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1.
顾枫华  章永梅  刘瑞萍  郑硌  孙玄 《岩石学报》2015,31(5):1374-1390
华北地台北缘乌拉山地区的沙德盖钾长花岗岩体中普遍发育以二长岩为主的暗色微粒包体,包体具塑性流变特征,与寄主岩的接触界线或为截然或为渐变过渡。岩相学观察表明,包体中发育多种反映岩浆混合作用的典型组构,如石英眼斑、环斑长石、镁铁质团块、钾长石巨晶的溶蚀、磷灰石的针柱状形貌、长石中的包体带以及钙长石的"针尖"结构等。造岩矿物的电子探针分析表明,岩浆混合在沙德盖岩体的形成中起了重要作用,寄主花岗岩浆主要来自下地壳,而暗色包体岩浆则主要为地幔来源。锆石LA-ICP-MS U-Pb同位素定年结果显示,沙德盖花岗岩及其暗色微粒包体的形成时代基本一致,分别为233.4±2.3Ma和229.7±1.5Ma(中三叠世),进一步佐证了该岩体是岩浆混合作用的产物。研究认为,当铁镁质岩浆与长英质岩浆混合时,早期基性岩浆的快速淬冷形成了边界清楚、具明显冷凝边且暗色矿物含量较高的包体;随着两种不同成分岩浆之间温差的减小以及组分的交换,进一步形成了颜色较浅、边界渐变过渡和无明显冷凝边的包体。  相似文献   

2.
Calc-alkaline, metaluminous granitoids in the north of Jonnagiri schist belt (JSB) are associated with abundant mafic rocks as enclave. The enclaves represent xenoliths of the basement, mafic magmatic enclaves (MME) and synplutonic mafic dykes. The MME are mostly ellipsoidal and cuspate shape having lobate margin and diffuse contact with the host granitoids. Sharp and crenulated contacts between isolated MME and host granitoids are infrequent. The MME are fine-grained, slightly dark and enriched in mafic minerals compare to the host granitoids. MME exhibits evidences of physical interaction (mingling) at outcrop scale and restricted hybridization at crystal scale of mafic and felsic magmas. The textures like quartz ocelli, sphene (titanite) ocelli, acicular apatite inclusion zone in feldspars and K-feldspar megacrysts in MME, megacrysts across the contact of MME and host and mafic clots constitute textural assemblages suggestive of magma mingling and mixing recorded in the granitoids of the study area. The quartz ocelli are most likely xenocrysts introduced from the felsic magma. Fast cooling of mafic magma resulted in the growth of prismatic apatite and heterogeneous nucleation of titanite over hornblende in MME. Chemical transfer from felsic magma to MME forming magma envisage enrichment of silica, alkalis and P in MME. The MME show low positive Eu anomalies whereas hybrid and host granitoids display moderate negative Eu-anomalies. Synplutonic mafic dyke injected at late stage of crystallising host felsic magma, display back veining and necking along its length. The variable shape, dimensions, texture and composition of MME, probably are controlled by the evolving nature and kinematics of interacting magmas.  相似文献   

3.
Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.  相似文献   

4.
The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ?Hf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ?Hf in two of the three analysed samples (8.8 and 10.1 ?Hf units) exceeds that expected from a single homogeneous population (～4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ?Nd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.  相似文献   

5.
In Bundelkhand Craton of central India, mafic dykes intruded when granitoids was partly crystallized. Cuspate–lobate boundary along the contact of granitoids and mafic magma indicates magma mingling in outcrop scale while textural evidence of mingling is represented by acicular apatite morphologies, titanite–plagioclase ocelli and ophitic–subophitic texture, mafic clots, resorbed plagioclase, and hornblende–zircon associations. Mingling also caused thermal exchange and fluid activity along the boundary between two coeval magmas. Crystal size distribution analyses for hornblende in the mafic rocks yield concave up curves which is also consistent with interaction of felsic and mafic magmas.  相似文献   

6.
Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.  相似文献   

7.
Miocene Ignimbrite ‘A’ on Gran Canaria containsthree compositional endmember fiamme types(two rhyolites andone trachyte) each of which crystallized distinct feldspar.Various textural and compositional criteria are interpretedas reflecting a complex scenario within the magma chamber inwhich the crystals formed. About 25–30% of the feldsparphenocrysts contain evidence for magma mixing in the form of(1) partial to severe dissolution–resorption rims, (2)distinct zones of drastically different compositions and (3)overgrowth textures on formerly resorbed crystals. Four majortypes of zoning in the oligoclase to anorthoclase feldsparsof ignimbrite ‘A’ include a normal and a reverselyzoned type and two complexly zoned types. The feldspars withnormal and reverse zonation show only minor compositional amplitudesbetween individual zones (  相似文献   

