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1.
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
2.
R. S. J. Sparks 《Contributions to Mineralogy and Petrology》1988,100(4):446-461
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
3.
The study area in the northwest Sinai represents one of the most significant regions in the Egyptian basement intensely invaded by post-orogenic calc-alkaline dyke swarms. Two post-orogenic dyke swarms have been recognized in NW Sinai namely: (1) mafic dykes of basalt, basaltic andesite and andesite composition and (2) felsic dykes of dacite, rhyodacite and rhyolite composition. These basaltic to rhyolitic dykes intruded contemporaneously and shortly after the intrusion of the post-orogenic leucogranite. The mafic and felsic dykes are enriched in incompatible elements, especially in the large ion lithophile elements (e.g. K, Rb, Ba) and depleted in high field strength elements with negative P, Ti and Nb anomalies. Major and trace element geochemistry indicates that investigated mafic and felsic magma types are not related via fractional crystallization. The protoliths of the mafic and felsic dykes appear to have evolved from different parental magmas. The incompatible trace element patterns favour a derivation of the mafic dykes from melting of a garnet peridotite source followed by fractional crystallization of olivine, clinopyroxene amphibole and zircon. The felsic dykes, on the other hand, could be generated by melting of garnet-free source modified subsequently by fractional crystallization of plagioclase, apatite and titanomagnetite. This implies variable source characteristics at the end of the Pan-African in the NW Sinai.The mafic and felsic dykes can be related to an intracontinental setting and that this was accompanied by a chemical evolution of the subcontinental lithosphere. Magma generation and ascent in the area was favoured by extensional movements, which is already known from other areas in NE Africa. 相似文献
4.
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
5.
The island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite). The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel. 相似文献
6.
Amphibole megacrysts as a probe into the deep plumbing system of Merapi volcano,Central Java,Indonesia 总被引：1，自引：0，他引：1
Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts’ parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts’ parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO2 in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6–2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H2 loss) and/or dehydration (H2O loss), as recorded by their variable H2O contents and D/H and Fe3+/Fe2+ ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi’s often erratic eruptive behaviour. 相似文献
7.
The present study deals with geochemical characteristics and petrogenesis of three younger granite varieties (coarse-grained biotite-muscovite granites (CBG), garnetiferous muscovite granites (GMG) and Abu Aggag biotite granites (AAG)) in El-Hudi area, east of Aswan, southeastern desert of Egypt. Mineral chemistry and whole rock chemistry data revealed that all granites have high SiO2 (70.8-74.7 wt.%), Al2O3 (12.8-14.3 wt.%), Na2O and K2O (>3.2 wt.%) contents with high Na2O/K2O ratios (～>1). Plagioclase feldspars range in composition from albite to oligoclase (An9-27) in CBG, oligoclase (An13-18) in GMG and albite (An2-6) in AAG. Potash feldspars are mainly perthitic microcline and exhibit chemical formulae as (Or93-96 Ab7-4 An0) in CBG, (Or95-98 Ab5-2 An0) in GMG and (Or82-98 Ab18-2 An0) in AAG. Biotites from CBG and GMG are enriched in (Mg and Ti) and depleted in (Al, Fe, Mn and K) compared with those of AAG. Biotites from CBG and GMG had been derived from calc-alkaline magma, whereas those from AAG had been derived from peraluminous magma. Chlorites from CBG and GMG are Mg-Fe bearing, while that from AAG is Fe-rich chlorite (chamosite). The CBG and GMG are Mg-rich monzogranites originated from high-K calc-alkaline magma with metaluminous to mildly peraluminous nature. The AAG are Fe-rich monzogranites to syenogranites generated from high-K calc-alkaline peraluminous magma. Both CBG and GMG are late- to post-orogenic granites, while the AAG are post-orogenic granites. All three granite varieties are considered as evolved I-type granites, formed under low to moderate water pressures (～ 0.5-7 kbars) and relatively high ranges of crystallization temperatures (～700-890°C). They were generated from partial melting of crustal materials at lower (CBG >30 km depth) and intermediate (GMG & AAG ～20-30 km depth) levels. The crystal fractionation was the predominant process during differentiation of parent magmas of these granites. Geochemical characteristics manifest that AAG represent the highly fractionated member of magma cycle differs from that produced CBG and GMG. The CBG are relatively enriched in both U and Th existing only within the accessory minerals such as zircon, sphene, and allanite. 相似文献
8.
Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California 总被引：13，自引：1，他引：13
Timothy L. Grove Julie M. Donnelly-Nolan Todd Housh 《Contributions to Mineralogy and Petrology》1997,127(3):205-223
Glass Mountain consists of a 1 km3, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust.
Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain.
Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H2O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional
crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated
at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5
km3) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and
provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled
from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two
distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of
the andesite at pressures of at least 200 MPa (6 km depth) under near H2O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing
magmatic inclusions indicates that crystallization under near H2O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence
indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction
with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the
crust.
Received: 26 March 1996 / Accepted: 14 November 1996 相似文献
9.
N. C. Stevens 《Australian Journal of Earth Sciences》2013,60(1):37-39
Two ring‐structures, one a ring‐dyke complex, the other a solitary ring‐dyke, have intruded gently‐dipping Mesozoic sedimentary rocks at Mt. Alford, S.S.W. of Brisbane. In the ring‐dyke complex, a central stock of porphyritic microdiorite with closely associated granophyre has dragged up the surrounding sandstones and tilted them to a vertical position around its northern margin. Narrow, steeply‐dipping ring‐dykes of rhyolite and trachyte have intruded the upturned sediments and andesite has invaded the stock, probably as a result of cauldron subsidence. Further dyke intrusions of rhyolite and trachyte and explosive activity were followed by a basaltic dyke swarm. The complex is adjoined on the south and east by stocks of rhyolite, a neck of bedded breccia with centroclinal dips and a ring‐dyke of alkaline rhyolite. It is suggested that the alternation of acid and basic magma is due to tapping of a magma reservoir which rises and falls in the crust, deriving acid magma from the sial. The present highest point in the complex is thought to be below the original surface at the time of intrusion (probably early Tertiary). 相似文献
10.
We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst
assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite,
plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with
a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with
only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved
water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst
assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a
wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition
with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures
below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the
andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages
for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of
initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite
parent by hornblende-plagioclase fractionation under the same hydrous conditions.
Received: 10 December 1996 / Accepted: 21 August 1997 相似文献
11.
Post-Collisional Miocene Alkaline Volcanism in the Oglakçi Region,Turkey: Petrology and Geochemistry
《International Geology Review》2012,54(7):640-660
Miocene volcanism of the Oglakci region (Sivrihisar, Eskisehir) in northwestern Central Anatolia, Turkey, is represented by basaltic and trachytic groups of rocks. Samples of both groups have been investigated using mineral-chemical data together with whole-rock major-, trace-element, and radiogenic Sr-Nd isotopic data. The basaltic volcanic rocks consist of mugearites and shoshonites, whereas the trachytic rocks include trachytes, latite, and rhyolite. Both groups are of alkaline character. The basaltic rocks contain plagioclase (An29-63), alkali feldspar (Or12-74), olivine, orthopyroxene (En64-67), clinopyroxene (Wo43-48), biotite (Mg#82-88), and Fe-Ti oxide phenocrysts, whereas the trachytic rocks contain plagioclase (An21-64), alkali feldspar (Or10-53), clinopyroxene (Wo41-49), amphibole (Mg#64-83), biotite (Mg#79-85), Fe-Ti oxide, titanite, apatite, and quartz phenocrysts. The measured 87Sr/86Sr ratios of basaltic samples range from 0.7045 to 0.7048, and those of trachytic samples from 0.7054 to 0.7056. The basaltic samples have 143Nd/144Nd ratios ranging from 0.512753 to 0.512737, and those of trachytic samples are 0.512713 to 0.512674. Isotopic, major-, and trace-element data suggest that the Oglakci volcanic rocks are products of postcollisional magmatism and originated from a complex interplay of crustal assimilation, magma mixing, and fractional crystallization processes following the demise of Neotethys. Trace-element characters also are consistent with an OIB-like mantle source. These volcanic rocks probably were associated with extensional tectonics, which occurred within the Anatolian plate as a result of collision of the Eurasian and Afro-Arabian plates during the neotectonic evolution of Turkey. 相似文献
12.
