from one magma to another, and direct mixing of compositionally distinct melt phases. Received: 22 September 1999 / Accepted: 4 April 2000  相似文献   

10.
Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H2O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage.  相似文献   

11.
The Neogene–Quaternary volcanic products, related to Arabian and Anatolian Plate collision along the Bitlis Suture Zone, cover wide areas on both plates. One of these volcanic exposures on the Arabian Plate is the Kepez volcanic complex (KVC). This study aims explain to petrogenesis of KVC. Although some examples display alkaline affinities, the majority of the volcanic rock is calc-alkaline and can be defined in three main groups. 40Ar/39Ar data obtained from dacite, basalt and andesite rock groups within the KVC yield ages of between 13.5 and 15.5 Ma. Geochemical and petrographical data show that the andesitic rocks are products of homogeneous mixing between basic end-member magmas and dacitic magmas which are the products of partial melting of lower crustal compositions. Basaltic products of KVC are asthenospheric mantle derived, while dacitic and andesitic volcanic rocks are crustal origin. High Sr and Nd isotope ratios may indicate that andesitic and dacitic rocks originated from continental crust. The lithospheric mantle, which is subducting underneath the Anatolian plate, must have experienced slab break-off processes 13–15 million years ago and sunk into the asthenosphere. KVC were produced with the collision between Arabian and Anatolian Plates and related uplift of the East Anatolia region.  相似文献   

12.
A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56–72 wt % SiO2) and the Strawberry Crater flows range from basalt to dacite (49–64 wt % SiO2). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO2, Al2O3, MnO, Na2O, K2O, and Zr and poorer in MgO, CaO, P2O5, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks, including the andesite xenoliths, contain xenocrysts of quartz, olivine and oligoclase, together with reversely zoned plagioclase and pyroxene phenocrysts. The abundance of intermediate volcanic rocks in the SFVF, as observed in detail at O'Leary Peak and Strawberry Crater, is due in part to crustal recycling, the result of basalt-driven crustal melting and the subsequent mixing of the silicic melts with basalts and derivative magmas.  相似文献   

13.
Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher 87Sr/86Srratios than the basalts but lower 143Nd/144Nd, 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.  相似文献   

14.
Rosanna De Rosa  Paola Donato  Guido Ventura   《Lithos》2002,65(3-4):299-311
Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO2 rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO2 rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO2 rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10−4 and 10−2 m. Morphological and compositional features of the Upper Pollara pumices result from turbulence.  相似文献   

15.
Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors.  相似文献   

16.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.  相似文献   

17.
 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   

18.
Geological mapping of the Tucumã area has enabled the identification of dike swarms intruded into an Archean basement. The disposition of these dikes is consistent with the well-defined NW-SE trending regional faults, and they can reach up to 20 km in length. They were divided into three main groups: (i) felsic dikes (70% of the dikes), composed exclusively of porphyritic rhyolite with euhedral phenocrysts of quartz and feldspars immersed in an aphyric felsite matrix; (ii) mafic dikes, with restricted occurrence, composed of basaltic andesite and subordinate basalt, with a mineralogical assembly consisting dominantly of plagioclase, clinopyroxene, orthopyroxene and olivine; and (iii) intermediate rocks, represented by andesite and dacite. Dacites are found in outcrops associated with felsic dikes, representing different degrees of hybridization or mixture of mafic and felsic magmas. This is evidenced by a large number of mafic enclaves in the felsic dikes and the frequent presence of embayment textures. SHRIMP U-Pb zircon dating of felsic dikes yielded an age of 1880.9 ± 3.3 Ma. The felsic dikes are peraluminous to slightly metaluminous and akin to A2, ferroan and reduced granites. The intermediate and mafic dikes are metaluminous and belong to the tholeiitic series. Geochemical modeling showed that mafic rocks evolved by pyroxene and plagioclase crystallization, while K-feldspar and biotite are the fractionate phases in felsic magma. A simple binary mixture model was used to determine the origin of intermediate rocks. It indicated that mixing 60% of rhyolite and 40% basaltic andesite melts could have generated the dacitic composition, while the andesite liquid could be produced by mixing of 60% and 40% basaltic andesite and rhyolite melts, respectively. The mixing of basaltic and andesitic magmas probably occurred during ascent and storage in the crust, where andesite dikes are likely produced by a more homogeneous mixture at high depths in the continental crust (mixing), while dacite dikes can be generated in the upper crust at a lower temperature, providing a less efficient mixing process (mingling). The affinities observed between the felsic to intermediate rocks of the Rio Maria and São Felix do Xingu areas and the bimodal magmatism of the Tucumã area reinforce the hypothesis that in the Paleoproterozoic the Carajás province was affected by processes involving thermal perturbations in the upper mantle, mafic underplating, and associated crustal extension or transtension. The 1.88 Ga fissure-controlled A-type magmatism of the Tucumã area was emplaced ∼1.0 to ∼0.65 Ga after stabilization of the Archean crust. Its origin is not related to subduction processes but to the disruption of the supercontinent at the end of the Paleoproterozoic.  相似文献   

19.
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   

20.
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   

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