8.
Summary The composite Yozgat batholith consists of a S-I-A-type granitoid association intruding the supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and medium- to high-grade metasedimentary rocks of the central Anatolian crystalline complex. These rocks are unconformably covered by Palaeocene to Early Eocene sedimentary rocks. The I-type granitoids are the most common rock association of this huge batholith. In an area between the towns of Şefaatli and Yerk?y, the southwestern part of the batholith can be subdivided into five mappable units: the Ak?akoyunlu quartz monzodiorite (mafic; hornblende K-Ar cooling ages of 77.6–79.3 Ma); the Cankılı monzodiorite (mafic; hornblende K-Ar cooling age of 71.1 Ma); the Adatepe quartz monzonite (mafic; hornblende K-Ar cooling age of 68.0 Ma); the Yassıağıl monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 69.9–79.8 Ma) and the Karakaya monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 71.3–77.0 Ma). All the lithological units, except the Karakaya monzogranite, include large K-feldspar megacrysts and various types of mafic microgranular enclaves in field outcrops, indicating mingling and mixing. In addition, microscopic textures showing the hybridization between the coeval mafic and felsic magma sources are present. Whole-rock major element geochemistry shows a high-K calc-alkaline, metaluminous, I-type composition with an aluminium saturation index (ASI) less than 1.10 and with CIPW diopside content in all the lithological units. Large ion lithophile elements (LILE), light rare earth elements (LREE), some high field strength elements (HFSE) (except Nb) enrichments and significant crustal contribution revealed by the oxygen and sulphur stable isotope compositions in the mafic and felsic I-type granitoid units are consistent with mafic lower crustal and metasomatized mantle sources the latter of which were metasomatized by earlier supra subduction zone (SSZ)-derived fluids during the development of the SSZ-type central Anatolian ophiolite. Supplementary material to this paper is available in electronic form at  相似文献   

9.
In the Variscan Western Tatra granites hybridization phenomena such as mixing and mingling can be observed at the contact of mafic precursors of dioritic composition and more felsic granitic host rocks. The textural evidence of hybridization include: plagioclase?CK-feldspar?Csphene ocelli, hornblende- and biotite-rimmed quartz ocelli, plagioclase with Ca-rich spike zonation, inversely zoned K-feldspar crystals, mafic clots, poikilitic plagioclase and quartz crystals, mixed apatite morphologies, zoned K-feldspar phenocrysts. The apparent pressure range of the magma hybridization event was calculated at 6.1?kbar to 4.6?kbar, while the temperature, calculated by independent methods, is in the range of 810°C?770°C. U-Pb age data of the hybrid rocks were obtained by in-situ LA-MC-ICP-MS analysis of zircon. The oscillatory zoned zircon crystals yield a concordia age of 368?±?8?Ma (MSWD?=?1.1), interpreted as the age of magma hybridization and timing of formation of the magmatic precursors. It is the oldest Variscan magmatic event in that part of the Tatra Mountains.  相似文献   

10.
Summary Pyroclastites erupted from the Upper Pollara magma chamber (13 ka, Salina Island, Aeolian Archipelago) resulted from mingling and mixing of rhyolitic and andesitic magmas. An experimental study has been conducted on the rhyolitic end-member to constrain the pre-eruptive conditions of the magma. In order to check for the role of mixing on the equilibrium phase assemblage, three different starting compositions, corresponding to three different mixing degrees, have been used. The crystallization experiments were conducted at two different oxygen fugacities and at variable temperature and fluid contents. The results indicate that the natural mineralogical assemblage can only be reproduced from a composition showing a certain degree of mixing. Assuming a pressure of 200 MPa (generally accepted for the Aeolian Islands), the pre-eruptive temperature of the magmas is estimated between 755 and 800 °C and the water content of the melt was higher than 4–4.5 wt.%. The Upper Pollara magma crystallized at relatively high fO2 (ΔlogfO2 = Ni–NiO + 1 log unit), compared to rhyolitic magmas from Lipari and Vulcano. As this difference has not been observed for the most primitive magmas the difference in fO2 could be related to different degassing processes operating in Salina and Lipari – Vulcano magmas.  相似文献   