M. Mattioli G. Serri E. Salvioli-Mariani A. Renzulli P. M. Holm P. Santi G. Venturelli 《Mineralogy and Petrology》2003,78(3-4):201-230
Summary ¶Fine- to coarse-grained plutonic nodules within the Petrazza pyroclastics (Paleo-Stromboli I period) consist of gabbroic rocks with variable amounts of interstitial material. They are characterised by cumulate textures and low pressure modal mineralogy formed by plagioclase (An96–87)+clinopyroxene (Mg-v 82–94)+olivine (Fo83–74)±amphibole±opaque minerals; the interstitial material consists of newly crystallised microlites (quenching) of plagioclase (An73–55)+amphibole+clinopyroxene±olivine±biotite±opaques and highly variable amounts of residual glasses that range in composition from shoshonite and high-K basaltic andesite to high-K andesite and latite. The interstitial material has a relatively high but variable degree of vesicularity. The whole rock incompatible element abundances are lower than – but the patterns are typical of – in subduction related magmas and the incompatible trace-elements are well correlated with the amount of the interstitial material. The Sr, Pb and Nd isotopic ratios resemble those of the extrusive rocks of Stromboli older series and the mineral chemistry of the gabbros is similar to that of the HKCA Paleo-Stromboli lavas. Modal mineralogy, mineral chemistry and chemical-isotopic whole rock compositions suggest that the cumulus portions of the gabbroic nodules crystallised from basaltic magmas compositionally compatible with those erupted by Stromboli volcano. The interstitial material does not represent the residual liquid after in situ crystallisation of the gabbros; it is also distinct from the juvenile host andesite magma. Textural evidence, Fe–Mg mineral/liquid partioning and mass balance calculations indicate that the interstitial material (quench crystals and vesicular glass) derived from infiltrated hydrous basaltic liquid undercooling and vesiculation of which occurred during the eruption of the Petrazza pyroclastics.Received April 17, 2002; revised version accepted November 14, 2002
Published online June 2, 2003 相似文献
13.
Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with XMg[=Fetot/(Fetot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, XMg[=Mg/(Mg+Fetot)]=0.70-0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab75−25An65−35) and as minor quartz. The compositions of the KFMs range from Or95Ab5An0 to Or82Ab17An1. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks.The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma. 相似文献
14.
河北武安坦岭多斑斜长斑岩的成因:冻结岩浆房活化机制 总被引：5，自引：3，他引：2
流变学实验表明,当岩浆中晶体体积分数达到约50vol%时,岩浆体实际上处于冻结状态,不再具有整体迁移的能力。但在自然界中仍存在含大量斑晶的浅成火成岩和火山岩。因此,富晶体岩浆的上升过程和侵位机制是近年来地球科学领域关注的热点之一。目前,冻结岩浆房的活化机制主要有二种:升温活化机制和流体活化机制。河北武安坦岭地区新发现的多斑斜长斑岩为揭示冻结岩浆房的活化提供了契机。野外观察和晶体粒度分布(CSD)分析表明,坦岭斜长斑岩中斜长石斑晶高达70vol%,基质为显微晶质结构。斜长石斑晶粒径分布均一,大小约为3.1×1.7mm;显微镜观察和背散射图像揭示,斜长石斑晶具环带结构,由宽广的斜长石核部+宽度可变的条纹长石边部组成,且无熔蚀现象;电子探针成分剖面分析表明,斑晶核部成分为更长石(An_(27)Ab_(71)Or_2),幔部为更长石(An_(13)Ab_(83)Or_4),边部为条纹长石。边部条纹长石的成分有一定变化,从内侧到外侧,主晶钠长石成分由Ab_(53)Or_(47)变为Ab_(99)Or_1,客晶钾长石成分由Ab_(48)Or_(51)变为Ab3Or97。斑晶斜长石核部存在细长条状或斑点状钾长石,且越靠近中心,钾长石斑点的数量越少。这些特点表明,边部条纹长石为交代成因。稀土和微量元素分析则显示,边部条纹长石具弱正Eu异常,相对富集LREE和K、Rb、Ba、Sr等大离子亲石元素,亏损Th、Zr、Nb的特点。CSD相关图解及以上特征表明,斜长石斑晶形成于稳定,封闭的结晶环境,并受到晚期碱交代作用的改造。基质主要由微粒钙质角闪石,条纹长石,石英,钾长石和钠长石组成,含少量自形-半自形磁铁矿和钛铁矿、磷灰石、榍石、金红石和锆石等11种矿物组成。11种矿物相和结构特征暗示基质形成于极端不稳定的结晶环境,与斜长石斑晶形成条件鲜明对照。根据基质的矿物组成,推测形成基质的岩浆具有富含K、Na、Fe、Si和挥发分的特征。这种特征与上述关于条纹长石环边形成条件的判断一致。据此,本文认为:产生斜长石斑晶的岩浆曾经在地壳深部作过长时间滞留,导致了斜长石的稳定结晶,增加了岩浆的粘度和密度,使岩浆处于冻结状态;富碱高铁熔体-流体流的注入大幅降低了岩浆的总粘度,并提高了岩浆的浮力,从而促使冻结岩浆房迅速活化和上升侵位;同时,富碱高铁熔体-流体流强烈交代了先存的斜长石斑晶,使其边部形成条纹长石;这种熔体-流体流则在快速排气,冷却过程中迅速结晶,形成了具有不平衡矿物组合的显微晶质基质。在岩浆侵入体较深部位,富碱高铁熔体-流体经历了很缓慢的固结过程,而相分离产生的流体有可能萃取携带岩浆中的铁质,形成富Fe流体流,后者可能对区内"铁矿浆"型铁矿的形成具有重要的贡献。 相似文献
15.
Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon 总被引：3，自引：0，他引：3
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
16.
Possible role of amphibole in the origin of andesite: some experimental and natural evidence 总被引：1，自引：1，他引：1
Experiments in the system high-A1 basalt (HAB)-water have been conducted in the melting range at pressures between 1 atm. and 10 kbar, defining the amphibole stability field and the composition of liquids which coexist with this amphibole. Plagioclase is the anhydrous liquidus phase between 1 atm. and 10 kbar but in the hydrous runs this role is taken by olivine at <7 kbar and then by clinopyroxene at higher pressures. Because amphibole is never on the high-A1 basalt liquidus it is not likely that andesite is derived from primary basalt by pure fractional crystallisation, although as we discuss, other mechanisms including equilibrium crystallisation might implicate amphibole. If primary basaltic magma undergoes closed-system equilibrium crystallisation, then the amphibole field will be intersected at between 50 and 100°C below the liquidus. The compositions of melts coexisting with amphibole alone do not match those of any of the natural andesite or dacitic lavas associated with the particular high-A1 basalt investigated. Like natural andesites, they become rapidly silica enriched, but they also become far more depleted in TiO2 and MgO. However, the compositions of liquids lying directly on the divariant amphibole-out reaction zone, where amphibole +liquid coexist with clinopyroxene or olivine (±plagioclase), do resemble those of naturally occurring low-silica andesites. With increasing temperature pargasitic amphibole breaks down via incongruent melting reactions over a narrow temperature range to form a large volume of relatively low-silica basaltic andesite liquid and a crystalline assemblage dominated by either clinopyroxene or olivine. Our important conclusion is that basaltic andesite liquid will be the product of reaction between cooling, hydrous mafic liquid and anhydrous ferromagnesian phases. The solid reactants could represent earlier cumulates from the same or different magma batches, or they could be peridotite wall-rock material. Because the amphibole-out boundary coexisting with liquid is one of reaction, it will not be traversed so long as the phases on the high temperature side remain. Thus, the assemblage amphibole+clinopyroxene±olivine±plagioclase+liquid is one in which the liquid is buffered (within limits), and results reported here indicate that this buffering generates melts of low-silica andesite composition. When tapped to lower pressures these liquids will rise, eventually to fractionate plagioclase-rich assemblages yielding silicarich andesite and dacite melts. Conversely, the partial melting of hornblende pyroxenite, hornblende peridotite or hornblende gabbro can also yield basaltic andesite liquids. The phase relationships suggested by these experiments are discussed in the light of naturally occurring phenocryst and xenolith assemblages from the east Sunda Arc. Primary magmatic additions to the lithosphere of volcanic arcs are basaltic and voluminous upper crustal andesite in these terranes, complemented by mafic and ultramafic crystalline deposits emplaced in the lower crust or close to the Moho. Together these components constitute total arc growth with a basaltic composition and represent the net accreted contribution to continental growth. 相似文献
17.