11.
We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30–40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (~70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.  相似文献   

12.
We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.  相似文献   

13.
 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   

14.
The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO2), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.  相似文献   

15.
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999  相似文献   

16.
Spinel-bearing peridotitic mantle xenoliths from the 1949 eruption on La Palma were modified mineralogically and chemically during prolonged reaction with their host magma. The magmatism that brought the peridotites to the surface caused two distinct generations of xenolith fractures: (1) Old fractures are characterized by crystalline selvages with cumulus textures towards the host magma, or by polymineralic veins. They are accompanied by 0.9–2 mm wide diffusion zones where peridotite olivine became less forsteritic through diffusive exchange with the host magma. Old fractures represent most of each xenolith's surface. (2) Young fractures show no selvages and only narrow diffusion zones of <0.02 mm width. Calculations based on a model of Fe-Mg interdiffusion give an age of 6 to 83 years and <4 days for old and young fractures, respectively. A combination of these data with fluid inclusion barometry indicates that selvages and veins formed during xenolith transport rather than representing wall-rock reactions or mantle metasomatism. The results provide ample evidence for prolonged storage of the xenoliths in the crust, constraining a multi-stage magma ascent: Years to decades prior to eruption, ascending magma ruptured peridotitic wall-rock possibly through hydraulic fracturing and stoping around magma reservoirs. Magma batches transported the peridotite xenoliths to the crust at ascent rates exceeding 0.2 ms−1. The xenoliths and their host magma stagnated during at least 6 years in possibly sill-like reservoirs at 7–11 km depth. The xenoliths became deposited and subsequently embedded in a mush of settled phenocrysts, while selvages and veins crystallized until the eruption commenced. At the end of the eruption, the xenoliths were finally transported to the surface within hours to days. Decompression during the rapid ascent induced internal stresses and caused renewed fragmentation of the xenoliths, producing the young fractures. Received: 25 August 1997 / Accepted: 25 November 1997  相似文献   

17.
This study documents the chemical and textural responses of zircon in the Elba igneous complex, with particular reference to the 7- to 7.8-Ma-old Monte Capanne pluton in relation to its coeval volcanic counterpart (Capraia), using BSE imaging and quantitative electron microprobe analyses. The Monte Capanne pluton displays multiple field and geochemical evidence for magma mixing. The samples we have investigated (including monzogranitic, mafic enclave and dyke samples) display similar zircon textures and are associated with an extremely large range of trace and minor element (Hf, Y, HREE, Th, U) compositions, which contrast with relatively simple textures and zoning patterns in zircons from a Capraia dacite. We have used a relatively simple textural classification (patchy zoning, homogenous cores, oscillatory zoning and unzoned zircon) as the basis for discussing the chemical composition and chemical variation within zircons from the Monte Capanne pluton. Based on these data and other works (Dini et al. 2004 in Lithos 78:101–118, 2004) , it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the Monte Capanne magma chamber. In particular, mixing was responsible for the development of the patchy-zoning texture in the zircon cores, which was associated with reactions between other accessory phases (including monazite, apatite, allanite), which we infer to have significantly affected the Th distribution in zircon. Zircons from the MC pluton displaying “homogeneous cores” have chemical affinities with zircons in the coeval Capraia volcanic system, consistent with the participation of a Capraia-like mantle end-member during mixing. Further zircon growth in the MC pluton produced the oscillatory zoning texture, which records both long-term (crystal fractionation) and transient (recharge with both silicic and mafic magmas) events in a hybrid magma chamber. It is inferred that Hf and the Th/U ratio cannot be used alone to infer magmatic processes due to their dependency on temperature, nor are they a diagnostic feature of xenocrystic grains. This study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system.  相似文献   