Ezzat Abdel Rahman 《Arabian Journal of Geosciences》2014,7(3):877-888
The Precambrian basement rocks exposed along Qift–Quseir asphaltic road, central Eastern Desert of Egypt, exhibit two contrasted tectonic units, each of which has its own lithology structural style and grade of metamorphism. They are intruded by dolerite and diorite dykes. The alkali (Na2O+K2O) and TiO2 contents increase whereas Al2O3, FeO, MgO, CaO and MnO decrease with increasing SiO2 from dolerites to diorites. The trace elements Ti, Zr, Cr, Y and Ni indicate that the dolerites are tholeiitic with slight tendency toward calc-alkaline and formed from basaltic magma in an active continental margin, while diorites are calc-alkaline and were formed by fractional crystallisation of high-alumina basaltic magma in an island arc and active continental margin tectonic environment where they probably represent the forerunner of G1 granites. The molecular ratios Mg values (MgO×100/MgO+FeO) of dolerites range from 47 to 49 while those of diorite range from 51 to 59, indicating that the dolerite and diorite have suffered mild fractionation. Mineral chemistry for the diorites shows that the amphiboles are classified as magnesiohornblende and the plagioclase composition is An39–42 (i.e. the narrow range indicate that the pluton has not suffered extensive fractional crystallisation). The Al content of amphibole displays significant variation with pressure and temperature, also the change of the Ca/(Ca+Na) ratio of plagioclase is dependent on temperature. The amphibole–plagioclase geothermobarometer suggested the P-T formation conditions of studied dykes as 2 kbar and 600 °C. 相似文献
18.
K. P. Sarma M. Venkateshwarlu S. K. Patil J. J. Laskar Niva Rani Devi J. Mallikharjuna Rao 《Journal of the Geological Society of India》2014,83(2):147-155
The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α95 = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite. 相似文献
19.
Abhishek Saha Avik Dhang Jyotisankar Ray Suvankar Chakraborty David Moecher 《Journal of Earth System Science》2010,119(3):365-381
Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair
(11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite
at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous
layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed
pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and
are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying
from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic
dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite
suite. 相似文献
20.
We studied the mineralogy, mineral chemistry, and compositions of 48 interior silicate inclusions and a large K-rich surface inclusion from the Colomera IIE iron meteorite. Common minerals in the interior silicate inclusions are Cr diopside and Na plagioclase (albite). They are often enclosed by or coexist with albitic glasses with excess silica and minor Fe-Mg components. This mineral assemblage is similar to the “andesitic” material found in the Caddo County IAB iron meteorite for which a partial melt origin has been proposed. The fairly uniform compositions of Cr diopside (Ca44Mg46Fe10) and Na plagioclase (Or2.5Ab90.0An7.5 to Or3.5Ab96.1An0.4) in Colomera interior inclusions and the angular boundaries between minerals and metal suggest that diopside and plagioclase partially crystallized under near-equilibrium conditions from a common melt before emplacement into molten metal. The melt-crystal assemblage has been called “crystal mush.” The bulk compositions of the individual composite inclusions form an array between the most diopside-rich inclusion and plagioclase. This is consistent only with a simple mechanical mixing relationship, not a magmatic evolution series. We propose a model in which partly molten metal and crystal mush were mixed together by impact on the IIE parent body. Other models involving impact melting of the chondritic source material followed by growth of diopside and plagioclase do not easily explain near equilibrium growth of diopside and Na plagioclase, followed by rapid cooling. In the K-rich surface inclusion, K feldspar, orthopyroxene, and olivine were found together with diopside for the first time. K feldspar (sanidine, Or92.7Ab7.2An0.1 to Or87.3Ab11.0An1.7) occurs in an irregular veinlike region in contact with large orthopyroxene crystals of nearly uniform composition (Ca1.3Mg80.5Fe17.8 to Ca3.1Mg78.1Fe18.9) and intruding into a relict olivine with deformed-oval shape. Silica and subrounded Cr diopside are present within such K-feldspar regions. Some enrichments of the albite component have been detected at the end of curved elongated nodules of K feldspar intruded into the mafic silicates. The textural relationships suggest that a K-rich melt was present. A K-rich melt is neither the first melt of a chondritic system nor a differentiation product of a Na-rich partial melt of chondritic material. The K-rich material may have originated as a fluid phase that leached K from surrounding materials and segregated by a mechanism similar to that proposed for the Na-rich inclusions. 相似文献