18.
Summary Fe-Ti-P-rich rocks (FTP) are unusual with respect to their mineralogy and bulk composition. Varieties of these rocks are mostly related to Proterozoic massif-type anorthosites and to a lesser extent to the upper parts of mafic-ultramafic intracratonic layered complexes and other igneous rock suites. We present results on the geology, mineralogy and geochemistry of a new occurrence of FTP, associated with mafic rocks in the northwestern part of Iran. The Qareaghaj mafic-ultramafic intrusion (QMUI) is a small igneous body situated between Palaeozoic sedimentary rocks and a Precambrian low grade metamorphic complex. The QMUI is composed mainly of non-mineralized mafic and apatite- and Fe-Ti oxide-rich ultramafic rocks. The mafic rocks, mainly coarse-grained gabbro, microgabbro and amphibolite, have a simple mineral assemblage (plagioclase + clinopyroxene + ilmenite) and based on field observations, mineralogy and chemical composition are comagmatic. The ultramafic rocks with high proportion of olivine (∼40–66 vol.%), apatite (∼0.1–16 vol.%), ilmenite (∼11–19 vol.%) and magnetite (∼2–13 vol.%), have unusual bulk compositions (e.g., SiO2 ∼ 21–30 wt.%, total iron expressed as Fe2O3 tot ∼ 26–42 wt.%, TiO2 ∼ 5–11 wt.%, MgO ∼ 9–20 wt.%, P2O5 up to 5.1 wt.%, Cr ∼ 40–160 ppm, Ni ∼ 7–73 ppm). The FTP forms numerous sill-like layers, ranging in thickness from ∼5 cm to few meters. These rocks, totally enclosed in mafic rocks with sharp and concordant contacts, show a magmatic lamination and follow the general NW–SE trend of QMUI. The apatite-rich ultramafic rocks makes up 90–95% of the total ultramafic outcrops and contain Mg-poor olivine (Mg# ∼ 40–58) and low-Mg spinel (Mg# ∼ 30–44) in contrast to apatite-poor ones (∼60–63 and ∼43–46, respectively). Field relationships, mineral compositions and geochemical data suggested that the FTP are not related to the mafic host rocks. On the contrary, they intruded latter into the gabbros during plastic, high temperature deformation in local shear zones. Fractional crystallization of P-rich ferrobasaltic parental magma at depth, probably in an open magmatic system, not far from the QMUI magma chamber, is considered as responsible for the formation of the evolved FTP in QMUI.  相似文献   

19.
王超  刘良  张安达  杨文强  曹玉亭 《岩石学报》2008,24(12):2809-2819
阿尔金造山带南缘玉苏普阿勒克塔格岩体中的似斑状中粗粒黑云钾长花岗岩发育有岩浆成因的暗色包体,并且该花岗岩被花岗细晶岩呈脉状侵入。该岩体含有丰富的岩浆混合作用特征: 如暗色包体中的碱性长石斑晶、针状磷灰石、长石的环斑结构、石英/斜长石主晶和榍石眼斑等。暗色包体、寄主花岗岩和花岗细晶岩代表了岩浆混合演化过程中不同端元比例混合的产物。地球化学特征上,钾长花岗岩和暗色包体的主要氧化物含量在Harker图解中多呈线性变化。暗色包体主要为闪长质,MgO、K2O含量高,为钾玄岩系列,总体上高场强元素不亏损,显示了岩浆混合中的基性端元信息,可能为幔源熔体结晶分异或壳幔物质的混合产物。寄主花岗岩均为准铝质,富碱,为高钾钙碱性系列,亏损Nb、Ta、Sr、P、Ti等高场强元素,高K2O/Na2O,富集高不相容元素,Ga含量高,显示了A型花岗岩的特征,Th/U 和Nb/Ta比值分别介于为6.67~10.96、8.99~11.94,代表了下地壳源区。花岗细晶岩均为钠质、过铝质,TiO2、MgO含量低, Na2O和CaO含量高,具有混合岩浆侵位后分异的特征。岩相学和地球化学特征说明岩浆混合作用对于环斑结构花岗岩的形成起到重要作用。花岗细晶岩中环斑长石的斜长石外环与钾长石内核的厚度比大于钾长花岗岩中的环斑长石,指示混合岩浆在一定的减压条件下更有利于环斑结构的形成。玉苏普阿勒克塔格岩体中的钾玄质暗色包体、高钾钙碱性花岗岩和中钾钙碱性花岗细晶岩代表了岩浆演化不同阶段的产物,反映了一个幔源岩浆和下地壳不断相互作用,引起地壳连续伸展减薄的过程,指示阿尔金南缘在早古生代末期存在造山后伸展背景下的幔源岩浆底侵作用。同一岩体中两种不同时代岩性的环斑结构显示了该岩体形成历史中的一定时空演化关系,代表了伸展过程中不同阶段的产物。  相似文献   

20.
Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg# clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing. The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano. The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites, such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central to quantifying the subcrustal mass fluxes beneath Popo. Received: 13 December 1999 / Accepted: 11 August 2000  相似文献   

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